Head Trip - Thought experiments in Scientific American of September 2015 - page 37

This document contains comments about the article Head Trip - Thought experiments by Sabine Hossenfelder In Scientific American of September 2015.
Einstein's thought experiments left a long and somewhat mixed legacy of their own


In both cases, Einstein crafted new theories about the natural world by using his mind's eye to push beyond the limitations of laboratory measurements.
In principle that is possible, however you have to be very carefull. Starting point to unravel the laws of nature should be always real observations or real experiments. To go outside this realm can cause troubles.
Today physicists regularly use thought experiments to craft new theories and to seek out inconsistencies or novel effects within existing ones.
IMO it is very tricky to unravel the way physical processes evolve just by thinking.
Suppose you are born in a cave and that is your world how can you imagine just by thinking that there are stars and planets? In fact in order to envision that you need observations and experiments.
Can thought alone sustain them? Where is the line between scientific intuition and fantasy.
I think it is better to find the line between science and speculation.
Einsteins legacy offers no certain answer. On one hand his reliance on the power of thought was a spectacular success. On the other many of his best knowb thought experiments were based on data from real experimentation such as the classic Michelson Morley experiment that first measured the constancy of the speed of light.
What the MM experiment only could measure is that the speed of light is or is not independent of its source.
It is impossible to measure locally that the speed of light is or is not the same globally.

The windowless elevator.

In his thought experiments, Einstein's genius was in realizing which aspects of experience were essential and which could be discarded.
All forms of experience i.e. facts or observations are important to unravel the laws of nature. The problem is that you easily can be wrong about the relations between the facts etc.
Einstein argued that inside a windowless elevator, a person cannot tell whether the elevator is at rest in a gravitational field or is instead being hauled up with constant acceleration.
The issue is that in both cases he feels the same pressure of his body weight on his feets.
In that situation when you cut the cable of the elevator or stop the hauling force both persons will start to "float".
He then conjectured that the laws of physics themselves must be identical in both situations.
I do not exactly understand what he means by that. The issue that there are similar processes. The mathematical equations that describe these processes are also similar. With similar meaning the parameters that describe these processes and the initial conditions. In the case of Newton's Law that are the number, masses, initial positions and velocities of the objects involved.
According to this "principle of equivalence" locally (in the elevator) the effects of gravitation are the same of that acceleration in the absence of gravity.
What is written above should be the other way around.
Something can never happening ("being the same") according to a principle. The principle is not the cause. The principle is the description of what is observed.
For more about the "equivalence principle" related to thought experiments, read this in the same issue of Scientific American: Head Trip - Thought experiments

Spooky action

Later in his career, Einstein fought hard against the tenets of quantum mechanics, particularly the uncertainty principle, which dictates that the more you know about one aspect of a fundamental particle, such as its position, the less you can know about another related aspect of that particle, such as its momentum - vice versa.
First of all the "uncertainty principle" is not a law of nature, because it makes a statement about what we humans know.
Secondly, IMO every(?) measurement disturbes or influences what you want to measure. That means most measurements can not be exactly be replicated. Except if you want to count something. I think that Einstein would agree with this.
Einstein thought that the uncertainty principle was a sign that quantum theory was deeply flawed.
IMO it is very important to know Einstein's definition of "the uncertainty principle" and of the quantum thery. When you don't know that you can not compare.
During a years-long exchange with Niels Bohr, Einstein conceived of a series of thought experiments meant to demonstrate that it is possible to violate the uncertainty principle, but Bohr dissected every one of them.
You must know the details. IMO the biggest problem is that you cannot perform thought experiments at quantum level and if you still want to perform them you must first make a list about all the points you agree about.
Skip some text:
The PR experiment unfolds as follows: imagine an unstable particle with a spin of zero decaying into two daughter particles, which speed off in opposite directions.
For an actual experiment this means that if one particle is ditected at one side always a particle should be detected at the other side.
Conservation laws dictate that the spins of those two daughter must add up to zero; one particle .. of "up", and the other .. of "down".
For an actual experiment this should be tested. That means you should test that if one particle is "up" in 100% of the cases the other one is "down". When you use this approach you do not use the conservation law but you demonstrate that the law is correct.
The laws of quantum mechanics dictate that in the absence of measurement, neither of the particle possesses a definite spin until one of the speeding entangled particles is measured.
That is the question.
What you should do is to test this 100% rule over a long distance and observe if it holds.
Such an experiment is extremely important When it holds you know that the state of the spin is stable and constant. It is easily possible that the state (spin) of the particle can become disturbed along its path by outside equipement (radar)
Once a measurement of one particle is made, the state of the other changes instantaneously, even if the particles are separated by vast distances!
Once a measurement of one particle is made, the state of that particle changes instantaneously and you know the state of the other particle
Einstein beleived this "spooky action at a distance" was nonsense.
I fully agree with him.
His own SR of relativity held that nothing could travel faster than light, so there was no way for two particles to communicate instantaneously.
SR has nothing to do with this experiment.
He suggested instead that the measurement outcomes must be determined prior to measurement by "hidden variables" that quantum mechanics failed to account for.
The problem is you should discuss actual experiments and explain (and demonstrate) what you mean by "hidden variables"

