# The One-Way Speed of Light: Speculation of a Tired Mind

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• Matthew_p
In summary: So physicists make it easy on themselves and assume the one way speed of light is the same in all directions, and they don't have to keep track of which way they decided was the fast direction. But if you really want to, you can assume it is different, and as long as you are consistent, you can make up a self-consistent set of equations that will work. But it is still completely arbitrary, and there is no way to tell whether it is right or wrong, because it is just a choice.
Matthew_p
TL;DR Summary
could we not mathematically prove a non-zero one-way speed of light relative to the speed of electrical transmission? (send a signal one way, turn a light on, and attempt to determine differences in transmission speeds?)
I perfectly understand that we cannot, as of our current methods, determine the one-way speed of light - I was, however, wondering if we could determine a relative speed using electrical transmission. the basis of which would be sending a laser light across a field into a sensor, starting a clock, and then waiting for an electrical signal to stop said timer. if we repeated the experiment with different orientations would it be possible to detect a shift, and - if present - use said to shift to determine the one-way speed of light relative to electrical transmission? (assuming electricity either suffers the same theoretical problems to a different degree or does not experience a speed change at all)

the video that peaked my interest into said topic -

No, the one way speed of light is a value you can choose and it cannot be measured without assuming your answer. If you pick an anisotropic one way speed of light you pick everything else to have anisotropic speeds too, including electric signals, in such a way that it makes no difference to your experimental results.

This isn't a thing that you can work your way around by sneaky experimental design. The problem is that "space" and "time" aren't truly separate things, they are both aspects of spacetime. How you choose to divide spacetime into space and time is just that - a choice. And the choice you make affects your definition of distance travelled and time taken, and hence your definition of speed (of anything, not just light) in such a way that the choice cancels out of measurements and remain in how you interpret your measurements. So in your proposal there is some fixed time for "light travel time plus electrical signal travel time" that is independent of the choice of light speed, but how you divide that total into its two components is your choice.

As long as you don't travel very fast, the flexibility in defining speeds doesn't make much difference. For example if you do a 100m race, I can only vary your time by about ##0.3\mathrm{\mu s}## by messing around with my definition of "space". That's in "who cares" territory for a 100m sprint, or even the fastest planes or cars, which is why we don't notice this stuff in every day life.

Last edited:
vanhees71, PeroK, Matthew_p and 3 others
Moderator's note: Thread moved to the relativity forum.

Matthew_p said:
TL;DR Summary: could we not mathematically prove a non-zero one-way speed of light relative to the speed of electrical transmission? (send a signal one way, turn a light on, and attempt to determine differences in transmission speeds?)

I perfectly understand that we cannot, as of our current methods, determine the one-way speed of light
It has nothing to do with our current methods. The one way speed of light is something that we define through our simultaneity convention. We fully determine it by making an arbitrary choice. It is not that it is unknown, but that it is something we are free to decide.

cianfa72, vanhees71, PeroK and 1 other person
vanhees71 and Orodruin
Ibix said:
As long as you don't travel very fast, the flexibility in defining speeds doesn't make much difference. For example if you do a 100m race, I can only vary your time by about 0.3ms by messing around with my definition of "space". That's in "who cares" territory for a 100m sprint, or even the fastest planes or cars, which is why we don't notice this stuff in every day life.
okay, speed dilation is then a major issue, but all in all this really doesn't matter in an applied physics sense then?

Matthew_p said:
okay, speed dilation is then a major issue, but all in all this really doesn't matter in an applied physics sense then?
It doesn’t matter in any physics sense.

Matthew_p said:
okay, speed dilation is then a major issue, but all in all this really doesn't matter in an applied physics sense then?
The maths is easier if you assume the one way speed of light is the same in all directions, and you don't have to keep track of which way you decided was the fast direction. But apart from that it really doesn't matter at all.

cianfa72 and Dale

## 1. What is the one-way speed of light?

The one-way speed of light is the speed at which light travels in one direction, without any interference or influence from external factors.

## 2. How is the one-way speed of light measured?

The one-way speed of light is typically measured using highly precise instruments, such as lasers and mirrors, in laboratory settings. These experiments involve sending a beam of light in one direction and measuring the time it takes to reach a specific destination.

## 3. Why is the one-way speed of light important in physics?

The one-way speed of light is a fundamental constant in physics and is used in various theories and equations, such as Einstein's theory of relativity. It also plays a crucial role in understanding the nature of space and time.

## 4. What is the "tired light" theory?

The "tired light" theory is a speculative idea that suggests the one-way speed of light may decrease over long distances due to the energy loss of photons. This theory is not widely accepted in the scientific community and has not been supported by empirical evidence.

## 5. How do scientists study the one-way speed of light in space?

Scientists use a variety of methods to study the one-way speed of light in space, including observations of distant objects and analyzing data from spacecraft missions. These studies have consistently shown that the one-way speed of light is constant and unaffected by factors such as gravity or the movement of objects in space.

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