1 PengKuan Em | How to test length contraction by experiment? | Monday 17 June 2019 |
2 Sylvia Else | Re :How to test length contraction by experiment? | Thursday 20 June 2019 |
3 Tom Roberts | Re :How to test length contraction by experiment? | Wednesday 26 June 2019 |
4 Eric Flesch | Re :How to test length contraction by experiment? | Sunday 30 June 2019 |
5 kunz | Re :How to test length contraction by experiment? | Sunday 30 June 2019 |
6 Tom Roberts | Re :How to test length contraction by experiment? | Monday 1 July 2019 |
7 Eric Flesch | Re :How to test length contraction by experiment? | Tuesday 2 July 2019 |
8 Tom Roberts | Re :How to test length contraction by experiment? | Thursday 4 July 2019 |
9 Nicolaas Vroom | Re :How to test length contraction by experiment? | Saturday 6 July 2019 |
10 PengKuan Em | Re :How to test length contraction by experiment? | Sunday 7 July 2019 |
11 Tom Roberts | Re :How to test length contraction by experiment? | Tuesday 9 July 2019 |
12 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 9 July 2019 |
13 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 9 July 2019 |
14 Nicolaas Vroom | Re :How to test length contraction by experiment? | Saturday 13 July 2019 |
15 Tom Roberts | Re :How to test length contraction by experiment? | Sunday 14 July 2019 |
16 Nicolaas Vroom | Re :How to test length contraction by experiment? | Monday 15 July 2019 |
17 Eric Flesch | Re :How to test length contraction by experiment? | Monday 15 July 2019 |
18 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 16 July 2019 |
19 Tom Roberts | Re :How to test length contraction by experiment? | Tuesday 16 July 2019 |
20 Phillip Helbig | Re :How to test length contraction by experiment? | Wednesday 17 July 2019 |
21 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 23 July 2019 |
22 Tom Roberts | Re :How to test length contraction by experiment? | Thursday 25 July 2019 |
23 Phillip Helbig | Re :How to test length contraction by experiment? | Friday 26 July 2019 |
24 Eric Flesch | Re :How to test length contraction by experiment? | Saturday 27 July 2019 |
25 Nicolaas Vroom | Re :How to test length contraction by experiment? | Sunday 28 July 2019 |
26 Phillip Helbig | Re :How to test length contraction by experiment? | Sunday 28 July 2019 |
27 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 30 July 2019 |
28 Sylvia Else | Re :How to test length contraction by experiment? | Tuesday 30 July 2019 |
29 Phillip Helbig | Re :How to test length contraction by experiment? | Tuesday 30 July 2019 |
30 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 30 July 2019 |
31 Nicolaas Vroom | Re :How to test length contraction by experiment? | Thursday 1 August 2019 |
32 Phillip Helbig | Re :How to test length contraction by experiment? | Thursday 1 August 2019 |
33 Tom Roberts | Re :How to test length contraction by experiment? | Thursday 1 August 2019 |
34 Tom Roberts | Re :How to test length contraction by experiment? | Saturday 3 August 2019 |
35 Tom Roberts | Re :How to test length contraction by experiment? | Sunday 4 August 2019 |
36 Ken Seto | Re :How to test length contraction by experiment? | Sunday 4 August 2019 |
37 Tom Roberts | Re :How to test length contraction by experiment? | Sunday 4 August 2019 |
38 Nicolaas Vroom | Re :How to test length contraction by experiment? | Monday 5 August 2019 |
39 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 6 August 2019 |
40 Tom Roberts | Re :How to test length contraction by experiment? | Tuesday 6 August 2019 |
41 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 6 August 2019 |
42 Nicolaas Vroom | Re :How to test length contraction by experiment? | Thursday 8 August 2019 |
43 Nicolaas Vroom | Re :How to test length contraction by experiment? | Monday 12 August 2019 |
44 Nicolaas Vroom | Re :How to test length contraction by experiment? | Tuesday 13 August 2019 |
How to test length contraction by experiment?
51 posts by 12 authors
https://groups.google.com/forum/?fromgroups=#!topic/sci.physics.research/JesOwTVZ-t4
Ideal direct experimental proof should contain the following steps:
1. Measure the tested object's length at rest, the value l0.
2. Put this object in motion.
3. Measure the object's speed, the value v.
4. Measure the object's length in motion, the value l.
5. Check if these 3 values verify length contraction law.
For doing this experiment, the difference of length l0 dl=80-l should be in measurable range. If the object is a chunk of matter, l0 dl=80-l is not measurable. For example, matter objects with the highest speed we can make are satellites, whose speed is generally 7.8 km/s. If a satellite is made of a string of 100 km long, the value of l0 dl=80-l would be 0.03 mm, which is absolutely not measurable from the ground. This is why contraction of length has never been measured.
Below I propose two experiments inspired from Bell's spaceship paradox and Ehrenfest paradox.
Please read the article at PDF: How to test length contraction by experiment? https://pengkuanonphysics.blogspot.com/2019/06/how-to-test-length-contraction-by.html or Word: https://www.academia.edu/39584663/How_to_test_length_contraction_by_experiment
> | Le mercredi 19 juin 2019 08:39:04 UTC+2, mfreem...@gmail.com a =C3=A9crit=C2=A0: |
>> | What does it even mean to measure the length of a moving body? Other than noting the time and position of the ends, which involves time dilation, how do you do it? |
> |
In books like < In the frame of the object, the length is at rest and measured. In the frame where it is moving, its both ends are noted at the same time. |
But now you are no longer just testing length contraction. Instead, you are looking at whether the entire experimental situation gives results that are consistent with relativity.
Sylvia.
> | Relativistic length contraction is theoretically predicted but not directly tested, which lead to incorrect interpretation of the theory illustrated by Bell's spaceship paradox and Ehrenfest paradox. |
I have no idea why you think that lack of a specific experimental test leads to "incorrect interpretations". Indeed, anyone who understands SR can correctly interpret the theory as applied to both of these paradoxes.
Note that "paradox" does not mean "contradiction". Here it means "a seemingly absurd or self-contradictory statement or proposition that when investigated or explained proves to be well founded or true".
> | But these paradoxes can help us designing experiments to test length contraction. |
Not here. Both of your attempts fail. See below.
> |
Ideal direct experimental proof should contain the following steps: 1. Measure the tested object's length at rest, the value l0. 2. Put this object in motion. 3. Measure the object's speed, the value v. 4. Measure the object's length in motion, the value l. 5. Check if these 3 values verify length contraction law. |
OK. Unfortunately, there is no hope of doing this with sufficient measurement resolution to distinguish between a) no contraction, and b) the contraction predicted by SR.
> | https://pengkuanonphysics.blogspot.com/2019/06/how-to-test-length-contraction-by.html |
In 2a you claim "There is no consensus over [Bell's spaceship paradox]". This is wrong, and today everybody who understands SR knows that the thread between the spaceships will break.
Your experiment in 2b fails, because the two electron guns are at rest in the lab, a distance L_rest apart, so the detectors will measure the electrons to be a distance L_rest apart, because the detectors are also at rest in the lab.
Your experiment in 3b also fails for essentially the same reason -- you set up the electrons a distance L apart in the lab, so the detectors will measure them to be a distance L apart, because the detectors are also at rest in the lab.
There is no escaping this: to directly test "length contraction" you must make distance measurements in two relatively-moving inertial frames. That is VERY difficult, and we simply do not have the technology to do it with sufficient resolution.
While there have been no DIRECT tests of "length contraction", there are indirect tests:
A. Magnetic fields
A current-carrying wire is observed to be electrically neutral in its
rest frame, and a nearby charged particle at rest in that frame is
unaffected by the current. A nearby charged particle that is moving
parallel to the wire, however, is subject to a magnetic force that is
related to its speed relative to the wire. If one considers the
situation in the rest frame of a charge moving with the drift velocity
of the electrons in the wire, the force is purely electrostatic due to
the different length contractions of the positive and negative charges
in the wire (the former are fixed relative to the wire, while the latter
are mobile with drift velocities of a few mm per second). This approach
gives the correct quantitative value of the magnetic force in the wire
frame. This is discussed in more detail in: Purcel, _Electricity_and_
_Magnetism_. It is rather remarkable that relativistic effects for such
a tiny velocity explain the enormous magnetic forces we observe.
B. Free-electron lasers
The wavelength of a free electron laser's light is proportional to
1/gamma^2, with gamma=1/sqrt(1-v^2/c^2) and v the speed of the electron
beam relative to the lab. One power of gamma comes from the Doppler
shift between the beam and lab frames, while the second comes from the
"length contraction" of the undulator (at rest in the lab) as seen in
the beam frame. This is verified experimentally to high precision.
C. Atmospheric muons
Muons created in the upper atmosphere are observed at the surface of the
earth. In the earth frame, this is due to "time dilation" of their
lifetime. In the muon frame it is due to "length contraction" of the
distance between their point of creation and the surface.
Note that there are many, many tests of SR, and NONE of them have differed significantly from the predictions of SR. The fact that not every prediction of SR has been tested does not in any way imply that SR is not valid.
Tom Roberts
> | Relativistic length contraction is theoretically predicted but not directly tested, which lead to incorrect interpretation of the theory illustrated by Bell's spaceship paradox and Ehrenfest paradox. But these paradoxes can help us designing experiments to test length contraction. |
I expect that there are uncertainties involved which will prevent such a measurement. A quick review: special relativity shows that arbitrarily large speeds are achievable within the constraint of the universal boundary condition "c" in that space travellers can cross the galaxy in a day as seen by themselves, whereas we observers see the crossing as taking 10^5 years. Because of the rule that what is seen to happen in one inertial frame is seen likewise in all other inertial frames, the spaceship is mapped (by we observers) into a foreshortened object which indeed travels faster-than-c in its own foreshortened frame except that the accompanying time dilation technically lowers that to within-c as seen by us.
However, can the foreshortening (i.e., length contraction) actually be observed, is the OP's question. Well, the length contraction is a calculational necessity which may however be enveloped by necessary uncertainties. Consider the Bohr-Einstein debates on quantum theory -- Bohr beat Einstein's argued paradoxes by showing that they were enveloped and thus nullified by the involved uncertainties. Now think about how hard it is to measure the relativistic length contraction -- I expect that physically required uncertainties will come into play in any such attempt. As a matter of fact, it's a sound speculation that the positional uncertainty will be found to be exactly half of the rest length of the object measured -- so that length contraction will forever be unmeasurable. That means that the required length contraction doesn't need to be reified into actual physical contraction. This is just typical of quantum uncertainties.
> |
What is sure in my paper is that no direct measurement of length
contraction has been done. This is what I'm trying to propose. Of cause
most people think like you and believe that Special Relativity is
flawless, because it has passed all tests it faced to until now.
However, science needs direct proof.
