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

[HansH]

That's not a contradiction. It's exactly what I said. Using Einstein synchronised clocks is using orthonormal coordinates and you have an isotropic speed of light. If you don't Einstein synchronise your clocks you aren't using orthonormal coordinates and you don't have an isotropic speed of light.

so to get things clear: do you then say that in that movie the one way speed of light is always c (that is what I thought isotropic speed of light means). in the move I don't see they don't talk specifically about ways to choose coordinates so I think they use standard orthonormal coordinates everyone uses in daily life.

 

[Ibix]

I haven't watched the movie so I've no idea what their argument is. As I have already said, if they are considering anisotropic one way speeds of light then they are considering non-orthonormal coordinate systems - whether they choose to say so explicitly or not.

 

[Dale]

am I right that the one way speed of light as presented in this movie can still be anything between c/2 and infinite ?

Yes. I may have understood your position incorrectly. I thought that your position was that the one way speed must be c.

but that then seems to be in contradiction with the remark of Ibix in #54 ?
'Choosing to use orthonormal coordinates on spacetime (not just space) is the same as choosing that the one way speed of light is isotropic, yes.'

That is not in conflict. @Ibix is correct

in the move I don't see they don't talk specifically about ways to choose coordinates so I think they use standard orthonormal coordinates everyone uses in daily life

Since the standard coordinates have the isotropic one way speed of light, and since they are discussing coordinates with non-isotropic one way speeds of light, it is clear that they are using non-standard coordinates. Even if they don’t talk about it explicitly.

By the way, this is why even correct videos are not as effective as a good textbook.

 

[cianfa72]

Using Einstein synchronised clocks is using orthonormal coordinates and you have an isotropic speed of light. If you don't Einstein synchronise your clocks you aren't using orthonormal coordinates and you don't have an isotropic speed of light.

In this discussion we have been assuming a flat spacetime so we are really talking about SR and not the general case of GR.

 

[HansH]

Yes. I may have understood your position incorrectly. I thought that your position was that the one way speed must be c.

no so far I saw no reason why it cannot be an arbitrary value between c/2 and infinite. However writing down the equations and generating the graphs as I posted here: https://www.physicsforums.com/threa...by-combined-measurements.1014053/post-6620171

, I realized that the requirements that resulted in these graphs are in my opinion not compatible with the other requirement that the 2 way speed of light must be c in every directon. (because that is a given fact) it could of course be that I made a mistake somewhere but otherwise my conclusion is that due to this compatibility issue the only option left is that the one way speed of light is also c in every direction. as said I will first give some room to our dutch forum to find a potential problem in my reasoning. but later I will post this probably as a new topic (if not clear yet then) as it is not directly related to the proposal that this topic is about.

 

[Ibix]

In this discussion we have been assuming a flat spacetime so we are really talking about SR and not the general case of GR.

I don't think I've said otherwise.

no so far I saw no reason why it cannot be an arbitrary value between c/2 and infinite.

It can be.

 

[HansH]

There is no one-way speed of light. Sorry to disappoint.

maybe you have 99 equations in 100 uniknowns. Same problem.

At this moment it is the other way around. describing the idea of 1 way lightspeed in a 2d situation I have more equations than variables and therefore a contradicton. so based on that I con only draw the conclusion that light cannot have a 1 way speed depending on direction for a 2 or 3 dimensional world, so if that is not what the physics world derived, then something should be wrong in my assumptions.

 

[Frabjous]

Such a material could be used to build a perpetual motion machine. So no.

Would you explain how? I am not familiar with this one.

 

[Ibix]

then something should be wrong in my assumptions.

...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, traveling to A or B, and being reflected to R.

The light leaves S at $x=0$, $y=0$, $t=0$.
It arrives at A/B at $x=\pm X$, $y=0$, $t=X/c$.
It arrives at R at $x=0$, $y=Y$, $t=X/c+(X/\cos\alpha)/c=\frac Xc(1+\sec\alpha)$.

Now let's work in a frame where the speed of light is not isotropic. To do this, we simply apply the coordinate transform $x'=x$, $y'=y$, $t'=t+\kappa x$, where $\kappa$ is a constant with dimensions of inverse velocity and $|\kappa|<\frac 1c$ so that our spatial planes remain spacelike.

The light leaves S at $x'=0$, $y'=0$, $t'=0$.
It arrives at A/B at $x'=\pm X$, $y'=0$, $t'=X/c\pm\kappa X$.
It arrives at R at $x'=0$, $y'=Y$, $t'=\frac Xc(1+\sec\alpha)$.

Note that the only actual difference from the Einstein case is the arrival time of the light at A and B.

From this we can deduce the speed of light along AR and BR. The distance traveled is $X\sec\alpha$ and the time taken is $\frac Xc(1+\sec\alpha)-(X/c\pm\kappa X)$ (i.e., the arrival time at R minus the arrival time at A/B), so the speed is $$c_\pm=\frac{c}{1\mp \kappa c\cos\alpha}$$There are a few interesting cases. First, if $\kappa=0$ this clearly reduces to the isotropic case. Second, if $\alpha=0$ then AR and BR are antiparallel and we have a 1d problem in which we get $c_\pm=c/(1\mp\kappa c)$. It's easy to see that the average speed, $c_{av}$, satisfies $\frac{2X}{c_{av}}=\frac{X}{c_+}+\frac{X}{c_-}$, and hence that $c_{av}=c$ for all $\kappa$ - i.e. that the two-way speed of light is preserved.

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

 

[PeroK]

then something should be wrong in my assumptions.

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

 

[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