4 ms·

The speed of light will always be seen to be the same no matter what, no matter where you are, no matter when you are. That's because we measure the speed of l

The speed of light will always be seen to be the same no matter what, no matter where you are, no matter when you are. That's because we measure the speed of light with light, and we measure distances using light or light-by-proxy (because the electronic interactions that make normal forces what they are... electronic and subject to the speed of light, as is everything else). Other constants might change, but it would be very surprising if the speed of light (as observed locally) could possibly vary.

mystified5016 2y ago
Speed of light can also be derived from the speed of a signal through a length of wire.
- jon_richards 2y ago
  I think the point is that if the speed of light changes, so does the length of the wire.
  - cryptonector 2y ago
    Basically. I suppose that ε0 and μ0 could change in different ways such that c remains the same, or in different ways such that c does change but the change might be noticeable.
oneshtein 2y ago
We can try to use gravitational waves (speed of gravitation propagation) to measure length.
- tsimionescu 2y ago
  Gravitational waves travel at the speed of light as well. So if the speed of light changes, so would the speed of gravity waves, presumably - otherwise, they would probably not be equal today (though of course it could always be a coincidence).
  - tzs 2y ago
    Gravitational waves and light travel at the same speed in a vacuum. What if you took a very very very long piece of glass and sent both gravity waves and light waves through it? The light will be slowed down. Google tells me that there is something analogous to the refractive index for gravity waves, so there should also be some slowing of the gravity waves, but would the optical refractive index and the gravitational refractive index be the same? I'd expect that it would not be the same. The optical refractive index if I recall correctly doesn't depend on the masses of the particles that make up the medium it is traveling through. Just charge and arrangement. Gravity waves should only depend on mass and arrangement.
    - tsimionescu 2y ago
      The speed of light in a medium is always some constant specific to that medium times c. If c changes, the constant will not be affected. So I don't see how you could use the speed of light in a medium to deduce anything about c. The same logic applies to gravitational waves. And as long as the speed of gravitational waves is always equal to the speed of light, the relative values of the two will still not change. Say old light speed in glass is k*c1, new speed is k*c2. Old gravity wave speed if n*c1, new is n*c2. How do you you use these numbers to find out if c1 == c2 or not?
mattashii 2y ago
Wouldn't a different speed of light impact the Schwarzschild radius of black holes of a given mass? Assuming that you can create a standard clock, and given a black hole of standard mass, you can then measure speed of light in black hole radii per unit of time, which will differ with different speeds of light.
- orlp 2y ago
  > Wouldn't a different speed of light impact the Schwarzschild radius of black holes of a given mass? No it wouldn't. Our fundamental unit of distance (the meter) is defined in terms of the speed of light, so the radius will stay exactly the same, in meters.
  - Zamicol 2y ago
    Black holes also frequently send ambassadors to the White House so they can comply with US law. Human leadership in physical law is well respected universally.
  - mattashii 2y ago
    > Our fundamental unit of distance (the meter) is defined in terms of the speed of light, so the radius will stay exactly the same, in meters. Why so? The Schwarzschild radius formula (r(s) = 2 * G * M / c²) uses c² as dividing constant. So when the speed of light is reduced by half, the radius of a black hole of the same mass will be larger by a factor 4. Add to that the reduced distance light travels in the same time, and you'll get that the time light takes to travel the distance equivalent to the schwarzschild radius of a black hole with iso-mass is 8x larger in a universe with a speed of light 1/2 that of ours, assuming no other constants have been modified.
    - Thorondor 2y ago
      You're ignoring the units of G. The gravitational constant has units N * m^2 / kg^2 = m^3 / (kg * s^2). That contains a length unit raised to the third power, which exactly balances the factor of 8 that you identified.
simpaticoder 2y ago
c is no longer measured, it is defined, and unless some contradiction to special relativity is discovered, c cannot change. If the speed of causality changes, then our measure of distance would change. For example, if c halves in some sense, then this means light travels half as far during N ticks of a clock, and the meter will halve (and all internet latencies will approx double!). If we keep the old meter, then we might say c has changed; it's truly a matter of definition (and practicality) at that point.
- nyc111 2y ago
  Same goes for their new thorium “clock”. They define it as their unit to measure everything else. They assume that all constants may be changing but not their thorium clock. I think this is an unjustified assumption.
  - JanisErdmanis 2y ago
    If they detect that the change there would be a strong incentive replicating the setup as well as to put into scrutinity. Academia even with its faults is a very good institution for that.
  - deleted 2y ago
    [deleted]

mystified5016 2y ago

Speed of light can also be derived from the speed of a signal through a length of wire.