I will give more elements later. |
I'd say the electromagnetic effect mentioned by Tom Roberts on 26 June is a direct measurent. We see the charge density of the wire increased (charges contracted) by measuring the coulomb force. There is no more direct relation than that between charge density and electric force.
This is much more direct than any attempt to apply rulers to moving objects, clock synch. etc.
> | What is sure in my paper is that no direct measurement of length contraction has been done. |
Yes. There are, of course, good reasons for this -- with current technology it simply is not possible to test "length contraction" directly (in the sense of measuring a length in two frames and comparing to the SR prediction).
> | This is what I'm trying to propose. |
Sure. But you must not only have a physical situation that demonstrates "length contraction", you must also have instrumentation that can distinguish between no contraction and the SR prediction, hopefully by more than five sigma, but certainly by at least three sigma. As I said before, at present no such instrumentation exists, for any implementable physical situation.
Tom Roberts
> | I expect that physically required uncertainties will come into play in any such attempt. As a matter of fact, it's a sound speculation that the positional uncertainty will be found to be exactly half of the rest length of the object measured -- so that length contraction will forever be unmeasurable. |
I must add the obvious, that objects travelling near c are seen to rotate so that we view not their sides, but the front end when approaching and the rear end when receding. You don't get to see or measure the length contraction -- nature does her elegant sleight-of-hand once again to confound us baryonoids. "Length contraction" works great for our models and experiments, but you'll never measure it -- it stays hidden behind the uncertainty curtain. .
[[Mod. note -- The author is (correctly) alluding to the Lampa-Terrell-Penrose effect: https://en.wikipedia.org/wiki/Terrell_rotation -- jt]]
> |
On 17 Jun 2019, PengKuan Em |
>> | Relativistic length contraction is theoretically predicted but not directly tested, [...] |
> |
I expect that there are uncertainties involved which will prevent such a measurement. |
There are, but not as you describe. The "uncertainties" are due to classical measurement resolutions, not quantum effects as you claim.
> | A quick review: special relativity shows that arbitrarily large speeds are achievable within the constraint of the universal boundary condition "c" in that space travellers can cross the galaxy in a day as seen by themselves, whereas we observers see the crossing as taking 10^5 years. Because of the rule that what is seen to happen in one inertial frame is seen likewise in all other inertial frames, the spaceship is mapped (by we observers) into a foreshortened object which indeed travels faster-than-c in its own foreshortened frame except that the accompanying time dilation technically lowers that to within-c as seen by us. |
This last sentence is frame hopping, and thus incorrect. We (on earth) only observe in our own frame, not using any sort of "foreshortened frame" of the space travelers. Relative to our frame, those travelers always travel with speed less than c -- neither "time dilation" nor "length contraction" is involved, we simply use instruments at rest in our own frame.
> | However, can the foreshortening (i.e., length contraction) actually be observed, is the OP's question. Well, the length contraction is a calculational necessity which may however be enveloped by necessary uncertainties. Consider the Bohr-Einstein debates on quantum theory -- Bohr beat Einstein's argued paradoxes by showing that they were enveloped and thus nullified by the involved uncertainties. |
Those are quantum uncertainties, which are irrelevant for considerations of special relativity using macroscopic objects (such as space travelers).
> | Now think about how hard it is to measure the relativistic length contraction -- |
Yes, it is so hard that nobody has succeeded in measuring it directly. But it is difficult for classical reasons (not quantum effects).
> | I expect that physically required uncertainties will come into play in any such attempt. As a matter of fact, it's a sound speculation that the positional uncertainty will be found to be exactly half of the rest length of the object measured -- so that length contraction will forever be unmeasurable. |
Such a speculation is not "sound" at all. For macroscopic objects there is no reason to expect that quantum uncertainties will be important (much less "half").
There are no quantum effects preventing us from measuring "length contraction", just technological limitations.
Indeed, as pointed out elsewhere in this thread, measuring a magnetic field is in essence measuring the "length contraction" of conduction electrons moving at a few cm/sec. No quantum effects prevent such a measurement or interpretation.
> | That means that the required length contraction doesn't need to be reified into actual physical contraction. This is just typical of quantum uncertainties. |
Typical for QUANTUM uncertainties, but not here. SR is a purely classical theory.
In a different post you said:
> | I must add the obvious, that objects travelling near c are seen to rotate so that we view not their sides, but the front end when approaching and the rear end when receding. |
That is true when using eyeballs (or equivalent) to look at the object via light rays. But it does not apply to the usual methods of measuring in SR using an array of assistants: they make measurements as an object passes nearby, and then communicate the measurements to a common observer who correlates them and makes conclusions. That method is used in gedankens, of course, to avoid the complexities of light propagation affecting the observations.
> | You don't get to see or measure the length contraction -- nature does her elegant sleight-of-hand once again to confound us baryonoids. |
You cannot SEE "length contraction" (i.e. with your eyeballs or equivalent), but with appropriate assistants or apparatus you can MEASURE it.
> | "Length contraction" works great for our models and experiments, but you'll never measure it -- it stays hidden behind the uncertainty curtain. |
Not so. There's no such "curtain", its just that we don't have any technology capable of measuring "length contraction".
Tom Roberts
> | Relativistic length contraction is theoretically predicted but not directly tested, which lead to incorrect interpretation of the theory illustrated by Bell's spaceship paradox and Ehrenfest paradox. But these paradoxes can help us designing experiments to test length contraction. |
If you want to understand length contraction you should not start by studying the Bell's spaceship paradox and the paradox. The first step is to agree that length contraction is something physical. That means when there is an effect we call length contraction and it is something physical we should be able to demonstrate this by means of an experiment. That does mean when the experiment fails that there is not something we call length contraction. It means that we need to modify this experiment.
The experiment I have in mind is the following take a very long locomotive. You can also take a train with wagons. In that case the wagons should be connected in such away that the whole train becomes a rigid object. Put the train on a very long straight track and put two markers near the track, one in front and one in the back such that the distance between the two markers is just slightly smaller that the distance of the train. The observer (video camera) should be situated a large distance from the center of the train. When that is the case the observer should see only one marker. When you move the train slightly the observer should see the other marker but never both markers. From the observers point of view the markers are behind the train or track. Now you bring the train completely at the far left end of the track. The experiment starts when the train starts to move towards the right at a high speed with the video camera on. The central question to answer is: when the train is in between the two markers will you observe both markers? Yes or no. When the answer is no, there is no length contraction involved. When the answer is yes, there is length contraction involved. In this experiment there are no clocks involved, which removes the issue of clock synchronisation. My guess is that the answer is no.
You can only perform the experiment in a slightly different ways. Construct two identical trains and put both side by side on a long track. The observer is a far distance away from the center of the trains. From that position the observer (video camera) can only see one train. At the beginning of the experiment you bring the train closest to you to the far left and the experiment starts when this train moves towards the right at high speed. The question is now when the moving train is in between the train at rest and the observer will the observer see (part of) the train at rest? When the answer is yes, there is length contraction involved.
Nicolaas Vroom
[Moderator's note: It is important to distinguish between purely relative effects (A sees B's clock run slow and vice versa), real effects (B's clock is behind A's when the two are compared at rest after B travels away and returns, this depending only on the length and speed of the journey and not on the acceleration and is explicable within the context of Special Relativity), and effects, such as the appearance of moving objects, which depend on the finite velocity of light -P.H.]
and I think yours is not performable because, as I have computed in my paper, the length of a satellite made of a 100 km thread will shrink 0.03 mm.
The only way to test length contraction is to use fast moving particles, which is described in my paper: https://pengkuanonphysics.blogspot.com/2019/06/how-to-test-length-contraction-by.html or https://www.academia.edu/39584663/How_to_test_length_contraction_by_experiment
The 2 electrons will be accelerated to relativistic speed and the distance between them will be measured. This distance should vary with speed, as the basketball in the video here: https://youtu.be/-Poz_95_0RA?t=530. If the outcome is true, then there is length contraction. Otherwise, there will be weird consequence.
> | [...] The first step is to agree that length contraction is something physical. |
Be careful, as the phrase "something physical" means different things to different people. Your sense (demonstrable via experiment) is OK, but many/most people would consider it to mean "a physical change in the object's length", which is wrong.
> | The experiment I have in mind is the following take a very long locomotive. You can also take a train with wagons. In that case the wagons should be connected in such away that the whole train becomes a rigid object. Put the train on a very long straight track and put two markers near the track, one in front and one in the back such that the distance between the two markers is just slightly smaller that the distance of the train. The observer (video camera) should be situated a large distance from the center of the train. When that is the case the observer should see only one marker. When you move the train slightly the observer should see the other marker but never both markers. From the observers point of view the markers are behind the train or track. Now you bring the train completely at the far left end of the track. The experiment starts when the train starts to move towards the right at a high speed with the video camera on. The central question to answer is: when the train is in between the two markers will you observe both markers? Yes or no. When the answer is no, there is no length contraction involved. When the answer is yes, there is length contraction involved. In this experiment there are no clocks involved, which removes the issue of clock synchronisation. |
While there are no clocks, you are assuming that the observations of the two ends are simultaneous in the frame of the rails and video camera. While that might seem obvious, is _IS_ an assumption you are making, and is an essential part of the measurement. Moreover, you are ignoring optical diffraction....
Put in some numbers: assume the train is 100 meters long, moving at the outrageous speed of mach 2 (686 m/s). The predicted "length contraction" is 3E-10 meters. The ends of the train cannot be made flat to that accuracy, and the experiment must fail (even ignoring diffraction).
Imagine a 1 meter projectile fired from a rail gun at the outrageous speed of mach 20 (6860 m/s). The predicted "length contraction" is also 3E-10 meters, with the same result.
Tom Roberts
> | Le samedi 6 juillet 2019 00:48:33 UTC+2, Nicolaas Vroom a écrit : |
> > | You can only perform the experiment in a slightly different ways. Construct two identical trains and put both side by side on a long track. The observer is a far distance away from the center of the trains. From that position the observer (video camera) can only see one train. At the beginning of the experiment you bring the train closest to you to the far left and the experiment starts when this train moves towards the right at high speed. The question is now when the moving train is in between the train at rest and the observer will the observer see (part of) the train at rest? When the answer is yes, there is length contraction involved. |
> |
If I have well understood, your experiment is equivalent to the Ladder paradox, which is explained here: https://en.wikipedia.org/wiki/Ladder_paradox |
My experiment is simpler as the Ladder paradox. In the Ladder paradox assumes IMO that there is length contraction involved and that this leads to a paradox. The Ladder paradox involves relativity of simultaneity which makes it rather complex. The experiment I have in mind should be as simple as possible (in theory). The purpose is only to answer the question: is there (physical) length contraction involved: yes or no. (Not how much) The most important tool you need is a very fast video camera to observe what happens.
Nicolaas Vroom.
> |
You can only perform the experiment in a slightly different ways.
