# Real Science Radio: One Way Speed of Light Measurement Proposal

#### Clete

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Real Science Radio: One Way Speed of Light Measurement Proposal

In this thread I want to discuss the measurement of the one way speed of light, which virtually all physicists will tell you cannot be done.
It would probably be a good idea for you to watch the Veritasium YouTube video on the topic so as to understand why they say it's impossible....

Why No One Has Measured the Speed of Light

Bob Enyart not only disputes the claim that it cannot be measured but also proposed a way of doing so which he and Fred Williams discuss in the episode linked to at the top of this post.

My initial reaction to the episode and to reading the material on the website was that I couldn't think of any reason why it wouldn't work but then the next morning it occurred to me that it was probably sneaking in the two way speed of light. I posted the following on another thread.....

So, the more I think about Bob's proposed experiment, the more I think it does sneak in the two-way speed of light.
The mirror that they have at one end of the bottle isn't the only mirror in the experiment. In fact, every particle of milk in the slightly milky water is a mirror and every photon of light that reaches those cameras is reflected light. As such, the experiment leaves us with the same basic problem as was described in the Veritasium video. There's no way to know whether the light travels at the same speed to the milk particle as it travels from the milk particle to the camera. And the fact that it goes from right to left and then from left to right doesn't help because the effect would simply be reversed but at exactly the inverse ratio and so it would look the same in both directions whether it actually was or not.​
Now, there is the issue of running the experiment again with water vapor instead of milky water. I understand that the speed of light is faster in water vapor than it is in water but I fail to see how this would solve the problem described above. Regardless of the medium, you'd still be using reflected light to take a measurement of its speed and it would therefore be a two-way speed, by definition.​
How am I wrong? (If I am wrong, it has something to do with the fact that the light is being reflected off to one side rather than straight back to the source. - I'm still letting this marinate in my brain for now.)​
Clete​
P.S. How do I collect my Chick-fil-A gift card?

Now, another day or so later, I think that the experiment might be salvageable and I think the saving grace does indeed have to do with the fact that the light is being reflected off to one side rather than straight back to the source as I alluded to above.

I think that the problem would be overcome by simply moving one of the cameras further away from the bottle. I see no need for three cameras, by the way, but there's no harm either. However, for the purposes of discussing the modified experiment, I'm going to assume only two cameras are being used. Also, the mirror at the end of the bottle is redundant as well and can be ignored.

So, the light travels from right to left from the source until it hits a particle of milk in the water and then some portion of the light is sent at 90° toward one of the cameras. If the total distance to one camera is significantly further away from the source than the other then any difference is the speed that the light is traveling to the left vs. it's speed coming toward the camera, would become apparent because one leg of the trip is much longer than the other.

Let's say I and friend of mine, we'll call him Bob, go on a trip. We take separate cars and we both go in one direction for half a mile and then make a left turn. After the turn, I go five miles and Bob goes ten and a half miles (total of exactly twice as far as my trip). On the first half mile leg of the trip, my friend and I go one speed but on the second leg we go some different speed. Let's say on the first leg we go .5 mph and on the second leg we go 1.0 mph.

Incidentally, the second leg of the trip wouldn't need to be at 90° to the first leg. Just so long as there are two legs going in different directions where one leg is longer than the other and at least two cameras at different points along the path of the second leg.

1st leg distance for both Bob and me = .5 miles
2nd leg distance for me = 5 miles
2nd leg distance for Bob = 10.5 miles

Total distance for me = 5.5 miles
Total distance for Bob = 11 miles

1st leg travel time for both Bob and me = 1 hr

2nd leg travel time for me = 5 hrs
2nd leg travel time for Bob = 10.5 hours

Total travel time for me (1 + 5) = 6 hrs
Total travel time for Bob (1 + 10.5) = 11.5 hours

6 x 2 ≠ 11.5 (i.e. Twice the distance but not twice the travel time)

Average speed for me (5.5 miles / 6 hrs) = .917 mph
Average speed for Bob (11 miles / 11.5 hrs) = .957 mph

.917 mph .957 mph

The bigger the difference in the speed traveled during one leg vs. the other leg, the bigger the discrepancy will be.

So, does this mean I get the Chick-fil-A gift card or not?

