Is the speed of light a constant? Or can it vary? If so, in what kinds of situations, or how much?

[Neogaia777]

and what evidence do you have thus far that the speed of light is constant in both directions? just asking...

None of us do. But it's the most logical assumption and I'm not going to be changing my mind until we find out otherwise.

 

[Arcangl86]

The subject of a one way speed of light has turned up in this forum before from a video by the physicist and science presenter Derek Muller.

The problem is the synchronisation of clocks from the light's source to its final destination. This would ordinarily require sending a signal at an infinite speed so the clocks are instantaneously synchronised.
Since this is impossible scientists have tried to work around this problem without much success so at this stage a one way speed of light is an unfalsifiable idea.
The speed of light based on a two way or round trip is constant and is supported by evidence in the form of interferometer measurements and for this reason we use Einstein's assumption the speed is isotropic.

A problem I found in the link was the reference to the speed of light being 10x faster in one direction compared to the other and length/time dilation needs to be considered.
Length and time dilation involves the Lorentz transformation and if light travels faster than c the equations breakdown as length and time take on imaginary values.
While complex numbers are used in QM they have no place in relativity or cosmology.

Finally a word on the Jason Lisle video, there is zero evidence for the speed of light being infinite in one direction and 0.5c in the other for the reasons given above and is not an observer frame of reference issue.

The link in the OP actually has Derek's video on it, otherwise I would have shared that because I think he does a great job explaining the issue.

 

[sjastro]

The link in the OP actually has Derek's video on it, otherwise I would have shared that because I think he does a great job explaining the issue.

I should become more familiar with his Veritasium channel.

 

[Arcangl86]

I should become more familiar with his Veritasium channel.

I'm a big fan. He is an excellent science communicator and his video on I helped me understand why it's important.

 

[sjastro]

The anisotropic nature of light also crops up in general relativity, one such case is the radial velocity of light and its peripheral or tangential velocity not being equal.

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In the case of the earth the difference is around Δv = 0.14 m/sec but this is a coordinate effect of the mathematics and not a real anisotropy.
I posed this problem to AI (Bing Chatbot) over a year ago and it failed miserably, today it is a piece of cake for AI to solve as given by GPT4-o's answer.
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[Neogaia777]

The anisotropic nature of light also crops up in general relativity, one such case is the radial velocity of light and its peripheral or tangential velocity not being equal.

View attachment 351947
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In the case of the earth the difference is around Δv = 0.14 m/sec but this is a coordinate effect of the mathematics and not a real anisotropy.
I posed this problem to AI (Bing Chatbot) over a year ago and it failed miserably, today it is a piece of cake for AI to solve as given by GPT4-o's answer.




​

So, what does all of this amount to?

Can light be coming at us infinitely fast one way, or at ten times faster at us one way, but be pretty much normal another way, or not?

Take Care.

 

[Neogaia777]

Ok, so I watched the video.

But time itself is different depending where you are at in the universe, etc. Depending upon where you are in the universe, that time is always "now", etc, and the universe is the same age equally everywhere, despite whatever images there is that you are seeing of them from where you are currently at in it currently, etc.

He brought up that what if the images we are seeing from where we are in the universe, what if those are not older, etc? And so I'm wondering if you could disprove that theory/hypothesis by proving that they are maybe, etc?

By proving that the images/radio signals themselves are older, but that the place itself is not actually, etc. Wouldn't that disprove the hypothesis that the one way speed of light could be different maybe, etc? Or would it not really, etc?

What do you think?

Take Care/God Bless.

 

[Neogaia777]

Also, isn't there a way to prove that light/radio signals are not coming back at us at half c? Or is there not really, etc?

 

[sjastro]

So, what does all of this amount to?

Can light be coming at us infinitely fast one way, or at ten times faster at us one way, but be pretty much normal another way, or not?

Take Care.

The example I gave shows the anisotropic speed of light is a mathematical issue which doesn't reflect reality.

As has been stated earlier in this thread it is impossible to measure the one way speed of light so it is pointless for young earth creationists to speculate on velocities.
A point to consider is Occam's razor, where the simplest explanation is usually the best where the speed is c in all directions.