Construct two identical trains and put both side by side on a long
track. The observer is a far distance away from the center of the
trains. From that position the observer (video camera) can only see one
train. At the beginning of the experiment you bring the train closest to
you to the far left and the experiment starts when this train moves
towards the right at high speed. The question is now when the moving
train is in between the train at rest and the observer will the observer
see (part of) the train at rest? When the answer is yes, there is length
contraction involved.
Nicolaas Vroom [Moderator's note: It is important to distinguish between purely relative effects (1) A sees B's clock run slow and vice versa), real effects (2) B's clock is behind A's when the two are compared at rest after B travels away and returns, this depending only on the length and speed of the journey and not on the acceleration and is explicable within the context of Special Relativity), and (3) effects, such as the appearance of moving objects, which depend on the finite velocity of light -P.H.] |
(1) A can only see B's clock run slow and vice versa, if this a
symmetrical experiment and the distance between both increases.
(2) B's clock is behind A's clock etc is a non-symmetrical experiment.
The explanation why the moving clock, using light signals, runs behind,
depends on the internal operation of the clock.
2a. That means when the light signals are perpendicular to direction of
motion the mathematics is the same as the Lorentz transformations. In
this case the mirrors are parallel to the direction of motion.
2b. When in lightsignals are in the same direction as the motion of the
clock the mathematics involved is slightly different. The mirrors are
perpendicular.
There are two different cases when (3) effects of moving objects are involved. 3a) When a train moves away from an observer the length seems shorter than the length of train really is. This is because the position observed from the front of the train shows the position of an earlier moment than from the back of the train. At that moment the front of the train was closer to the observer, as such the train appears to look shorter. All of this depends on the speed of light. 3b) When a train moves towards an observer the length seems longer than the length of train really is. This is because the position observed from the back of the train shows the position of an earlier moment than from the front of the train. At that moment the back of the train was further away from the observer, as such the train appears to look longer.
Nicolaas Vroom.
> | On 7/5/19 5:48 PM, Nicolaas Vroom wrote: |
> > | [...] The first step is to agree that length contraction is something physical. |
> |
Be careful, as the phrase "something physical" means different things to different people. Your sense (demonstrable via experiment) is OK, but many/most people would consider it to mean "a physical change in the object's length", which is wrong. |
Let us perform an even simpler experiment. (in stead of rod you can also read train) Take one reference rod of for example 100m. Position the observer a certain distance away from the centre of the rod, perpendicular on the direction of the rod. To make this simpler you can also place two lights at both end of the reference rod. Take a second rod of 50m. Place this rod between the observer and the reference rod just as close as possible against the reference rod. What that means the observer can see both ends of the reference rod. Now you perform the experiment the same as before: You place the second rod completely towards the left and you move the rod with a constant speed completely towards the right. The question is: when second rod is straight in front of the observer, will the observer see both ends of the reference rod? IMO the most obvious answer should be: yes. That means (and that is important) in this way you have an experiment which demonstrates that the length of the second rod or moving rod is physical different as the reference rod. The next step is to make the second rod longer (but still smaller as the reference rod) and or increase the speed of the second and try to find out what the limits are that the observer can still observe both ends of the reference rod. The important point is that you have an experiment at hand to test under which physical conditions, the physical length of a moving rod versus a rod at rest are different.
The final step is to perform this experiment with two rods, with the same length at rest. We know from the first experiment it is principle possible to test if rods are of different length. The question here is if it is in principle possible that the outcome of this experiment shows that the two rods have a different length. I have my doubts. The problem is not so much the practical limitations to perform this experiment, but the physical explanation.
> | While there are no clocks, you are assuming that the observations of the two ends are simultaneous in the frame of the rails and video camera. While that might seem obvious, is an assumption you are making, and is an essential part of the measurement. |
You are correct. But there are two issues involved: The physical issue if the moving rod (or train) becomes shorter and the second issue how to demonstrate that this is the case (or not). This second part requires observations i.e. involve the speed of light, but light is almost of no importance related to the physical issue. The same is also the case if you want to understand celestial mechanics (or the behaviour of all free moving objects, like bullets). The first step is observations, which involve the speed of light and the second step is to investigate the physical behaviour in space, which does not involve the speed of light, but the speed of gravitation. The fact that the two are quantitative the same is of less importance than the physical differences between the two.
> | Moreover, you are ignoring optical diffraction.... |
Nicolaas Vroom
> | [...] The final step is to perform this experiment with two rods, with the same length at rest. |
This experiment will fail, for the same reason as before: you cannot make the ends of the rods flat to the accuracy required to observe the "length contraction" of the moving rod.
> | The physical issue if the moving rod (or train) becomes shorter |
But SR predicts that the moving rod (or train) does NOT "become shorter". It also predicts the moving rod WILL BE OBSERVED to "look" shorter. While this does not negate your proposed experiment, your repeated use of this incorrect phrase does indicate a fundamental misunderstanding on your part.
Tom Roberts
[[Mod. note -- Note that in the final paragraph above, the author is using "will be observed" in the technical sense of special relativity, i.e., this refers to observations made by a CO-LOCATED observer so that there are no speed-of-light propagation delays.]] -- jt]]
> | On 7/12/19 5:11 PM, Nicolaas Vroom wrote: |
> > | [...] The final step is to perform this experiment with two rods, with the same length at rest. |
> |
This experiment will fail, for the same reason as before: you cannot make the ends of the rods flat to the accuracy required to observe the "length contraction" of the moving rod. |
That can be a practical reason when there actual is physical length contraction involved. But the experiment can also fail because there is no physical LC.
> > | The physical issue if the moving rod (or train) becomes shorter |
> |
But SR predicts that the moving rod (or train) does NOT "become shorter". |
> | It also predicts the moving rod WILL BE OBSERVED to "look" shorter. |
IMO if we agree both about the new proposals there is a problem, because the two predictions are in conflict which each other. IMO what counts are the results of an actual experiment.
[[Mod. note -- The reason for the apparent conflict between the two statements is that they are answering different questions. To elucidate this, it's important to be very precise about just what question we are considering. That is, a phrase such as "the measured physical length of a moving rod" is not sufficiently precise: it's necessary to specify precisely who (what observer) is making that measurement, and how that measurement is made.
Suppose that you are an inertial observer, i.e., that you observe that Newton's 1st law holds for motion measured relative to you. And for simplicity, let's suppose that the rod is also in inertial motion, i.e., an observer riding on the rod also observes that Newton's 1st law holds for motion measured relative to her. And (again for simplicity) let's say the rod is oriented horizontally in your reference frame, and is moving lengthwise from your left to your right.
Then the simplest case is one where the observer-on-the-rod measures the length of the rod. In this case, measuring the rod's length is easy (for her): she measures the position of the left end of the rod
If she does this, then (according to special relativity) she will find the rod's length to be unchanged from its "basic" length (its length before it was in motion with respect to you). (We're assuming that the accelerations involved in setting the rod into motion were sufficiently gentle to not permanently distort it.) This means, for example, that if the "rod" is actually made up of a number of eggs placed end-to-end, that she will NOT observe these eggs to have been crushed.
This framework -- defining "the length of the rod" as that measured by an observer riding along with the rod (this is sometimes called a "proper observer"), is the basis for the statement you quoted above
> | But SR predicts that the moving rod (or train) does NOT "become shorter". |
Now let's consider what *you* might observe. That is, you see the rod moving from left to right, and as it's moving you (somehow) measure it's length. What will you measure?
As I noted above, answering this question requires being very precise about just how the measurement is made. (It turns out that this case requires rather more precision than was the case for the proper-observer measurement.)
One obvious way for you to measure (define!) the length of the rod is for you to measure the position of the left end of the rod at some time t, and for you to measure the position of the right end of the rod at the same time t, and then for you to subtract the two positions.
The position of an end of the rod at a time t is then measured by that (unique) sub-observer who is coincident with that end of rod at her clock's time t. Since that sub-observer is coincident with her end of the rod, there are no light-travel-time delays from the moving rod to the sub-observer.
[It's precisely this avoidance of light-travel-time effects that's the motivation for introducing the infinite set of sub-observers.]
After making their measurements, the sub-observers then send their observations back to some central collation point for analysis.]
In relativity, "time" is observer-dependent, so we need to be specific about whose (which reference frame's) "time t" we're using. The simplest choice is to say that since *you* are making this measurement (in *your* inertial reference frame), then you should measure the left and right rod ends' positions at times which are simultaneous in *your* inertial reference frame, i.e., your sub-observers should each measure the rod ends' positions at the same clock reading (t).
That is, to recap,
* the subobserver who is coincident with the left end of the rod at her
clock reading t sends a message back to the central collation point
to that effect (the message includes the subobserver's position, say
x = x_left, in your inertial reference frame)
* the subobserver who is coincident with the right end of the rod at her
clock reading t sends a message back to the central collation point
to that effect (the message includes the subobserver's position, say
x = x_right, in your inertial reference frame)
* the central collation point computes L_you := x_right - x_left
This (L_you) is a reasonable operational definition of "the length of the rod as measured by you".
According to special relativity, if you do this, you will find that this length will show Lorentz contraction. Aswering *this* question is the basis of the other statement you quoted above,
> | It also predicts the moving rod WILL BE OBSERVED to "look" shorter. |
It should now be clear(er) that the two apparently-contradictory statements you quoted above, are in fact the answers to two *different* questions... so it's no longer surprising or contradictory that their answers are different. -- jt]]
> | [[Mod. note -- Note that in the final paragraph above, the author is using "will be observed" in the technical sense of special relativity, i.e., this refers to observations made by a CO-LOCATED observer so that there are no speed-of-light propagation delays.]] -- jt]] |
IMO physical length contraction becomes a paramount problem, considering an experiment with a set of identical rods flying or moving freely through the entire universe all with different speeds. Is there any length contraction involved? This problem involves to calculate the two positions (front and back) of each rod at regular intervals. This is specific tricky when the distances between the front and the end to a central observation point are different. (This situation is easier when all the rods move in a storage ring in which all the rods have the same speed, which an adjustable speed for all rods)
The same problem exists when you want to observe the positions of the planets around the Sun. You need in some way a set of accurate clocks which all run synchronuous to monitor the positions, such that speed of light propagation delays can be eliminated.
IMO the most important question to answer is: what is the physical explanation of length contraction. The explanation can not be in the way the rods are observed, assuming light signals are involved.
Nicolaas Vroom
> | On 6/30/19 2:53 AM, Eric Flesch wrote: |
>> | That means that the required length contraction doesn't need to be reified into actual physical contraction. This is just typical of quantum uncertainties. |
> |
Typical for QUANTUM uncertainties, but not here. SR is a purely classical theory. |
Sure, but my point was that there isn't an actual physical contraction. I see that you're making that same point elsewhere in this thread. So you could have put more emphasis on where we agree instead of simply rebutting.
