As humanity’s furthest reach into the Universe so far, the two Voyager spacecraft’s well-being is of utmost importance to many. Although we know that there will be an end to any science…

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.

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.

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:

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.

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.

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.

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.

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.

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.

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.

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.

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–Morley_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

provethat 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’redoing. 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

insistinganything. I’m saying that, based oneverything 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.

Whywould 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

isdifferent, only that it’s impossible to prove one way or the other. Light traveling the same speed in all directions, and light traveling at 2xcaway from an observer and instantaneously on the return, and every other alternative that averages out tocfor the round trip, areindistinguishableto any experiment we can conduct.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.

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

infiniteand it’scausality/informationthat is limited toc. 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

sloggingthrough at points.