Undergrad Can we determine the one way speed of light by combined measurements?

[PeroK]

So what is then the goal of this forum if people need to know all details first because then no questions are needed anymore of course?

The basics are important. It's like asking us to explain a game of chess, but refusing to learn the rules.

 

[Dale]

it looks counterintuitive. So that was for me the reason to ask this question. So now the conclusion probably is that there are no consequences for this laserbeam but I still cannot understand the details of that

The reason it is counterintuitive is because you don’t understand the right fundamental concepts. The details are not relevant once the underlying principles are understood. There are two important concepts:

1) the laws of physics and the outcomes of all physical experiments are independent of the coordinate system used. This is called covariance.

2) the one way speed of light is purely an aspect of your chosen coordinate system. Changing your coordinate system changes the one way speed of light.

Please focus your attention on these two correct concepts. Once you understand them then it will be intuitive.

So this means I first need to understand the full theory of relativity and how the 1 way speed of light fits into that before coming back with a question at all. So what is then the goal of this forum if people need to know all details first because then no questions are needed anymore of course?

That is a strawman. Nobody said that. We are saying that your approach of “please poke holes in this scenario” is ineffective. That in no way implies that you need to understand everything before coming here with a question. There are many other strategies in between.

What this actually means is that you need to listen to the answers you have received. We have told you that the specific approach you are using is ineffective and we suggested effective approaches. We have explained the correct principles which, if you didn’t understand those explanations, is an invitation to ask follow up questions about those correct principles.

That is what this forum is useful for: asking experts who understand this material and are interested in sharing and then actually using their answers to gain understanding.

 

[Nugatory]

So this means I first need to understand the full theory of relativity and how the 1 way speed of light fits into that before coming back with a question at all.

No, but it does mean that until you understand the theory you will be unable to construct effective challenges to the predictions of the theory. Sure, you can spend the rest of your life thinking up ever more complicated and creative ways of trying to measure the one-way speed of light - but every one must fail because every one will be a more or less clever way of hiding the same false underlying assumption about absolute simultaneity. This is not an effective way of learning anything, and eventually people will tire of answering questions of the form “I know this can’t work, but tell me where I hid the simultaneity assumption this time?”.

But if instead your questions come from trying to understand the theory, they will be welcome and will receive many helpful answers from the army of unpaid volunteers here who believe that helping people understand physics is a worthy cause.

Edit: @PeroK’s two-sentence response in #92 is the example that I was looking for.

 

[HansH]

@ Dale, thanks. This again shines a different light on what I thought 1 way speed of light means. I will digg into your advice first before coming back.

to whom it concerns: The basics depend on your own relative reference. For me that is for sure not general relativity at this moment as I had to keep to other priorities in life which is too short to do all. But I keep being interested.

Of course I do not refuse to learn the rules, but that takes time and will not be done by tomorrow. I realize asking so much questions to speed up the learning curve also asks much of the team. (but your answers help for sure). 90 responses I really appriciate. Thanks for your patience. But perhaps better indeed not to experiment with working out my thoughts to reconstruct the theory and test that with you, because that is not so effective and also costs me a lot of time. That is at least what I also learned from this topic.

For me it remains difficult to grasp the knowledge from the internet or books in a logical order because one first need to now already a lot to be able to determine that logical order needed to be effective.

It is already quite a challenge for example to get clear what is actually meant by 1 way speed of light. (not sure even if I understand that now as I understood first that it was a degree of freedom and it has no effect on what you measure, but based on last input of Dale I should conclude that it is no degree of freedom at all but only 1 on 1 related to your coordinate choice. I think I asked that also several times during the discussions, but still confusing)

 

[Sagittarius A-Star]

For me that is for sure not general relativity at this moment

For the topic of this thread, basics of SR (flat spacetime) are sufficient. GR (curved spacetime) is not needed.

as I had to keep to other priorities in life which is too short to do all. But I keep being interested. Of course I do not refuse to learn the rules, but that takes time and will not be done by tomorrow.

What you need to understand as basics for the topic are the Einstein clock synchronization and the definition of a standard inertial coordinate system:

The basic principle of clock synchronization is to ensure that the coordinate description of physics is as symmetric as the physics itself. For example, bullets shot off by the same gun at any point and in any direction should always have the same coordinate velocity dr/dt . Because of the light-postulate, photons serve particularly conveniently as such bullets in SR.
...
We should, strictly speaking, differentiate between an inertial frame and an inertial coordinate system, although in sloppy practice one usually calls both IFs. An inertial frame is simply an infinite set of point particles sitting still in space relative to each other. For stability they could be connected by a lattice of rigid rods, but free-floating particles are preferable, since keeping constant distances from each other is also a criterion of the non-rotation of the frame. A standard inertial coordinate system is any set of Cartesian x,y,z axes laid over such an inertial frame, plus synchronized clocks sitting on all the particles, as described above. Standard coordinates always use identical units, say centimeters and seconds.