> | On Sunday, 14 July 2019 02:53:29 UTC+2, Tom Roberts wrote: |
> > |
But SR predicts that the moving rod (or train) does NOT "become shorter". It also predicts the moving rod WILL BE OBSERVED to "look" shorter. |
> |
[[Mod. note -- The reason for the apparent conflict between the two statements is that they are answering different questions. That is, a phrase such as "the measured physical length of a moving rod" is not sufficiently precise: it's necessary to specify precisely who (what observer) is making that measurement, and how that measurement is made. |
I agree.
> | Then the simplest case is one where the observer-on-the-rod measures the length of the rod. In this case, measuring the rod's length is easy (for her): she measures the position of the left end of the rod then she measures the position of the right end of the rod and she subtracts the two positions. |
The question is how exactly does she makes this measurement? Does she performs this measurement using a set of standard rods?
> | If she does this, then (according to special relativity) she will find the rod's length to be unchanged from its "basic" length (its length before it was in motion with respect to you). |
I assume you mean its rest length? The question is how is this rest length measured? Is this measurement performed using a set of standard rods?
I expect if both measurements are performed in the same way (Using both standard rods) no length contraction will be measured because if there is any length contraction both her moving rod and the moving standard rods will change, making it disappear.
> | This framework -- defining "the length of the rod" as that measured by an observer riding along with the rod (this is sometimes called a "proper observer"), is the basis for the statement you quoted above |
> > | But SR predicts that the moving rod (or train) does NOT "become shorter". |
> |
Now let's consider what *you* might observe. That is, you see the rod moving from left to right, and as it's moving you (somehow) measure it's length. What will you measure? As I noted above, answering this question requires being very precise about just how the measurement is made. (It turns out that this case requires rather more precision than was the case for the proper-observer measurement.) |
I agree. I think finally both require the same precision, specific is no length contraction is observed (which could be the case in my proposal)
> | One obvious way for you to measure (define!) the length of the rod is for you to measure the position of the left end of the rod at some time t, and for you to measure the position of the right end of the rod at the same time t, and then for you to subtract the two positions. |
That is what I'm proposing in my experiment. I want to measure (compare) the positions when the center of the rod is exactly in between the rod at rest. That is moment t.
> |
The position of an end of the rod at a time t is
then measured by that (unique) sub-observer who is
coincident with that end of rod at her clock's time t.
Since that sub-observer is coincident with her end of
the rod, there are no light-travel-time delays from
the moving rod to the sub-observer.
[It's precisely this avoidance of light-travel-time effects that's the motivation for introducing the infinite set of sub-observers.] After making their measurements, the sub-observers then send their observations back to some central collation point for analysis.] In relativity, "time" is observer-dependent, so we need to be specific about whose (which reference frame's) "time t" we're using. |
See above how that moment is defined in my proposed experiment.
> |
The simplest choice
is to say that since *you* are making this measurement (in *your*
inertial reference frame), then you should measure the left and right
rod ends' positions at times which are simultaneous in *your* inertial
reference frame, i.e., your sub-observers should each measure the
rod ends' positions at the same clock reading (t).
That is, to recap, * the central collation point computes L_you := x_right - x_left According to special relativity, if you do this, you will find that this length will show Lorentz contraction. Answering *this* question is the basis of the other statement you quoted above, |
> > | It also predicts the moving rod WILL BE OBSERVED to "look" shorter. |
> |
It should now be clear(er) that the two apparently-contradictory statements you quoted above, are in fact the answers to two *different* questions... so it's no longer surprising or contradictory that their answers are different. -- jt]] |
When I understand everything correct at every moment t there is always one observer at the front end of the rod and one at the back end of each rod such that we can measure the length of two rods, one at rest and one moving in the same (inertial) frame simultaneous. What I also understand that the measured length of the moving rod is shorter than the measured length rod at rest and a function of the speed v of the moving rod. That means this demonstrates length contraction.
This leaves open the question what is the physical explanation.
What is difficult to understand the experiment, which is a thought experiment, should be in agreement, as SR predicts, while the experiment I propose, which is much simpler, is supposed to physical fail (?) IMO every experiment is very difficult to perform in reality
Nicolaas Vroom.
> | IMO the most important question to answer is: what is the physical explanation of length contraction. |
This is really geometrical: both "length contraction" [#] and "time dilation" [#] are simple geometrical projections. No physical properties of anything are changed, but when you measure such properties from frames relative to which an object is moving, you get values different from when it is at rest.
These projections are directly analogous to "length contraction" and "time dilation", with the line's angle analogous to relative velocity of inertial frames, except the y axis is the time axis, and the geometry is hyperbolic (not Euclidean).
> > | Then the simplest case is one where the observer-on-the-rod measures the length of the rod. In this case, measuring the rod's length is easy (for her): she measures the position of the left end of the rod then she measures the position of the right end of the rod and she subtracts the two positions. |
> |
The question is how exactly does she makes this measurement? Does she performs this measurement using a set of standard rods? |
> > |
If she does this, then (according to special relativity) she will find the rod's length to be unchanged from its "basic" length (its length before it was in motion with respect to you). |
> |
I assume you mean its rest length? The question is how is this rest length measured? Is this measurement performed using a set of standard rods? I expect if both measurements are performed in the same way (Using both standard rods) no length contraction will be measured because if there is any length contraction both her moving rod and the moving standard rods will change, making it disappear. |
From the point of view of another observer, yes, you could think of it this way. But from the point of view of the observer moving with the rod, the better answer is that of course there is no difference, because there is no way to determine whether she is moving or at rest. That's the whole point of "relativity": inertial motion is relative.
You might be implying that there is some physical change, but it affects both the thing measured and the measuring device. Nope. There is no physical change.
(In the "twin paradox", where two observers meet up later and compare clocks, then there IS a physical difference: the elapsed time is different. This is easy to understand in the context of relativity, but it seems like that something must have physically affected the clocks---all clocks, whether mechanical, biological, atomic, whatever.)
> | On 7/14/19 7:56 PM, Nicolaas Vroom wrote: |
> > | IMO the most important question to answer is: what is the physical explanation of length contraction. |
> |
This is really geometrical: both "length contraction" [#] and "time dilation" [#] are simple geometrical projections. No physical properties of anything are changed, but when you measure such properties from frames relative to which an object is moving, you get values different from when it is at rest. |
Personally I prefer the method described by JT in which in principle you use one (inertial) reference frame and describe (measure) the complete state of the universe simultaneous.
> | Think about it: it must be possible for multiple observers in multiple frames to observe a given object, and they all get different values. |
I agree with you when a rod is considered because in all these measurements different physical time delays are involved, caused by the light travel time of the photons along different path length. However and that is important: these physical measurements don't cause any physical change within the rod. The length does not physical change.
> | Geometrical projections do just that, but ask yourself how any "physical change" of the object could do it.... |
But geometrical projections is mathematics ......
The problem is time dilation of moving clocks. If you want to know the time of moving clock which moves away from you, you get a time count that is always earlier than the actual time at the moment when you receive that information. The reason is simple communication time between sending and receiving any message (which is a function of distance) Sending and receiving does not cause any physical effect on the clocks in use.
What causes a physical effect is the internal operation of a clock if you compare a moving clock versus a clock at rest. The moving clock runs physical slower compared to a clock at rest. Depending about the internal operation of the clocks the mathematics which describe this physical behaviour are the Lorentz Transformations
Nicolaas Vroom
> | But geometrical projections is mathematics ...... |
Geometry, and specifically geometrical projections, can have physical consequences:
You can carry a ladder through a narrow doorway in some orientations but not others -- the geometrical projection of ladder's length onto doorway's width determines this.
Yes, collisions between ladder and doorframe are the cause, but the geometrical projection determines whether such a collision will occur.
> | The problem is time dilation of moving clocks. If you want to know the time of moving clock which moves away from you, you get a time count that is always earlier than the actual time at the moment when you receive that information. The reason is simple communication time between sending and receiving any message (which is a function of distance) Sending and receiving does not cause any physical effect on the clocks in use. |
You are confusing Doppler shift with "time dilation". The usual approach to separate them is to use a set of assistants arrayed along the path of the moving clock, each at rest in the observer's frame with a clock synchronized to the observer's clock; the assistants note the time on the moving clock as it passes, and also their own clock, and they send these notes to the observer, who can then measure "time dilation" without any light delays or Doppler shift. (The moderator already mentioned this approach in this thread.)
> | What causes a physical effect is the internal operation of a clock |
No. The internal operation of a clock is unaffected by its motion (relative to anything) -- that's the first postulate of SR.
The laws of physics govern the internal operation of every clock, and the first postulate says they are the same in every locally inertial frame.
> | The moving clock runs physical slower compared to a clock at rest. |
Only if the first postulate is wrong. Zillions of experiments show that it is correct.
The observer will OBSERVE the moving clock to run slower than her own clock, but the moving clock ITSELF is unaffected (i.e. it does NOT run "physically slower").
Tom Roberts
> > | What causes a physical effect is the internal operation of a clock |
> |
No. The internal operation of a clock is unaffected by its motion (relative to anything) -- that's the first postulate of SR. The laws of physics govern the internal operation of every clock, and the first postulate says they are the same in every locally inertial frame. |
> > |
The moving clock runs physical slower compared to a clock at rest. |
> |
Only if the first postulate is wrong. Zillions of experiments show that it is correct. The observer will OBSERVE the moving clock to run slower than her own clock, but the moving clock ITSELF is unaffected (i.e. it does NOT run "physically slower"). |
I think that everyone understands purely illusory effects: A sees B's clock running slower and vice versa. It is clear that these are only apparent effects and not "real". The question of what is observed is trickier, because the finite speed of light has to be taken into account. There WAS some real confusion about this, but it was cleared up by Terrell and Penrose a long time ago.
[[Mod. note -- Note that in the previous paragraph, the author is using the word "observed" NOT in the technical-special-relativity sense (of measurements made by instantaneously co-located sub-observers so as to eliminate light-travel-time effects), but rather in the sense of "what a camera image would show". That is, the author's usage of "observed" INCLUDES light-travel-time effects. -- jt]]
What is difficult to understand is the twin paradox: After A goes away and comes back while B stays at home and they then compare clocks at rest, EVERYONE agrees that A's clock has ticked less. Recent discussion here shows that acceleration is not the "cause", since the effect depends on the length of the journey, and not on the acceleration. Since all clocks (mechanical, electronic, atomic, biological, nuclear) are equally affected, it is a) hard to imagine that some mechanism affects them all equally and b) no-one has any idea what such a mechanism could be. (A similar effect exists if A spends more time in a deeper gravitational potential. This is really equally difficult to understand, though since there IS an obvious cause, the gravitational potential, fewer people have a problem with it. And perhaps the equivalence people suggests to them that in the SR twin paradox the acceleration must have something to do with it.) Yes, one can adopt the "shut up and calculate" approach and calculate the strength of the effect, and it agrees with observations, but this is not the same as understanding. The question is whether such an understanding is possible. GR is much more complicated mathematically, though conceptually I find it easier to digest, agreeing with Peter Ustinov here (bonus points if you spot the reference).
