# Light Speed

## Main Question or Discussion Point

The Light Speed has been proved to be around 300.000Km per second.
So, Before continuing to expose my real thread, i need to know if it is really impossible to an object to travel faster than Light Speed.

## Answers and Replies

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malawi_glenn
Science Advisor
Homework Helper
This is one of the most asked questions.

The speed of light in VACUUM is around 300.000Km per second. Nothing can travels faster than that.

But in a medium, glass for instance, the speed of light is decreased by a factor 1.3, and particles can travel faster than light in that perticular media. But not in vacuum.

Ok then.

So now what I wanted to say in the first place.

Picture a Giant Circle in vacuum, which radius is aproximately 47600Km. This would make its perimeter to be around 300 000 km if im not mistaken.
What if the Giant Circle would take exactly 1 second to make a complete spin? Then the speed of rotation in the Circle edge would be equivalent to the light speed.

Now, what if our circle had a bigger radius? Then the speed in the edge would be greater than the light speed, now wouldnt it?

Doc Al
Mentor
If your "Giant Circle" is a material body, you won't be able to get it spinning fast enough.

Rotating reference frame

x2thay's post reminds me of an issue I still heve not resolved. I hope I am not diverting the focus of this thread, but perhaps someone here knows the answer.

What's the relativistic transformation from inertial reference frame to rotating reference frame?

For instance:
Suppose I stand in the middle of x2thay's giant circle with me and the circle at rest in an inertial reference frame. Then I transform to a reference frame rotating at 2 revolutions per second. According to classical physics, I would declare that an object on the rim of the circle had a tangential velocity with magnitude twice the speed of light with respect to me.

Yeah, when you're standing on the Earth, it looks like the stars are rotating around you at many thousands of times the speed of light doesn't it!!

Well, they're not. :) You're the one that's rotating, and the reason you know that is because rotation involves acceleration, and all observers do agree on who is accelerating and who is not.

Rotating Reference Frame

So there is no Relativistic revsion of the transformation of reference frames from inertial to rotating. And its OK to have relative velocities greater than 'c' when the velocities are with respect to a rotating reference frame.

If your "Giant Circle" is a material body, you won't be able to get it spinning fast enough.
It was all hypotherical but, why wouldn't one be able to move it at 1 spin per second?

Anyway, is it right that an object approaching the speed of light, becomes heavier and heavier and turns into pure energy when hitting light speed itself?? If so, my Giant Circle, would transform in energy the ring beyond the 47 600Km radius.

Doc Al
Mentor
It was all hypotherical but, why wouldn't one be able to move it at 1 spin per second?
Because such a spin rate implies that the material would be moving at light speed. Not going to happen.

Anyway, is it right that an object approaching the speed of light, becomes heavier and heavier and turns into pure energy when hitting light speed itself??
No. But as an object's speed approaches the speed of light it does become harder and harder to increase its speed, requiring more and more energy.

Because such a spin rate implies that the material would be moving at light speed. Not going to happen.
I realise it is known that nothing can ever travel faster than the speed of light, but that's just so confusing to me. Can i be given some proof? ^^

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malawi_glenn
Science Advisor
Homework Helper
it would require infinite energy to make something move at the speed of light in vacuum.

Making the atoms, at the outer position of the wheel of yours, move at the speed of light with respect to an arbitrary reference will require an infinite amount of energy.

$$E = \sqrt{(pc)^2 + (mc^2)^2}$$

$$p = \gamma mv$$

$$\gamma = \frac{1}{\sqrt{1-(v/c)^2}}$$

If one space ship moves against a point x from the left at speed 0.75c, and another space ship moves against the same point from the right with speed 0.75c, their relative speed with respect to the point x is NOT NOT NOT 1.5c, it is less, always less. Always.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html#c2

It's normal that, in certain cases, the apparent speed be faster than light c, in example the speed of the nodes of a electromagnetic wave passing through a surface with n>1, but it's always apparent and we can always verify that, in reality, the effective speed of light is again c.

Rotation Observer

Chatman, I hope you are not referring to the example I extended off x2thay’s post. For that example, peter0302 agreed that the velocity of an object relative to a rotating observer may be greater than c.

But absolutely not, is an apparent effect only!
However it's not real at all!
I'd like to see one of you that show me an object that runs "effectively" faster than light.

Beyond that it seems to be by our perception or impression.

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Well, wonder a spaceship which moves around at 250.000km per second. On it, someone flashes a light beam, which speed is c. Wonder the ship is long enough to the beam to travel for a few seconds (sure is a big ship). Wonder the spaceship disappears suddenly and the beam goes free into the vaccum where the ship were. Then its speed, rises up to 550.000km per sec, I would think...Or maybe it wont...at least according to malawi's demonstration over there...

Is there any physic experiment in these matters or is everything just theoretical?

Doc Al
Mentor
Well, wonder a spaceship which moves around at 250.000km per second. On it, someone flashes a light beam, which speed is c. Wonder the ship is long enough to the beam to travel for a few seconds (sure is a big ship). Wonder the spaceship disappears suddenly and the beam goes free into the vaccum where the ship were. Then its speed, rises up to 550.000km per sec, I would think...Or maybe it wont...at least according to malawi's demonstration over there...
Every observer will measure the speed of the light beam to be c in his frame. That includes observers in the spaceship that flashes the beam and observers on earth or anywhere else. I have no idea why you think the spaceship's disappearance would affect the speed of the light beam.
Is there any physic experiment in these matters or is everything just theoretical?
This is just one of several consequences of relativity, which has oodles of experimental support. Look here: https://www.physicsforums.com/showthread.php?t=229034"

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