Source:
http://www.scholarpedia.org/article...nematics#Galilean_and_Lorentz_transformations

 

[cianfa72]

It is that given an inertial frame if we do a measurement of two-way speed of light the result is always the same regardless of the state of motion of the light source employed w.r.t the given inertial frame.

I would like to discuss further the following point. Consider two light sources, the first (A) at rest and the second (B) moving with constant velocity $v$ w.r.t. a given inertial reference frame. Suppose the two-way speed of light emitted from the stationary source (A) is isotropic with fixed speed $c$. Then from the principle of relativity it follows that the two-way speed of light emitted from source B measured in the inertial frame in which it is at rest must be isotropic with fixed value $c$ as well.

Note that the principle of relativity does not imply at all that the two-way speed of light as measured in an inertial frame may not depend on the state of motion of the source. So the two-way speed of light emitted from source B measured in the first inertial frame (i.e. the rest frame of source A) might not be isotropic and/or with constant speed $c$.

 

[Sagittarius A-Star]

Note that the principle of relativity does not imply at all that the two-way speed of light as measured in an inertial frame may not depend on the state of motion of the source.

But the second postulate of SR implies that the two-way speed of light as measured in an inertial frame does not depend on the state of motion of the source.

 

[cianfa72]

But the second postulate of SR implies that the two-way speed of light as measured in an inertial frame does not depend on the state of motion of the source.

Yes of course, as you said it is the second postulate of SR and it is logically indipendent from the the principle of relativity (the first principle).

 

[Dale]

The basics depend on your own relative reference. For me that is for sure not general relativity

I agree. I apologize if I gave that impression. I was just asking about your background because if you did happen to already know the math of GR (tensors) then the explanation is very easy. But this problem is not itself a GR problem and does not require knowledge of GR.

It is already quite a challenge for example to get clear what is actually meant by 1 way speed of light

When you decide to pursue this topic further, that might be the place to start.

 

[cianfa72]

But the second postulate of SR implies that the two-way speed of light as measured in an inertial frame does not depend on the state of motion of the source.

Just to highlight that assuming both the first and the second postulate force us to give up galilean transformation between inertial reference frames (i.e. global inertial coordinate charts for flat spacetime) in favour of Lorentz transformations.

 

[HansH]

For the topic of this thread, basics of SR (flat spacetime) are sufficient. GR (curved spacetime) is not needed.
What you need to understand as basics for the topic are the Einstein clock synchronization and the definition of a standard inertial coordinate system:Source:
http://www.scholarpedia.org/article...nematics#Galilean_and_Lorentz_transformations

Thanks again (also the other people who responded) for investing additional energy in this. I think this could help not only me but also others not having deep knowledhge of he matter. I think I know the principle of Einstein clock synchronization and the definition of a standard inertial coordinate system (that should be basic high school level?) as I now have looked to that film given at the start of the topic several times. I even looked in detail at all the ways mentioned to synchronize clocks in that film including the one to start with both clocks together. But my main problem remains to recognize that things like laser beams interfering and lightbeams reflecting with an angles in a mirror should behave different if the speed of light really would vary in different directions. So using the synchronisation of clocks in mind and using simple standard inertial coordinate system, my only conclusion could be that this cannot explain different speeds than c so if it is different than c it must be some kind of calculation trick. so if this really does not influence whatever measurable effect, then there should be something underlying that I do not catch at the moment. so how can we get that clear?

 

[Sagittarius A-Star]

But my main problem remains to recognize that things like laser beams interfering and lightbeams reflecting with an angles in a mirror should behave different if the speed of light really would vary in different directions.

Physics does not change if you only transform the mathematical description to a different coordinate system, like:

Given any inertial coordinate system x,y,z,t, we are free to apply a coordinate transformation of the form

$$ x'=x \ \ \ \ \ y'=y \ \ \ \ \ z'=z \ \ \ \ \ t'=t+\frac{kx}{c}$$
where k = 2ε – 1. In terms of the primed coordinates the speed of light is then dependent on the angle of the light ray with respect to the x axis.

