# The speed of lights independence of the speed of the observer?

Chipper Q
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### RE: So it is possible to

Message 59262 in response to message 59261

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So it is possible to exceed the speed of light? What happens to the mass of a particle that exceeds light speed? I thought it would get infinitely heavy before that happened. Perhaps that is more in the realm of quantum physics where things really get wierd.

Well, the speed of light in vacuum is different than the speed at which it moves through a material (like glass, or water). In vacuum the speed is â€œcâ€? (~300,000 kilometers/second), but the speed of light that's propagating through water is about 0.75c, (or about 75% of the speed in vacuum). This is because the light is interacting with the water molecules. But a particle like an electron doesn't interact with water molecules in quite the same way as the photons of light, and so it's possible for an electron to be moving faster than 0.75c in water. When this happens you get the Cerenkov radiation, but the electron is not moving faster that â€œcâ€?, the speed of light in vacuum. Hopefully this makes a little more sense...

Devilogic
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### RE: If no absolute rest

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If no absolute rest state exists, how does the Universe know that two reference frames can not move apart faster than the speed of light? Does that not mean that a two frames though millions of light years apart would have to know instantaneously that they can not move apart faster than light? That implies that time means nothing to the universe. The ole "spooky action at a distance".

Maybe this thought experiment will help a little: Imagine you were standing on Earth (a train station perhaps ;), and you had two pen lasers with you. You decide to point them at each other and turn them on. New imagine you were to pick (at random) one photon from the laser in your left hand, and one photon from the laser in your right hand.

Now ask yourself: 1. How fast is the left photon relative to me? (c), 2. How fast is the right photon relative to me? (c), 3. What is their relative velocity according to MY inertial frame? (2c), 4. And what is their relative velocity according to the reference frame of the LEFT (/right) photon? (c !)

Why is the answer to question 4 different to the one to question 3? Because normal Galilean transfromations between inertial frames don't work anymore at such speeds! They are instead replaced by Lorentzian transformations, which basically say that when you switch your frame of reference (from "me" to "photon"), the notions of time and distance change (and accordingly also the notion of velocity) - the left photon percieves a different distance between him and the right photon than me (and also percieves time differently than me) - all of this means that according to the left photon the right photon is NOT moving at 2c but meerly at 1c (and there is no spooky action at a distance needed for that).

All of this changing of time and distance can then be (maybe a bit misleadingly) summed up in the sentence: "Nothing can move faster than the speed of light" (in an inertial reference frame atleast). The important thing to take from all of this is that normal (Galilelian) rules of thinking don't apply anymore when even basic notions such as time and distance change depending on where you decide to describe the situation from.

Simplex0
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### Is the theory of relativity

Is the theory of relativity says that noting can move faster than light or is it just say that nothing can be measured to move faster than the light?

Based on the 3'd part observer scenario which can be found here math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#2 it seams that you can move faster than light, you are just not aloud to measure speed in that way.

Erik
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### Forgive me for posting off

Forgive me for posting off topic but I hate to start a thread just for a random question...

Anyway, how fast are the theoretical gravity waves? Approx. Sunday driver speed, light speed, Ludicrous speed, etc.? Is there any speculation or educated guesses?

tullio
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### RE: Forgive me for posting

Message 59266 in response to message 59265

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Forgive me for posting off topic but I hate to start a thread just for a random question...

Anyway, how fast are the theoretical gravity waves? Approx. Sunday driver speed, light speed, Ludicrous speed, etc.? Is there any speculation or educated guesses?

According to Einstein, nothing can move faster than light.
Tullio

Otubak
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### RE: (...) how fast are the

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(...) how fast are the theoretical gravity waves? Approx. Sunday driver speed, light speed, Ludicrous speed, etc.? Is there any speculation or educated guesses?

Gravitational waves progress with speed of light, just as gravity itself does.

(By the way: Don't confuse gravity and gravitational waves, they are completely different things).

Erik
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### RE: (By the way: Don't

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(By the way: Don't confuse gravity and gravitational waves, they are completely different things).

Ah, it was an unfortunate mix up when I typed the initial post. I did mean gravitational waves and not gravity (ie. fluid dynamics).