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

[Hans Blaster]

No - the context of our conversation was seeing planets and seeing stars in space from earth....generally speaking larger planets would be easier to see from earth from great distances then smaller planets.

This star is the size of a minimal mass main sequence star (M9). They have temperatures of more than 2000 K and are much brighter than a planet.

The only thing I've claimed that is "special" about reflected light is that 2 way light (reflected) has been measured - 1 way light (direct) has not been measured. Do you dispute that?

Since the one way speed of light can't be measured what is the point of this?

 

[Warden_of_the_Storm]

Just from own reading on this thread and from my (very limited understanding on the topic), but by the nature of light and observing light, is it not impossible for there to be an observance of "one way light", because by the nature of light, to see it, the light must be reflected, thus becoming two way light?

 

[BNR32FAN]

It doesn't work as the process of moving the clock to the destination introduces time dilation and the clocks are no longer synchronized.
Getting around this problem by moving the clock slowly to the destination results in another problem in the case for anisotropic light, the clocks can no longer detect time differences between light moving in opposite directions.

If the speed of light is anisotropic the time dilation τ equation is:

View attachment 352265

ϒ is the Lorentz factor, κ is the anisotropic factor which varies from -1 to +1 and v is velocity of moving the clock.
If the velocity is very small;

View attachment 352266

This is the inverse Lorentz factor which crops up in special relativity and only applies to two way light or light which is isotropic.

Ok I have no idea what any of that means but are you saying that if I were to travel to say mars for example not at light speed but a speed that is actually realistic to what we could accomplish today that my digital clock would speed up or slow down? I don’t see why that would happen.

 

[BNR32FAN]

It doesn't work as the process of moving the clock to the destination introduces time dilation and the clocks are no longer synchronized.
Getting around this problem by moving the clock slowly to the destination results in another problem in the case for anisotropic light, the clocks can no longer detect time differences between light moving in opposite directions.

If the speed of light is anisotropic the time dilation τ equation is:

View attachment 352265

ϒ is the Lorentz factor, κ is the anisotropic factor which varies from -1 to +1 and v is velocity of moving the clock.
If the velocity is very small;

View attachment 352266

This is the inverse Lorentz factor which crops up in special relativity and only applies to two way light or light which is isotropic.

Ok I looked up time dilation and it does not stop time, it only alters a person’s perception of time because they can’t see the hand of a clock moving which is completely different. Just because we can’t see something happening doesn’t mean it’s not happening. If I’m watching a video of a clock and I pause the video that doesn’t mean that I’ve stopped time. Time dilation isn’t about time actually stopping it’s about a difference in perception. So if I have a digital clock on my spaceship that is synchronized with a clock on earth and I fly to mars and stop and they turn on a light at 12:00 noon I can wait and record what time I see the light according to my clock which would still be synchronized with the clock on earth.

 

[Tinker Grey]

Just from own reading on this thread and from my (very limited understanding on the topic), but by the nature of light and observing light, is it not impossible for there to be an observance of "one way light", because by the nature of light, to see it, the light must be reflected, thus becoming two way light?

Well, I, too, have limited understanding on the topic.

It isn't that one-way light cannot be seen, it is, it's that its speed cannot be measured. We don't need light bouncing off the moon to see the sun. (Unless one wants to conflate reflecting in the eye-ball.)

We see light as it arrives from stars and there is no "two-way" about it (AFAIK). We can measure red-shift. We can measure intensity. We just cannot measure the speed at which that light gets to us. We make assumptions based on Einstein's theories.

I think it was @sjastro up-thread that said that for there to be different speeds of light there must be a special frame of reference. This would a violation of Einstein's theories. While we cannot measure the one-way speed of light, there are, in fact, experiments that are consistent with no-special-frame-of-reference and thus it is reasonable to conclude that the one-way speed of light is the same as the two-way speed of light.

And, thus, we know how far star-x is from us.

Did I summarize that right?

 

[BNR32FAN]

Just from own reading on this thread and from my (very limited understanding on the topic), but by the nature of light and observing light, is it not impossible for there to be an observance of "one way light", because by the nature of light, to see it, the light must be reflected, thus becoming two way light?