 

[Neogaia777]

The example I gave shows the anisotropic speed of light is a mathematical issue which doesn't reflect reality.

As has been stated earlier in this thread it is impossible to measure the one way speed of light so it is pointless for young earth creationists to speculate on velocities.
A point to consider is Occam's razor, where the simplest explanation is usually the best where the speed is c in all directions.

I would like to try to prove or disprove it though?

Is there anyway to do that?

What if you could travel towards a destination, let's say Mars from here basically, etc, and let's say you shot a light beam at it and the exact same time you left or took off for it, etc, and let's say that you could travel at the speed of light, or some other speed closer to but lesser than that, etc, (really doesn't matter, but you would have to compensate, or count that into your calculations, etc), but were both (you and the beam of light) proceeding to travel or go away from Earth towards Mars at the exact same time, etc?

If you could record the images on your way there both behind you and also ahead of you and as you were traveling to there or were headed towards there, etc, and they (the images) slowed down behind you (when traveling toward Mars and away from Earth at some speed lesser than the speed of light) or stopped for a time behind you (in the case of traveling to or towards Mars and away from Earth at the exact speed of light, etc) or for the time while you were on your way traveling there, but sped up or went faster for a time for what was ahead of you, but then returned to normal once you stopped, or once you got there, and the time dilation/duration/difference was still exactly 10 minutes, and then also exactly 10 minutes back if you were to do the same thing back, etc? (Though the images would now be for Mars behind you, and the Earth ahead of you, etc).

Anyway, wouldn't that disprove this theory that they could be different maybe, etc?

Take Care/God Bless.

 

[AlexB23]

Alright, this was a fun thread, cos science is pretty cool. God bless, folks.

 

[Neogaia777]

...

I would like to try to prove or disprove it though?

Is there anyway to do that?

What if you could travel towards a destination, let's say Mars from here basically, etc, and let's say you shot a light beam at it and the exact same time you left or took off for it, etc, and let's say that you could travel at the speed of light, or some other speed closer to but lesser than that, etc, (really doesn't matter, but you would have to compensate, or count that into your calculations, etc), but were both (you and the beam of light) proceeding to travel or go away from Earth towards Mars at the exact same time, etc?

If you could record the images on your way there both behind you and also ahead of you and as you were traveling to there or were headed towards there, etc, and they (the images) slowed down behind you (when traveling toward Mars and away from Earth at some speed lesser than the speed of light) or stopped for a time behind you (in the case of traveling to or towards Mars and away from Earth at the exact speed of light, etc) or for the time while you were on your way traveling there, but sped up or went faster for a time for what was ahead of you, but then returned to normal once you stopped, or once you got there, and the time dilation/duration/difference was still exactly 10 minutes, and then also exactly 10 minutes back if you were to do the same thing back, etc? (Though the images would now be for Mars behind you, and the Earth ahead of you, etc).

Anyway, wouldn't that disprove this theory that they could be different maybe, etc?

Take Care/God Bless.

@sjastro and others in this thread.

We wouldn't necessarily have to be going anywhere even close to the speed of light to be able to do this or test this maybe, etc. Even if we were only going 1/100, or 1/1000, or 1/10,000 the speed of light, depending on the sensitivity of our instruments, etc, we should be able to do this, or test this, or prove or disprove this, etc.

If we were just going 1/10,000 of c, etc, then if the images either ahead of us or behind us were always off or were always being altered/affected by that much time-wise either way, etc, then we should be able to test this, or prove or disprove this, etc.

And I'm always thinking about cameras, and images from a visual camera, etc, but we might also be able to do it with radio waves/transmissions also as well maybe, etc?

Though I know a lot less about that, other than they travel/transmit at the speed of light as well, etc.

Take Care.

 

[Hans Blaster]

We wouldn't necessarily have to be going anywhere even close to the speed of light to be able to do this or test this maybe, etc. Even if we were only going 1/100, or 1/1000, or 1/10,000 the speed of light, depending on the sensitivity of our instruments, etc, we should be able to do this, or test this, or prove or disprove this, etc.

