1 quantum teleportation and time reversal

Chris

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Quantum mechanics tells us what results we'll get. It doesn't say anything about how it works, and popular science descriptions are usually just misleading.

Sylvia.

[*] Spell check says that this isn't a word, but what does it know?

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>	In popular descriptions of quantum teleportation I see time and again that it is explained by spooky action at a distance which is also simultaneous.

There is something "spooky" about entanglement for other (intrinsic) reasons, but it does not violate relativity since no information is transferred. Indeed, quantum teleportation, beside entangled particles, needs a classical channel to transmit classical information from the origin site to the destination site, where that information is needed to reconstruct the state. As WP puts it:

4 quantum teleportation and time reversal

Chris,

I don't believe that is quite correct. Events that are on each others light cone are effectively simultaneous (the metric separation is zero), and that is true for all observers. It is only spatially separated events that have an ambiguous time sequence, and temporally separated events have an ambiguous spatial sequence.

Rich L.

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Yes, for such pairs of events the metric separation is zero, and that applies for all observers (it is really independent of observer and coordinates).

But that is not at all what "simultaneous" means. Rather, that is a lightlike interval.

Simultaneous means at the same time, and that is a coordinate-dependent concept (because a coordinate system defines what one means by "time") -- for a given coordinate system all events with the same value of the time coordinate are simultaneous with each other. In an inertial coordinate system, for each such value the locus of events is necessarily a 3-D spacelike hypersurface. In general, for different coordinate systems these loci are completely different.

Tom Roberts

6 quantum teleportation and time reversal

I don't like the term "spooky action at a distance". It implies something non-sensical, yet there is all this evidence of something happening that is very different from our normal macroscopic based intuition.

>	Quantum mechanics tells us what results we'll get. It doesn't say anything about how it works, and popular science descriptions are usually just misleading.

You are correct, of course, that QM does not offer any mechanism for this behavior, and I agree that the popular descriptions are little or no help. But I do think that this is something worth thinking about in order to get a deeper understanding, perhaps beyond QM. I've been studying QM since the early 1970's, and it is only in the last decade or so that there has been much interest in the "interpretation" of QM and what it implies about the mechanisms of particle propagation. I think this is long over due.

>	Sylvia. [*] Spell check says that this isn't a word, but what does it know?

BTW, I like "actee" as a word, even if it isn't in the dictionary!

Rich L.

7 quantum teleportation and time reversal

On 14/06/2022 17:15, Tom Roberts wrote:

>	Simultaneous means at the same time, and that is a coordinate-dependent concept (because a coordinate system defines what one means by "time") -- for a given coordinate system all events with the same value of the time coordinate are simultaneous with each other.

No, that is fundamentally upside down:

The "time" we experience is *proper* time, which is the (physical) time clocks and everything tick *locally* (i.e. at and per themselves), and is *universally* (an invariant of the geometry and) one and the same for every observer, clocks and everything, i.e. not even restricted to inertial motion or observers. So, it is *proper* time that is (physical) "time" and, not per chance, modulo more speculative research on time itself, it is the primary parameter of any dynamical system.

OTOH, coordinate-time is an artefact of coordinate systems, i.e. of a choice of reference frame, and it is a *relative* notion that introduces distortions such as time dilation and length contraction for the trajectory of any particle that is not simply at rest in that frame. It is indeed overall "just" related to *simultaneity*, i.e. a class of procedures to *measure* things (distances, speeds, etc.) that are not local. But, indeed, as we should very well know and never forget: time dilation is not actually happening on board that ship...

Julio

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Rich L.

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No, it is not. It is what these words mean, and what is required for determining simultaneity for spatially-separated objects. That is what this (sub-)thread was about.

>	The "time" we experience is *proper* time, which is the (physical) time clocks and everything tick *locally* (i.e. at and per themselves), and is *universally* (an invariant of the geometry and)

Sure, though you use an unusual and useless meaning for the word "universal". Yes, the proper time of a given observer or object is independent of coordinates (aka invariant). But it is not, and cannot be, "universal" in the usual sense of the word: applying to everything -- it only applies to the specific observer, object, of clock in question.

>	one and the same for every observer, clocks and everything,

No. Each observer, object, or clock's proper time is completely independent of the proper time for other observers, objects, or clocks. Proper time is a property of a worldline, separate for each worldline.

>	i.e. not even restricted to inertial motion or observers.

Yes.

>	So, it is *proper* time that is (physical) "time" and, not per chance, modulo more speculative research on time itself, it is the primary parameter of any dynamical system.

Hmmm. This depends on what one wants to do. Certainly the standard relativistic equation of motion for a given object refers to its proper time. The ticking of any clock displays its proper time.

But the subject was simultaneity, for objects separated spatially. For that, the proper time of any object is useless -- a given observer, object, or clock can only apply its proper time to events located along its worldline, which does not include objects separated spatially.

For spatially separated objects, one must DEFINE what one means by "simultaneous", and that means setting up a coordinate system. As I said.

