“We don’t really know the speed of light”

[saac]

2 hours ago, Deadlake said:

I’ve not read the whole post, however ways of measuring the velocity of light are 

1) Reflection, where a mirror is used to return the light.

2) Synchronised clocks reporting the velocity of light between two points?

And the aim is to measure the velocity of light one way?

Well one way (no pun) of measuring the one way velocity is to have light bend back on itself, just need a black hole to bend time/space and light will loop around and come back. The astrophysics are using this technique to find out new physics around the photon rings around back holes….

Isn't that the problem though, to contrive to measure a one way flight path - no reflected journey, no traveling back on itself.  Synchronised clocks - how do you tell the 2nd clock to start?  That is the problem.  Black holes , I can see a few problems there    It sure is a frustrating problem conceiving how this could be done for sure. 

Jim 

Edited February 27, 2023 by saac

[Deadlake]

32 minutes ago, saac said:

Isn't that the problem though, to contrive to measure a one way flight path - no reflected journey, no traveling back on itself.  Synchronised clocks - how do you tell the 2nd clock to start?  That is the problem.  Black holes , I can see a few problems there    It sure is a frustrating problem conceiving how this could be done for sure. 

Jim 

A one way journey, no mirror, no clock synchronisation. 
 

The is called a light ring around a black hole, I leave the expert to it

 

[saac]

33 minutes ago, Deadlake said:

A one way journey, no mirror, no clock synchronisation. 
 

The is called a light ring around a black hole, I leave the expert to it

 

Lol and that's is the rub, a modern take on "use a sky hook"   Nearest black hole is how far away?  Gravitational distortion will do what to any instrument package?  Trust an engineer to think about the practicalities. Hey, maybe we can make one in the lab.  

Not to mention what the distorted gravitational field may mean/influence any anisotropic properties of the local space? How would you know what affect has influenced what?

Cool video, these things really are fascinating -  thanks for link. 

Jim

Edited February 27, 2023 by saac

[andrew s]

Light going around any closed loop in space has to go in both directions so it won't measure the one way speed of light.

Formally, the claim is about the local speed of light. Things get very complex in curved spacetime where you can't uniquely add vectors (like velocity). 

As an example consider two cars on the equator on opposite sides of the globe both heading north at 10mph.

What is there relative speed? You calculate this by bring their velocity vectors together via a process called parallel transport.

Well if you parallel transport them round the equator they are equal so relative velocity is 0 mph.

However,  if you parallel transport them along a great circle throuh them and the poles they are travelling in opposite directions so their relative velocity is 20 mph. 

Fun isn't it 

Regards Andrew 

 

Edited February 27, 2023 by andrew s

[saac]

3 minutes ago, andrew s said:

Fun isn't it 

Regards Andrew 

 

My maths teacher used to say that, and yes it is, but also frustrating in equal measure    It's like one of those perpetual motion proposals, everything looks plausible at first, but it doesn't stay that way for long. 

Jim 

[Moonshed]

16 minutes ago, saac said:

My maths teacher used to say that, and yes it is, but also frustrating in equal measure    It's like one of those perpetual motion proposals, everything looks plausible at first, but it doesn't stay that way for long. 

Jim 

I wonder if anyone knows what has come the closest to being perpetual motion? (in the real world not theory).

[Ouroboros]

6 hours ago, Moonshed said:

I wonder if anyone knows what has come the closest to being perpetual motion? (in the real world not theory).

I guess it depends what we are willing to include and what we considered perpetual. A flywheel on a levitating magnetic bearing under vacuum will spin for a very long time. I don’t know what the record is. 

Then there are various natural phenomena.  A planet in orbit around a star for example. Not perpetual but it’s been going for a blooming long time.  Electrical current will flow indefinitely in a loop of superconducting wire. Would that count?  Light has been in motion since the universe became transparent over 14 billion years ago.

 

[saac]

8 hours ago, Moonshed said:

I wonder if anyone knows what has come the closest to being perpetual motion? (in the real world not theory).

Oh great question. I vaguely recall some sort of flywheel device that baffled investigators for some time but I might be getting that mixed up with gyroscopes proporting to be used to generate lift. Pretty sure now all such proposals are blocked by patent office at application stage - 2nd law thermodynamics violation. Not really sure what the definition is any way, for example does it qualify if it cannot drive an external load, ie it has to do work on the external environment.

Jim

 

[andrew s]

To honour the original I would propose it has to be a man made mechanical device that operates (I.e. has moving parts) continuously without any energy input.

Regards Andrew 

PS this seems close https://en.m.wikipedia.org/wiki/Beverly_Clock

Not been wound since 1864. However  it uses changes in air temperature and pressure as energy input so does not really count.

Edited February 28, 2023 by andrew s

[EarthLife]

11 hours ago, saac said:

Isn't that the problem though, to contrive to measure a one way flight path - no reflected journey, no traveling back on itself.  Synchronised clocks - how do you tell the 2nd clock to start?  