Clete

P.S. If this holds up to scrutiny, I would seriously like to get this idea in front of Fred Williams if anyone here has access to him. This one way speed of light issue is genuinely a big deal. It would be cool to be part of solving it!

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#### Jefferson

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If this holds up to scrutiny, I would serious like to get this idea in front of Fred WIlliams if anyone here has access to him. This one way speed of light issue is genuinely a big deal. It would be cool to be part of solving it!
You should be able to call into the Real Science Radio show and talk to Fred. 800-8Enyart or 303-463-7789

#### Clete

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I had the math wrong when I first wrote the opening post. It didn't not alter the overall point but I have fixed it.

#### Clete

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Okay, so I just realized that what I've described in the opening post is a modified version of the Michelson–Morley experiment which compared the speed of light in perpendicular directions and found no significant difference between the speed of light in either direction. The significant difference being that the Michelson–Morley experiment was set up such that the distance the light travels in both directions is exactly the same whereas my proposal wants very different distances. In fact, in this regard, Bob's proposed experiment is even more similar to the Michelson–Morley experiment.
I'm not sure that it matters though because you can't measure speed without measuring distance, by definition, and using a light interference pattern is a very very precise way of making the measurement. In other words, if there was even the slightest difference in the directional speed, it would have shown up in that experiment. The fact that it didn't with such a precise measuring methodology, means that there very likely isn't any difference between the speed of light in either direction.

So....

If my thinking on this is right, it would mean that Michelson and Morley already measured the one way speed of light back in July of 1887 and every physicist between then and now just over looked it.

Not likely the case - to say the least!

I am surely wrong on this somehow!

#### Clete

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Moments after posting the above post, it struck me how my thinking there is wrong!

The Michelson–Morley experiment measured the two-way speed of light in perpendicular directions!

This will always be the case when you try to do this with one final target and where every leg of the trip is the same length. For my proposal to work, it requires one light source, a reflector in one location and TWO cameras set at different distances from the reflector.
Technically, this is still two separate measurements of the two-way speed of light but since both trips share only one common leg, the comparison will still betray any difference in the directional speed of light.

#### Clete

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You should be able to call into the Real Science Radio show and talk to Fred. 800-8Enyart or 303-463-7789
When is the best time to call?

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#### Jefferson

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When is the best time to call?
The show starts at 5pm Eastern, so I would call in about 5 minutes before that.

#### JudgeRightly

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The show starts at 5pm Eastern, so I would call in about 5 minutes before that.

They haven't done the shows live for some time because the device they used to do that broke a long time ago, and I'm not sure if they ever replaced or repaired it.

#### Stripe

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Okay, so I just realized that what I've described in the opening post is a modified version of the Michelson–Morley experiment which compared the speed of light in perpendicular directions and found no significant difference between the speed of light in either direction. The significant difference being that the Michelson–Morley experiment was set up such that the distance the light travels in both directions is exactly the same whereas my proposal wants very different distances. In fact, in this regard, Bob's proposed experiment is even more similar to the Michelson–Morley experiment.
I'm not sure that it matters though because you can't measure speed without measuring distance, by definition, and using a light interference pattern is a very very precise way of making the measurement. In other words, if there was even the slightest difference in the directional speed, it would have shown up in that experiment. The fact that it didn't with such a precise measuring methodology, means that there very likely isn't any difference between the speed of light in either direction.
The Michelson and Morley experiment made a couple of assumptions that distanced it from a time/distance measurement. They were trying to measure Earth's orbital velocity by looking at fringe shifts with the expectation that they would prove the existence of an aether, which requires a bit of technical know-how to explain.

However, their results are almost always misrepresented. They did not find a result of zero fringe shift. They found enough to give an Earth orbital velocity of about one-quarter what would have been expected had they devised an accurate method.

The numbers they got are usually chalked up to mechanical imprecision, that is, the proponents of a "no aether" reality were expecting a 0kms result but got 8kms and celebrated because that is less of a difference to overcome than the "yes aether" people faced, ie, 8kms versus 30kms.

But the point is: Michelson and Morley did not get a zero result. A difference did show up. It must gets swept under the rug.

#### Clete

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The Michelson and Morley experiment made a couple of assumptions that distanced it from a time/distance measurement. They were trying to measure Earth's orbital velocity by looking at fringe shifts with the expectation that they would prove the existence of an aether, which requires a bit of technical know-how to explain.