On Fri, 26 Jul 2019 08:57:08 PDT, "Phillip Helbig (undress to reply)"
> | What is difficult to understand is the twin paradox: After A goes away and comes back while B stays at home and they then compare clocks at rest, EVERYONE agrees that A's clock has ticked less. Recent discussion here shows that acceleration is not the "cause", since the effect depends on the length of the journey, and not on the acceleration. Since all clocks (mechanical, electronic, atomic, biological, nuclear) are equally affected, it is a) hard to imagine that some mechanism affects them all equally and b) no-one has any idea what such a mechanism could be. |
> | Yes, one can adopt the "shut up and calculate" approach and calculate the strength of the effect, and it agrees with observations, but this is not the same as understanding. The question is whether such an understanding is possible. |
I wonder if Mach's Law is involved. Nobody understands that either, but both issues involve an unseen reference frame of some kind. Maybe it's the same one.
> | I think that everyone understands purely illusory effects: A sees B's clock running slower and vice versa. |
I doubt if it is that simple. You must know the whole physical situation from start to end of this experiment, because it involves both physical (mechanical?) and optical effects.
There is a difference if A sees B's clock running slower or running behind. Running behind means that the difference in clock readings is constant during a certain period is constant. This can only be when the distance is fixed. As soon as any clock moves (which requires a force) this experiment becomes like the twin experiment with moving clocks. See below.
> | It is clear that these are only apparent effects and not "real". |
Length contraction is simpler to study
> | The question of what is observed is trickier, because the finite speed of light has to be taken into account. There WAS some real confusion about this, but it was cleared up by Terrell and Penrose a long time ago. |
What physicists should do is only to discuss measurements and how these measurements are made. When I see or observe something this are also measurements, but they are relatif and involve my position or my opinion.
When I observe a train which moves away from me and this train all of a sudden stops, then the first thing I will observe (measure) later is that the back of the train stops. At that moment the front is still moving away. A little later the front. This means the length of train during a small period becomes longer. This is an optical effect and not a physical effect. (later implies light-travel-time effects)
The same thing happens when a train approaches me. In that case I will observe (see, measure) that when the train suddenly stops, that the front stops first. At that moment the back is further away. A little later the back stops and during that small period the train becomes shorter, optical. This is I think what Terrell meant.
> | [[Mod. note -- Note that in the previous paragraph, the author is |
When you use this method, and eliminate all light-travel-time effects we will get instantaneous measurements for all objects involved (in one reference frame) The measurements will indicate that the physical length of all moving rods will be the same (and not change).
But when identical clocks are involved free moving though space, which initially all show the same time at position (p,t), after a certain time all will be at different positions and show different clock readings. What the measurements also could show is, that the clocks do not undergo any form of length contraction.
> | What is difficult to understand is the twin paradox: After A goes away and comes back while B stays at home and they then compare clocks at rest, EVERYONE agrees that A's clock has ticked less. Recent discussion here shows that acceleration is not the "cause", since the effect depends on the length of the journey, and not on the acceleration. Since all clocks (mechanical, electronic, atomic, biological, nuclear) are equally affected, it is a) hard to imagine that some mechanism affects them all equally and b) no-one has any idea what such a mechanism could be. |
To explain the above each clock should intially undergo a different force (in a different direction) or a temporarily acceleration which will change the speed of each clock differently, including its internal operation, which is based on (the direction of) the speed of the clock and (the direction of) the speed of light inside the clock. The overall result will be that the # of ticks will be different, which can be demonstrated when they meet again at one point.
Nicolaas Vroom
> | On Friday, 26 July 2019 17:57:12 UTC+2, Phillip Helbig wrote: |
> > |
I think that everyone understands purely illusory effects: A sees B's clock running slower and vice versa. |
> |
I doubt if it is that simple. You must know the whole physical situation from start to end of this experiment, |
By "purely illusory effects" I mean those which arise solely from the relative, unaccelerated motion. These are well documented, easily understood in SR, and no mystery at all.
> | because it involves both physical (mechanical?) and optical effects. |
Not in the case of purely relative motion.
> | There is a difference if A sees B's clock running slower or running behind. |
It runs behind because it runs slow.
> > | What is difficult to understand is the twin paradox: After A goes away and comes back while B stays at home and they then compare clocks at rest, EVERYONE agrees that A's clock has ticked less. Recent discussion here shows that acceleration is not the "cause", since the effect depends on the length of the journey, and not on the acceleration. Since all clocks (mechanical, electronic, atomic, biological, nuclear) are equally affected, it is a) hard to imagine that some mechanism affects them all equally and b) no-one has any idea what such a mechanism could be. |
> |
To explain the above each clock should intially undergo a different force (in a different direction) or a temporarily acceleration which will change the speed of each clock differently, including its internal operation, which is based on (the direction of) the speed of the clock and (the direction of) the speed of light inside the clock. The overall result will be that the # of ticks will be different, which can be demonstrated when they meet again at one point. |
In the classic twin paradox, only one clock is accelerated. While it is clear that the accelerated clock runs slower (depending on the length of the journey and not on the magnitude of the acceleration!), it is less clear that acceleration "causes" this in the sense of physically affecting the operation of the clock.
> |
By "purely illusory effects" I mean those which arise solely from the relative, unaccelerated motion. These are well documented, easily understood in SR, and no mystery at all. |
When I walk away from a building the observed height from my point decreases. This is a pure optical effect and has no physical implications i.e. the height of the buiding does not change.
> > | because it involves both physical (mechanical?) and optical effects. |
> |
Not in the case of purely relative motion. |
> > |
There is a difference if A sees B's clock running slower or running behind. |
> |
It runs behind because it runs slow. |
Consider the following clock readings, both clocks are placed in front of me.
Case 1 Clock A 10:00 11:00 12:00 13:00 14:00 Clock B 10:00 10:30 11:00 11:30 12:00In this case Clock B runs behind but Clock B also runs slower.
Case 2 Clock A 10:00 11:00 12:00 13:00 14:00 Clock B 6:00 7:00 8:00 9:00 10:00In this case Clock B runs behind but Clock B does not run slower.
In the above two examples no motion is involved.
Case 1 can also be used as an example that there is motion involved
i.e. that clock B moves away from clock A.
But the cause is now both optical (light travel time) and physical
(clock B runs slower). This is the case with the twin paradox.
> | In the classic twin paradox, only one clock is accelerated. |
> | While it is clear that the accelerated clock runs slower (depending on the length of the journey and not on the magnitude of the acceleration!), it is less clear that acceleration "causes" this in the sense of physically affecting the operation of the clock. |
The physical change in the performance (ticking rate) of the moving clock
is caused by the forces enforced on the clock.
If the direction of the internal lightsignal, which creates oscillations
(each cylce defines one count), is in the direction of the moving clock,
then the path length will become longer and the clock rate decreases.
The path length is defined as the distance between two mirrors, one in the
back and one in the front.
If the lightsignal starts in the back and moves toward the front mirror,
then When this mirror moves away, the time for the lightsignal to reach
the mirror in front increases.
The faster the clock speed, the longer this distance.
In theory when the speed approaches the speed of light, this time
will reach infinite and the clock will stop counting. All this is physics.
If you drop an hourly glass, which internal operation is based on gravity, during the fall the internal glass particles will not move towards the other container, thereby increasing the cycle time i.e. the time that one container becomes empty and the hourly glass has to be turned over. As a consequence the hourly glass will run slower (just like a moving clock). All this is physics
What is also interesting is that the behaviour of a clock is influenced by the speed of light (photons) and an hourly glass by the speed of gravity (gravitons). Both are also influenced by external (temporary induced) forces.
Nicolaas Vroom.
> |
If you drop an hourly glass, which internal operation is based on gravity, during the fall the internal glass particles will not move towards the other container, thereby increasing the cycle time i.e. the time that one container becomes empty and the hourly glass has to be turned over. As a consequence the hourly glass will run slower (just like a moving clock). All this is physics |
An hours glass just doesn't work when it is in free fall. While certainly a consequence of physical law, its failure has nothing to do with the relativistic slowing (in another frame) of an otherwise functioning clock. I can make the other clock stop by hitting it with a hammer. This has similarly nothing to do with relativity, but serves to emphasise that real clocks are designed to function within certain constraints, and cannot be expected to give valid results outside those constraints, with one typical constraint entailing not being hit with a hammer.
> |
What is also interesting is that the behaviour of a clock is influenced by the speed of light (photons) and an hourly glass by the speed of gravity (gravitons). Both are also influenced by external (temporary induced) forces. |
The behaviour of the clock in a different frame depends on the universal constant c. Light travels at that speed, but photons are not causing the relativistic effects.
I have no idea what is meant by "induced forces". It sounds like something that has been made up.
Sylvia.
> | The physical change in the performance (ticking rate) of the moving clock is caused by the forces enforced on the clock. |
In the twin paradox, the only force is the force of acceleration of the twin who journeys. But the magnitude of the effect is greater the greater the journey FOR THE SAME ACCELERATION. Also, if there was some physical change, it should be possible to measure it in some other way.
> | On 30/07/2019 4:49 pm, Nicolaas Vroom wrote: |
> > |
If you drop an hourly glass, which internal operation is based on gravity, during the fall the internal glass particles will not move towards the other container, thereby increasing the cycle time i.e. the time that one container becomes empty and the hourly glass has to be turned over. As a consequence the hourly glass will run slower (just like a moving clock). |
> |
An hours glass just doesn't work when it is in free fall. |
I agree.
> | While certainly a consequence of physical law, its failure has nothing to do with the relativistic slowing (in another frame) of an otherwise functioning clock. |
I comparing performance of an hourly glass with a clock in extreme. That means the hourly glass falls (with an average speed v) and the clock moves almost at the speed of light (c). Result both don't work. However if the speed of the hourly glass is 0.1*v and the speed of the clock 0.1*c both will function (operate) however as a clock each will run slower.
[[Mod. note -- In relativity, we need to (implicitly or explicitly) qualify statements of the form "X moves with speed V" with the reference frame with respect to which the speed is defined. E.g., "if the speed of the hourly [sic] glass is 0.1*v" needs to be expanded into something like "if the speed of the hour glass WITH RESPECT TO SUCH-AND-SUCH AN OBSERVER is 0.1*v".
And, suitable clocks (e.g., the decay of unstable elementary particles) work just fine when moving almost at the speed of light with respect to a laboratory. -- jt]]
> > | What is also interesting is that the behaviour of a clock is influenced by the speed of light (photons) and an hourly glass by the speed of gravity (gravitons). Both are also influenced by external (temporary induced) forces. |
> |
The behaviour of the clock in a different frame depends on the universal constant c. Light travels at that speed, but photons are not causing the relativistic effects. |
Photons are the physical reason that a clock operates i.e. functions as an oscillator and ticks.
For an hourly glass this are the glass particles. A human is required to take care that the hourly glass becomes an oscillatot i.e. for time keeping.