 

Two clocks with laser TX/RX's, each with a mirror for the other clock to use as a reflector.

Each clock measures the round trip time and then remembers the average round trip time of the two measurements to use as a reference round trip time.

Both clocks start continuously TX'ing their pulses (not to often though).

Clock 2 phase locks it's TX pulse to the RX'ed pulse from clock 1, it then offsets it's TX pulse by half the pre-determined (the averaged) round trip time taken at the start - this will phase lock (synchronize) the clocks TX pulse with the other clocks TX pulse - IF the speed of light is the same in both directions.

If the speed of light is actually the same in both directions, clock 1 should RX the pulse from clock 2 at exactly half the averaged round trip time measured at the start (and visa-versa).
If either clock see's a difference in the two pulses (TX'ed pulse and RX'ed pulse), then the speed of light is different between the directions.

Yes ?

ps. a simple 2D animation would make it much more easy to see the method/result, but I don't have an easy way to create the animation.

Edited February 28, 2023 by EarthLife
Amended the original method to overcome an error

[Moonshed]

1 hour ago, andrew s said:

To honour the original I would propose it has to be a man made mechanical device that operates (I.e. has moving parts) continuously without any energy input.

That’s what I had in mind, I should have made it clear. We know it’s impossible of course because of the 2nd law of thermodynamics but I’m sure some weird gadgets have come pretty close. The Beverly clock is a good example, I hadn’t come across that one before.

[andrew s]

19 minutes ago, EarthLife said:

Both clocks start continuously TX'ing their pulses.

Clock 2 then phase locks their TX pulse to the RX'ed pulse from clock 1.
ie, clock 2 synchronizes it's TX pulse with the pulse it RX's from clock 1.

If the speed of light is actually the same in both directions, clock 1 should RX the pulse from clock 2 at the exact same time it TX's it's own pulse.
If clock 1 see's a difference in the two pulses (TX'ed pulse and RX'ed pulse), then the speed of light is different between the directions.

Yes ?

You need to be clear what your trying to measure.  One way speed requires the clocks to be synchronized  anisotropy of the speed does not. Regards Andrew 

[EarthLife]

2 minutes ago, andrew s said:

You need to be clear what your trying to measure.  One way speed requires the clocks to be synchronized  anisotropy of the speed does not. Regards Andrew 

Have amended the method to correct a flaw.

[EarthLife]

33 minutes ago, andrew s said:

You need to be clear what your trying to measure. 

To measure the time it takes for light to travel in each individual direction.

[iantaylor2uk]

I don't know what you think the physical basis is for why the speed of light would differ in direction.

If you look at Maxwell's equations, there is a single speed c, predicted for electromagnetic waves, and as far as I'm aware the c that appears in those equations does not depend on direction, and electromagnetism agrees well with experiment. 

In addition, there have been many published papers where general relativity has been applied to pairs of spinning black holes, particularly for black hole collisions, where as I understand it the simulations agree well with the measured data. 

I think if the speed of light were different on the two separate paths application of GR to spinning systems would have shown up as discrepancies with experiment. 

[EarthLife]

1 minute ago, iantaylor2uk said:

I don't know what you think the physical basis is for why the speed of light would differ in direction.

If you look at Maxwell's equations, there is a single speed c, predicted for electromagnetic waves, and as far as I'm aware the c that appears in those equations does not depend on direction, and electromagnetism agrees well with experiment. 

In addition, there have been many published papers where general relativity has been applied to pairs of spinning black holes, particularly for black hole collisions, where as I understand it the simulations agree well with the measured data. 

I think if the speed of light were different on the two separate paths application of GR to spinning systems would have shown up as discrepancies with experiment. 

I myself aren't saying C can/is different in any direction, but I'm guessing someone is.

Finding a practical method to physically prove anything in physics can only help with confirming our ideas of how we as a race of beings is telling itself how the universe appears to work/be (ignoring the fact that the whole thing is most certainly beyond anything we could ever truly comprehend anyway). 

[andrew s]

44 minutes ago, iantaylor2uk said:

I don't know what you think the physical basis is for why the speed of light would differ in direction.

If you look at Maxwell's equations, there is a single speed c, predicted for electromagnetic waves, and as far as I'm aware the c that appears in those equations does not depend on direction, and electromagnetism agrees well with experiment. 

In addition, there have been many published papers where general relativity has been applied to pairs of spinning black holes, particularly for black hole collisions, where as I understand it the simulations agree well with the measured data. 

I think if the speed of light were different on the two separate paths application of GR to spinning systems would have shown up as discrepancies with experiment. 

You can have an eletrodynamics with an anisotropic speed of light but the equations are much more complex. 

The consensus is that you can measure the anisotropy but not the one way speed.