If someone points a flashlight at you then turns it on you’re not seeing the light reflecting you’re seeing it come straight at you. The light from stars and the sun is not reflected, it’s coming straight towards us.

 

[Warden_of_the_Storm]

If someone points a flashlight at you then turns it on you’re not seeing the light reflecting you’re seeing it come straight at you. The light from stars and the sun is not reflected, it’s coming straight towards us.

Ah. I had a dumb moment. Physics was never my forte.

 

[KevinT]

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

Interesting question. Let me reply by asking how would one go about finding an answer to this question? One could do lots of experiments here on earth, but then one might ask how things are acting on the other side of our galaxy? Or one can look out at all the other galaxies using telescopes and see how things are working there. But since it took light a long time to travel from those galaxies, what were are seeing is from a long time ago. Perhaps something has changed since then.

So the short answer is that there will always be some uncertainty about blanket statements about the speed of light being constant in ALL locations in the universe. It would make sense that it is the same everywhere and for all time, but short of a pronouncement from the Creator, we won't know for sure.

But so far, all our measurements have showed that the speed of light is constant in all frames of reference.

Now I'll add another wrinkle. Speed is a RATIO. It is distance per time. And it is the variance of time flow that makes the speed seem constant in all frames of motion. That is why a man who speeds away on a rocket for 1 yr out and 1 yr back will find that he is ever so slightly younger than those that stayed home on earth.

Stephen Wolfram is working on a theory of cellular automata, where the universe is composed of tiny parts that interact with each other. He calls this a "graph", but I like to think of it like pixels in the Conway's Game of Life. Each part or cell or pixel just interacts with its neighbors, following pre-set rules. It is the vast sea of these elements that comprise the quantum fields, which in turn, carry the energy of the electromagnetic fields which we perceive to be light. And in an interview on YouTube (I can't find the link right now), Wolfram described that it seems that each automata seems to allow motion (moving energy along the grid), OR time flow, or some ratio of the two. Thus the faster something is passed from node to node, the slower time will progress for that entity. What is time flow? It is the working of the laws of physics that allows a clock to tick, or a radioactive atom to decay.

My point with all this is to say, that if there was a pocket in the universe where the action of the universal grid was slower, then both changes in distance, AND changes in time could be affected. And thus the ratio (the speed) would appear the same to observers existing in that grid.

tl;dr -- the speed of light is constant, but there is likely a deeper reason for that.

KT

 

[Neogaia777]

Interesting question. Let me reply by asking how would one go about finding an answer to this question? One could do lots of experiments here on earth, but then one might ask how things are acting on the other side of our galaxy? Or one can look out at all the other galaxies using telescopes and see how things are working there. But since it took light a long time to travel from those galaxies, what were are seeing is from a long time ago. Perhaps something has changed since then.

So the short answer is that there will always be some uncertainty about blanket statements about the speed of light being constant in ALL locations in the universe. It would make sense that it is the same everywhere and for all time, but short of a pronouncement from the Creator, we won't know for sure.

But so far, all our measurements have showed that the speed of light is constant in all frames of reference.

Now I'll add another wrinkle. Speed is a RATIO. It is distance per time. And it is the variance of time flow that makes the speed seem constant in all frames of motion. That is why a man who speeds away on a rocket for 1 yr out and 1 yr back will find that he is ever so slightly younger than those that stayed home on earth.

Stephen Wolfram is working on a theory of cellular automata, where the universe is composed of tiny parts that interact with each other. He calls this a "graph", but I like to think of it like pixels in the Conway's Game of Life. Each part or cell or pixel just interacts with its neighbors, following pre-set rules. It is the vast sea of these elements that comprise the quantum fields, which in turn, carry the energy of the electromagnetic fields which we perceive to be light. And in an interview on YouTube (I can't find the link right now), Wolfram described that it seems that each automata seems to allow motion (moving energy along the grid), OR time flow, or some ratio of the two. Thus the faster something is passed from node to node, the slower time will progress for that entity. What is time flow? It is the working of the laws of physics that allows a clock to tick, or a radioactive atom to decay.

My point with all this is to say, that if there was a pocket in the universe where the action of the universal grid was slower, then both changes in distance, AND changes in time could be affected. And thus the ratio (the speed) would appear the same to observers existing in that grid.

tl;dr -- the speed of light is constant, but there is likely a deeper reason for that.

