Talk:Time travel

Talk about the problems of time travel and causality loops.

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This brings another objection. According to special relativity, traveling faster than light is equivalent to traveling backwards in time, according to some observers. In particular, if faster than light travel is possible without too many arbitrary restrictions, it is possible to have events in the future cause events in the past. This is called a causality loop.

In relativity there are two kinds of intervals, time-like and space-like, the former corresponding to sublight speeds and that latter to supralight speeds. The two are non-equivalent so faster than light objects won't have a rest frame. This is not the same thing as traveling backwards through time, but I'm not really sure how this paragraph should be edited, without trying to explain the whole theory here.

There are also light-like intervals, which correspond to (!) light speed.

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For anything travelling faster than light, there exists a reference frame in which it goes back in time. This does not cause paradox unless it or a signal from it returns to the starting point before the start of travel, thereby creating a causality loop.

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In science fiction and elsewhere, I've encountered 3 different types of time machine. Please improve the suggested names of each of these types.

• Vehicular type This time machine travels with the traveller(s) to the destination time and place (e.g. the TARDIS of Doctor Who).
• External type This time machine does not travel with the traveller(s). The travellers arrive at the destination without the time machine (e.g the Time Tunnel[?] or the time machine of Quantum Jump).
• History Type This time machine does not travel in time with the traveller(s), but the traveller(s) always emerge from the time machine at some time in its history (e.g. wormhole time machine). The travellers can not go further back in time than the earliest time in the time machine's history or go anywhere where the time machine was not located.

An interesting case is the H. G. Wells Time Machine. It looks like a History type machine, because it does not move, during the time travel. However, it is however moved in the far future and upon return to 1900, it arrives outside the house, rather than inside the house where it was in 1900. This makes it a vehicular type.

--- Karl Palmen

One way to have a "paradoxical" universe is to have two time dimensions. For example, model the entire present universe as the internal state of a computer. In that case, when someone goes into the "past", that just tells the computer to rewind and alter a backup copy of the universe's state. In this case, there's the time of the computer and the time inside of the computer. And of course, this is exactly how Star Trek and other such shows model time travel.

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Not sure about this.

First, exceeding the speed of light wouldn't result in a reversal of time.

Second, this isn't true if you have an object of a rest mass of zero.

This would lead one to posit that exceeding the speed of light would result in a reversal of time. While this is mathematically sound, as one approaches the speed of light, exponentially more force must be applied to accelerate, until finally an infinite amount of energy must be applied to reach the speed of light exactly. The only people who suggest that we must only develop technology to exceed the speed of light to develop time travel are people who do not understand the concept of mathematical limits.

But that's exactly what it is: a mathematical limit, in a mathematical model of reality. Mathematical models are constrained by what we know of the universe. Reality isn't. :)

Objects of zero mass have to travel at the speed of light. There can be no exception.

Objects that go faster than the speed of light to travel backwards in time. And while the reasoning above is valid, it doesn't deal with the possibility of something that comes into existence travelling faster than the speed of light.

What is the difference between the Revelation of John[?] and the Book of Revelation? - Zoe

Stuffed if I know, Zoe. I was just trying to remove the preaching implicit in the last version. -Robert Merkel 07:18 May 7, 2003 (UTC)

An important issue for time travel is the movement of information, and here it crosses over with prophecy. Harry Potter