> | I have no idea what is meant by "induced forces". It sounds like something that has been made up. |
In order to operate properly, a clock or a hourly glass should stay at rest
and not be moved.
Moving a clock (or hourly glass) will influence this normal operation.
(And the clock will start ticking slower)
To move a clock (or hourly glass) you need an external induced (applied?)
force. Dropping an hourly glass mimics such a force.
[[Mod. note -- Again, phrases like "should stay at rest and not be moved" are ambiguous, because they don't specify a reference frame. For example, does "stay at rest and not be moved" mean (a) "not be moved with respect to a laboratory located at the Earth's south pole"? Does "stay at rest and not be moved" mean (b) "not be moved with respect to a laboratory located on the Earth's equator"? (Since the Earth's equator moves at about 460 meters/second relative to the Earth's south pole, it's impossible to be at rest with respect to both of them simultaneously.)
And that said, some types of clocks (e.g., radioactive decay)
don't need any external forces to operate. -- jt]]
The same problem exists when you place a chronometer on board of a ship. Special design of such a chronometer is required to minimise the influence of the waves. All of that is physics.
Nicolaas Vroom
> |
I comparing performance of an hourly glass with a clock in extreme. That means the hourly glass falls (with an average speed v) and the clock moves almost at the speed of light (c). Result both don't work. However if the speed of the hourly glass is 0.1*v and the speed of the clock 0.1*c both will function (operate) however as a clock each will run slower. [[Mod. note -- In relativity, we need to (implicitly or explicitly) qualify statements of the form "X moves with speed V" with the reference frame with respect to which the speed is defined. E.g., "if the speed of the hourly [sic] glass is 0.1*v" needs to be expanded into something like "if the speed of the hour glass WITH RESPECT TO SUCH-AND-SUCH AN OBSERVER is 0.1*v". |
I agree with you. When you try to quantify physical processes you place yourself on icy ice. The general idea is to place two identical clocks (or hourly glasses) side by side. When you move one (this always requires some force) its behaviour will change and the clock will run slower.
> | And, suitable clocks (e.g., the decay of unstable elementary particles) work just fine when moving almost at the speed of light with respect to a laboratory. |
My first guess is that a clock based on the decay of unstable elementary particles (its counting rate) is not constant over a long period.
> | -- jt]] |
> |
In order to operate properly, a clock or a hourly glass should stay at rest
and not be moved.
Moving a clock (or hourly glass) will influence this normal operation.
(And the clock will start ticking slower)
To move a clock (or hourly glass) you need an external induced (applied?)
force. Dropping an hourly glass mimics such a force.
[[Mod. note -- Again, phrases like "should stay at rest and not be moved" are ambiguous, because they don't specify a reference frame. For example, does "stay at rest and not be moved" mean (a) "not be moved with respect to a laboratory located at the Earth's south pole"? Does "stay at rest and not be moved" mean (b) "not be moved with respect to a laboratory located on the Earth's equator"? (Since the Earth's equator moves at about 460 meters/second relative to the Earth's south pole, it's impossible to be at rest with respect to both of them simultaneously.) |
Also here I agree with you. What exactly means at rest? In most experiments (in books) one clock stays on earth and in other clock is ejected in space with a speed of 0.3 * c. The observer on earth is considered in his reference frame at rest. If that is commonly accepted, I have no problem. Maybe it is better to consider the observer always hypothetical at the centre of the earth.
> | And that said, some types of clocks (e.g., radioactive decay) [That is, our "clock" is a mass of some radioactive isotope, and we measure time by counting decays/second and using the isotope's known half-life to determine elapsed time since we set up the clock.] don't need any external forces to operate. |
I fully agree that such a clock does not need any force to operate. That is the same for a clock using lightsignals (photons) or an hourly glass (except to turn the clock when one container is empty)
Consider you have two identical clocks and one clock is moved (is influenced
by an external force). The question is: does this have consequence
for the performance of the moving clock versus the not moving clock.
For a clock based on light signals the answer is yes, because the path
length of the photons will become longer.
For an hourly glass the answer is also yes. The issue is gravity.
Dropping an hourly glass a couple of times (or shaking carefully)
will increase the path length.
I don't know how constant clocks based on radioactive decay are, while
under going speeds close to the speed of light, compared to an identical
clock which stays on earth.
More technical detail about specific radioactive material used is required.
https://en.wikipedia.org/wiki/List_of_radioactive_isotopes_by_half-life
The more decays/second we want the larger mass is required.
Part of the problem I see is, that to count each radio active decay
instant is more difficult using very high speeds.
More technical information is required how such a clock is build.
For a clock using light signals it is easy to operate under laboratory conditions. To use that same clock in a space ship when accelerations are involved is much more complex. This also depends on the direction of the lightsignal versus the direction of the movement of the clock.
Nicolaas Vroom
> | The general idea is to place two identical clocks (or hourly glasses) side by side. When you move one (this always requires some force) its behaviour will change and the clock will run slower. |
First, this has nothing to do with relativity. Second, I could imagine the hour glass tilted slightly in the direction of motion. When accelerated, it would then run faster.
> | My first guess is that a clock based on the decay of unstable elementary particles (its counting rate) is not constant over a long period. |
I'm not sure what you mean here. Radioactive decay is an exponential process, often described by the half life. We can observe that increasing for particles moving quickly.
> | In most experiments (in books) one clock stays on earth and in other clock is ejected in space with a speed of 0.3 * c. The observer on earth is considered in his reference frame at rest. If that is commonly accepted, I have no problem. Maybe it is better to consider the observer always hypothetical at the centre of the earth. |
The Earth is moving around the Sun, the Sun is moving around the galaxy. What matters in the twin paradox is who has the strongest acceleration.
> | In article <85b8886b-41e2-43fd-bb70-953712d8fbb2@googlegroups.com>, |
>> | On Friday, 26 July 2019 17:57:12 UTC+2, Phillip Helbig wrote: |
>>> | I think that everyone understands purely illusory effects: A sees B's clock running slower and vice versa. |
> |
By "purely illusory effects" I mean those which arise solely from the relative, unaccelerated motion. These are well documented, easily understood in SR, and no mystery at all. |
Yes, "time dilation" does not affect the moving clock being observed, but it is not really "illusory", because it can have real and measurable physical consequences.
>> | There is a difference if A sees B's clock running slower or running behind. |
> |
It runs behind because it runs slow. |
This is just plain not true, and I wish physicists who OUGHT to know better would stop repeating such errors. The clock does NOT "run slow", it runs at its usual and natural rate -- the first postulate of SR would be violated if this were not so.
Yes, in the twin paradox the traveling twin's clock accumulates less elapsed proper time than the earthbound twin's clock. But it does NOT "run slow" -- rather it travels a shorter path through spacetime.
>>> | What is difficult to understand is the twin paradox: After A goes away and comes back while B stays at home and they then compare clocks at rest, EVERYONE agrees that A's clock has ticked less. [...] Since all clocks (mechanical, electronic, atomic, biological, nuclear) are equally affected, it is a) hard to imagine that some mechanism affects them all equally and b) no-one has any idea what such a mechanism could be. |
This is an example of the confusion created by incorrect statements about "clocks running slow". The resolution is simple: the clocks do NOT "run slow", so no "mechanism" is needed.
> | In the classic twin paradox, only one clock is accelerated. While it is clear that the accelerated clock runs slower [...] |
Again, stop making such erroneous and confusing claims. The clock does NOT "run slower"; rather, it accumulated less elapsed proper time (and did so while ticking at its usual and natural rate). The reason for this is geometrical: the traveling twin followed a shorter path through spacetime than did the earthbound twin.
> | When I walk away from a building the observed height from my point decreases. This is a pure optical effect and has no physical implications i.e. the height of the buiding does not change. |
But it CAN have physical implications. For instance, if you want to photograph the building, close up it won't fit in the camera's aperture, but far away it will.
Note this is a geometrical projection, which is directly analogous to "time dilation" and "length contraction". For them, relative motion varies the projection, while for your example it is distance that varies the projection.
Tom Roberts
>> | I comparing performance of an hourly glass |
An hourglass is NOT a clock. Hourglass+earth is the clock, and if you change the relationship between the hourglass and the earth you have a completely different clock. Of course no hourglass is accurate enough to demonstrate any relativistic effects. Ditto for a pendulum or grandfather clock.
> | Consider you have two identical clocks and one clock is moved (is influenced by an external force). The question is: does this have consequence for the performance of the moving clock versus the not moving clock. For a clock based on light signals the answer is yes, because the path length of the photons will become longer. |
No! You keep making the same mistake: not specifying the inertial frame used for reference.
If you want to discuss a clock's tick rate, you must use its own rest frame. For a light clock, in its rest frame the light path lengths are constant; regardless of which frame it is at rest in, its tick rate does not change (remember the speed of light is also the same in all inertial frames).
If you want to discuss a clock's tick rate as measured in some inertial frame other than its rest frame, you MUST say so. For any good clock, it is only when measured from an inertial frame relative to which it is moving that the observed tick rate changes.
Or when observed from a different altitude in a gravitational field.
As I keep saying, discussing relativity requires precision in thought and word. Your statements repeatedly fall short of what is required, and you MUST improve the precision of your thinking in order to understand this.
> | For an hourly glass the answer is also yes. The issue is gravity. Dropping an hourly glass a couple of times (or shaking carefully) will increase the path length. |
Dropping an hourglass changes its relationship to the earth and you have a completely different clock. This is useless.
> | I don't know how constant clocks based on radioactive decay are, |
There are measurements of "time dilation" using radioactive ions in a beam moving at an appreciable fraction of c, but I have lost the references. Here are references for a "clock" made from the decay rate of muons:
Bailey et al., =E2=80=9CMeasurements of relativistic time dilation for positive and negative muons in a circular orbit,=E2=80=9D Nature 268 (July 28, 1977) pg 301. Bailey et al., Nuclear Physics B 150 pg 1=E2=80=9379 (1979).
They stored muons in a storage ring and measured their lifetime. The measured decay rate is consistent with the prediction of SR.
When combined with measurements of the muon lifetime at rest this becomes a highly relativistic twin scenario (v ~0.9994 c), for which the stored muons are the traveling twin and return to a given point in the lab every few microseconds. While being stored in the ring they were subject to a proper acceleration of approximately 10^18 g (1 g =3D 9.8 m/s2).
Tom Roberts
> |
Relativistic length contraction is theoretically predicted but not directly
tested, which lead to incorrect interpretation of the theory illustrated by
Bell's spaceship paradox and Ehrenfest paradox. But these paradoxes can help
us designing experiments to test length contraction.
