Swancon 07 Science of Time-travel Panel

This synopsis was written by one of the panellists who doesn't have a physics degree, so take it with a grain of salt!

Panellists

We are all moving forward through time at the rate of one second per second. It's definitely possible to speed up that progression, but can we move backwards? If we could, what would the consequences be?

What is Time?

According to Einstein's theory of General relativity, time and the three spacial dimensions (up/down, left/right, forward/back) form a four-dimensional "space-time", the fabric of the universe. The equations of general relativity tell us that spacetime can be warped and changed by gravity and the relative speed of an object.

Some physicists interpret them as saying we can travel backwards through time, the same as we can travel backwards through space. Others say that the forward arrow of time is an illusion created by the human mind, which means that either the past and future are both fixed (we have no free will) or the past and future are both malleable (this hurts my brain :)).

Types of Time Travel

Cryogenics: Plausible

It doesn't really count but I thought I should mention it anyway :) This also includes stuff like "bobbling", where you're sealed off in an unchanging...thing for a set period of time and emerge unchanged in the future.

Time Dilation: Proven to exist!

With special relativity, the faster you move the more time appears to slow down for you to an outside observer, so that objects travelling at the speed of light would appear to be frozen in time.

This allows fast travellers to go forward in time at a drastically increased rate. For example, if you blasted off from earth at around 99% of the speed of light and travelled in a big circle for an hour, when you got back 7 hours would have passed on earth.(I could have calculated this incorrectly!)

This is a genuine effect: for example clocks on some planes are infinitesimally behind those on earth due to their extended fast trips. Unfortunately to get any significant time dilation you have to go really fast for a very long time, which requires more energy than we have to hand, since the closer you get to the speed of light the heavier you get (this doesn't affect light, since it has no mass) Also you can't slow time or go backwards, just speed it up.

More information about time dilation

Faster than light travel: Impossible?

One of the side effects of special relativity is that because people moving at different speeds ("in different reference frames") experience time differently, they have different perceptions of the order in which events occur. A side effect of this is that what appears to be faster than light in one reference frame will appear to be moving backwards in time from a different reference frame.

For example, Person A and Person B both own faster-than-light communicators, so that they can send messages to each other at say double the speed of light.
B gets in a space ship and travels off at a bit less than the speed of light.
A sends B a message. B gets the message, then a little while later sees A sending the message (since the message is travelling faster than light)
B sends a reply. Because of the way spacetime works, A will get this reply before they send their first message.
Thus if you can send messages faster than light, you can send them back in time. If you can send people faster than light then you can send them back in time too.

The simple objection to this is that the same equations which say the time travel occurs also say that it requires infinite energy to accelearate past the speed of light.

Wormholes: Possible in theory but probably not in practice

"Shortcuts" through space time. Because time and space are intrinsically linked, they also allow travel through time. Much of the theory of wormholes has been developed by the physicist Kip S Thorne.

How to make one:: Virtual wormholes pop in and out of existence all over the place but don't last long enough to be useful. To make a stable wormhole you'd need two solar system sized balls of metal with huge and opposite electric charges. When a wormhole happened to from going from one to the other the electric arc would run through the wormhole making it go from virtual to real. To keep it from collapsing in on itself you'd need to ring the centre of the wormhole with "exotic matter", stuff with negative energy,

How to use it to travel through time:: If you take one end of a wormhole and make it travel in a circle very fast (as in the time dilation example above) then when it comes back it will be rooted in an earlier time than the other end of the wormhole. So for example if we take the "front end" and use time dilation so that it is an hour behind the "back end" then anyone travelling from the back to the front will end up an hour backwards in time. By going through repeatedly you could travel back an arbitrary number of hours, but not to before the "time machine" was formed.

Why this is impossible: One serious problem with this theory is the fact that there seems to be no way to make a stable wormhole without "exotic matter", stuff with negative energy, and as far as anyone can tell exotic matter doesn't exist. Also, new simulations have shown that any wormhole will become unstable due to energy (such as a single photon of light) looping back and adding to itself, quickly creating an infinite amount of energy which will collapse the device.

Cosmic Strings

I know nothing about cosmic strings, go read this page. But from what I can remember of Andrew's description: they're infinitely long lines of infinite energy formed at the boundaries between sections of space with different field direction, kind of like cosmic Ley lines. Noone knows if they exist, or how would we use them, but infinite energy sounds like it could have theoretical applications, right?

Paradoxes and The Trousers of Time

One problem with time travel is the possibility of paradoxes: what happens to you if you go back and kill your own grandmother? In Back to the Future and other fiction you seem to have some sort of grace period to set things right before you vanish, but afaict this is not a part of any accepted physical model :)

Anyway, there are two generally accepted schools of thought on how to remove the possibility of pardoxes.

Multiple universes: The trousers of time

This theory says that when you travel back in time a new timeline splits off, so that whatever you do in your current, new timeline doesn't affect the original timeline you came from (it also means you're permanently trapped in a parralel universe). Some theories say that new timelines/universes split off every time anything happens.

Single Universe

This model tends to say that the history is set in stone, both the past and present. There are three flavours of this: There is also a "single universe" model which says that time is an illusion, and that every action we take sends ripples back through the future and the past. I do not understand this concept remotely well enough to explain it :)

Links and References

Links

Non-Fiction references

Fictional References

Books: Movies: