# Having velocity of light is like having infinite Velocity

1. Aug 26, 2009

### I_am_learning

Having velocity of light is like having infinite Velocity!!!

At first When I heard that We can't travel at speed greater than c, I thought it put a barier on how far humans will be able to travel in their lifetime. So, They could never hope to reach those thousands of light year far stars.
But as I learned more, I found that its not the case. As we travell at speed closer and closer to c, the distance will be shorter and shorter and we can travel any distance in given time provided we can travel as close to c as required. So Travelling at C in relativistic physics is like travelling at infinite velocity in Classical physics. Moreover at velocity of light in relativistic phyics we have infinite kinetic Energy just like as we have infinite kinetic Energy at infinite velocity in classical physics.

So what I am wondering is stating that "Nothing can travel faster than light " in Relativistic physics is just like saying that "Nothing can travel faster than Infinite Velociy !!!" in Classical physics.
What is point of all these?

Last edited: Aug 26, 2009
2. Aug 26, 2009

### Naty1

Re: Having velocity of light is like having infinite Velocity!!!

Your local time, say in a space ship, ticks along at the "everyday" rate....no change. If it did change you could then gauge your absolute velocity...which can't be done. It is from another frame, say earth, that your space ship time appears to run slower....Only when you return to earth and compare clocks do you realize you are younger than those left behind.

Not really. No relative slowing of time in classical physics for example. Nor length contraction. But since neither is possible there is little point in trying to make comparisons.

and both are impossible as far as is known.

lots of differences because classical and relativity are different.

having some level of understanding of how are universe works...being able to create mathematical expressions to make predictions...and then confirm those predictions with measurements where possible.

3. Aug 26, 2009

### Force1

Re: Having velocity of light is like having infinite Velocity!!!

Not any distance in a given time, but a finite distance calculated by our speed relative to our initial rest frame times the duration of our given time.
No, the velocity is still some percentage of the invariant speed of light.
That would be true but it would take infinite energy to accelerate you to the speed of light so we begin to be getting into philosophy .
It hurts my pea brain to contemplate that. .
Not the same thing. The speed of light is not infinite velocity so there is still a point to special relativity.

4. Aug 26, 2009

### HallsofIvy

Re: Having velocity of light is like having infinite Velocity!!!

One important difference- if you travel at .9c to a distant star (speed and distance measured at rest on your home planet), turn around and go back home at .9c, the trip willk seem very short to you- dividing the distance you traveled (again measured at rest on you home planet) by the time of the trip (measured in your frame of reference) then you will get a result much larger than c. (That doesn't really qualify as "faster than light" because you are mixing frames.) But when you get home, everyone will be much older (or, more likely, their great-great-grandchildren will be).

5. Aug 26, 2009

### Ich

Re: Having velocity of light is like having infinite Velocity!!!

You know what A.E. wrote, 1905?

6. Aug 26, 2009

### I_am_learning

Re: Having velocity of light is like having infinite Velocity!!!

Thanks. Then I think I am reasoning like what A.E. expected !!! Cheers!!!

7. Aug 26, 2009

### A.T.

Re: Having velocity of light is like having infinite Velocity!!!

Look up "proper velocity". It goes to infinity when coordinate velocity goes to c.

8. Aug 26, 2009

### robphy

Re: Having velocity of light is like having infinite Velocity!!!

Rapidity (the analogue of angle) is a more useful quantity than "proper velocity" (the analogue of sine) [ and "spatial velocity" (the analogue of slope or tangent) ].

9. Aug 26, 2009

### I_am_learning

Re: Having velocity of light is like having infinite Velocity!!!

Though I had heard proper acceleration as the real acceleration i.e. the acceleration given by a = F/m when the Forces considered are all real.

What now is this proper velocity?

10. Aug 27, 2009

### DrGreg

Re: Having velocity of light is like having infinite Velocity!!!

Velocity is distance divided by time.

In the standard definition of velocity, both distance and time are measured by the same observer who is calculating the velocity.

In "proper velocity" the observer measures the distance, but the time is measured by the object whose speed is being calculated. I don't really like the name "proper velocity" because it doesn't quite fit in with the other uses of "proper" in relativity. I prefer the alternative name "celerity". At low speeds, celerity and velocity are almost the same. The celerity of light is infinite. There is no upper limit to a massive object's celerity. Momentum of a massive object is proportional to celerity. p = mw where m is (rest) mass and w is celerity.

Rapidity has a different definition, but also has the properties that at low speeds, rapidity and velocity are almost the same (apart from a conversion factor of c), the rapidity of light is infinite, and there is no upper limit to a massive object's rapidity. Rapidity also has the useful property that, for one-dimensional motion, you can add two rapidities together (unlike velocity) and that the rate of change of an object's rapidity with respect to the object's own proper time is equal to the object's own proper acceleration (as measured by an accelerometer) (again, with a conversion factor of c). These properties, however, apply in only one spatial dimension and can't be generalised to vectors.

11. Aug 27, 2009