Ideal direct experimental proof should contain the following steps: For doing this experiment, the difference of length l0 ï=80- l should be in measurable range. If the object is a chunk of matter, l0 ï=80- l is not measurable. For example, matter objects with the highest speed we can make are satellites, whose speed is generally 7.8 km/s. If a satellite is made of a string of 100 km long, the value of l0 ï=80- l would be 0.03 mm, which is absolutely not measurable from the ground. This is why contraction of length has never been measured. Below I propose two experiments inspired from Bell's spaceship paradox and Ehrenfest paradox. Please read the article at PDF: How to test length contraction by experiment? https://pengkuanonphysics.blogspot.com/2019/06/how-to-test-length-contraction-by.html or Word: https://www.academia.edu/39584663/How_to_test_length_contraction_by_experiment |
There is no material length contraction. A meter stick will return the same length after a journey. What the LT length contraction mean: The light-path-length (LPL) of a moving meter stick is predicted to be contracted by a factor of 1/gamma. This prediction by the observer is based on the assumption that the LPL of the observer’s meter stick is its material length.
> | What matters in the twin paradox is who has the strongest acceleration. |
No. What matters is who has the shortest path through spacetime.
It is easy to set up a twin scenario in which two twins orbit a mass, one in circular orbit and one in a highly elliptical orbit, such that they meet periodically. Both have zero proper acceleration. It can be arranged so either one has the larger elapsed proper time between meetings.
One can also set up a twin scenario using clocks on the rims of rotating disks, with the disks' centers at rest in some inertial frame, arranged so they meet periodically. By varying the diameter and rotation rate of the disks, one can arrange for the clock with the larger proper acceleration to have either larger or smaller elapsed proper time between meetings. In this case, of course, it is the speed of the clock that matters, not its acceleration (both relative to the inertial frame).
Tom Roberts
> | On 7/28/19 2:09 PM, Phillip Helbig (undress to reply) wrote: |
> > > | I doubt if it is that simple. You must know the whole physical situation from start to end of this experiment, |
> > |
By "purely illusory effects" I mean those which arise solely from the relative, unaccelerated motion. These are well documented, easily understood in SR, and no mystery at all. |
> |
Yes, "time dilation" does not affect the moving clock being observed, but it is not really "illusory", because it can have real and measurable physical consequences. |
Nothing affects any physical processes to be observed, to be measured. This is not exactly physical true. If you put a thermometer in water, the water 'heats' up if the T of thermometer is higher than the T of water. As such the length of rod does not physical change being observed. The same for any clock (its physical dimensions) being observed. The problem with any clock is that its physical performances changes when a clock undergoes movement.
At page 393 from the book Gravitation we can read: "However, in any other context (e.g on a rocket journey to the moon), a pendulum clock should be far from ideal. Wildly changing accelerations or no acceleration at all will make it worthless" For more see this link: https://www.nicvroom.be/Book_Review_GRAVITATION_by_MTW.htm#page393 At page 393 we can also read: "Rather, one must ask the laws of physics themselves what types of rods and clocks will do the job." This is a rather self fullfilling question. We must know the physical laws, which are descriptions of physical processes and which we initially don't know, how the physical processes behave.
[[Mod. note -- Yes, that's usually the case in physics. We usually need some minimal understanding of the natural world in order to figure out what to observe and how to go about making those observations. Then we can use those observations to refine our understanding of the natural world, and in particular, to try to infer some of the physical laws governing the natural world.
A beautiful example of this in a relatively simple context is Galileo's elucidation of what we now call Newton's first law. This involved (among other things) introducing and providing an operational definition of the concepts of *instantaneous velocity* and *instantaneous acceleration*. There's an excellent discussion of this in chapter 2 of Arnold B Arons's classic book "A Guide to Introductory Physics Teaching" (Wiley, 1990, ISBN 0-471-51341-5). -- jt]]
> |
For instance, charged pions have a proper lifetime of 26 nanoseconds, which at 0.999999 c corresponds to traveling only 7.8 meters. Fermilab and CERN have had pion beamlines over a kilometer long because the "time dilation" of high-energy pions permits such long beamlines to work -- this is no "illusion". |
But this process has nothing to do with a clock which operates using light signals.
[[Mod. note -- On the contrary, it has a great deal to do with such a clock. One of the key idealizations in special relativity is that of an *ideal clock*, which is approximated by various actual physical clocks.
The decay of charged pions provides a useful example of an actual physical clock: in the pion's rest frame they decay with a half-life of 26 nanoseconds. This means that we can use the decay of pions as a clock: if we start with some large number N1 of pions and later measure some somewhat-less-large number N2 of pions (without the pions having significantly interacted with anything else), then we say that (in the pion's rest frame) the elapsed time from the N1 measurement to the N2 measurement is (log(N1/N2) / log(2)) * 26 nanoseconds . -- jt]]
> >> | There is a difference if A sees B's clock running slower or running behind. |
> > |
It runs behind because it runs slow. |
> |
This is just plain not true, and I wish physicists who OUGHT to know better would stop repeating such errors. |
In the book 'Subtle is the Lord' by Abraham Pais at page 145 we can read:
"(g) Einstein rather casual mentioned that if two synchronous clocks C1 and
C2 are at the same initial position and if C2 leaves A and moves along a
closed orbit, then upon return to A, C2 will run slow relative to C1,
as often observed since in the laboratory. He called this result a theorem
and cannot be held responsible for the misnomer clock paradox, which is
of later vintage. Indeed, as Einstein himself noted later, "no contradiction
in the foundations of the theory can be constructed from the result"
since C2 but not C1 has experienced acceleration."
It is important to mention that C2 undergoes acceleration, and in principle
can undergo continuous acceleration, like the earth, implying that an
observer attached to the clock is not in an inertial frame.
> |
Yes, in the twin paradox the traveling twin's clock accumulates less
elapsed proper time than the earthbound twin's clock. But it does NOT
"run slow" -- rather it travels a shorter path through spacetime.
Please remember how English works -- in the sentence "That clock runs slower than this one", only the clocks are mentioned; in SR this is simply false because the laws of physics that govern both clocks' ticking are the same in the rest frames of both clocks. To make a correct statement about this you MUST mention how its tick rate is measured. A clock moving relative to inertial frame S does not "tick slow", but S will measure it to tick slower than identical clocks at rest in S. |
All of this is tricky. The problem is when moving clocks are involved you have to be carefull to possible correct for errors. IMO we should study celestial mechanics only with one set of clocks in one reference frame. It is already difficult enough.
> > | In the classic twin paradox, only one clock is accelerated. While it is clear that the accelerated clock runs slower [...] |
> |
Again, stop making such erroneous and confusing claims. The clock does NOT "run slower"; rather, it accumulated less elapsed proper time (and did so while ticking at its usual and natural rate). The reason for this is geometrical: the traveling twin followed a shorter path through spacetime than did the earthbound twin. |
See the text by Einstein above.
Nicolaas Vroom.
> | On 7/28/19 2:09 PM, Phillip Helbig (undress to reply) wrote: |
> > > | I doubt if it is that simple. You must know the whole physical situation from start to end of this experiment, |
> > |
By "purely illusory effects" I mean those which arise solely from the relative, unaccelerated motion. These are well documented, easily understood in SR, and no mystery at all. |
> |
Yes, "time dilation" does not affect the moving clock being observed, but it is not really "illusory", because it can have real and measurable physical consequences. |
Nothing affects any physical processes to be observed, to be measured. This is not exactly physical true. If you put a thermometer in water, the water 'heats' up if the T of thermometer is higher than the T of water. As such the length of rod does not physical change being observed. The same for any clock (its physical dimensions) being observed. The problem with any clock is that its physical performances changes when a clock undergoes movement.
At page 393 from the book Gravitation we can read: "However, in any other context (e.g on a rocket journey to the moon), a pendulum clock should be far from ideal. Wildly changing accelerations or no acceleration at all will make it worthless" For more see this link: https://www.nicvroom.be/Book_Review_GRAVITATION_by_MTW.htm#page393 At page 393 we can also read: "Rather, one must ask the laws of physics themselves what types of rods and clocks will do the job." This is a rather self fullfilling question. We must know the physical laws, which are descriptions of physical processes and which we initially don't know, how the physical processes behave.
> |
For instance, charged pions have a proper lifetime of 26 nanoseconds, which at 0.999999 c corresponds to traveling only 7.8 meters. Fermilab and CERN have had pion beamlines over a kilometer long because the "time dilation" of high-energy pions permits such long beamlines to work -- this is no "illusion". |
But this process has nothing to do with a clock which operates using light signals.
> >> | There is a difference if A sees B's clock running slower or running behind. |
> > |
It runs behind because it runs slow. |
> |
This is just plain not true, and I wish physicists who OUGHT to know better would stop repeating such errors. |
In the book 'Subtle is the Lord' by Abraham Pais at page 145 we can read:
"(g) Einstein rather casual mentioned that if two synchronous clocks C1 and
C2 are at the same initial position and if C2 leaves A and moves along a
closed orbit, then upon return to A, C2 will run slow relative to C1,
as often observed since in the laboratory. He called this result a theorem
and cannot be held responsible for the misnomer clock paradox, which is
of later vintage. Indeed, as Einstein himself noted later, "no contradiction
in the foundations of the theory can be constructed from the result"
since C2 but not C1 has experienced acceleration."
It is important to mention that C2 undergoes acceleration, and in principle
can undergo continuous acceleration, like the earth, implying that an
observer attached to the clock is not in an inertial frame.
> |
Yes, in the twin paradox the traveling twin's clock accumulates less
elapsed proper time than the earthbound twin's clock. But it does NOT
"run slow" -- rather it travels a shorter path through spacetime.
Please remember how English works -- in the sentence "That clock runs slower than this one", only the clocks are mentioned; in SR this is simply false because the laws of physics that govern both clocks' ticking are the same in the rest frames of both clocks. To make a correct statement about this you MUST mention how its tick rate is measured. A clock moving relative to inertial frame S does not "tick slow", but S will measure it to tick slower than identical clocks at rest in S. |
All of this is tricky. The problem is when moving clocks are involved you have to be carefull to correct for errors. IMO we should study celestial mechanics only with one set of clocks in one reference frame. It is already difficult enough.
> > | In the classic twin paradox, only one clock is accelerated. While it is clear that the accelerated clock runs slower [...] |
> |
Again, stop making such erroneous and confusing claims. The clock does NOT "run slower"; rather, it accumulated less elapsed proper time (and did so while ticking at its usual and natural rate). The reason for this is geometrical: the traveling twin followed a shorter path through spacetime than did the earthbound twin. |
See the text by Einstein.
Nicolaas Vroom.
> | On Thursday, 1 August 2019 21:34:33 UTC+2, Tom Roberts wrote: |
>>>> | There is a difference if A sees B's clock running slower or running behind. |
>>> | It runs behind because it runs slow. |
>> |
This is just plain not true, and I wish physicists who OUGHT to know better would stop repeating such errors. [... further discussion] |
> |
In the book 'Subtle is the Lord' by Abraham Pais at page 145 we can read: "(g) Einstein rather casual mentioned that if two synchronous clocks C1 and C2 are at the same initial position and if C2 leaves A and moves along a closed orbit, then upon return to A, C2 will run slow relative to C1, as often observed since in the laboratory. [...] |
Einstein did not know better when he wrote, as this had not yet been fully understood. Pais is not a physicist and is writing for a general audience, so it is no surprise that he does not know better.
