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

[Dale]

so far I saw no reason why it cannot be an arbitrary value between c/2 and infinite. … the only option left is that the one way speed of light is also c in every direction

You are contradicting yourself.

As I showed above it is almost trivial to show the conventionality of the one way speed of light.

 

[HansH]

...as we have been telling you for some time now.

Let's do this properly. See the diagram in post #1. In the usual Einstein coordinates we describe light leaving S,

If you get different equations from these you are doing something wrong.

You refer to my post #01 while I already posted an updated version based on what I learned in the meantime, see https://www.physicsforums.com/threa...by-combined-measurements.1014053/post-6620171 leading to a different conclusion probably in line with what you state, but I need to dive into that first. so the discussion about the idea of post #1 is solved already Isuppose.

 

[HansH]

You are contradicting yourself.

As I showed above it is almost trivial to show the conventionality of the one way speed of light.

Right if you put these 2 sentences after each other. But the first one was before my further analysis and the second was after, so I had to change my point of view between the first end second line. (which analysis is not yet presented yet) as said I will post this in a separate topic (If I am allowed) as I assume this current topic as clear now and allowed to close.

 

[cianfa72]

If you are implicitly assuming a diagonal metric, this is equivalent to assuming an isotropic speed of light.

Yes, since starting from the expression of the metric and setting $ds^2=0$ we get a quadratic equation for the coordinate speed of light $v$. Now in order to get two equal and opposite solutions for the one-way speed of light (one for direction) the mixed term in the equation (i.e. the linear term in $v$) must vanish (i.e. the metric is diagonal).

 

[Dale]

But the first one was before my further analysis and the second was after

Ok, that clarifies. Your first one was correct. Your further analysis has taken you from a correct “before” to an incorrect “after”.

I will post this in a separate topic

There is no point in posting your analysis, I would recommend against it. It would be much better for you to actually learn the correct analyses that have been presented here. The correct analysis is almost trivial to understand, so please start there.

 

[HansH]

And if that's still the case after 70 posts, then perhaps you need to rethink your approach to learning physics?

to be clear: I am not a physics student, but an electrical engineer with a profession in a big semiconductor company as architect in integrated circuit design and systems with 35+ year experience. I do physics for hobby and hope to use this forum to learn with a steep learning curve and get answers on things I do probably wrong as this allows for such a fast learning curve. This could also cause my view to change in time. Hope that is not too much asked? something should be wrong in my assumptions could therefore also mean my latest assumptions, so not the assumptions of 70 posts ago.

 

[Dale]

an electrical engineer with a profession in a big semiconductor company as architect in integrated circuit design and systems with 35+ year experience. I do physics for hobby

Cool! I am also an engineer doing physics as a hobby.

You will still be better off focusing on learning the correct concepts and not on dissecting your mistakes.

 

[HansH]

Cool! I am also an engineer doing physics as a hobby.

You will still be better off focusing on learning the correct concepts and not on dissecting your mistakes.

yes of course. But to do that I need to do a university study physics I suppose. learning from books and the internet turned out not to be a very steep curve so far. and engineering is also very challenging. but that is offtopic. So I hope this forum helps a bit.

 

[Dale]

But to do that I need to do a university study physics I suppose

I didn’t need to. I did engineering type physics in college, and then the rest has been online. I do use Mathematica a lot and actually frequently work out problems that people post even if I don’t post my answer. I think that is the main thing for me.

The issue with learning the correct concept instead of focusing on dissecting your argument is psychological. A mind abhors a vacuum. If we help poke a hole in your arguments and you don’t understand the correct concept, then that will make a mental vacuum. You will have an empty place where you used to think you knew something, and nothing to fill it. That will make you very reluctant to actually discard the incorrect concept.

In contrast, if you learn the correct concept then you will be able to see the hole in your own argument and since you will already be able to fill it you will not have trouble discarding the incorrect concept.

 

[HansH]

Ok, that clarifies. Your first one was correct. Your further analysis has taken you from a correct “before” to an incorrect “after”.

There is no point in posting your analysis, I would recommend against it. It would be much better for you to actually learn the correct analyses that have been presented here. The correct analysis is almost trivial to understand, so please start there.

Ok I will do in a few days as said. I think there is an interesting point of view in this analysis that could be revolutionary or stupid, so could help to make a big step for me or for physics. (I suppose it is for me) I am also waiting for valuable FB from the Dutch forum, so therefore it can take a few days.

 

[HansH]

You refer to my post #01 while I already posted an updated version based on what I learned in the meantime, see https://www.physicsforums.com/threa...by-combined-measurements.1014053/post-6620171 leading to a different conclusion probably in line with what you state, but I need to dive into that first. so the discussion about the idea of post #1 is solved already Isuppose.

what you describe is indeed what I also did in the udated version as far as I can see. https://www.physicsforums.com/threa...by-combined-measurements.1014053/post-6620171
so that should not be a point of discussion anymore. Basically it says that the time you gain in 1 direction due to the 1 direction light speed you loose in another direction such that it does not make a difference which path you take or which one way light speed you take, you always get the same 2 way light speed for a certain total pathlength followed. My point is still that I think i have reason to believe that the options for selecting a 1 way light speed are very limited. But I will start a new topic for that to prevent confusion. Would be great to get that clear.

 

[Vanadium 50]

Would you explain how?

Consider the limiting case where the speed of light is zero left to right and non-zero right to left. Energy can move to the left but not to the right. So the left side heats up. If the bar starts at temperature T, eventually there is a temperature differential across the bar, which can be used to drive an engine. The energy for that work comes from cooling the bar - you are spontaneously turning heat into work, and that's a thermodynamic no-no.

But that's a side-track. For the reasons I gave, this is equivalent to claiming there is a solution of one equation in two unknowns. That puts it in the same category as the angle trisectors and circle squarers.

 

[PeroK]

to be clear: I am not a physics student, but an electrical engineer with a profession in a big semiconductor company as architect in integrated circuit design and systems with 35+ year experience. I do physics for hobby and hope to use this forum to learn with a steep learning curve and get answers on things I do probably wrong as this allows for such a fast learning curve. This could also cause my view to change in time. Hope that is not too much asked? something should be wrong in my assumptions could therefore also mean my latest assumptions, so not the assumptions of 70 posts ago.

Physics is a hobby for me too. The issue here, IMO, is that you do you not understand the problem with an unambiguous one-way speed measurement because you have not mastered the basics of relativistic spacetime. If you knew the basics, then this problem would be simple.

Not learning the basics allows such problems to remain a tricky puzzle that you can endlessly enjoy.

Those are the two approaches. Starting from the basics alllows you to develop a solid understanding of the subject. Whereas, jumping in with a specific problem allows you to wander about making one false assumption after the other and relying on us to continually tell you when you have gone wrong.

 

[HansH]

Consider the limiting case where the speed of light is zero left to right and non-zero right to left. Energy can move to the left but not to the right. So the left side heats up. If the bar starts at temperature T, eventually there is a temperature differential across the bar, which can be used to drive an engine. The energy for that work comes from cooling the bar - you are spontaneously turning heat into work, and that's a thermodynamic no-no.

But that's a side-track. For the reasons I gave, this is equivalent to claiming there is a solution of one equation in two unknowns. That puts it in the same category as the angle trisectors and circle squarers.

But zero is not a possible solution as I understood, because then you cannot meet the requirement for 2 way speed=c as for one side speed is infinite, for the opposite speed then you need at least c/2. so then you should be able to tell something about energy and temperature in relation to this allowed range.

 

[HansH]

what you describe is indeed what I also did in the udated version as far as I can see. https://www.physicsforums.com/threa...by-combined-measurements.1014053/post-6620171
so that should not be a point of discussion anymore.

My point is still that I think i have reason to believe that the options for selecting a 1 way light speed are very limited. But I will start a new topic for that to prevent confusion. Would be great to get that clear.

We made a significant progress on the Dutch forum today. In my description I assumed both a degree of freedom for the 1 way light speed in both x and y direction that led to the contradiction making it impossible to fullfill the requirement of 2 way speed of light in opposite direction being c for all directions. I only got a valuable answer for a few directions.
Finally it turned out that there is a relation between 1 way light speed in both x and y direction (as proposed by another menber) in order to fulfill all requirements. changing the speed equation in my calculation showed that now all requirements were fulfilled. so conclusion is that I had introduced one degree of freedom too much and adding an additional relation solved that problem.

 

[HansH]

Still one final remark as result of another input [1] posted on the dutch forum: What about a laserbeam reflecting back in in a mirror and causing interferencepatterns on the way in between source and mirror. What if the speed in 2 directions would be different? Then I would suppose you get no interference patterns anymore or at least different?. So isn't that a proof that the speed of light must be equal in all directions?
[1] https://www.wetenschapsforum.nl/viewtopic.php?p=1168776#p1168776

 

[PeroK]

Still one final remark as result of another input [1] posted on the dutch forum: What about a laserbeam reflecting back in in a mirror and causing interferencepatterns on the way in between source and mirror. What if the speed in 2 directions would be different? Then I would suppose you get no interference patterns anymore or at least different?. So isn't that a proof that the speed of light must be equal in all directions?
[1] https://www.wetenschapsforum.nl/viewtopic.php?p=1168776#p1168776

All your arguments are based on the implicit assumption of absolute Newtonian spacetime, where you have absolute global simultaneity.

The only problem with your ideas is that they are incompatible with relativity. They are basically classical ideas, which do not apply in this universe. Even in the Netherlands!

 

[Dale]

Then I would suppose you get no interference patterns anymore or at least different?. So isn't that a proof that the speed of light must be equal in all directions?

I think this is now the third time that I have told you that there is no possible experiment that will depend on the one way speed of light. That means no possible measurement or observation of any physical phenomenon will depend on the one way speed of light. I also explained why in terms of the fact that the laws of physics are covariant and can be written in a manifestly covariant form.

This is why it is useless to address all of the different possible wrong scenarios. As long as you only shoot down wrong ideas and don’t learn the right ideas you will continue coming up with other wrong scenarios. The only useful approach is to learn the correct information. I am not sure why you are avoiding that.

 

[HansH]

I think this is now the third time that I have told you that there is no possible experiment that will depend on the one way speed of light. That means no possible measurement or observation of any physical phenomenon will depend on the one way speed of light. I also explained why in terms of the fact that the laws of physics are covariant and can be written in a manifestly covariant form.

This is why it is useless to address all of the different possible wrong scenarios. As long as you only shoot down wrong ideas and don’t learn the right ideas you will continue coming up with other wrong scenarios. The only useful approach is to learn the correct information. I am not sure why you are avoiding that.

Yes I am fully aware that that remarks was made, but difficult to get the consequences for such a situation as a reflected laserbeam, because 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. 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?

 

[Sagittarius A-Star]

So isn't that a proof that the speed of light must be equal in all directions?

No. If a one-way speed is isotropic depends on the definition of simultaneity of events at distant locations. Einstein formulated this as follows:

an observer placed at the mid-point $M$ of the distance $AB$
...
That light requires the same time to traverse the path $A\rightarrow M$ as for the path $B\to M$ is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity.

Source:
https://en.wikisource.org/wiki/Rela..._I#Section_9_-_The_Relativity_of_Simultaneity

You can for example use the primed 4-dimensional reference coordinate system in the following article and get an anisotropic one-way speed of light:
https://www.mathpages.com/home/kmath229/kmath229.htm

 

[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).