I Synchronizing clocks at different locations to measure speed of light

[ESponge2000]

Yes, it would, because "the time showing on Andy's clock when I answered Andy's call" is a convention.

What would not be a convention would be "the time showing on Andy's clock when he emitted the light signal that I receive at a certain time on my clock". But that is not the same thing as "the time showing on Andy's clock when I answered Andy's call". Think carefully.


That's true, this is not a convention, because it's a time at Andy's location of an event at Andy's location. But it's not the same thing as "the time showing on Andy's clock when I answered Andy's call"--the latter is a convention, because it's assigning a time on Andy's clock to an event that is at a different location. Any such time is a convention.


No, it's not an "absolute distance". There is no such thing in relativity.


Not as far as physics goes, no. The physics of how the cop's radar detector works is the same whether the judge accepts your argument or not.

here’s what’s still tricky . Distances are not absolute, time is not absolute, velocities are not absolute , even a directional speed of light is not absolute, even the roundtrip apparent speed of light isn’t absolute unless it travels through no medium. But all reference frames can use SI or can they? What we sent out on the Golden record on Voyager One Space probe is still universally accurate , correct ? Because if there is intelligent life they would be able to model earth and earth at rest and then use our own space-time diagram for calculating lengths and time, right?

 

[PeterDonis]

all reference frames can use SI or can they?

If you mean SI units, the choice of units is part of the choice of frame. So not, not "all" frames use SI, because some frames choose different units. SI units are a convention.

What we sent out on the Golden record on Voyager One Space probe is still universally accurate , correct ?

I have no idea what you think this has to do with relativity or the issue under discussion in this thread.

 

[Nugatory]

In fact the prosecutor if they brought in a physicist as an expert witness , The physicist would testify to the court that...

Or the physicist could testify that the car's coordinate velocity using a frame in which the roadway is at rest (which is what the statute means by "speed") is unaffected by all this stuff so no relativity/frame dependence hanky-panky can be relevant to the facts of the case... and they're pretty sure that they were only brought into the case because someone still doesn't understand what relativity does and doesn't do.

 

[weirdoguy]

here’s what’s still tricky

What's tricky is that it seems that you don't belive (maybe subconsciously) that relativity is correct. What's the point of this long dissertations? As a teacher, I can assure you, they are not helping, even though you may think otherwise. Go get a textbook on relativity and use your time usefully.

 

[ESponge2000]

Am I correct to say nothing in relativity is not compatible with QE because we already have that 2 locations don’t have a simultaneity and QE does not attempt to assign a time at more than one location of a measurement, which is already consistent with relativity there’s no way of knowing a time in another location correlated to a choice of action at a different location

But maybe that’s not even the main reason both QE and SR compatible the main reason is SR concerns casually connected events in spacetime but does not impede on events that are correlated but not causally connected , correlations that don’t have meaningful cause-effect are an exception to the communication velocity limit since communication isn’t transmitted. To establish simultaneity in a quantum collapse you must have FTL communication since that contradicts relativity we also have no simultaneity framework for quantum collapses either, knowing it or a true simultaneity here is incompatible with both frameworks.

The frameworks don’t contradict each other but both are incomplete when applying GR to quantum theory

 

[PeterDonis]

QE

What is QE?

 

[Nugatory]

Am I correct to say nothing in relativity is not compatible with QE….

I think that by “QE” you mean quantum entanglement? If so what you’ve written is kinda OK as a B-level answer to how we reconcile relativity with the apparent action at a distance behavior of entanglement.

But be aware that there are some subtleties here…

If you aren’t working from a graduate level textbook, it is a near certainty that you’ve been reading about non-relativistic quantum mechanics. Absolute time is baked into this non-relativistic formulation - the $t$ that appears in Schrodinger’s equation is the reading of a mythical giant clock in the sky to which everyone’s clock is synchronized, inconsistent with relativity.

 

[ESponge2000]

I think that by “QE” you mean quantum entanglement? If so what you’ve written is kinda OK as a B-level answer to how we reconcile relativity with the apparent action at a distance behavior of entanglement.

But be aware that there are some subtleties here…

If you aren’t working from a graduate level textbook, it is a near certainty that you’ve been reading about non-relativistic quantum mechanics. Absolute time is baked into this non-relativistic formulation - the $t$ that appears in Schrodinger’s equation is the reading of a mythical giant clock in the sky to which everyone’s clock is synchronized, inconsistent with relativity.