Rather then figure out exactly how to respond to you question, I will note that we already are.

The orbital speed of the Earth is about 1/10,000 the speed of light and the particles of the Solar wind travel 10-20 times faster (or about 1/1000 the speed of light). The Sun orbits the center of the Milky Way at about 1/1200 the speed of light. These are very pedestrian speeds in astronomical situations.

 

[Neogaia777]

Rather then figure out exactly how to respond to you question, I will note that we already are.

Please let me know how it goes if you can please, ok, because I am very, very interested in the for sure or not answer to this question, as I have been seeking it out for quite a long time now, etc.

Much, much thanks in advance if you can keep me apprised/updated, ok.

Take Care.

 

[Hans Blaster]

I would like to try to prove or disprove it though?

Is there anyway to do that?

What if you could travel towards a destination, let's say Mars from here basically, etc, and let's say you shot a light beam at it and the exact same time you left or took off for it, etc, and let's say that you could travel at the speed of light, or some other speed closer to but lesser than that, etc, (really doesn't matter, but you would have to compensate, or count that into your calculations, etc), but were both (you and the beam of light) proceeding to travel or go away from Earth towards Mars at the exact same time, etc?

If you could record the images on your way there both behind you and also ahead of you and as you were traveling to there or were headed towards there, etc, and they (the images) slowed down behind you (when traveling toward Mars and away from Earth at some speed lesser than the speed of light) or stopped for a time behind you (in the case of traveling to or towards Mars and away from Earth at the exact speed of light, etc) or for the time while you were on your way traveling there, but sped up or went faster for a time for what was ahead of you, but then returned to normal once you stopped, or once you got there, and the time dilation/duration/difference was still exactly 10 minutes, and then also exactly 10 minutes back if you were to do the same thing back, etc? (Though the images would now be for Mars behind you, and the Earth ahead of you, etc).

Anyway, wouldn't that disprove this theory that they could be different maybe, etc?

Take Care/God Bless.

I'm not sure about this (the behind and ahead seem hard to measure), but if you placed a high-precision clock on a spacecraft and sent it on a one-way deep space mission it might work.

The experiment would be to regularly transmit a timing signal from the ground station to the spacecraft that would record the signal and the spacecraft's time at arrival of the signal. At the same time the relative velocities could be measured with a fixed frequency signal by Doppler shift. The space craft would report the arrival times and frequencies of the signals back to the ground station. Also, the spacecraft would transmit its timing/frequency signal back to the ground station with a spacecraft time stamp, so the ground station could see how long it took the signal to travel from spacecraft to Earth.

Unless I am missing something about the clocks in accelerating frames or anisostropic light speeds, the full data set should show agreement on the distance between and the delay times between the clocks would indicate the speed of light any anisotropy. When V'Ger was being built 50 years ago, the kinds of clocks needed were far too bulky, so I suspect they were engineered to resync their time from Earth-sourced signals.

 

[sjastro]

I would like to try to prove or disprove it though?

Is there anyway to do that?

What if you could travel towards a destination, let's say Mars from here basically, etc, and let's say you shot a light beam at it and the exact same time you left or took off for it, etc, and let's say that you could travel at the speed of light, or some other speed closer to but lesser than that, etc, (really doesn't matter, but you would have to compensate, or count that into your calculations, etc), but were both (you and the beam of light) proceeding to travel or go away from Earth towards Mars at the exact same time, etc?

If you could record the images on your way there both behind you and also ahead of you and as you were traveling to there or were headed towards there, etc, and they (the images) slowed down behind you (when traveling toward Mars and away from Earth at some speed lesser than the speed of light) or stopped for a time behind you (in the case of traveling to or towards Mars and away from Earth at the exact speed of light, etc) or for the time while you were on your way traveling there, but sped up or went faster for a time for what was ahead of you, but then returned to normal once you stopped, or once you got there, and the time dilation/duration/difference was still exactly 10 minutes, and then also exactly 10 minutes back if you were to do the same thing back, etc? (Though the images would now be for Mars behind you, and the Earth ahead of you, etc).

