# Dumb question (time travel)

1. Oct 24, 2008

### 39ster

If we have two balls, a red and blue ball, and the red ball passes the blue ball at 99.9% the speed of light, would "time" for the red ball pass slower than it does for the blue ball? If it does, and if speed is just relative, couldn't we say the blue ball is passing the red ball at 99.9% the speed of light, but just in the opposite direction? Wouldn't this cause the blue ball and red ball to age at exactly the same speed and make "time travel" impossible?

2. Oct 24, 2008

### Fredrik

Staff Emeritus
Yes, yes and no. I assume that what you have in mind when you say "time travel" is just the lame kind where you travel very fast for a while, and when you stop, a lot more time has passed on Earth.

The red and blue balls are both correct when they think "right now, the other ball is aging at 60% of my aging rate". This is not a contradiction. The reason has to do with the fact that "now" refers to a different set of events in different coordinate systems, and these balls aren't using the same coordinate system.

The attempt to prove that this is a contradiction is called the "twin paradox", but it really is nothing more than a mistake in a calculation. If you're interested, check out e.g. #3 and #142 in this thread.

3. Oct 24, 2008

### 39ster

Hmmm, i'm not sure if i get it or not. Can you verify? Is the time only different for an object that is accelerating, rather than just having a higher velocity? (as in, does the object only experience this slow down of time during the period that it's speed increases from 60Km/h to 100Km/h, and once it's speed has reached 100Km/h, and it doesnt increase any further, would their time rate be the same?). Because that's the only difference i can see. Once the velocity of the red ball stops increasing, they would be identical..?

4. Oct 24, 2008

### JesseM

If both are moving at constant velocity relative to one another, both measure the other one to be aging more slowly, in their own coordinate system (their own rest frame). Each one's own coordinate system is defined in terms of measurements on rulers and synchronized clocks at rest relative to themselves (note that in relativity the meaning of 'synchronized' is relative to one's coordinate system too, clocks that are synchronized in their own rest frame will be out-of-sync in a different frame--see here and here). For an illustration of how each system can measure clocks in the other system to be slowed down without this leading to any disagreements about what any pair of clocks read at the moment they pass next to one another, take a look at the example I wrote up on this thread.

5. Oct 25, 2008

### 39ster

I was wondering about a certain situation and i'm not sure if it has anything to do with time dilation. Let's say that i have a device that could slow down the time for everything around me, but time for me passes at normal speed. If i were to walk from one point to another, everyone around me would see me walk much faster than i actually am, and from their view, i would get to my destination much faster. To them it would look like i was in fast forward. Would that effect be time dilation?

6. Oct 25, 2008

### George Jones

Staff Emeritus
You have to pin down the science underlying the scenario. Stating "I have a device" does not do this. I could state "I have a device that makes miniaturizes objects." This might make for a good movie or novel (Fantastic Voyage), but that doesn't mean that there is real science behind the statement.

This forum discusses real science.