However, their results are almost always misrepresented. They did not find a result of zero fringe shift. They found enough to give an Earth orbital velocity of about one-quarter what would have been expected had they devised an accurate method.

The numbers they got are usually chalked up to mechanical imprecision, that is, the proponents of a "no aether" reality were expecting a 0kms result but got 8kms and celebrated because that is less of a difference to overcome than the "yes aether" people faced, ie, 8kms versus 30kms.

But the point is: Michelson and Morley did not get a zero result. A difference did show up. It must gets swept under the rug.
Yes, I was aware that their result wasn't zero but it is also true that their non-zero result was within the margin of error and nowhere near where it would have been expected to be if the stationary eather that they were looking for existed. Their experiment didn't actually prove that there was no eather at all but only that if it does exist, it isn't sufficiently stationary to present a meaningful result in their experiment. Some have suggested that perhaps it behaves something like a viscous fluid and the Earth moving through it would be analogous to a spoon moving through honey where the fluid near the surface of the object clings to the surface and moves along with the object.

Also, the Laser Interferometer Gravitational-Wave Observatory (LIGO) effectively performs the Michelson and Morley experiment every day with more precision than Michelson and Morley could have imagined. Still no eather!

None of which has anything to do with the modern issue of Einstein's synchronicity convention and the supposed impossibility of measuring the one-way speed of the light. The similarity in the experiments is more or less coincidental.

Also, the experiment, as proposed in the opening post, would NOT be measuring the one-way speed of light but only comparing the speed of light in one direction to the speed of light in another. I can't wrap my head around the issues involved clearly enough to have figured out yet whether it would even be possible to tell which leg was traveled faster than the other (presuming an affirmative result, of course). It might be possible to figure that out but I sort of have the feeling that it wouldn't be and that the experiment would give a simple "Yes" or "No" to the question, "Does light travel the same speed in both directions?"

Clete

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#### Stripe

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Yes, I was aware that their result wasn't zero but it is also true that their non-zero result was within the margin of error and nowhere near where it would have been expected to be if the stationary eather that they were looking for existed.

That the result wasn't close to the opposing idea is not evidence that the no aether people were right.

Statistically speaking, their non-zero result was well outside any reasonable confidence level that the aether idea should be upheld.

Their experiment didn't actually prove that there was no eather at all but only that if it does exist, it isn't sufficiently stationary to present a meaningful result in their experiment. Some have suggested that perhaps it behaves something like a viscous fluid and the Earth moving through it would be analogous to a spoon moving through honey where the fluid near the surface of the object clings to the surface and moves along with the object.

I think the aether discussion — and the timespace one — is a discussion about gravity.

Also, the Laser Interferometer Gravitational-Wave Observatory (LIGO) effectively performs the Michelson and Morley experiment every day with more precision than Michelson and Morley could have imagined. Still no either!
Using a built-in relativity assumption.

None of which has anything to do with the modern issue of Einstein's synchronicity convention and the supposed impossibility of measuring the one-way speed of the light. The similarity in the experiments is more or less coincidental.
It might be tangential, but it is good to have an understanding about how these experiments work.

Also, the experiment, as proposed in the opening post, would NOT be measuring the one-way speed of light but only comparing the speed of light in one direction to the speed of light in another. I can't wrap my head around the issues involved clearly enough to have figured out yet whether it would even be possible to tell which leg was traveled faster than the other (presuming an affirmative result, of course). It might be possible to figure that out but I sort of have the feeling that it wouldn't be and that the experiment would give a simple "Yes" or "No" to the question, "Does light travel the same speed in both directions?"

I'm in the same boat.

#### Idolater

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That the result wasn't close to the opposing idea is not evidence that the no aether people were right.

Statistically speaking, their non-zero result was well outside any reasonable confidence level that the aether idea should be upheld.

I think the aether discussion — and the timespace one — is a discussion about gravity.

Using a built-in relativity assumption.

It might be tangential, but it is good to have an understanding about how these experiments work.