But that’s a fault of quantum mechanics using a “t” in a way that isn’t the same usage as in SR .
The t in quantum should be outside of any local clock and property of no point in space , a quantum field … The way this was explained to me is you have stochastic probability wave yes like schrodingers cat, at measurement nothing is created nor altered , a past present future is revealed that wasn’t revealable otherwise …
In quantum mechanics there’s no relevance to the question “who measured first”, It just is what the correlation is at measurement and can’t be before so. In this framework there’s also no simultaneity
But the “t” just like in SR a convention is used for relativity of simultaneity … You can perhaps say the quantum mechanics conventions are the quantum system’s inertial frame defines the clock, as if it’s one object , and also the same Einstein convention we use in relativity of simultaneity …. Those assumptions are would create a framework where a single t value holds

Where terminology is arbitrary :

Didn’t Einstein also create a confusion by we say objects with mass increases with kinetic energy. But in E=MC^2 Einstein means Only “rest mass” not total mass , Which makes perfect sense because kinetic energy is “relative” ! The total mass of an object if altered by its velocity is relative to the object it is in relative motion with , so the only meaningful value of an object’s mass is its mass at rest . But then we have .::: photons have energy , blue light photons in fact have more energy than red light photons …. Light has energy
But … photons travel at c because they are massless … energy and mass are interchangeable… contradiction?

Einstein redefined mass to mean rest mass only . That means we have non-zero mass and require infinite energy and would be total mass Infinity to accelerate to the speed of light, hence impossible. Einstein M though is our rest mass which doesn’t change when we accelerate

Photons travel only at c , they have zero rest mass, but in motion at c the effect is a convergence on a finite , not infinite , total energy. Am I understanding this right ?,

 

[PeterDonis]

that’s a fault of quantum mechanics using a “t” in a way that isn’t the same usage as in SR .

It's a fault of non-relativistic QM using "t" that way. But non-relativistic QM is just an approximation anyway.

Quantum field theory does not have any such issue; it is perfectly consistent with SR.

The t in quantum should be outside of any local clock and property of no point in space , a quantum field

No, it isn't. Please do not try to speculate. QFT is a complex subject, and if you want to understand how QM is reconciled with SR, you need to take the time to learn it.

 

[PeterDonis]

The way this was explained to me

By whom? From what source?

 

[PeterDonis]

you have stochastic probability wave yes like schrodingers cat, at measurement nothing is created nor altered , a past present future is revealed that wasn’t revealable otherwise …

Some QM interpretations might work this way, but not all of them do.

In quantum mechanics there’s no relevance to the question “who measured first”

There is if the measurement events are timelike or null separated. Only if the measurements are spacelike separated is there no invariant time ordering.

Discussion of QM belongs in the QM forum, not this one. And discussion of QM interpretations, which is what you're getting into here, belongs in the QM interpretations subforum. And any such discussion would need to be based on a valid reference, not just a vague "someone told me".

 

[ESponge2000]

I agree and I’ll stop here. You would know more about quantum and SR than me.

 

[cianfa72]

Ultimately, it's just a choice of whether the imaginary coordinate grid you draw over spacetime is made of imaginary orthogonal lines (isotropic one-way speed) or imaginary non-orthogonal ones (non-isotropic one-way speed).

Just to take it simple, consider a 2D spacetime in the realm of SR (no gravity at all). We want to draw coordinate lines over it. As first set of coordinate lines, take a flock of identical, free-falling clocks filling the entire space such that:

• their (timelike) worldlines do not intersect
• round-trip time of light signals traveling from any clock to any other and back doesn't change (as measured from the clock sending and receiving back the light signal)

That the above construction results to be feasible, follows from SR flat Minkowski geometry.

Next, we need to draw over the other set of (spacelike) coordinate lines. To do that, we can use for instance Einstein's synchronization procedure/convention starting from a "master clock". This way we label each event with a time coordinate. Now draw over the lines connecting all events with the same "assigned time label": they define the coordinate lines in the other set.

From the above procedure, we get a coordinate grid (i.e. global chart) drawn over the 2D flat spacetime made of Minkowski-orthogonal coordinate lines. In this chart/coordinates the one-way speed of light is, by very construction/definition, isotropic with the same speed.

P.s. note that free-falling is a physical/invariant definition (i.e. zero proper acceleration/zero reading of accelerometers attached to each of those clocks).

 

[cianfa72]

From the above procedure, we get a coordinate grid (i.e. global chart) drawn over the 2D flat spacetime made of (Minkowski) orthogonal coordinate lines. In this chart/coordinates the one-way speed of light is, by very construction/definition, isotropic with the same speed.

Conversely, we can pick a synchronization procedure different from Einstein's one giving us a different synchronization convention in which the one-way speed of light turns out to be no longer isotropic.

