# What is speed?

1. Sep 15, 2007

### Skhandelwal

How do define if something is moving or not? I mean isn't it friction, attraction, repulsion, etc. which causes something to slow down or speed up? Also by reference pt., we make judgment. But if there is an object, only by itself in the space, how would you know if its moving at 3x the speed of light or not?

Why is moving at the speed of light impossible?(I know it requires infinite energy but why? B/c it gains more mass? But why does it gain more mass? the faster something travel, the thinner and longer and slimer it gets, why? Why does time slows down the faster we move?

2. Sep 15, 2007

### Staff: Mentor

By definition, speed is timed displacement of an object with respect to another frame of reference (another object). So an object isolated in space, with no reference but itself has no speed. It could use inertial guidance to create a frame of reference (it would work just like dropping a buoy) and measure it's speed by monitoring it's acceleration, but that's it.

I'll leave the second part to experts in Relativity...

3. Sep 16, 2007

### Skhandelwal

What about through Kinetic E? Can't someone tell at what speed you are going by how much Kinetic E you have?

4. Sep 16, 2007

### JesseM

Kinetic energy depends on your choice of frame too, it's not something you can measure in an objective frame-independent way. And it's important to understand that in relativity there is no notion of absolute speed--speed is only defined relative to other objects.

5. Sep 16, 2007

### matteo16

if an object is isolated and there aren't other frames of reference we can't speack of speed. moreover we can speack about something only if this is a measurable thing.
Einsteing has started saying that the speed of the light(a measurable thing) is the greatest speed.
so does not existe a material thing with a greater speed than the speed of the light because this is impossible to measure(for now)

6. Sep 16, 2007

### Skhandelwal

Why does the light travels at the same speed to a person who is standing, and to another person who is accelerating?

7. Sep 16, 2007

### JesseM

Do you mean another person who is moving inertially relative to the first? Usually in SR we just use inertial frames, not accelerating ones (it is possible to define accelerating coordinate systems but it makes things a lot more complicated). If we're just talking about two observers moving inertially, basically it has to do with the fact that "speed" is defined in terms of distance/time, and in relativity each observer is supposed to use rulers and clocks at rest relative to themselves to measure distance and time, and since each person observes the other person's rulers to be shrunk relative to their own and the other person's clocks to be slowed relative to their own, that means that speeds don't add the same way in relativity that they do in Newtonian physics. For example, if I measure a rocket to be moving at 0.4c to the right relative to me, and you measure me to be travelling at 0.2c to the right relative to yourself, then you will not measure the rocket to be moving at 0.4c + 0.2c = 0.6c as in Newtonian physics, instead we must use the formula for addition of relativistic velocities found here to see that in your frame the rocket is moving at (0.2c + 0.4c)/(1 + 0.2*0.4) = 0.6c/1.08 = 0.56c. By the same token, if I measure a light beam to be moving at 1c to the right relative to me, and you measure me to be traveling at 0.2c to the right relative to yourself, then in your frame the light beam is moving at (0.2c + 1c)/(1 + 0.2*1) = 1.2c/1.2 = 1c.

8. Sep 17, 2007

### Skhandelwal

I dont get the link you gave me....In the section, "How can that be right?". When he says, "sure enough he finds that A is moving at a speed v and C is moving at speed u." Well, how the hell can C be moving at speed u??? Isn't C the speed of light? However, if you think it will be easier for you to explain it your way instead of getting rid of this block from the article, I could care less, I just want to understand it, don't care how.

9. Sep 17, 2007

### Staff: Mentor

On that page, capital (upper-case) C is the name of an object (like A and B). Lower-case c is the speed of light.

10. Sep 17, 2007

### tabchouri

In Relativities theories, Einstein postulated that light is the fastest thing in the universe, and its speed is unreachable by matter (unless supplied infinite energy). He also postulated that speed has a constant velocity, NO MATTER the reference frame (at rest, moving, or accelerating as compared to other frames).
This is the guideline of the Relativity theory, and it implies the beautiful velocity transformation formulas, among other results, which are verified experimentally, or at least no experiment contradicted them until now (for the constancy of the speed of light for that matter).

As for why is it so (constant velocity of light no matter what reference) ? I dont remember where i read that, but is it considered as a fundamental property of light, of nature and the universe itself.
The three universal fundamental properties are (if i am right) : the speed of light c (and its constancy), the Plank time, and the chage of an electron e. Now i also remember that those are fundamental elementary units, their values must be the same in any imaginable universe (with the respect to the measuring units).

Last edited: Sep 17, 2007