# Faster than light?

1. Sep 27, 2014

### Anoop Koushik

Everywhere we see that nothing can go faster than light. Light is the fastest.
Is there any accepted law that states that nothing else can be faster than light?
Or is it just that humans are unable to perceive things which is faster than light?
There might be something faster than light, giving rise to whole new dimension and a different interpretation on understanding the universe.
And also
From E=mc^(2)
Taking c=3x10^(8), m=1kg
E can never exceeded 9x10^(16) for mass of 1kg?
Why can't it exceeded ?
How is that you limit speed of light?
(By 'light' I mean EM waves)

2. Sep 27, 2014

### phinds

There is a universal speed limit which has been found empirically to be obeyed my massless objects and impossible to reach for objects with any mass. This is not a limitation on our ability to measure things.

3. Sep 27, 2014

### julcab12

...Special relativity. Speed varies depending on the observer in relation to a medium--let say, a vacuum or in a blackhole. We assume it is C in a flat spacetime.

http://johanw.home.xs4all.nl/PhysFA...ments.html#Measurements_of_the_Speed_of_Light

4. Sep 27, 2014

### HallsofIvy

Staff Emeritus
Actually, c is by definition the speed of light in vaccum- that is not an assumption.

5. Sep 27, 2014

### Staff: Mentor

No. $E = mc^2$ is only valid for objects at rest; an object in motion can have arbitrarily high energy--its energy increases without bound as its speed approaches the speed of light. The correct formula for moving objects is $E = \gamma m c^2$, where $\gamma = 1 / \sqrt{1 - v^2 / c^2}$; obviously $\gamma$ increases without bound as $v \rightarrow c$, so $E$ does as well.

6. Sep 27, 2014

### Anoop Koushik

Then they formula gives indeterminate valie when v=c ?

What is the cause for this speed limitation ?
If you are observe A and I am observer B moving very slowly compared to you, from a fixed point. Light travels at same speed for both of us if we measure?? (If light is originated from that fixed point)

7. Sep 27, 2014

### phinds

Exactly. Nothing with mass can travel at c.

Don't know. It is what it is.

Exactly. Light travels at c in ALL inertial frames of reference. If you are moving at .9c towards me and turn on a flashlight, I measure the light as moving at c. If you are going AWAY from me at .9c and shine a flashlight towards me, I measure the light as moving at c.

Now in the first case, the light will blue shifted and in the second case it will be red shifted, but it will be traveling at c in all cases.

8. Sep 28, 2014

### julcab12

Yes. My wording is used very loosely here sorry lol. C constant is postulate of relativity. We can guarantee C in a flat spacetime. GR C is local. In QM, Speed of light varies as photon can travel slower or faster statistically but constant on average.

9. Sep 28, 2014

### Fredrik

Staff Emeritus
What we know is that theories that include this speed limit make much better predictions about results of experiments than theories that don't. We don't really know why one theory makes better predictions than another. The only thing that can answer that question is a better theory. If there was such a theory, you could ask why that theory is so good, and to answer that, we'd need an even better theory...and so on.

Yes.

10. Sep 28, 2014

### Anoop Koushik

What if I use only one frequency of light? How will the two cases be ?
And if the relative velocity is very huge, then the shift will be greater, but greater to what extent ?

11. Sep 28, 2014

### phinds

Regardless of the frequency that is emitted, the received frequency will be red shifted or blue shifted by an amount that depends on the relative speed of the source and receiver, not on the emitted frequency. The ABSOLUTE frequency will depend on the emitted frequency but the % shifting won't.

The larger the speed, the larger the shift.