Source:
https://www.mathpages.com/home/kmath229/kmath229.htm

lightbeams reflecting with an angles in a mirror

Short argument for the angle is not different: In the above cited transformation, only the time coordinate gets transformed. The spatial coordinates stay the same and therefore also the reflection angle. The long argument of @Dale is redundant to that short argument.

 

[Dale]

then there should be something underlying that I do not catch at the moment. so how can we get that clear

Perhaps (not certainly) the issue is that you don’t fully recognize the anisotropic one-way speed of light as a simple coordinate transform. You appear to have some proficiency with mathcad. Perhaps you should implement the coordinate transform posted previously and play around with it to convince yourself that it does indeed change the one way speed of light but not the two way speed of light.

 

[Sagittarius A-Star]

things like laser beams interfering

There is the same argument: In the above cited transformation, only the time coordinate gets transformed. The spatial coordinates stay the same and therefore also the (x,y,z) coordinates of each part of a static interference pattern.

 

[Sagittarius A-Star]

I think I know the principle of Einstein clock synchronization

If you define your 4D-coordinate system based on an Einstein clock synchronization, then you have an isotropic one-way speed of light (because you have stipulated it this way).

If you define your 4D-coordinate system based on a non-Einstein clock synchronization, then you have an anisotropic one-way speed of light (because you have stipulated it this way).

 

[PeroK]

If you define you 4D-coordinate system based on an Einstein clock synchronization, then you have an isotropic one-way speed of light (because you have stipulated it this way).

If you define you 4D-coordinate system based on a non-Einstein clock synchronization, then you have an anisotropic one-way speed of light (because you have stipulated it this way).

I think the OP's assumption is that these must represent two different physical realities. He's assuming that they can't both be valid, so it must be one or the other. And, is trying to devise an experiment to reveal which one is correct.

The OP does not see how both these conventions can be possible in the same universe.

Perhaps (not certainly) the issue is that you don’t fully recognize the anisotropic one-way speed of light as a simple coordinate transform. You appear to have some proficiency with mathcad. Perhaps you should implement the coordinate transform posted previously and play around with it to convince yourself that it does indeed change the one way speed of light but not the two way speed of light.

And, the OP's assumption is that such a "coordinate transformation" changes the physics (as it changes the one-way speed of light).

And, since it changes the physics, there must be an experiment that proves that this coordinate transformation is not physically valid.

Finally, I was thinking of an analogy. We know that classical projectile motion problems can be done with the acceleration of gravity being $g = \pm 9.81 \ m/s^2$. I.e. we can set up a problem with "up" being positive and $g = -9.81 \ m/s^2$. Or, we can do a coordinate transformation so that "up" is negative and $g = 9.81 \ m/s^2$.

The OP's assumption would be that "up" is either positive or negative and there must be an experiment to determine which is correct.

 

[HansH]

at this moment I am mainly digesting the info I get from you, but perhaps it makes sense for the proper understanding to verify if next fragment of this movie is correct:

because of they say that you receive the light from one direction istantly and from the other direction at c/2 you see the complete universe as is it now in one direction and a it was far in the past in the opposite direction. so is this a coordinate transformation or real speed of light using same physical coordinates (as he looks on both directions using coordinates as we measure on Earth so suppose they are the same)

edit: because if it is only based on a coordinate transformation, I could use another transformation and this would mean for the movie that what I saw in one direction first seeing Mars instantly then changes into seeing Mars 20 minutes ago, so that would means I invented a time machine so could not be true.

 

[Dale]

so is this a coordinate transformation

Yes, despite the wonderment in his voice as he talks about this topic, it is nothing more than a coordinate transformation. There is no physics involved.

This is one of his worst videos ever. Not that what he says is wrong, but he leaves out the most important fact (that it is just a coordinate transform) and speaks in a way that makes the audience think he is talking about some mysterious unknown physics instead of some completely understood and mostly useless math.

if it is only based on a coordinate transformation, I could use another transformation and this would mean for the movie that what I saw in one direction first seeing Mars instantly then changes into seeing Mars 20 minutes ago, so that would means I invented a time machine so could not be true.

Well, one of the mathematical requirements of a coordinate system is that it must be a one-to-one mapping from spacetime to R4. So while you could change from seeing Mars instantly to seeing it 20 min ago, you would have to do so gradually in a way that avoids mapping the same event to two different coordinates. Thus you would not get a “time machine”

 

[Ibix]

you see the complete universe as is it now in one direction and a it was far in the past in the opposite direction.

This is because he has applied a coordinate transform such that "stuff hapenning now" is defined to be exactly what he sees in that one direction (and stuff he hasn't seen yet in others).

