OT: System Clock Loses Hour

[Phoenix-D]

Which doesn't contradict me at all, he just didn't explain it the same way.

[narf poit chez BOOM]

Hmm, yes, then he didn't answer my question, because I specifically asked the difference between the speed of light here and at jupiter *While both are moving*. Not at rest.

[Fyron]

The speed of light is the same here as it is near Jupiter. It is not relative to the nearest large body. That is the point; from all reference frames, the speed of light is the same.

This contradicts what you would expect, given the behavior of matter. eg: If I throw a ball, its speed of travel is just the speed at which I threw it, when we are discussing the reference frame of sitting here an Earth. However, the ball is really moving a lot faster than that. Its absolute speed is the speed at which I threw it, plus the speed of the Earth's rotation, plus the speed of the Earth's orbit around the sun, plus the speed of the suns movement in the galaxy, plus the movement of the galaxy... (though we really should be talking about velocities, not speeds.)

But with light, none of that is relevant; it travels at the same speed regardless of reference frame. Shine a flashlight from the surface of the earth, the light moves at c. Shine a light from (a stable, geosynchronous) orbit, the light moves at c. Shine a light from orbiting Jupiter, where you are moving at a different speed than orbiting Earth, the light still moves at c.

[narf poit chez BOOM]

Ah, well, thanks. That answers my question and brings up a different interesting thought - That, using the speed of light, it should be possible to calculate an absolute frame of reference.

Anybody done that yet?

[AngleWyrm]

Might be able to do something like this with red/blue doppler shifts seen in surrounding stars.

For instance, take a given star and measure it's color-band shifts in spring when the earth is travelling in one direction, then again in fall when the earth is travelling in another.

[narf poit chez BOOM]

Why not just measure the speed of the earth relative to the speed of light?

[capnq]

narf poit chez BOOM said:
Why not just measure the speed of the earth relative to the speed of light?

Because that results in the inconveniently small number (0.001% of c) that AngleWyrm mentioned further upthread.

[douglas]

narf poit chez BOOM said:
Ah, well, thanks. That answers my question and brings up a different interesting thought - That, using the speed of light, it should be possible to calculate an absolute frame of reference.

Anybody done that yet?

The whole point of relativity is that an absolute frame of reference simply does not exist. No matter what frame of reference you use, light travels at c in that frame of reference. If you measure the speed of a planet as 300 km/s and then measure the speed of light relative to that planet in the same direction of movement, you will get c - 300 km/s. In the opposite direction you will get c + 300 km/s. If you then change your reference frame by landing on the planet so you are moving along with it and the planet is at rest from your perspective, repeating your measurements would tell you that light is moving at c relative to the planet in all directions.

[se5a]

sooo...
the speed of light IS a frame of reference?

if I have a torch (flashlight) moving at 0.5c and I turn it on, the light from the torch would be moving at c right?
but, ralitive to the torch the beam is moving at 0.5c or 1.5c depending on weither the torch is facing forwards or backwards.

what narf is saying, is if you use said torch, and mesure the amount of time it takes for light to get from the torch to an known distance, you should be able to work out how fast, and in what direction the torch is moving, and therefore be able to say what "still" is.

[douglas]

se5a said:what narf is saying, is if you use said torch, and mesure the amount of time it takes for light to get from the torch to an known distance, you should be able to work out how fast, and in what direction the torch is moving, and therefore be able to say what "still" is.

What I'm saying is that this exercise will just tell you that whatever you had already decided was "still" for purposes of calculation was, in fact, correct. Simply stating that something is moving at speed X inherently requires a definition of what is at rest. The point of relativity is that this definition can be absolutely anything and the speed of light will be the same regardless.

For example:
Suppose you are on Earth trying to carry out that torch experiment. You carefully measure the distance between the torch and a mirror, you light the torch, and you time how long it takes for the reflected light to get back to you. You will calculate from this data that you (actually your measuring device) are at rest.

Next, you move to Mars and repeat the whole experiment. You will again calculate that your measuring device is at rest even though it is clearly moving at a different velocity than it was on Earth. Consideration of this simple fact is how the entirety of the Theory of Relativity was derived.