KT

You should probably review the rest of the thread.

But thanks for the comment though.

God Bless.

 

[sjastro]

Ok I looked up time dilation and it does not stop time, it only alters a person’s perception of time because they can’t see the hand of a clock moving which is completely different. Just because we can’t see something happening doesn’t mean it’s not happening. If I’m watching a video of a clock and I pause the video that doesn’t mean that I’ve stopped time. Time dilation isn’t about time actually stopping it’s about a difference in perception. So if I have a digital clock on my spaceship that is synchronized with a clock on earth and I fly to mars and stop and they turn on a light at 12:00 noon I can wait and record what time I see the light according to my clock which would still be synchronized with the clock on earth.

To use your example suppose your spaceship can travel at relativistic speeds, before lift off from Earth your spaceship clock is synchronized with an Earth based clock.
You travel to Mars and back and will find the clocks are no longer synchronized, the clock on your spaceship has been ticking at a different rate than the Earth based clock.

This is because there are two types of time that need to considered, the spaceship’s clock proper time and its coordinate time.
The proper time is the time elapsed as measured by spaceship’s clock travelling to Mars and back, while the coordinate time is the time elapsed according to the Earth based clock.

The elapsed proper time ΔT and coordinate time Δt are not the same and are defined according to the Lorentz transformation when the spaceship is travelling at a velocity v.

If we replace the clocks with twins we come across the famous twin paradox of special relativity, since there is no special frame of reference a spaceship moving away from Earth is no different from the spaceship being stationary and the Earth moving away instead.
According to the paradox each twin should therefore be older than the other when they meet up again.
The resolution of the paradox also provides an insight into what happens to proper time and coordinate time.

 

[BNR32FAN]

To use your example suppose your spaceship can travel at relativistic speeds, before lift off from Earth your spaceship clock is synchronized with an Earth based clock.
You travel to Mars and back and will find the clocks are no longer synchronized, the clock on your spaceship has been ticking at a different rate than the Earth based clock.

This is because there are two types of time that need to considered, the spaceship’s clock proper time and its coordinate time.
The proper time is the time elapsed as measured by spaceship’s clock travelling to Mars and back, while the coordinate time is the time elapsed according to the Earth based clock.

The elapsed proper time ΔT and coordinate time Δt are not the same and are defined according to the Lorentz transformation when the spaceship is travelling at a velocity v.

View attachment 352287

If we replace the clocks with twins we come across the famous twin paradox of special relativity, since there is no special frame of reference a spaceship moving away from Earth is no different from the spaceship being stationary and the Earth moving away instead.
According to the paradox each twin should therefore be older than the other when they meet up again.
The resolution of the paradox also provides an insight into what happens to proper time and coordinate time.

Again this is only referring to perspective not actual differences in time. It’s like looking at a star and understanding that just because I see the light at a particular position in space doesn’t mean that’s actually where it is in reality. And saying that the clock on the ship will tick slower doesn’t actually explain anything and if the clock was ticking slower then that would mean that the ship would actually be traveling faster since it would be covering the same distance in a shorter amount of time. If the Twin’s clock is ticking at half the speed traveling at 86% the speed of light that would mean she isn’t traveling at 86% the speed of light she’d be traveling at 172% the speed of light because she’s traveling twice as far per light year. None of that video makes any sense at all to me. Where do they get the idea that time slows to half at 86% light speed? I’m guessing it’s based on Einstein’s theory of relativity but here the problem with that idea. Just because time appears to have stopped because you can’t see the hand move because you’re continuously seeing that one frame of light doesn’t mean that the hand isn’t still moving, it only means that you can’t see it moving. Sight has absolutely nothing to do with actual real time. If the clock was connected to a bomb that was set to explode at exactly 12:05 and you were traveling at the speed of light watching the clock showing 12:00 the bomb would still explode even though you didn’t see it change to 12:05 because the speed of light has nothing to do with the internal mechanisms of the clock. If you were standing in front of a bus traveling at the speed of light you would never see the bus coming but it would still hit you regardless of your inability to see it, just like bullets shot from a long distance will hit their target before the shot is heard. I just don’t see how that video has anything to do with actual time. They didn’t explain why the clock on the ship would slow other than from the person’s perspective on earth which is irrelevant to what is actually happening in reality. To me it seems as if their whole argument is based on optical illusions and that optical illusions have an actual effect on time itself which is ridiculous.