He hasn't changed what he sees, he's chosen to change how he interprets what he sees.

 

[Ibix]

I should add that I think the $c_+=\infty$, $c_-=c/2$ case is on rather dubious ground because one of your "spatial" coordinates is lightlike. There's nothing wrong with it mathematically, but it has even fewer of the properties of a frame (depending on your definition of "frame", of course) than the $c<c_+<\infty$ cases have.

 

[PeroK]

At about 15:45 in this video he asks: "If it makes no difference to physics, then what is the point of talking about it?". Good question!

 

[Vanadium 50]

You had the stamina to sit through the video? My hats off to you.

 

[PeroK]

You had the stamina to sit through the video? My hats off to you.

No. The link started at 15 minutes!

 

[HansH]

This is one of his worst videos ever. Not that what he says is wrong, but he leaves out the most important fact (that it is just a coordinate transform) and speaks in a way that makes the audience think he is talking about some mysterious unknown physics instead of some completely understood and mostly useless math.

Thus you would not get a “time machine”

I think this explains a lot and could be the reason why this topic gets so long. Because this movie triggered me to find out what is going on. it sounded very strange to me and even worse, put me completely on the wrong leg by this 'looking into the past or not' explanation. so simple coordinate transformation, nothing special and no actual speed of light being different than in the way most people think of the speed of light.

 

[PeroK]

I think this explains a lot and could be the reason why this topic gets so long. Because this movie triggered me to find out what is going on. it sounded very strange to me and even worse, put me completely on the wrong leg by this 'looking into the past or not' explanation. so simple coordinate transformation, nothing special and no actual speed of light being different than in the way most people think of the speed of light.

There is a similar issue with the expanding universe. In comoving coordinates we have galaxies essentially stationary and space expanding. The galaxies have only a subluminal peculiar velocity relative to expanding space.

But, change to different coordinates and we have the galaxies moving faster-than-light (as a coordinate speed). The assumption in GR is the locally-measured (two-way!) speed of light is always $c$. And, we do not have expanding space in these coordinates.

We can't read too much into this, because these are coordinate-dependent statements. The invariant statement is that universe overall has a certain spacetime curvature, which may manifest itself as expanding space, coordinate speeds greater than $c$ or whatever else - depending on the choice of coordinates.

 

[HansH]

There is a similar issue with the expanding universe.

these are coordinate-dependent statements. The invariant statement is that universe overall has a certain spacetime curvature, which may manifest itself as expanding space, coordinate speeds greater than $c$ or whatever else - depending on the choice of coordinates.

I think you mean the statemanent that the whole universe once was a big as an orange? but you cannot talk about the size of an orange because the size itself to measure with was also reduced with the same factor? same way to put people on the wrong leg I assume. I think I get it.

 

[Ibix]

I think you mean the statemanent that the whole universe once was a big as an orange?

No - this is a common but totally incorrect statement. The observable universe was once the size of an orange (if classical gravity is valid that early, anyway), but the whole universe was always infinite as far as we are aware.

I think the point @PeroK is making is that you can make galaxy velocities vary wildly by a simple coordinate change. This is not solely a light phenomenon.

 

[cianfa72]

The assumption in GR is the locally-measured (two-way!) speed of light is always $c$.

So, regardless of the coordinate chart employed to cover locally a limited region of spacetime, the outcome of experiments designed to measure locally the two-way speed of light is always the same: it is isotropic with fixed speed $c$.

 

[Ibix]

So, regardless of the coordinate chart employed to cover locally a limited region of spacetime, the outcome of experiments designed to measure locally the two-way speed of light is always the same: it is isotropic with fixed speed $c$.

Yes. Locally, all GR spacetimes look like Minkowski spacetime for the same reason your kitchen floor looks Euclidean even though it's part of the curved surface of the Earth.

 

[HansH]

Yes, despite the wonderment in his voice as he talks about this topic, it is nothing more than a coordinate transformation. There is no physics involved.

Just one question about this movie as a check for me. at a certain moment they compare both situations left and right where left is the standard with normal Einstein synchronisation I suppose. but the question is what they are doing in the right picture: is this the coordinate transformation you talked about several times or are they showing here something else. What I would think is that they make a space time diagram with positive light speed c/2 and negative light speed infinite, but with the original horizontal end vertical coordinates (horizonta; is the distance between Earth and Mars), vertical is the time between sending a signal and receiving back the signal from Mars. but I am not sure. Also the time scale for the right picture is different for the left vertical line (20 minutes) and right vertical line (10 minutes). So it this correct what happens here?