 

[pervect]

In the context of my favorite easy-to-understand (though not necessarily most accurate) test of relativity described in "The Ulitmate Speed", , if the speed of light is anisotropic, so should the speed of a relativistic electron beam. For a sufficiently high energy, in fact, we should observe that relativity predicts they both move at essentially the same speed for the same energy. So any games you play with the speed of light will also affect the speed of the electron beam.

 

[PAllen]

Something worth noting in these discussions is that isotropy can be assumed for some physical process unrelated to light, and this will then force the consequence of one way light speed isotropy. For example, suppose you assume that moving two collocated identically constructed clocks away from each other with identical acceleration profile in opposite directions leaves them synchronized - no matter what direction you move them. Then this is equivalent to Einstein synchronization and forces you to also choose light speed isotropy. This also gets at why everyone makes this choice. It would be quite cumbersome to do physics with a convention that makes this clock assumption false.

 

[cianfa72]

For example, suppose you assume that moving two collocated identically constructed clocks away from each other with identical acceleration profile in opposite directions leaves them synchronized - no matter what direction you move them.

Sorry, how can one check/ascertain that they stay synchronized?

 

[Ibix]

Sorry, how can one check/ascertain that they stay synchronized?

You assume it. It's equivalent to an assumption that $\gamma$ is not a function of direction, i.e. Einstein synchronisation. So you can verify against a flock of inertial Einstein synchronised clocks that their proper times have relationships to coordinate time.

 

[cianfa72]

You assume it. It's equivalent to an assumption that $\gamma$ is not a function of direction, i.e. Einstein synchronisation. So you can verify against a flock of inertial Einstein synchronised clocks that their proper times have relationships to coordinate time.

As far as I can understand, your point is: take two colocated clocks that initially read/show the same time.Then accelerate away from each other with the identical profile of proper acceleration in opposite directions. Now you can check the reading of each of them at the same time as defined by the coordinate time "shared" by a flock of inertial Einstein's synchronizated clocks.

Now, the "to be synchronizated" condition for them amounts to show/read the same time value (i.e. showing the same proper time value).

 

[jbriggs444]

Now, the "to be synchronizated" condition for them amounts to show/read the same time value (i.e. showing the same proper time value).

As I understand things...

Definition: two clocks are said to be "synchronized" if their readings when located at simultaneous events are equal. Conversely, for synchronized clocks, their readings can only be identical if the clocks are located at a pair of simultaneous events.

Obviously, the state of being synchronized in this sense depends on the chosen simultaneity convention.

In the case at hand, the assumed simultaneity convention is such that pairs of clocks prepared and accelerated as described remain always synchronized.

The claim is that this simultaneity convention is viable and matches Einstein synchronization. That claim seems obviously correct.

 

[pervect]

As I think about the issue more, I h ave to note that my calculations were NOT based on an experimental defintion for energy (or momentum), but rather on symmetry principles, i.e. Noether's theorem.

Most active posters in this thread are already familiar, but for any lurkers who may not be, I'll point to the wiki article, https://en.wikipedia.org/wiki/Noether's_theorem.

I do believe tying the theory to experiment is still a great goal - I just don't think my approach necessarily realizes that goal yet.

The theoretical approach I used is based on space-time symmetries, and it is open to questions of how we break space-time into space + time. Clock syncrhonization defines which set of points are considered to be "at the same time", so it's integral to the issue, but not something I've thought about enough yet.

Specifically, it seemed clear to me that my approach tied energy and momentum to a specific way of dividing space-time into space+time, and it's unclear how this affects the experiments.

 

[cianfa72]

As I understand things...

Definition: two clocks are said to be "synchronized" if their readings when located at simultaneous events are equal. Conversely, for synchronized clocks, their readings can only be identical if the clocks are located at a pair of simultaneous events.

Obviously, the state of being synchronized in this sense depends on the chosen simultaneity convention.

In the case at hand, the assumed simultaneity convention is such that pairs of clocks prepared and accelerated as described remain always synchronized.

I.e. they remain always synchronized w.r.t. the definition you gave (namely their readings are identical if they are located at a pair of simultaneous events -- in the case at hand simultaneous w.r.t. the simultaneity convention given by Einstein's synchronization procedure applied to a flock of inertial clocks at rest each other).

 

[PAllen]

Note, an invariant fact is that if you move the clocks away and then back together with identical acceleration profile, they will be in synch. The part subject to choice is whether or consider them synchronized while apart. There is no way to check this without some other convention. If you check them apart using light signals assuming light speed isotropy, you find them in synch. If you check them assuming anisotropy, you find them out of synch.