 

[BNR32FAN]

To use your example suppose your spaceship can travel at relativistic speeds, before lift off from Earth your spaceship clock is synchronized with an Earth based clock.
You travel to Mars and back and will find the clocks are no longer synchronized, the clock on your spaceship has been ticking at a different rate than the Earth based clock.

This is because there are two types of time that need to considered, the spaceship’s clock proper time and its coordinate time.
The proper time is the time elapsed as measured by spaceship’s clock travelling to Mars and back, while the coordinate time is the time elapsed according to the Earth based clock.

The elapsed proper time ΔT and coordinate time Δt are not the same and are defined according to the Lorentz transformation when the spaceship is travelling at a velocity v.

View attachment 352287

If we replace the clocks with twins we come across the famous twin paradox of special relativity, since there is no special frame of reference a spaceship moving away from Earth is no different from the spaceship being stationary and the Earth moving away instead.
According to the paradox each twin should therefore be older than the other when they meet up again.
The resolution of the paradox also provides an insight into what happens to proper time and coordinate time.

So in Einstein’s theory of relativity if you’re traveling away from the clock at the speed of light the clock would appear to stop but what if you’re traveling towards it. Now you’re actually seeing the light at twice the speed. Does that mean that the hand on the clock moves twice as fast now? No it just means you have twice the frame rate basically. The light has absolutely no effect on the mechanisms inside the clock. It’s like the difference between looking at a monitor at 60hz and 120hz. I would compare Einstein’s theory to the effect that a strobe light has on a ceiling fan. You can adjust the frequency of the strobe light to make the fan appear to stop but that doesn’t mean that you can put your hand in it and not get hit by the blades. You can still feel the air being blown by the blades regardless of what the light is doing.

 

[Neogaia777]

So in Einstein’s theory of relativity if you’re traveling away from the clock at the speed of light the clock would appear to stop but what if you’re traveling towards it.

@sjastro can correct me if I am wrong about this, etc. But if your moving at the speed of light everything seems to stop for you no matter what, and no matter what direction your heading, but time everywhere else is unaffected, but proceeds normally, etc. So if you traveled ten light minutes away from Earth at light speed, but time froze or stopped for you while you were traveling, then you immediately turned around without pause and immediately headed back to earth again at light speed, the time elapsed on earth would be 20 minutes no matter what when you got back, but zero time would have passed for you actually, etc.

And in the case of moving directly away from earth at light speed, the clock on earth would appear to freeze as you went away if you could observe it while you were traveling directly away from earth at light speed, but on your immediate direct return trip back at light speed, the clock on earth, if you could observe it while you traveling to it now, etc, would now be elapsing at double the speed, resulting in a total time elapsed on earth of 20 minutes when you got back, etc, but because time froze for you both ways when you were moving in both directions at light speed, no time would have actually passed for you or on your clock actually, so you'd be 20 minutes younger than those on earth are now actually, etc. But once you stopped moving, time in both places would return to normal for both of you now actually, etc. But the time or rate of time on earth never really actually changed ever actually, but only just your observations of it temporarily actually, except that time actually froze or stopped for you, which means you'd be 20 minutes younger than those on earth now actually, etc.

God Bless.

 

[sjastro]

Again this is only referring to perspective not actual differences in time. It’s like looking at a star and understanding that just because I see the light at a particular position in space doesn’t mean that’s actually where it is in reality. And saying that the clock on the ship will tick slower doesn’t actually explain anything and if the clock was ticking slower then that would mean that the ship would actually be traveling faster since it would be covering the same distance in a shorter amount of time. If the Twin’s clock is ticking at half the speed traveling at 86% the speed of light that would mean she isn’t traveling at 86% the speed of light she’d be traveling at 172% the speed of light because she’s traveling twice as far per light year. None of that video makes any sense at all to me. Where do they get the idea that time slows to half at 86% light speed? I’m guessing it’s based on Einstein’s theory of relativity but here the problem with that idea. Just because time appears to have stopped because you can’t see the hand move because you’re continuously seeing that one frame of light doesn’t mean that the hand isn’t still moving, it only means that you can’t see it moving. Sight has absolutely nothing to do with actual real time. If the clock was connected to a bomb that was set to explode at exactly 12:05 and you were traveling at the speed of light watching the clock showing 12:00 the bomb would still explode even though you didn’t see it change to 12:05 because the speed of light has nothing to do with the internal mechanisms of the clock. If you were standing in front of a bus traveling at the speed of light you would never see the bus coming but it would still hit you regardless of your inability to see it, just like bullets shot from a long distance will hit their target before the shot is heard. I just don’t see how that video has anything to do with actual time. They didn’t explain why the clock on the ship would slow other than from the person’s perspective on earth which is irrelevant to what is actually happening in reality. To me it seems as if their whole argument is based on optical illusions and that optical illusions have an actual effect on time itself which is ridiculous.

I suggest you look at the video again as the travelling twin is not moving at 172% speed of light.
The distance travelled in twin's frame of reference is reduced by the Lorentz factor ϒ while time dilation in the stationary twin's frame of reference increases by the factor 1/ϒ.

Since you are struggling to understand how time dilation works here is a simple geometrical explanation.
Coordinate time in special relativity is easily explained with a thought experiment.

​

In the spaceship’s frame of reference, the proper time is the time taken for a light beam to bounce between the mirrors in a vertical trajectory.
In the observer’s frame of reference on earth since the spacecraft is travelling in a transverse direction, the light beam is no longer vertical, but follows an oblique longer path resulting in a longer coordinate time for the light beam to bounce between the mirrors.

This is not a case of perspective as time dilation and length contraction are real observer effects.
Muons are short lived particles formed in upper atmosphere when cosmic rays collide with air molecules.
Their proper time is defined by their half life and the half life determines the survival rate of muons reaching the Earth's surface.
If there was no length contraction and time dilation a survival rate of 0.3 muons per million would reach the surface.
However muons travel at 98% the speed of light c corresponding to ϒ = 5, and the predicted survival rate increases to 49,000 per million.

​

In the muon's frame of reference it does not travel 10 km but only 10/ϒ = 10/5 = 2 km due to length contraction, while the halflives in the observers frame of reference increases by 4.36.ϒ = 4.36 x 5 = 21.8.
The predicted survival rate of muons due to relativistic effects is supported by measurements of the survival rate.

 

[Neogaia777]

@BNR32FAN

And again, I will always invite @sjastro to correct me if I am wrong about this actually, etc.

If your moving directly away from earth at the speed of light that makes time 50% slower for you at that percentage of light speed (86% I think it was?) Anyway, if you went directly away from earth at that speed, to approach a way point that was 10 light minutes away from earth at light speed, then time would go 50% slower for you actually, and the clock on earth, if you could observe it, would appear to be slowed by a rate of 86% while you were moving away from it actually, etc. And if you immediately turned around once you reached the way point without pause and immediately headed back to earth at that speed, etc, time would still be elapsing at 50% for you, but if you could observe the clock on earth, it would now be appearing to now be elapsing at 172% normal speed so that when you got back to earth, time would have elapsed or passed on their clock 20 minutes again, etc, but this time plus the extra 1.14% multiplied by 2 of extra minutes of time it took you to travel towards the ten minute light year away way point and back both ways at 86% light speed, etc, so 22.8 minutes would have elapsed total when you got back to earth maybe, etc, this will also slightly alter the percentages at which you saw time elapse on earth's clock back at earth also when you were observing it both ways, etc, but I can't get all of this math 100% exactly correctly right right now, as I was never good at math, and it is straining my brain, etc, and this is only an illustration, etc, but know that it will only just slightly alter those previous percentages at which you would have seen earth's clock going or elapsing on it both ways, etc, but slower somewhere close to 86% going directly away from it at 86% percent light speed one way, and headed back to it, or earth, etc, somewhere close to 172% coming back or headed back to earth the other way, etc. Either way, when you got back to earth the flow of time there never really would have actually changed on earth ever, etc, but just your observations of it temporarily both ways, etc, and when you got back to earth 22.8 minutes would have elapsed total on earth's clock either way, but time for you was 50% slower the whole time, so you would be a little bit less than half as young as them now either way, etc, maybe 45%.to 40% younger than them now maybe, etc. Like I said, I can't do all of this math exact right now, but I know I'm close enough to give an illustration, etc. But again, if you stop traveling at that speed, time returns to normal for both of you, even though the actual passage of time on earth, or the rate at which time elapsed/elapses on earth never really actually ever changes, or ever really changed ever either way, etc, but just what you could see of it as you were in motion or moving directly away from it, and immediately right directly back towards it at that same speed is the only thing that really ever changed/changes ever either way, or both ways, etc.