>	OTOH, coordinate-time is an artefact of coordinate systems, i.e. of a choice of reference frame, and it is a *relative* notion

Yes. Necessarily so. "Simultaneous" is a relative concept. (Simultaneity is of course only one aspect of a coordinate system.)

>	that introduces distortions such as time dilation and length contraction for the trajectory of any particle that is not simply at rest in that frame.

Hmmm. This is garbled. The coordinate system does not induce "time dilation and length contraction" -- they are generated by certain measurement procedures applied to objects moving relative to an inertial coordinate system.

Tom Roberts

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>

You are correct, of course, that QM does not offer any mechanism for this behavior, and I agree that the popular descriptions are little or no help. But I do think that this is something worth thinking about in order to get a deeper understanding, perhaps beyond QM. I've been studying QM since the early 1970's, and it is only in the last decade or so that there has been much interest in the "interpretation" of QM and what it implies about the mechanisms of particle propagation. I think this is long over due.

Part of the problem is that the thinking can not go much beyond pure speculation until and unless we get experimental results that are either inconsistent with QM, or are consistent with it, but not fully described by it, in the latter case indicating that QM is incomplete.

Sylvia.

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Agreed, but part of the problem has been that there has been little thinking (except by a very few) about what quantum mechanics implies about our concepts of time and causality, nor how the Born Rule works. There are two ways to discover deeper theories about such things. One is by an experiment that gives an unexpected result (e.g. the Lamb shift) that inspires a theory. The other is a speculation that permits an experimental test. (And then there are the speculations that are inherently untestable, e.g. the so called many worlds interpretation of QM.)

I suspect out current impasse in physics is due to an unexamined assumption in our theory. While it would be nice for an experiment to lead us in that direction, until then we can speculate and try to test those ideas. But first we have to admit that there is something that needs a deeper understanding.

Rich L.

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Perhaps QM has reached the bottom level, and that's just how the universe is.

Sylvia.

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That is certainly a possibility, but I'd argue that there must still be some deeper layers that we should be curious about. For example, in something like the two slit experiment, how does the photon end up in only one spot always, as opposed to only statistically on average one spot? For a given photon we calculate a wave function that can be used to predict the probability distribution of photon destinations. If the wave function represents some aspect of something real, then why don't we sometimes see two photons arriving in different places? (NOTE: I'm not suggesting that the wave function IS a real thing, it is a mathematical object that we use to calculate probabilities. Rather I'm suggesting that the wave function appears to capture some aspect of something that is real.)

Now one way around that question is to say that the photon DOES follow one path, and thus does end up in one location. But that suggests that there is some mechanism that ensures that the photon is guided so that it ends up, after many trials, reproducing the probability distribution. This is the pilot wave interpretation of QM.

But even this has a problem with entanglement experiments and Bells Inequality. It appears that the results of a measurement at one location depends on a measurement at another spatially separated location. This coordination of results is very hard to understand if we still believe special relativity and locality. This reasoning is why I strongly suspect that there is a deeper mechanism that we would like to understand.

Of course this deeper mechanism, if it exists, might be too subtle for us to discern experimentally. In that case, as I believe you've indicated, this level is not scientifically accessible. But there are enough hints that our physics is not complete yet that I'm hopeful that we can still dig a little bit deeper.

Rich L.

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IMO simultaneity means that at any moment in the universe millions of events take place and that all these events are happening simultaneous. Each of these events can be linked to a spatially-separated object. This physical implies that none of these events can influence any of the other events directly.

You can only claim that the primary cause of these events lies in the past, and that the results of these events (what happens next) can only influence each other in the future.

The reason why I use this approach is that this is the easiest way to simulate the evolution of our galaxy. That means one coordinate system and one clock.

> >	one and the same for every observer, clocks and everything,

>	No. Each observer, object, or clock's proper time is completely independent of the proper time for other observers, objects, or clocks.

Suppose there exists a same type of civilisation, like ours, near another star at a planet which resembles ours. Suppose we both want to predict the future of our galaxy. Is that possible? IMO the biggest problem is time keeping and to try to synchronise our clocks with their clocks.

>	But the subject was simultaneity, for objects separated spatially. For that, the proper time of any object is useless -- a given observer, object, or clock can only apply its proper time to events located along its worldline, which does not include objects separated spatially.

IMO to have two have observers, each with their own coordinate system each considering their system at rest, creates havoc.

> >	OTOH, coordinate-time is an artefact of coordinate systems, i.e. of a choice of reference frame, and it is a *relative* notion

>	Yes. Necessarily so. "Simultaneous" is a relative concept. (Simultaneity is of course only one aspect of a coordinate system.)

I don't understand what you mean with relative concept. My understanding is that each observer, at the origin of his coordination system, with his clock, based on his observations of 10 events, will have different ideas which events are simultaneous or not. There could also be a third observer. Which one is correct?

>	The coordinate system does not induce "time dilation and length contraction" -- they are generated by certain measurement procedures applied to objects moving relative to an inertial coordinate system.

The problem starts when objects are linked to different coordinate systems moving relative to each other and using their own clocks.

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>	For example, in something like the two slit experiment, how does the photon end up in only one spot always, as opposed to only statistically on average one spot?