# Time travel in the past

The past weeks I watched some episodes from "Through the Wormhole" with Morgan Freeman.

When talking about traveling back in time, they said you can only travel back up to the date when the first time travel occured.

So, if for instance time travel will be discovered in 2045, all time travelers(from any given time) won't be able to travel in time earlier than 2045.

Why is this? Can anyone explain this please? I know this is just theoretical but still I would like to know the concept behind it.

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I think it's worth expanding a bit on Hal's answer to try and make it a bit less technical. We denote a point in spacetime as $(t, x, y, z)$ i.e. both the position $x, y, z$ and the time $t$. In the absence of time machines we can only pass through a spacetime point once. Of course you can go back to the point in space $x, y, z$ but only at a later time so you can't get back to the point $(t, x, y, z)$. If it were possible to pass through $(t, x, y, z)$ go somewhere else then get back to $(t, x, y, z)$ your trajectory would form a loop, and we call this a closed timelike curve (or CTC). It's closed because it's a loop and timelike is a technical term that means you don't have to travel faster than light to go round the loop.

For any particular CTC there will be some earliest time that lies on the loop, so by going round the loop you can only get as far back in time as this earliest point. The point that Morgan Freeman is making is that for all the types of time machine we know about this earliest point corresponds to the creation of the time machine. So the statement is true for all the time machines we know about. I don't know if there is a general rule that says it must be true for all time machines, but I suspect not.

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Nice. This was really clear and comprehensive for a non-physicist like myself. Thanks. –  Fofole Jan 28 '13 at 12:38

The basic idea is generated from solutions to general relativity that permit Closed Timelike Curves as first discovered by Kurt Godel. One thing to understand is that solutions to the Field Equations of General Relativity are metrics. Godel discovered a metric called the Godel Metric which permitted CTCs. If one is willing accept General Relativity as part of a final theory, one has to find a mechanism which eliminates CTCs in areas where we have experimental evidence that they don't occur (e.g. the normal experience of everyday life).

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## protected by Qmechanic♦Mar 17 '13 at 21:44

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