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# Explanation of the fact that if you reached a certain speed you'd go back in time?

## Explanation of the fact that if you reached a certain speed you'd go back in time? - Physics Forum

### Explanation of the fact that if you reached a certain speed you'd go back in time? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
07-31-2009, 01:16 PM
Pipette Filler

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Explanation of the fact that if you reached a certain speed you'd go back in time?

I know it's impossible for you to ever reach that speed but I'd just like someone to explain why that would happen. I remember my Physics teacher saying something about it when I went to school but I the explanation behind it. I just totally can't get my head around how that would work, so yer if anyone can explain it to me?
#2
10-20-2009, 02:25 AM
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Re: Explanation of the fact that if you reached a certain speed you'd go back in time

very nice post.
i ahve accepted you post informations.
#3
10-20-2009, 09:51 AM

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Re: Explanation of the fact that if you reached a certain speed you'd go back in time

i was part of physics forums from school, got this from that... Hope you understood that time only APPEARS/SEEMS to slow down..

Time appears to pass more slowly for faster-moving bodies. In other words the faster you travel, the slower time will move for you.

When you reach the speed of light time stops. If you travel faster than the speed of light it is assumed you can go back in time.

All these natural explanations of time seems harmonically sound..........until you get to how you can go into the future.

Why does time speed up as a approach the speed of light? Doesn't the violate the "Time appears to pass more slowly for faster-moving bodies" principle. Time going slower as you go faster seems to logically make more sense. Time all of a sudden speeding up instead of slowing down as you approach the speed of light (as will as some other aspects of time dilation) do not logically make much sense.

Instead, it would be more logical to go forward in time with negative speed (t). However we all know that you cannot go at a negative speed, and in this alternative way you cannot go forward in time. This alternative law of nature is similar to the fact that you can't go over the speed of light and therefore you cannot go back in time.
#4
10-20-2009, 02:16 PM
Post-Doc

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Re: Explanation of the fact that if you reached a certain speed you'd go back in time

It doesn't work that way. As you accelerate towards the speed of light (c) several things happen - time slows down, the apparent mass of the moving object increases, the object's dimensions change, and a bunch of other weird stuff.

All of these factors change by a set amount, called "gamma" (actually, the greek symbol for gamma), which is 1/(sqrt(1-(v^2/c^2)), where
-sqrt = square root
-v^2 = the objects speed squared
-c^2 = the speed of light squared

As you get closer and closer to the speed of light, the value of gamma increases exponentially. For example, at 99.999999995% the speed of light, gamma is 100,000. Meaning the mass of your spaceship (or whatever) will be 100,000 times heavier than at rest. Meaning you need 100,000 times more energy to maintain acceleration. At the speed of light, gamma becomes infinite, meaning you ship will have infinite mass, and require infinite energy to accelerate past c.

Since there isn't a infinite amount of energy available, you cannot exceed c.

B
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Last edited by Warthaug; 10-20-2009 at 02:46 PM. Reason: bad grammar

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