The observation mentioned is really that C2 showed less elapsed proper time than C1; the tick rates of the two clocks were NOT compared, and "running slow" was NOT actually observed.
This is merely another example of insufficiently precise wording: the notion that C2 "runs slow" compared to C1 implicitly assumes a) using the inertial frame of A (and C1), and b) the difference in final displayed times depends ONLY on the clocks' tick rates. But a) if you are talking about the tick rate of C2 you MUST use its own rest frame, and b) the final difference also CLEARLY depends on the clocks' paths.
Bottom line: C2 does not "run slow". But C2 does run slow RELATIVE TO THE FRAME OF C1. Note carefully the difference in English wording, which reflects an essential difference in meaning.
>>> | In the classic twin paradox, only one clock is accelerated. While it is clear that the accelerated clock runs slower [...] |
>> |
Again, stop making such erroneous and confusing claims. The clock does NOT "run slower"; rather, it accumulated less elapsed proper time (and did so while ticking at its usual and natural rate). The reason for this is geometrical: the traveling twin followed a shorter path through spacetime than did the earthbound twin. |
> |
See the text by Einstein above. |
Einstein did not know better -- he had the excuse that this had not yet been fully understood. Today there is no such excuse.
I repeat: if a given clock actually does "run slow", then the first postulate of SR would be violated , and the entire edifice of modern physics would be overthrown. But no such violation has ever been observed.
Tom Roberts
> | On 8/4/19 7:14 PM, Nicolaas Vroom wrote: |
> > |
In the book 'Subtle is the Lord' by Abraham Pais at page 145 we can read: "(g) Einstein rather casual mentioned that if two synchronous clocks C1 and C2 are at the same initial position and if C2 leaves A and moves along a closed orbit, then upon return to A, C2 will run slow relative to C1, as often observed since in the laboratory. [...] |
> |
Einstein did not know better when he wrote, as this had not yet been fully understood. Pais is not a physicist and is writing for a general audience, so it is no surprise that he does not know better. The observation mentioned is really that C2 showed less elapsed proper time than C1; the tick rates of the two clocks were NOT compared, and "running slow" was NOT actually observed. This is merely another example of insufficiently precise wording: the notion that C2 "runs slow" compared to C1 implicitly assumes a) using the inertial frame of A (and C1), and b) the difference in final displayed times depends ONLY on the clocks' tick rates. But a) if you are talking about the tick rate of C2 you MUST use its own rest frame, and b) the final difference also CLEARLY depends on the clocks' paths. Bottom line: C2 does not "run slow". But C2 does run slow RELATIVE TO THE FRAME OF C1. |
That means if Abraham Pais would have written: "and if C2 leaves A and moves along a closed orbit, then upon return to A, C2 will run slow relative to THE FRAME of C1, as often observed since in the laboratory. [...]" then everything is okay? Maybe that is what he (and Einstein) meant?
But is all off this that important? Original both C1 and C2 are resident of the same frame (state). When C2 leaves A a certain force is exerted on C2. In order to travel a closed loop more forces are exerted on C2. At the end again a force is exerted on C2 to bring him back in the frame of C1. (What this implies is that C2 during his travelling, under going accelerations, is not part of one particular frame at rest) Finally what is observed, that C2 shows less counts than C1. (relative to the frame of C1). The cause of this lies in all these extra forces, (which physical affect the internal operation of the clock) which results that the path length of C2 travelled is much larger than C1.
Nicolaas Vroom
> | On 8/1/19 2:33 PM, Phillip Helbig (undress to reply) wrote: |
> > | What matters in the twin paradox is who has the strongest acceleration. |
> |
No. What matters is who has the shortest path through spacetime. It is easy to set up a twin scenario in which two twins orbit a mass, one in circular orbit and one in a highly elliptical orbit, such that they meet periodically. Both have zero proper acceleration. It can be arranged so either one has the larger elapsed proper time between meetings. |
The answer lies in the details of this experiment. IMO it is important to describe this experiment in more detail. The mass involved is the earth with observer C1 at rest. For the rest the universe is considered empty. C2 orbits around the earth in one year (relative to C1). C2 will make 10 revolutions around the earth. C3 orbits around the earth in 10 years (relative to C1) C3 will make 1 revolution around the earth. After 10 years (earth time) they will all meet again on earth. This is a general rule: The duration (earth time) for all observers is the same.
When you perform such an experiment the observer closest to the earth (C2) will be subject to the largest speed*, largest acceleration*, its path length will be the longest* and its clock will run the slowest relative to the frame of C1. (*) If my calculations using Newton's law are correct i.e. r = T^(2/3)
For a description of the details for each observer follow this link: Spaceship parameters
In a real experiment extra travel time should be added for C2 and C3 going into orbit at the predetermined distance and speed. This requires skilled labour at the control panel of each spaceship. A similar extra travel time should be included to bring each spaceship savely back to earth. Both these 'sequence of events' involve extra forces and accelerations.
The purpose of this posting is that such an experiment is rather complex. The movement of the objects involves 'gravity' (Newton's Law or GR) and extra forces in order to control the movement of the spaceships. ONLY in order to describe the behaviour of the clocks the speed of light is involved.
Nicolaas Vroom.
On Sunday, 4 August 2019 12:18:00 UTC+2, Tom Roberts wrote:
> | On 8/1/19 2:33 PM, Phillip Helbig (undress to reply) wrote: |
> > | What matters in the twin paradox is who has the strongest acceleration. |
> |
No. What matters is who has the shortest path through spacetime. It is easy to set up a twin scenario in which two twins orbit a mass, one in circular orbit and one in a highly elliptical orbit, such that they meet periodically. Both have zero proper acceleration. It can be arranged so either one has the larger elapsed proper time between meetings. |
The answer lies in the details of this experiment.
IMO it is important to describe this experiment in more detail.
The mass involved is the earth with observer C1 at rest.
For the rest the universe is considered empty.
C2 orbits around the earth in one year (relative to C1).
C2 will make 10 revolutions around the earth.
C3 orbits around the earth in 10 years (relative to C1)
C3 will make 1 revolution around the earth.
After 10 years (earth time) they will all meet again on earth.
This is a general rule:
The duration (earth time) for all observers is the same.
[[Mod. note -- I don't know what you mean by that last sentence.
In Tom Roberts' scenario we are comparing /proper time/, i.e., readings of clocks carried by the various observers. That is, we first synchronize clocks #1, #2, and #3. Then we leave #1 on the Earth, put #2 in some one-year orbit, and put #2 in some 10-year orbit. (These orbits may be elliptical.) Then we wait a while, until (by clever arrangement of the orbits) all 3 clocks are back in the same place again and clock #1 (stationary on the Earth) says 10 years have elapsed. In this scenario the elapsed time shown by clocks #1 (= 10 years), #2, and #3 will in general all be different.
(To tie this together with the traditional "twin paradox", note that the aging of a person's body is a type of clock. It's not the most precise of clocks, but that doesn't matter for our /gedanken/ purposes.) -- jt]]
When you perform such an experiment the observer closest to the earth (C2)
will be subject to the largest speed*, largest acceleration*, its path
length will be the longest* and its clock will run the slowest
relative to the frame of C1.
(*) If my calculations using Newton's law are correct i.e. r = T^(2/3)
For a description of the details for each observer follow this link: Spaceship parameters
In a real experiment extra travel time should be added for C2 and C3 going into orbit at the predetermined distance and speed. This requires skilled labour at the control panel of each spaceship. A similar extra travel time should be included to bring each spaceship savely back to earth. Both these 'sequence of events' involve extra forces and accelerations.
The purpose of this posting is to mention that any experiment which tries to demonstrate the importance or influence of acceleration is rather complex. The movement of the objects involves 'gravity' (Newton's Law or GR) and extra forces in order to control the movement of the spaceships. ONLY in order to describe the behaviour of the clocks the speed of light is involved.
Nicolaas Vroom.
> | On Sunday, 4 August 2019 12:18:00 UTC+2, Tom Roberts wrote: |
> > | It is easy to set up a twin scenario in which two twins orbit a mass, one in circular orbit and one in a highly elliptical orbit, such that they meet periodically. Both have zero proper acceleration. It can be arranged so either one has the larger elapsed proper time between meetings. |
This experiment is not very clear. That is why I have setup a different experiment.
> |
The answer lies in the details of this (each) experiment.
C2 orbits around the earth in one year (relative to C1).
C3 orbits around the earth in 10 years (relative to C1)
C3 will make 1 revolution around the earth. C2 10 revolutions.
After 10 years (earth time) they will all meet again on earth.
This is a general rule: The duration (earth time) for all observers is the same. [[Mod. note -- I don't know what you mean by that last sentence. |
The whole idea is that the experiment for all observers takes the same time i.e. 10 years (central mass or earth based) time.
> | In Tom Roberts' scenario we are comparing /proper time/, i.e., readings of clocks carried by the various observers. That is, we first synchronize clocks #1, #2, and #3. |
The clocks for all observers are all only reset at the start of the experiment. During the trip no clock synchronization is performed. This is the only (?) correct way to study the behaviour of the individual clocks.
> | (To tie this together with the traditional "twin paradox", note that the aging of a person's body is a type of clock. It's not the most precise of clocks, but that doesn't matter for our /gedanken/ purposes.) |
It is the behaviour of the (moving) clocks during the experiment to mimic the aging of the twins from the start to the end.
> |
-- jt]]
The purpose of this posting is to mention that any experiment which tries to demonstrate the importance or influence of acceleration is rather complex. The movement of the objects involves 'gravity' (Newton's Law or GR) and extra forces in order to control the movement of the spaceships. ONLY in order to describe the behaviour of the clocks the speed of light is involved. |
The above mentioned experiment is important because it shows that identical clocks under different circumstances i.e. different path through space, different forces, different accelerations, will behave differently. In this particle setup the forces are rocket propulsion forces and gravity forces. The propulsion forces are used to bring the spaceship (clock) in orbit at the correct distance and with the correct speed. Secondly to bring the spaceship and clock back home, back to earth. The force of gravity is used to keep the speceship in orbit. The same experiment can also be done without a central mass. In that case propulsion forces have to be used to keep the spaceship in 'orbit' In both cases the result will be the same. The spaceship in the closest orbit will run the slowest, relative to the frame of the earth.
It also should be mentioned that clocks are not governed by laws. All clocks function partly mechanical partly electronic. https://nawcc.org/index.php/just-for-kids/about-time/how-does-it-work In some sense each clock is described by its own set of mathematical equations or laws when it is at rest on earth. Moving clock relative to this rest frame will run slower.
Nicolaas Vroom.
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