# Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec

1. Oct 5, 2008

### Bible Thumper

Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Correct me if I'm wrong, but when something is going light-speed, all inertial reference frames are likewise observed going this speed, right? Knowing that, then it would appear as if all observed inertial frames of reference are actually moving at the same rate, correct? And if this is so, isn't that a bit like everything going zero kilometers per second? I mean, everything is going the same rate, right?

2. Oct 5, 2008

3. Oct 5, 2008

### JesseM

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

No, things moving at light speed do not have their own inertial rest frames, so it isn't really meaningful to ask what is "observed" by something moving at light speed.

4. Oct 5, 2008

### sr_philosophy

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Agreed. It is not meaningful to say what one 'observes' while moving light speed. There is a maximum velocity, the velocity of light, which cannot be surpassed by any moving material body.

5. Oct 6, 2008

### Bible Thumper

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Then replace "light speed" with "near-light speed". Read the barn-rod paradox thread this forum. They're doing it over there! :)

6. Oct 6, 2008

### JesseM

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

But if you do that your initial post doesn't make sense either. You said:
This isn't right, if you are moving at 0.999c relative to the Earth, it's true that in your own rest frame the Earth will be moving at 0.999c, but other inertial frames will be moving at other speeds relative to you--you can find one moving at 0.1c relative to you, another at 0.34c, another at 0.732c, and so on for any possible sublight speed. So, it's not true that "all" inertial frames are observed going at some near-light speed.

7. Oct 7, 2008

### Staff: Mentor

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Excellent! That should be the first reply for all "POV of photon" or "I'm travelling at c" threads.

8. Oct 7, 2008

### Bible Thumper

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

So you're saying that at 0.99c (relative to Earth) there will be situations when objects in relative motion to me can appear going only 0.1c? Since it's a well-known fact that Maxwell's constancy of the velocity of light law is maintained for any frame of reference that's going that speed, then what you're saying is that:
1. At 0.999c relative to Earth, there can be some objects that, relative to me, appear to go 0.1c. And,
2. Once my speed goes from 0.999c to 1.0c (relative to Earth or any reference, for that matter), that object that was going 0.1c relative to me now takes an awesome jump in velocity to 1.0c?
Is this correct? I can't see how something can appear to go from 0.1c relative to me to 1.0c relative to me just because I made an incremental increase in my velocity, to 1.0c.

9. Oct 7, 2008

### Bible Thumper

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

In case you haven't noticed, I changed the game to eliminate the lazy. We are no longer considering light speed, but a sub-light speed that's very near to c.
If they can do this in the Barn-Pole Paradox thread, then we can do it here.

10. Oct 7, 2008

### JesseM

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Any frame of reference that's going what speed? Again, there is no valid inertial frame that's moving at exactly light speed, but it's true that the speed of light is always measured to be c in the frame of any observer that's moving at any speed less than c.
Sure. Using the formula for the addition of relativistic velocities, if you are moving at 0.999c away from the Earth as measured in the Earth's frame, and you measure an object moving at 0.1c away from the Earth in your frame, in the Earth's frame of reference the object will be moving at (0.999c + 0.1c)/(1 + 0.999*0.1) = 1.099c/1.0999 = 0.999181744c, slightly faster than you. On the other hand, if you are moving away from the Earth but the object is moving towards the Earth at 0.1c in your frame, then in the Earth's frame the object will be moving away from Earth at (0.999c - 0.1c)/(1 - 0.999*0.1) = 0.899c/0.9001 = 0.998777914c, slightly slower than you.
No, it's impossible for you to move from 0.999c to 1.0c, and even if you could do that, you would no longer have your own inertial rest frame at 1.0c so it would be meaningless to ask how fast any object is moving "relative to you" (since that phrase is normally used as shorthand for 'how fast the object is moving in your rest frame'). But if you like we can look at that object that was moving at 0.1c towards the Earth in your frame when you were moving at 0.999c in the Earth's frame, and ask how fast it will be moving in your frame if you jump up to moving at 0.999999999c in the Earth's frame. I showed above that the object would be moving at 0.998777914c in the Earth's frame, and after your change in speed the Earth will be moving at 0.999999999c in your frame, so we can apply the velocity addition formula again to conclude the object will now be moving at (0.999999999c - 0.998777914c)/(1 - 0.999999999c*0.998777914) = 0.001222085c/0.001222087 = 0.999998c, very close to light speed.
The incremental increase in your velocity from 0.999c to 0.999999999c in my example is accompanied by very large changes in the time dilation factor on your clocks and the Lorentz contraction factor on your ruler, as measured in the Earth frame (the clocks are slowed down by, and the rulers shrunk by, a factor of $$\sqrt{1 - v^2/c^2}$$). And each observer measures the "speed" of any object in their own rest frame using rulers and clocks at rest in their frame, in order to calculated (change in position)/(change in time) for two points on the object's path.

Last edited: Oct 7, 2008
11. Oct 7, 2008

### matheinste

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Quote:-

Correct me if I'm wrong, but when something is going light-speed, all inertial reference frames are likewise observed going this speed, right?

Wrong.

Change this to :-Correct me if I'm wrong, but when something is going NEAR light-speed, all inertial reference frames are likewise observed going this speed, right?

Still wrong.

Matheinste

12. Oct 7, 2008

### phyti

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Using the addition formula, the speed of .1c relative to you translates to .9992c relative to a your common reference frame, so the change is not as large as you think.

13. Oct 7, 2008

### JesseM

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

But as I showed, Bible Thumper is right in that there can be a large change in these situations--in my example, the ship was originally moving at 0.999c relative to the Earth and then jumped to 0.999999999c relative to the Earth, and this meant that an object which was moving at 0.1c relative to the ship before the jump would be moving at 0.999998c relative to the ship after the jump.

14. Oct 7, 2008

### Staff: Mentor

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Frankly I find this rather rude, particularly considering the fact that you are so new to the forum.

For your information your questions (both the original and modified versions) are incredibly common. Search engines should have eliminated them, so forgive me if I am pleased to see a well-done reference cited for a change.

15. Oct 7, 2008

### Bible Thumper

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Sorry if I did come off as rude-sounding (I admit--it's a little rude), but I was just trying to eliminate rubber-stamp replies, that's all. My error was in making the comment to your post specifically. For that I apologize.
As for the search engine eliminating inquiries like this, well... The search engine landed me here. At Physics Forums...

16. Oct 7, 2008

### Bible Thumper

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

Whenever I think of these kinds of questions, I immediately fall on two premises:
1. James's Law of the Constancy of the Velocity of Light (in the Vacuum). And,
2. Einstein's Principle of Relativity.
The former is self-explanatory; the latter merely states that all physical laws must hold regardless of motion states. Thus, a conservation of energy experiment must hold true, even if I'm seeing the experiment being performed on a very fast train.
Maxwell's Law of the Constancy of Light must also hold. These two are all we need.

17. Oct 8, 2008

### phyti

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

If B is moving at .1c relative to A, in the same direction as A at .999c, B is moving relative to earth faster than A. That's why the addition formula, to determine the speed of B relative to earth.

18. Oct 8, 2008

### JesseM

Re: Speed of light c = 0 Km/s. Not 300K, but a big, fat zero Km/sec!!!

"In the same direction" in whose frame? In A's frame A is at rest, so A is not moving in any direction. Even if B is moving towards the Earth at 0.1c in A's frame, in the Earth's frame both A and B will be moving away from the Earth in the same direction, with B's speed slightly less than A's in the Earth's frame--that's the situation I was analyzing.