jon_richards 2y ago
I think the point is that if the speed of light changes, so does the length of the wire.
- cryptonector 2y ago
  Basically. I suppose that ε0 and μ0 could change in different ways such that c remains the same, or in different ways such that c does change but the change might be noticeable.

oneshtein 2y ago

We can try to use gravitational waves (speed of gravitation propagation) to measure length.

tsimionescu 2y ago
Gravitational waves travel at the speed of light as well. So if the speed of light changes, so would the speed of gravity waves, presumably - otherwise, they would probably not be equal today (though of course it could always be a coincidence).
- tzs 2y ago
  Gravitational waves and light travel at the same speed in a vacuum. What if you took a very very very long piece of glass and sent both gravity waves and light waves through it? The light will be slowed down. Google tells me that there is something analogous to the refractive index for gravity waves, so there should also be some slowing of the gravity waves, but would the optical refractive index and the gravitational refractive index be the same? I'd expect that it would not be the same. The optical refractive index if I recall correctly doesn't depend on the masses of the particles that make up the medium it is traveling through. Just charge and arrangement. Gravity waves should only depend on mass and arrangement.
  - tsimionescu 2y ago
    The speed of light in a medium is always some constant specific to that medium times c. If c changes, the constant will not be affected. So I don't see how you could use the speed of light in a medium to deduce anything about c. The same logic applies to gravitational waves. And as long as the speed of gravitational waves is always equal to the speed of light, the relative values of the two will still not change. Say old light speed in glass is k*c1, new speed is k*c2. Old gravity wave speed if n*c1, new is n*c2. How do you you use these numbers to find out if c1 == c2 or not?

mattashii 2y ago

Wouldn't a different speed of light impact the Schwarzschild radius of black holes of a given mass? Assuming that you can create a standard clock, and given a black hole of standard mass, you can then measure speed of light in black hole radii per unit of time, which will differ with different speeds of light.

orlp 2y ago
> Wouldn't a different speed of light impact the Schwarzschild radius of black holes of a given mass? No it wouldn't. Our fundamental unit of distance (the meter) is defined in terms of the speed of light, so the radius will stay exactly the same, in meters.
- Zamicol 2y ago
  Black holes also frequently send ambassadors to the White House so they can comply with US law. Human leadership in physical law is well respected universally.
- mattashii 2y ago
  > Our fundamental unit of distance (the meter) is defined in terms of the speed of light, so the radius will stay exactly the same, in meters. Why so? The Schwarzschild radius formula (r(s) = 2 * G * M / c²) uses c² as dividing constant. So when the speed of light is reduced by half, the radius of a black hole of the same mass will be larger by a factor 4. Add to that the reduced distance light travels in the same time, and you'll get that the time light takes to travel the distance equivalent to the schwarzschild radius of a black hole with iso-mass is 8x larger in a universe with a speed of light 1/2 that of ours, assuming no other constants have been modified.
  - Thorondor 2y ago
    You're ignoring the units of G. The gravitational constant has units N * m^2 / kg^2 = m^3 / (kg * s^2). That contains a length unit raised to the third power, which exactly balances the factor of 8 that you identified.

simpaticoder 2y ago

c is no longer measured, it is defined, and unless some contradiction to special relativity is discovered, c cannot change. If the speed of causality changes, then our measure of distance would change. For example, if c halves in some sense, then this means light travels half as far during N ticks of a clock, and the meter will halve (and all internet latencies will approx double!). If we keep the old meter, then we might say c has changed; it's truly a matter of definition (and practicality) at that point.

nyc111 2y ago
Same goes for their new thorium “clock”. They define it as their unit to measure everything else. They assume that all constants may be changing but not their thorium clock. I think this is an unjustified assumption.
- JanisErdmanis 2y ago
  If they detect that the change there would be a strong incentive replicating the setup as well as to put into scrutinity. Academia even with its faults is a very good institution for that.
- deleted 2y ago
  [deleted]