This Wiki article is a good starting point 

https://en.m.wikipedia.org/wiki/One-way_speed_of_light

Regards Andrew 

PS This paper https://arxiv.org/abs/1011.1318  reviews the isotropy of the speed of light see table 5. Current experiments show it is isotropic at the 1 in 10^-10 level.

Edited February 28, 2023 by andrew s

[saac]

4 hours ago, andrew s said:

To honour the original I would propose it has to be a man made mechanical device that operates (I.e. has moving parts) continuously without any energy input.

Regards Andrew 

PS this seems close https://en.m.wikipedia.org/wiki/Beverly_Clock

Not been wound since 1864. However  it uses changes in air temperature and pressure as energy input so does not really count.

I wasn't aware of that one, amazing, must be very finely balanced and excellent bearings. I wonder if the night shift cleaner is winding it each night  

Jim 

[saac]

1 hour ago, andrew s said:

You can have an eletrodynamics with an anisotropic speed of light but the equations are much more complex. 

The consensus is that you can measure the anisotropy but not the one way speed.

This Wiki article is a good starting point 

https://en.m.wikipedia.org/wiki/One-way_speed_of_light

Regards Andrew 

PS This paper https://arxiv.org/abs/1011.1318  reviews the isotropy of the speed of light see table 5. Current experiments show it is isotropic at the 1 in 10^-10 level.

Relativity effects of time dilation, length contraction etc are predicated on Einstein's postulates (Galilean invariance holding true and speed of light being fixed).  If the speed of light was dependant upon direction would that not show in our measurements of relativity effects?  Or is the magnitude of the speed less important than light being compelled to travel at its maximum speed whatever that would be in either direction?  

Jim 

[saac]

3 hours ago, iantaylor2uk said:

I don't know what you think the physical basis is for why the speed of light would differ in direction.

If you look at Maxwell's equations, there is a single speed c, predicted for electromagnetic waves, and as far as I'm aware the c that appears in those equations does not depend on direction, and electromagnetism agrees well with experiment. 

In addition, there have been many published papers where general relativity has been applied to pairs of spinning black holes, particularly for black hole collisions, where as I understand it the simulations agree well with the measured data. 

I think if the speed of light were different on the two separate paths application of GR to spinning systems would have shown up as discrepancies with experiment. 

That was my initial thoughts Ian but I think what the issue is with that is that it represents a "derived" value, assumption are in effect baked in. The question arises from the possibility that space may not be isotropic in nature with respect to the passage of light.  So the challenge for the experimentalist is to find a way to verify the one way light path and it seems impossible. 

Jim 

[andrew s]

40 minutes ago, saac said:

Relativity effects of time dilation, length contraction etc are predicated on Einstein's postulates (Galilean invariance holding true and speed of light being fixed).  If the speed of light was dependant upon direction would that not show in our measurements of relativity effects?  Or is the magnitude of the speed less important than light being compelled to travel at its maximum speed whatever that would be in either direction?  

Jim 

The isotropy of the speed of light is built into SR through the Einstein synchronisation procedure. It's not the only possible procedure and you can develop a nonstandard relativity based on them. However,  while the equations are more complex you get the same predictions as SR.

As the paper I linked to show the measured limits of any anisotropy are very tight.

In my view and that of the SI board it's a done deal and the isotropy is built into the SI standards.

Regards Andrew 

[saac]

I found some homework for tonight.

[2012.12037] The One-Way Speed of Light and the Milne Universe (arxiv.org)

Jim 

[andrew s]

1 minute ago, saac said:

That was my initial thoughts Ian but I think what the issue is with that is that it represents a "derived" value, assumption are in effect baked in. The question arises from the possibility that space may not be isotropic in nature with respect to the passage of light.  So the challenge for the experimentalist is to find a way to verify the one way light path and it seems impossible. 

Jim 

Just to be clear you can measure the anisotropy without trying to measure the one way speed. This has a tradition going back to the MM attempts to detect the eather. 

Regards Andrew 

[saac]

2 minutes ago, andrew s said:

The isotropy of the speed of light is built into SR through the Einstein synchronisation procedure. It's not the only possible procedure and you can develop a nonstandard relativity based on them. However,  while the equations are more complex you get the same predictions as SR.

As the paper I linked to show the measured limits of any anisotropy are very tight.

In my view and that of the SI board it's a done deal and the isotropy is built into the SI standards.

Regards Andrew 

I suppose that is reflected (no pun indented) in the classic representation of Einstein's though experiment with his light clock - it uses a return path. So is he in effect saying in his postulate the return speed of light is fixed, hence built in.  I must admit before this thread I was never even aware of the notion that it was a legitimate question. Then again, the comment on birefringence  should have made the possibility less strange a question.  

Jim 

[EarthLife]

As light is affected/bent by gravity, then the speed of a photon moving away from the center of gravity (say the Earth) will be different to the speed of a photon moving directly towards the center of gravity - one way that the speed in each direction can be asymmetric.