# Time dilation formula

1. Jul 2, 2012

### moatasim23

As according to formula:
t=t"/(1-u2/c2)
If we travel at speed c will time for stationary observer amount to infinity?

2. Jul 2, 2012

### ghwellsjr

Where'd you get that formula?

In any case, we cannot travel at speed c so the question is meaningless.

3. Jul 2, 2012

### harrylin

That's the wrong formula I think (at least, it is at odds with the Lorentz transformations).

Anyway, if we could travel at almost speed c (we can never reach it), then our "time" will nearly stop according to a stationary observer. Consequently, it may in principle be possible (not sure about acceleration though) to reach a far-away star in almost "no time" for us, so that travel to that star (and reaching it before you die) could be possible in principle.

"the velocity of light in our theory plays the part, physically, of an infinitely great velocity"
- http://www.fourmilab.ch/etexts/einstein/specrel/www/

4. Jul 2, 2012

5. Jul 2, 2012

### harrylin

I interpreted "for" in the sense of "as seen by", and he or she didn't specify whose time - that's the kind of ambiguity that is common for such questions.

6. Jul 2, 2012

### ghwellsjr

Yes, meaningless and ill-formed questions ought not to be answered, they need to be revised by the questioner until they make sense. Otherwise, we end up with a whole bunch of guesses as to what the questioner's issue is.

7. Jul 2, 2012

### moatasim23

I think I got the square root missing.Otherwise the forumula is correct .Isnt it?

8. Jul 2, 2012

### ghwellsjr

I asked you where you got the formula. I have never seen a formula like that even with a square root somewhere. That's why I'm asking you where you got it. You can either answer that question or you can describe what the variables mean, otherwise, how should we determine if it is correct or not?

In any case, we cannot travel at the speed of light so what difference does it make if the formula is correct or not?

9. Jul 2, 2012

### moatasim23

http://www.salamandersociety.com/spacedoctrine/050318mathew_time_dilation.gif
I used this formula.

10. Jul 2, 2012

### Staff: Mentor

I think he means $$t = t' \gamma = \frac{t'}{\sqrt{1-\frac{u^2}{c^2}}}$$
with the velocity u and a strange notation (u^2 would be better for "u squared").

Edit: Too slow. And t' <-> t is relative anyway :).

11. Jul 2, 2012

### ghwellsjr

This is the formula that you linked to:

But your formula from post #1 started out t=t"...

I'm just curious--why did you change t'=t... to t=t"...?

As I look at the webpage from which that formula came and at the webpage that it linked to, I don't see any discussion about traveling at speed c or that anything is infinity and as harrylin pointed out in post #3, we can never reach c so you need to modify your question to make it meaningful. If you do that, does your issue go away?

12. Jul 2, 2012

### Naty1

"Eternity is no time at all for a photon."

13. Jul 3, 2012

### ghwellsjr

That's a cute phrase but it can be misunderstood. It is true in the sense that:

"No time is for a photon" or "Time is not for a photon" or "Time doesn't exist for a photon".

I'm afraid that it could be misunderstood to mean that what takes an infinite amount of time for a stationary observer takes a time of zero for a photon.

According to Einstein's theory of Special Relativity, time is what we measure with a clock. A clock requires massive objects or particles in its construction. A clock cannot be built exclusively out of photons because they have no mass. Massive objects or particles cannot travel at the speed of light. Since a photon cannot have a clock traveling with it, the concept of time has no meaning for a photon. It is just as incorrect to say that a time is infinite for a photon as it is to say that a time is zero or anything in between for a photon. What is correct is to emphasize that time doesn't apply for a photon.