God Bless.

 

[sjastro]

@sjastro can correct me if I am wrong about this, etc. But if your moving at the speed of light everything seems to stop for you no matter what, and no matter what direction your heading, but time everywhere else is unaffected, but proceeds normally, etc. So if you traveled ten light minutes away from Earth at light speed, but time froze or stopped for you while you were traveling, then you immediately turned around without pause and immediately headed back to earth again at light speed, the time elapsed on earth would be 20 minutes no matter what when you got back, but zero time would have passed for you actually, etc.

And in the case of moving directly away from earth at light speed, the clock on earth would appear to freeze as you went away if you could observe it while you were traveling directly away from earth at light speed, but on your immediate direct return trip back at light speed, the clock on earth, if you could observe it while you traveling to it now, etc, would now be elapsing at double the speed, resulting in a total time elapsed on earth of 20 minutes when you got back, etc, but because time froze for you both ways when you were moving in both directions at light speed, no time would have actually passed for you or on your clock actually, so you'd be 20 minutes younger than those on earth are now actually, etc. But once you stopped moving, time in both places would return to normal for both of you now actually, etc. But the time or rate of time on earth never really actually changed ever actually, but only just your observations of it temporarily actually, except that time actually froze or stopped for you, which means you'd be 20 minutes younger than those on earth now actually, etc.

God Bless.

Objects with mass cannot travel at the speed of the light, so the question becomes what happens to a massless particle such as a photon travelling at the speed of light.
At the speed c, the Lorentz factor is ϒ = 0, which means due to length contraction a single photon is 'everywhere' and doesn't experience time.
Technically a photon does not exist in a frame of reference as there is no frame where a photon can be at rest when observed.
It travels at the speed of light to all inertial observers.

 

[KevinT]

You should probably review the rest of the thread.

But thanks for the comment though.

God Bless.

Doooh! I missed that there were more pages. I read the initial posts and scrolled to the end of the page and missed that this thread has been going on for awhile. Wow! People are passionate about the one way vs two way speed of light....




KT

 

[Neogaia777]

Objects with mass cannot travel at the speed of the light, so the question becomes what happens to a massless particle such as a photon travelling at the speed of light.
At the speed c, the Lorentz factor is ϒ = 0, which means due to length contraction a single photon is 'everywhere' and doesn't experience time.
Technically a photon does not exist in a frame of reference as there is no frame where a photon can be at rest when observed.
It travels at the speed of light to all inertial observers.

Yes, I do know that traveling at the exact speed of light, or very close to it, is impossible, or next to impossible right now for anything with mass, and might always be, etc.

But explaining the previous example, even if it was only in theory, was a heck of a lot easier to do, and probably a whole heck of a lot easier for other people to try and get and comprehend and understand maybe, than maybe the last example I just gave at 86% of light speed, or time at 50% maybe, etc.

Although I did my best with both, etc.

Take Care @sjastro

God Bless.

 

[Neogaia777]

Doooh! I missed that there were more pages. I read the initial posts and scrolled to the end of the page and missed that this thread has been going on for awhile. Wow! People are passionate about the one way vs two way speed of light....




KT

Yes, they apparently are.

And I still am a little bit still also, etc.

I'd maybe still like to try and see it resolved for sure anyway, etc.

And/or anyway,

God Bless.

 

[Platte]

Since the one way speed of light can't be measured what is the point of this?

You asked what was special about reflective light. That was the point. Reflective is the result of direct light reflecting off an object and generated everything we see. Everything we see is reflective light. The colors, the shapes, the details. Reflective light has special and unique properties that we don’t see in direct light.