Alice and Bob

Today some of the most significant thought experiments in physics etc
Consider, for instance, the widely discussed black hole information paradox.
If you combine GR and quantum field theory, then, you find that black holes evaporate slowly radiating away their mass because of quantum effects.
The issue is are the masses of BH's decreasing? This does not seem logical, because than they should not exist.
You also find that this process is not reversible, regardless of what formed the BH, the evaporating BH always produces the same featureless bath of radiation from which no information about its contents can be retrieved.
If a BH produces radiation than a BH is not black.
But such a process is prohibited in quantum theory, which states that any occurence can, in principle , be reversed in time.
All of this seems to me very vaque.
What means reversed in time? Suppose a hugh comet collides which our Sun, can the time of this event be retrieved based on present day observations? I completely doubt that.
If that is true you can calculate all what happened bassed on present day observations.
For instance, according to the laws of QM, the left overs of a burned book still contain alll the information necessary to reassemble the book even though this information is not easily accessible.
This seems to me impossible. Laws should always be a description of actual processes.
Not so for evaporating black holes.
All of this is not very clear.
And so we arrive at a paradox, a logical inconsistency.
This whole paradox is not clear, and sorry not very scientific.
A little further:
We must be making some mistake - but where.
The mistake is that you should not use thought experiments as your starting point but real experiments.

Reflection 1 - Spooky action at a distance.

Spooky action at a distance for example means that you get a random selected message from source A while at the same time you also receive the same message from source B which is at a further distance. To explain this discrepancy faster than light signals are involved.

In order to demonstrate spooky action at a distance you should perform the following experiment. Imagine an unstable particle at point O with a spin of zero decaying into two daughter particles, which speed off in opposite directions. You measure the spin of each at a certain distance (at the points A and B at equal distance from point O) and you send the results back via een video line to point O. At point O you will observe that when at A they measure "Up" at point B they measure "Down". The correlation is -1.
At the same time also clocks are involved inorder to measure what is happening. For example the particle decays at 0.00 at point O, reaches point A at 0.10 and this is observed at 0.20 at point O. For point B the same three measurements exists: 0.00 0.10 and 0.20. Next one change is done: the measurement point A is moved closer to point O. The result will be the measurements via path A will be earlier (visible at O) For example for experiment via A we get now the three measurements 0.00 0.08 and 0.16 For the path via point B there is no difference 0,00 0.10 and 0.20.
The the correlation between A and B should remain as -1.
I do not expect that that will change. If there is a change than also point B should be moved closer to O and again the correlation should be tested.

The important point is that in this experiment no difficult mathematics is involved.

A whole different issue is why do you need entangled particles to demonstrate "Spooky action at a distance." Suppose at point A we have a source which transmits either a 1 or 0 signal to point O. At point B (at the same distance from O as A) we also have a source which transmits either a 1 or a 0 signal to point O. The expectancy is that the correlation between the two signals is zero.
It is the chalenge for the experimenter at point B to develop a device such that that is possible. IMO it is impossible.

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Created: 5 September 2015

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