this post was submitted on 15 Jun 2024
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Here's a fun fact that I think of every time I read about light delay.
We assume the speed of light is the same in all directions but there's no way to prove that it is.
It could be light speed is instantaneous in one direction, and half the speed we think it is in the reverse. Any test we could devise depends on information traveling in two directions, nullifying any discrepancies in light speed.
... but there is a way, and it has been proven.
One of the more memorable physics classes I've had went into the history of discoveries that led to our understanding of relativity. The relevant story here, starts with how sound travels though air.
Let's say you're standing at the bottom of a building shouting to your friend peeking out a window on the 5th floor. On a calm day, that friend will hear you at pretty much the same time as someone standing the same distance away, but on the street. However, if it's windy, the wind pushes around the air through which the sound of your voice is traveling, the friend up in the window will have a slight delay in receiving that sound. This can of course be verified with more scientific rigor, like a sound sent in two perpendicular directions activating a light.
Scientist at the time thought that light, like sound, must travel though some medium, and they called this theoretical medium the Aether. Since this medium is not locked to Earth, they figured they must be capable of detecting movement of this medium, an Aether wind, if you will. If somehow the movement of this medium caused the speed of light in one direction to be faster than another due to the movement of this medium, measuring the speed in two directions perpendicular to each other would reveal that difference. After a series of experiments of increasing distances and measurement sensitivities (think mirrors on mountain tops to measure the time for a laser beam to reflect), no change in the speed of light based on direction was found.
Please enjoy this wikipedia hole: https://en.m.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment , and please consider a bit of caution before you refer to things as facts in the future!
As far as I'm aware, what you cited only proves that there is no ether that acts on light in a way such that the round trip time in the direction of ether travel is different from the round trip time in the direction perpendicular to ether travel.
It's not merely that:
Instead, it's that the speed of light must be different in the two directions in a way such that their round trip times don't average out to the same average as in the other direction.
The theories of ether at the time predicted such a round trip difference because of the wind like interactions that you say.
I believe that this in no way proves anything about the one way speed of light. The Michaelson Morley inteferometer only measures difference in round trip time.
(Insert comment about the irony of your last statement). See https://en.m.wikipedia.org/wiki/One-way_speed_of_light
How does the double slit experiment work into this?
The speed of light in a vacuum unaffected by external forces such as gravity should be the same no matter what direction it is in. I'm not sure why it wouldn't be. That's like saying a kilometer is longer if you go East than if you go West.
However, it's actually far more complicated than that, and much of it beyond my understanding.
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html
That said, direction should not matter.
There's no reason it wouldn't be. The point is that it's impossible to prove that it is. There is no conceivable experiment that can be performed to prove the two-way speed of light is symmetric.
That's not how anything works. It's impossible to prove that the universe wasn't created last Thursday with everything in place as it is now. There's no point in assuming anything that can't be proven has validity.
It's just a thought exercise. There are several reputable YouTube videos on this topic. None of them claim that the speed of light isn't the speed of light. They're just demonstrating that we can't prove it with current technology. Similar to the difficulty it took to finally prove that one plus one equals two. We know that's correct, but it took years to prove it.
...but that's exactly what you're doing. The fact that light travels at the same speed in all directions cannot be proven. You're the one insisting that it does.
I'm not insisting anything. I'm saying that, based on everything we know, the direction of light has no bearing on its speed.
Suggesting that it does just because we don't have evidence that it doesn't is no different, as I said, as claiming the universe was created last Thursday.
Maybe the speed of light doubles when it goes through the exact right type of orange. You can't prove it doesn't.
This is slighlty different though, we only know the two-way speed of light, not the one way speed of light.
We only know that this trip, to and back, takes x seconds. We cannot prove that the trip to the mirror takes the same length of time as the way back.
The special theory of relativity for example does not depend on the one way speed of light to be the same as the two way speed of light.
Wiki
With a detector and very accurate clocks, it would be easy to say "I'm going to send a pulse at 2pm, record when you receive it" that's measuring it in one direction
The very accurate clock needed in this case is physically impossible as far as we know, there's no way to measure it as far as our current understanding of physics goes.
Though if you can figure out a way you should publish a paper about it.
Can you cite some literature to back up that claim? Stating that something like acceptable clock synchronisation (a well established and appreciated method in the measurements of physical effects) is impossible in and of itself is something so bold that no one can just take your word for it.
It is impossible to synchronize the clocks in such a way that you can actually measure the speed of light with it due to time dilation unless you define beforehand how fast the speed of light is to calculate that time dilation.
See also This or, more accessibly "Synchronization conventions"
The clocks involved in gps are accurate enough that they have to take relatively into account for gps to be accurate. That's far more accurate than you need to measure the speed of light.
And to calculate the offset needed to get them all synced up involves calculating time dilation, which involves knowing/assuming the speed of light. These synchronizations work just as well if the two way speed of light is different than the one way speed of light.
To know the speed of light you assume the speed of light is c, but you're trying to calculate c so all those clocks aren't verified synced.
Just read through the wiki or Harvard's books if you'd like, this is an unsolved "problem" in physics for a reason or do you think no one cares about how fast c is?
See also This or, more accessibly "Synchronization conventions"
I read all those and every test has reduced the amount that the speed of light could be anisotropic. From "it could be twice as fast in this direction to the other" to "it could be a small fraction of the relativistic effect of moving a clock through space." Every improvement in measurement trends towards isotropic.
Why would the one-way speed be different? For what reason? Just because you think it's possible?
For no reason. No one is saying that it is different, only that it's impossible to prove one way or the other. Light traveling the same speed in all directions, and light traveling at 2x c away from an observer and instantaneously on the return, and every other alternative that averages out to c for the round trip, are indistinguishable to any experiment we can conduct.
And it's impossible to prove that just the exact right type of orange will double the speed of light.
But there's no reason to speculate either thing without a reason for the speculation. Your reason seems to be "I think it would be cool."
I don't think you realize it, but this is a very similar argument to "you can't prove God doesn't exist."
Take 30 seconds to at least glance at the article the other user posted. It's not just myself, there are plenty of very interested physicists who also find the unprovability of the one-way speed of light interesting.
I'm also not sure what your point about orange is supposed to be. Are you suggesting that there is a particular spectra of light that we cannot test?
My reason for being interested isn't just that I think it's "cool". I think it's fascinating that a fundamental underpinning of physics has such a gap in its experimental verifiability.
No, I'm saying it's just another version of Russel's Teapot.
https://en.wikipedia.org/wiki/Russell%27s_teapot
It kind of is. It's just the thing being asserted without proof is the one-way speed of light. That you don't seem to find that interesting I guess is where we differ.
Synchronise two high-precision clocks at different locations. Transmit the signal from A to a receiver at B and then send a signal back (or reflect the initial signal) from B to A. Both locations will record the synchronised time that their sensors picked up the transmission. Then, compare their clocks.
How would you sync them... ? Seems to beg the premise.
Sync them right next to each other, then move one of them. The other way you could test this theory is to have one clock tell the other the time over an optical link and then have the other do the same. If the speed of light was different in different directions. Each would measure a different lag.
Well, moving them is out of the question, since, you know, motion will change the clocks time. If you re-sync them, you bake the "error" into your framework. If you try a timer, the timer is offset. If you try and propagate a signal, the signal is offset. And eventually, you have to compare the two times, which muddies the waters by introducing a third clock.
Basically, there is no way to sync two clocks without checking both clocks, ergo, no way of proving or disproving. That's the premise.
In practicality, I assume it is constant, but it's like N=NP. You can't prove it within the framework, even if you really, really want to believe one thing.
If you move one clock very slowly away from the other, the error is minimised, perhaps even to a degree that allows for statistically significant measurements.
To cite the Wikipedia entry that one of the other commenters linked:
"The clocks can remain synchronized to an arbitrary accuracy by moving them sufficiently slowly. If it is taken that, if moved slowly, the clocks remain synchronized at all times, even when separated, this method can be used to synchronize two spatially separated clocks."
One-Way Speed of Light
And further down:
Yes, I understand that part, but it doesn't disprove that such an experiment could show isotropy. Instead, it says that it would always indicate isotropy, which is not entirely useful either, of course. I'll dig deeper into the publication behind that section when I have the time. Nonetheless, my original point still stands. With a highly synchronised clock, you could measure the (an)isotropy of the one-way speed of light. To determine whether the time dilation issue is surmountable I'll have to look at the actual research behind it.
Except if you continue reading beyond your Quote, it goes on to explain why that actually doesn't help.
That the measurements from the slow clock transport synchronisation method are equivalent to the Einstein synchronisation and its isotropic speed of light can be interpreted to show that the one-way speed of light is indeed isotropic for a given set-up and not anisotropic. The problem with this is that anisotropy could not even be measured if it were to exist in this context. But this is definitely not a clear-cut zero sum game, there's no evidence suggesting anisotropy while there are observations that would at least suggest isotropy, but neither possibility can be ruled out. However, my initial point was that, could you have ultra-synchronised clocks, you could potentially be able to draw a reliable conclusion. But I'll dig into the publication the Wiki entry cites for the time dilation part in the slow clock section when I have the time.
Couldn't we send out two devices in different directions, wait a decade, have them shine light at eachother simultaneously, record when they receive the light, then send the times back to earth?
Your question is good. You're missing understanding of time dilation and frame of reference. An explanation of the theory of relativity is at least pages long.
The first book I ever read on the subject, and IMO the best introductory text for any non-physiscist, is Stephen Hawking's "A Brief History of Time". But, any introduction to relativity should answer your question.
Another interesting way to conceptualize it is that the speed of light is infinite and it's causality/information that is limited to c. You shine a light at the moon and it takes 1.3 seconds for the "fact" that the light was turned on to propagate that far.
Cixin Liu imagines exactly that towards the end of the Three Body series. Among other things, which make the series worth absolutely slogging through at points.