Anyway, wouldn't that disprove this theory that they could be different maybe, etc?

Take Care/God Bless.

You are not going to travel at the speed of light because you have non zero mass.
The issue will always be an inability to synchronize clocks at the source and destination, the clock at the destination must know exactly when light is emitted from the source as measured by the clock at the source.
To do this a signal at the source travelling at an infinite speed must be emitted at the same time as the light and instantaneously reach the clock at the destination after which both clocks are synchronized.

Instead of large distances consider the source and destination are only separated by a metre.
Light in a vacuum travels approximately 3 x 10⁻⁹ seconds over a distance of one metre and since physics is about approximations 3 x 10⁻⁹ seconds is close enough to being instantaneous.
Combining this with the use of Optical lattice - Wikipedia clocks that have measurement uncertainties of the order of 10⁻¹⁸ one might think measuring one way speed of light is possible but synchronization is still a problem.

 

[Neogaia777]

You are not going to travel at the speed of light because you have non zero mass.
The issue will always be an inability to synchronize clocks at the source and destination, the clock at the destination must know exactly when light is emitted from the source as measured by the clock at the source.
To do this a signal at the source travelling at an infinite speed must be emitted at the same time as the light and instantaneously reach the clock at the destination after which both clocks are synchronized.

Instead of large distances consider the source and destination are only separated by a metre.
Light in a vacuum travels approximately 3 x 10⁻⁹ seconds over a distance of one metre and since physics is about approximations 3 x 10⁻⁹ seconds is close enough to being instantaneous.
Combining this with the use of Optical lattice - Wikipedia clocks that have measurement uncertainties of the order of 10⁻¹⁸ one might think measuring one way speed of light is possible but synchronization is still a problem.

Well, forget about the speed of light for a minute. Couldn't you travel at almost any much lesser speed (see post #32) and just run a camera or something on your "ship", or whatever it is, etc, and see if the speed at which the camera records as you're moving along speeds up a bit for what is ahead of you, or for what you are headed to or towards, as you were moving towards it, or slows down a bit for what is behind you, or for what you are recording as you were receding away from it?

Ok, Say it's 1/10,000 of c, well if the camera speeds up 1/10,000 of t as you were headed towards something, but slows down 1/10,000 of t as you were moving away from the opposite thing away from it, or whatever you were leaving at that speed, etc...?

Anyway, would that do anything at all for us really? Or maybe not maybe, etc?

If the speed is exactly 1/10,000 of c, and the change in the camera's speed of recording is exactly 1/10,000 t faster for what's being recorded ahead of you as you were moving to or towards it, and exactly 1/10,000 t slower for what was being recorded for what's behind you as you were moving or receding away from it...?

Anyway, does that do anything for us really? Or really not really, etc?

If the figures always match exactly, etc.

Does that do anything for us really?

If the fractions or percentages of c were always exactly equal to the changes in the camera's recording speed being faster or slower as it was moving along, or t, etc?

Take Care.

 

[Neogaia777]

Well, forget about the speed of light for a minute. Couldn't you travel at almost any much lesser speed (see post #32) and just run a camera or something on your "ship", or whatever it is, etc, and see if the speed at which the camera records as you're moving along speeds up a bit for what is ahead of you, or for what you are headed to or towards, as you were moving towards it, or slows down a bit for what is behind you, or for what you are recording as you were receding away from it?

Ok, Say it's 1/10,000 of c, well if the camera speeds up 1/10,000 of t as you were headed towards something, but slows down 1/10,000 of t as you were moving away from the opposite thing away from it, or whatever you were leaving at that speed, etc...?

Anyway, would that do anything at all for us really? Or maybe not maybe, etc?

If the speed is exactly 1/10,000 of c, and the change in the camera's speed of recording is exactly 1/10,000 t faster for what's being recorded ahead of you as you were moving to or towards it, and exactly 1/10,000 t slower for what was being recorded for what's behind you as you were moving or receding away from it...?