 

[cianfa72]

Note, an invariant fact is that if you move the clocks away and then back together with identical acceleration profile, they will be in synch. The part subject to choice is whether or consider them synchronized while apart. There is no way to check this without some other convention.

Ok yes, definitely.

If you check them apart using light signals assuming light speed isotropy, you find them in synch. If you check them assuming anisotropy, you find them out of synch.

Sorry, could you explain in detail how you plane to check those clocks when they are apart, by using light signals ? Thanks.

 

[PAllen]

Ok yes, definitely.


Sorry, could you explain in detail how you plane to check those clocks when they are apart, by using light signals ? Thanks.

At time t, clock 1 sends a signal to clock 2, triggering clock 2 to send its time reading to clock 1. We are assuming the clocks mutually stationary after having separated with identical acceleration profile. Also, that they started out in synch before separation. At some time on clock 1: t+k, the clock 2 reading arrives at clock 1. The clock 2 reading will be t+(k/2). All of this, so far, is invariant physics.

Now assumptions come in. If you assume one way light speed is isotropic, the clock 2 reading is what you expect for separated clocks that are synchronized. So the clocks are still synchronized under this model. If, instead you assume (for example) that the outbound light speed is 1.5c, inbound .75c (one of the allowed combinations), then, for properly synchronized clocks you expect the clock 2 reading to be t+(k/3). Since it is not, you conclude that the separation process has desynchronized the clocks ( because time dilation is anisotropic, in this model). So you are correctly observing the desynchronization predicted under this model.

 

[cianfa72]

At time t, clock 1 sends a signal to clock 2, triggering clock 2 to send its time reading to clock 1. We are assuming the clocks mutually stationary after having separated with identical acceleration profile. Also, that they started out in synch before separation. At some time on clock 1: t+k, the clock 2 reading arrives at clock 1. The clock 1 reading will be t+(k/2). All of this, so far, is invariant physics.

I believe you meant: when the light signal arrives back at clock 1 from clock 2 (at clock 1's own time t+k), the clock 2's sent encoded own time will be t +(k/2). This will be the reading/information that clock 1 "extracts" from the signal received back from clock 2.

 

[PAllen]

I believe you meant: when the light signal arrives back at clock 1 from clock 2 (at clock 1's own time t+k), the clock 2's sent encoded own time will be t +(k/2). This will be the reading/information that clock 1 "extracts" from the signal received back from clock 2.

Yes, fixed now.

 

[cianfa72]

At time t, clock 1 sends a signal to clock 2, triggering clock 2 to send its time reading to clock 1. We are assuming the clocks mutually stationary after having separated with identical acceleration profile. Also, that they started out in synch before separation. At some time on clock 1: t+k, the clock 2 reading arrives at clock 1. The clock 2 reading will be t+(k/2). All of this, so far, is invariant physics.

Yes, that is an invariant fact. We can check it taking the viewpoint of the inertial frame with coordinates $(x,t)$. The worldlines of the two clocks (proper) accelerating apart with the same acceleration profile are represented in those inertial coordinates by the same timelike curve symmetric about a straight line parallel to the $t$ axis. As you pointed out, the two clocks are assumed to be mutually stationary after receding apart with the same acceleration profile (as measured/assessed by constant round-trip travel time for the light signals exchanged between them). That's means, I believe, from that point on their worldlines became parallel straight lines when represented/drawn in inertial coordinates. By drawing in this chart the relevant light cones' boundaries representing the paths taken from light signals exchanged between those clocks, we can check your "invariant fact" claim.

 

[PAllen]

Yes, that is an invariant fact. We can check it taking the viewpoint of the inertial frame with coordinates $(x,t)$. The worldlines of the two clocks (proper) accelerating apart with the same acceleration profile are represented in those inertial coordinates by the same timelike curve symmetric about a straight line parallel to the $t$ axis. As you pointed out, the two clocks are assumed to be mutually stationary after receding apart with the same acceleration profile (as measured/assessed by constant round-trip travel time for the light signals exchanged between them)

No, as measured locally by an accelerometer, or by design with preprogrammed propulsion systems. The aim is to demonstrate:
1) There exist simultaneity conventions having nothing to do with light that are equivalent to the Einstein convention.
2) You only get to make one choice for isotropy (in cases where there is a choice not constrained by physics). If you assume it for one such case, then consistency forces the same choice for all other cases that have a choice. There is, in a sense, only one parameter controlling all such cases.

That's means, I believe, from that point on their worldlines became parallel straight lines when represented/drawn in inertial coordinates. By drawing in this chart the relevant light cones' boundaries representing the paths taken from light signals exchanged between those clocks, we can check your "invariant fact" claim.