4 ms·

I get what you are saying, it suggests there is no preferred direction, but it is just an inference. The point is one cannot create an experiment to measure the

I get what you are saying, it suggests there is no preferred direction, but it is just an inference. The point is one cannot create an experiment to measure the speed of light other than a round trip average speed.

CamperBob2 4y ago
Wasn't that the whole idea behind Michelson-Morley ( https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...)?
- db48x 4y ago
  Yes, that is an example of a measurement of the two-way speed of light. What is impossible, however, is a measurement of the one-way speed of light.
  - CamperBob2 4y ago
    M-M compared the speed in one direction to the speed at right angles. That's not quite the same thing as a one-way measurement, but it's also not the same thing as a two-way measurement along the same path.
    - db48x 4y ago
      Right, but the way that they measured the speed in any direction was by making a two-way measurement. They sent a pulse of light out to be reflected and timed how long it took for it to return.
      - CamperBob2 4y ago
        I imagine that a ring-laser gyro would return some pretty strange results in a universe where c was anisotropic.
        db48x 4y ago
        Nope. A ring–laser gyro sends two light waves in opposite directions around a loop. Some of the light from both beams is let out at one point on the perimeter and the interference between them is measured. It can be any shape you like, as long as the beams trace out the perimeter of some area. When the gyro is rotating, one of the beams has to travel around the perimeter plus some extra distance due to the rotation, while the other beam has to travel the perimeter less that same distance. This causes the interference pattern to shift. The amount of the shift is proportional to the area inside the perimeter, the amount of rotation, the speed of light, and the frequency of the light that you are using. When the speed of light is anisotropic, we have to replace the constant c with some horrible integral which makes the math a lot worse. However, it is important to recognize that the overall time to go around the perimeter clockwise and counterclockwise is the same: both beams go the same distance to the left as they go to the right. As long as the leftwards speed of light and the rightward speed of light average out to the usual value, then the result will work out to be the same as when the speed of light is a constant with the usual value in all directions. Any time you send a pulse of light out and back, the time to travel the distance d will be αcd+βcd=2cd, where α and β show the relationship between the speeds of light in those two directions. We generally assume that α=β=1.0, but as long as α+β=2.0, then everything we can measure will work out to be exactly the same.
  - gumby 4y ago
    I don't remember that experiment measuring the speed, but merely showing (via lack of a beat pattern) that there was no difference in the speed, whatever it was, in two paths at right angles to each other.
  - kergonath 4y ago
    This does not imply that the speed of light could depend on direction, though. If that were the case, we could see it by rotating a simple Michelson interferometer.
kolinko 4y ago
Red shift would be different in various directions then. We would also see fewer young galaxies / stars in the direction in which light flies faster.
- lovich 4y ago
  That would imply an axiom that forces/light/whatever are on a gradient we operate against and does not change based on reference point. I think the heart of the what the GP is getting at is no matter how you define it, you can measure things smaller than your measuring equipment can detect.

CamperBob2 4y ago

Wasn't that the whole idea behind Michelson-Morley ( https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...)?

db48x 4y ago
Yes, that is an example of a measurement of the two-way speed of light. What is impossible, however, is a measurement of the one-way speed of light.
- CamperBob2 4y ago
  M-M compared the speed in one direction to the speed at right angles. That's not quite the same thing as a one-way measurement, but it's also not the same thing as a two-way measurement along the same path.
  - db48x 4y ago
    Right, but the way that they measured the speed in any direction was by making a two-way measurement. They sent a pulse of light out to be reflected and timed how long it took for it to return.
    - CamperBob2 4y ago
      I imagine that a ring-laser gyro would return some pretty strange results in a universe where c was anisotropic.
      - db48x 4y ago
        Nope. A ring–laser gyro sends two light waves in opposite directions around a loop. Some of the light from both beams is let out at one point on the perimeter and the interference between them is measured. It can be any shape you like, as long as the beams trace out the perimeter of some area. When the gyro is rotating, one of the beams has to travel around the perimeter plus some extra distance due to the rotation, while the other beam has to travel the perimeter less that same distance. This causes the interference pattern to shift. The amount of the shift is proportional to the area inside the perimeter, the amount of rotation, the speed of light, and the frequency of the light that you are using. When the speed of light is anisotropic, we have to replace the constant c with some horrible integral which makes the math a lot worse. However, it is important to recognize that the overall time to go around the perimeter clockwise and counterclockwise is the same: both beams go the same distance to the left as they go to the right. As long as the leftwards speed of light and the rightward speed of light average out to the usual value, then the result will work out to be the same as when the speed of light is a constant with the usual value in all directions. Any time you send a pulse of light out and back, the time to travel the distance d will be αcd+βcd=2cd, where α and β show the relationship between the speeds of light in those two directions. We generally assume that α=β=1.0, but as long as α+β=2.0, then everything we can measure will work out to be exactly the same.
- gumby 4y ago
  I don't remember that experiment measuring the speed, but merely showing (via lack of a beat pattern) that there was no difference in the speed, whatever it was, in two paths at right angles to each other.
- kergonath 4y ago
  This does not imply that the speed of light could depend on direction, though. If that were the case, we could see it by rotating a simple Michelson interferometer.

kolinko 4y ago

Red shift would be different in various directions then. We would also see fewer young galaxies / stars in the direction in which light flies faster.

lovich 4y ago
That would imply an axiom that forces/light/whatever are on a gradient we operate against and does not change based on reference point. I think the heart of the what the GP is getting at is no matter how you define it, you can measure things smaller than your measuring equipment can detect.