Anyway, does that do anything for us really? Or really not really, etc?

If the figures always match exactly, etc.

Does that do anything for us really?

If the fractions or percentages of c were always exactly equal to the changes in the camera's recording speed being faster or slower as it was moving along, or t, etc?

Take Care.

And I know, the camera itself is not actually recording any faster or slower, etc, but is only recording slightly less amount of frames as it is moving to or towards something, and slightly more when it is moving away from something, giving it the appearance of recording faster or slower, if you were to watch the film or images, etc, but in the case of something ahead, it is doing that in order to catch up to the time there, when you get there, etc, and once you hit the brakes when you get there, or got there, then the camera would go right back to recording normally again once you were no longer in motion or moving at whatever set fraction or percentage of c, which would be the same as t, or the same as the fraction or percentage of the speed difference of the camera's recording speed when it was in motion or was moving at whatever fraction or percentage of c, which would always be exactly equal to or always the same as t, etc.

Right, etc?

Or am I missing something here?

Because I will have to admit, I very, very well could be maybe, etc?

Take Care.

 

[sjastro]

Well, forget about the speed of light for a minute. Couldn't you travel at almost any much lesser speed (see post #32) and just run a camera or something on your "ship", or whatever it is, etc, and see if the speed at which the camera records as you're moving along speeds up a bit for what is ahead of you, or for what you are headed to or towards, as you were moving towards it, or slows down a bit for what is behind you, or for what you are recording as you were receding away from it?

Ok, Say it's 1/10,000 of c, well if the camera speeds up 1/10,000 of t as you were headed towards something, but slows down 1/10,000 of t as you were moving away from the opposite thing away from it, or whatever you were leaving at that speed, etc...?

Anyway, would that do anything at all for us really? Or maybe not maybe, etc?

If the speed is exactly 1/10,000 of c, and the change in the camera's speed of recording is exactly 1/10,000 t faster for what's being recorded ahead of you as you were moving to or towards it, and exactly 1/10,000 t slower for what was being recorded for what's behind you as you were moving or receding away from it...?

Anyway, does that do anything for us really? Or really not really, etc?

If the figures always match exactly, etc.

Does that do anything for us really?

If the fractions or percentages of c were always exactly equal to the changes in the camera's recording speed being faster or slower as it was moving along, or t, etc?

Take Care.

There is a variation to your set up that has been around since 1911, when photography was first used to calculate the parallax of stars.

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These day CCDs are used which are vastly more sensitive in picking up brightness variations.
Parallax measurements are done six months apart, suppose you took a CCD image in June when the earth is moving towards the distant fixed stars then six months later when it is moving away in its orbit.
If the speed of light in anisotropic it is no longer constant to inertial observers, one would expect distant fixed stars to be of a different brightness in the CCD image in June compared to December as the photon flux density hitting the CCD depends on the relative velocity of light.
No change in brightness is measured.

The answer like your example is that we are not measuring a one way speed of light but a two way where special relativity applies and the speed of light is constant for all inertial observers.

 

[Platte]

There is a variation to your set up that has been around since 1911, when photography was first used to calculate the parallax of stars.

View attachment 352030
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These day CCDs are used which are vastly more sensitive in picking up brightness variations.
Parallax measurements are done six months apart, suppose you took a CCD image in June when the earth is moving towards the distant fixed stars then six months later when it is moving away in its orbit.
If the speed of light in anisotropic it is no a longer constant to inertial observers, one would expect distant fixed stars to be of a different brightness in the CCD image in June compared to December as the photon flux density hitting the CCD depends on the relative velocity of light.
No change in brightness is measured.

The answer like your example is that we are not measuring a one way speed of light but a two way where special relativity applies and the speed of light is constant for all inertial observers.

I would consider one way speed of light as direct light. And the 2 way speed of light as reflective. We can’t measure the speed of direct light but we can measure the speed of reflective light. Einstein made the assumption that direct light was the same speed as reflective light since we could actual measure direct light speed. When asked how could he make that assumption he simply said it was his theory (Relativity) and he could make any assumption he wanted.