# Can someone help me to clear my mind!

1. Mar 19, 2005

### bayan

Speed is = to distance over time.

That is fine. but what happenes when you reach speed of light? your time would =0 therefore you would have 0 infinet speed.

These are just the things that I of last night whlest I was delivering Pizza.

Can some one clear that up for me please?

2. Mar 19, 2005

interesting

this is interesting, it seem that the speed of light should be undefined, however, i dont think that your time is 0, since the speed of light is a speed, it cannot be instant, 299,792,458 meters per second is the speed if im not mistaken, well, go 299,792,458 meters in a second, a second will still have passed from your point of veiw, you til will have aged 1 second, so time does not =0, i dont think it ever can.

3. Mar 19, 2005

### bayan

What about relativity and time? It says that if you speed up your time slows down. and also using time dilation factor we get t prime to be 0 if V is the same as C.

This is what I don't get. If the speed is mesuared by the time of abserver then time is =0 then this means there is no V for you from your frame of refrence.

4. Mar 19, 2005

### SpaceTiger

Staff Emeritus
This is why objects with mass can't move at the speed of light. Keep in mind, however, that in addition to time dilation, there is also length contraction (proportional to the same gamma factor).

5. Mar 20, 2005

### KingNothing

And? That's exactly how it is. Speed isn't necessarily measured from anyone's point of view. From our perspective, the number is c.

6. Mar 20, 2005

### Pengwuino

You end up with c+v/0 if v is the speed of light. Divisible by 0 doesnt mean 0 though so your speed isnt 0.

7. Mar 20, 2005

### Integral

Staff Emeritus
No, it does not, it says that your time AS OBSERVED BY A STATIONARY OBSERVER slows down. Time as observed by the traveler does NOT change.
Like wise the traveler observes NO CHANGE in length or mass in HIS FRAME of Reference.
So the answer is that Length and time contraction only happen to OBSERVED frames of reference, never to that of the observer.

Last edited: Mar 20, 2005
8. Mar 20, 2005

### Ivan Seeking

Staff Emeritus
We also have to consider whose reference frame we are talking about. If you are approaching the speed of light, this would be as measured by some observer who is defined to be at rest. The observer sees your clocks running slowly, but his clocks are running as normal. So your speed is distance over time as measured in his frame of reference, just like we do every day. Now turn things around. In this case we can argue that the observer is moving and you are at rest. According to SR there is no preferred obvserver. So you would measure his rate of motion, which is again normal from your frame of reference since you are the one defined to be at rest. You would both agree on the speed measured on the other but not the direction of motion. You would both agree that the other person's clocks are running slowly.

9. Mar 20, 2005

### SpaceTiger

Staff Emeritus
I think you mean slows down.

My understanding of his question was this exact point. That is, if someone is observing something move at the speed of light, that object does not age (from the stationary point of view). However, the moving object sees everything as normal, so there's an infinite chasm of sorts between the moving object and the observer. The solution would then be that nothing massive can move exactly at the speed of light.

Another way of putting it is that the gamma factor diverges:

$$\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}} \to \infty$$

10. Mar 20, 2005

### Ivan Seeking

Staff Emeritus
Whoops, nearly simultaneous posts..

11. Mar 20, 2005

### Integral

Staff Emeritus
Yes I did, edit in place.

12. Mar 22, 2005

### bayan

Ohhh! OK it is becoming more clear now. THanx.

One more question if I may ask. lets say that a mass was able to travel @ C! Then his time relative to a stationary person would equal to 0. Is that right? If so Would he/she reach eternity in a flash of a sec(if the stationary person lived that long) from stationary persons frame of refrence?

I will just make the question more clear as my english is still in its 3rd year.
A moving @ C and B @ 0. Would A reach the end of time in a flash of a second from B's frame of refrence?(assuming B lives that long)

13. Mar 22, 2005

### DaveC426913

Yes. (And yes, B will live that long.) If a man is a spaceship were able to reach almost the speed of light, the universe would grow old and die while we flew. From a stationary observer's point of view, the man in the spaceship will appear to have frozen stiff for all eternity.

Note that it is not necessary to go fast to accomplish this. As Einstein points out, acceleration due to force and acceleration due to gravity are one and the same.

If the spaceman, rather than trying to reach the speed of light, instead aims his ship into a black hole, the same thing will happen. As he descends toward the event horizon, the universe around him will speed up, and age and die while he falls. From outside he will appear to slow down and ultimately stop, poised on the edge of the event horizon for all eternity.

14. Mar 22, 2005

### Staff: Mentor

Big picture: Relativity actually helps our odds for interstellar travel, at least as far as time is concerned. It means that if you can get close enough to the speed of light, the universe becomes a very small place and you can travel to places that seem (to us on earth) to be too far away to reach in a lifetime.

Incidentally, that's the premise of "Planet of the Apes," which I saw for the first time (the original) two nights ago...

15. Mar 22, 2005

### Danger

That's kilometers per second...
Look at the gravity equivalent of velocity. If you fall to the event horizon of a black hole, it's all over in a flash for you. To someone watching, you remain suspended there forever.

16. Mar 22, 2005

### SpaceTiger

Staff Emeritus
You can get anywhere in a lifetime (assuming you can accelerate to the appropriate speed), but it takes a ridiculous amount of time to communicate any information back to your home planet.

17. Mar 23, 2005

### Danger

Ignore that stupid remark. There are some burned-out pixels in my monitor. When I first read that figure, the rightmost comma looked like a decimal point.

18. Mar 23, 2005

### bayan

So person B would see A as still for ever! and A would actually see what had happened before(travels back in time)? And after a while he may be able to see "BigBang"?

19. Mar 23, 2005

### DaveC426913

No. He was right. 299,792,458 meters per second = 299,792.458 kilometers per second

Yes, although, while you are falling to the event horizon, you will (theoretically) 'see' the universe age and die.

More practically, if you manage a trajectory that skirts the black hole, near-grazing the event horizon, but with enough residual velocity to escape again, you will emerge into a universe that is old and dying.

I can point you at some good books that explore ideas like this. My fave is Larry Niven's "A World out of Time". The character goes on a round trip to the galactic core and returns to an Earth that is 3 million years older.

(Cats have evolved to be legless, like a snake - just a furry head attached to a 6 foot tail. More loveable; you "wear" them like a feather boa. )

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