I'm in the same boat.
idk. It seems like, in order to measure time over a distance, you need a way to couple a start time and a stop time, from two clocks. Couldn't we just do what we think is simultaneously record the time at the clock at the start, and the clock at the end? Between a long distance? Collect this data and analyze it, use it as a baseline for when we are trying to synchronize them. Now, rig the clock at the end to record the time only when it detects light from the start. And rig the clock at the start to record the time right when the light is emitted. And then compare these measurements with your measurements when you think you're triggering a simultaneous record. It doesn't seem statistically impossible to do this. Given enough data points with sensitive enough instruments you ought to be able to get a good measure of the one-way speed of light, with a little luck.

#### Stripe

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idk. It seems like, in order to measure time over a distance, you need a way to couple a start time and a stop time, from two clocks. Couldn't we just do what we think is simultaneously record the time at the clock at the start, and the clock at the end? Between a long distance? Collect this data and analyze it, use it as a baseline for when we are trying to synchronize them. Now, rig the clock at the end to record the time only when it detects light from the start. And rig the clock at the start to record the time right when the light is emitted. And then compare these measurements with your measurements when you think you're triggering a simultaneous record. It doesn't seem statistically impossible to do this. Given enough data points with sensitive enough instruments you ought to be able to get a good measure of the one-way speed of light, with a little luck.
The problem is that the measurements are never made of whatever light is in a direct manner. With a car, you just stick your measuring device on the car, or trigger it when the car arrives. With light, it's always measuring some artifact of light.

And by measuring in this indirect manner, you're always getting the round trip (even when it's carefully disguised as a one-way trip).

For example, with your two clocks, someone has to read the clocks. Those two people have to read them from equal respective distances. There are all sorts of problems with this, one being that if you are a relativist, you had to accelerate one of the clocks away from the other and they are in unique gravity environments.

#### Clete

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idk. It seems like, in order to measure time over a distance, you need a way to couple a start time and a stop time, from two clocks. Couldn't we just do what we think is simultaneously record the time at the clock at the start, and the clock at the end? Between a long distance? Collect this data and analyze it, use it as a baseline for when we are trying to synchronize them. Now, rig the clock at the end to record the time only when it detects light from the start. And rig the clock at the start to record the time right when the light is emitted. And then compare these measurements with your measurements when you think you're triggering a simultaneous record. It doesn't seem statistically impossible to do this. Given enough data points with sensitive enough instruments you ought to be able to get a good measure of the one-way speed of light, with a little luck.
The problem is that if you want to measure the one way speed of light you have to have synchronized clocks but in order to synchronize your clocks you have to know the one way speed of light, which is what you're trying to measure.

Why No One Has Measured the Speed of Light

#### Clete

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Since the cameras are acting as clocks where each frame is counted as a "tick" of the clock, I wonder whether the same problem that exists for synchronizing clocks in other experiments would apply to the synchronizing of the cameras in my experiment?

#### Yorzhik

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Since the cameras are acting as clocks where each frame is counted as a "tick" of the clock, I wonder whether the same problem that exists for synchronizing clocks in other experiments would apply to the synchronizing of the cameras in my experiment?

I thought there was a scheme someone proposed using entangle particles?

#### Clete

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I thought there was a scheme someone proposed using entangle particles?
I don't think that entangled particles do what people think they do.
I'm no expert but I'm reasonably certain that there is no way to transfer information faster than the speed of light, even with quantum entangled particles.

#### JudgeRightly

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I don't think that entangled particles do what people think they do.
I'm no expert but I'm reasonably certain that there is no way to transfer information faster than the speed of light, even with quantum entangled particles.

To explain: When two or more entangled particles are generated, if you modify one of them, its entanglement with the other(s) will be broken, while the other(s) will remain unchanged.

#### Clete

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To explain: When two or more entangled particles are generated, if you modify one of them, its entanglement with the other(s) will be broken, while the other(s) will remain unchanged.
You can measure the quantum state of an entangled particle, which then instantly determines the quantum state of the particle it's entangle with, regardless of how far away it is, but as soon as you make that measurement, the entanglement goes away and, since there's no way for you to know the quantum state of either particle until you measure it, there's no way to use the system to send information because you can't even tell if someone has already measured the quantum state of the other particle until you measure it on your particle. Did you collapse the entanglement by measuring it or had it already been collapsed by the person you're trying to send a message too? You can't tell.