Saturday, October 08, 2011

Time travel

(This is a non-political weekend post.) This quiz got me thinking about time machines, as seen in the movies. If you dialed such a device to take you into the past, wouldn't you end up floating in the void, about eight seconds away from death? After all, it's a time machine, not a time and space machine. The Earth moves. So does the solar system.
Science-fiction author Larry Niven says that there can be no time travel, because--if anyone ever invented time travel--someone would eventually go back and change history so that there was no time travel.
I think the TARDIS is a time and space machine unlike HG Wells's which was time only. The time turners in Harry Potter were cool as they were used for 1-3 hours top. Some of the best time traveling stories are in cartoons: Futurama of course and even SpongeBob SquarePants with its chrome plated scary future.
Even the Harry Potter thing proves my point. Three hours' difference is still enough to put you inside a wall or a rock, or half-way into the earth. Or maybe eight feet off the ground.
Moves relative to what? :)
Well, the Earth moves relative to the sun and the solar system moves in relation to the center of the Milky Way. And they both would move, over time, in relation to the spot where you are sitting now.

Right? At least I THINK that's right.
Maybe the term time machine has become shorthand for time and space machine - rolls off the tongue more easily?

Movies on that theme (a favourite of mine) I found most memorable:
Millennium (Kris Kristofferson),
Primer (a low budget one), and the recent time Traveller's Wife, (where no device was needed). I couldn't get my head around the book, the movie was a bit easier, but still mind-boggling.
Edgeoforever got it: TARDIS stands for Time and Relative Dimension in Space. This is why The Doctor never ends up in the void, eight seconds from death. Of course, even after 900 years, he can't seem to get a chameleon circuit repaired.
Yours is an intuitive take on how time works, and I agree your conclusion seems sound. If time is just like another of the 3-dimensions of space, linear, etc., then one can imagine moving only along its axis without also moving along the 3 spatial dimensions.

Ignoring our solar system travel entirely (which isn't right), even considering only the rotation of the earth might find you somewhere else on the world when the time travel is done. Realistically, given the travel of the earth around the sun, you might end up half an orbit away from the earth.

To avoid such effects, a different kind of temporal dimension has to be imagined, where the travel in the time dimension also involves travel in the 3 dimensions of space in the exactly correct total direction to still be where you 'were' when you 'left' our time (relative to an earthly grid location).

I can think of no reason this would be true, but again, no reason why it might not be true. Could be. As we have only the fictional accounts to rely on, that they generally don't find this problem may be only an oversight in the writers' imaginations.

Some physicists believe there's no such thing as time, that all configurations of all atoms in the universe exist at once, and we only perceive the configurations in such a way that there seems to be movement and there seems to be cause and effect, and there seems to be time.

Can't say as I understand it, and it sounds way too much like predestination to me, but it's what some really smart people believe.

Carolyn Kay
If you really,really want to get into this, dig up some old issues, circa 60's, of Analog magazine and read the Lab articles and Letters to the Editor. The fiction's good, too.
Wow, the solar system does not move around the center of the galaxy? I was misinformed.

Seems to me that a machine which transports you instantaneously from one position in space to another would be even harder to come up with than a time machine. Of course, you could put the airlines out of business.

Steven King wrote about such a device in his story "The Jump." Scared the bejeebers out of me, that one did.
Questions like the one you present about time and space remind me of the debates the Catholic Church used to have on how many angels could fit on the top of a pin. The way I understand, time is change as perceived by the mind. If nothing changed, the idea of time would not exist. But I'm a Zen Buddhist (cultist), who sees discussions like this one to be nonsensical.
Here is a Zen koan:

"Two monks were standing in the Temple grounds, arguing about a flag that was flapping wildly in the strong winds, that were blowing that day.

One said, '' The flag is moving. ''

The other replied, " No, it is the wind that is moving."

Just then Hui Neng happened to pass by.

He told them,

'' Not the wind, not the flag. Mind is moving.''

"Well, the Earth moves relative to the sun and the solar system moves in relation to the center of the Milky Way. And they both would move, over time, in relation to the spot where you are sitting now.

Right? At least I THINK that's right.

I was trying to say that the spot where I'm sitting now can only be defined relatively. So I might as well define a frame of reference in relation to my own location - i.e. assume that I stay still. By this reckoning, I'm always at the same point in space. When I supposedly jump forwards, it's really the room jumping backwards.

OK you don't like that. So if you are thinking of defining points in space using Cartesian coordinates (x,y,z) (and whatever coordinate system you use, you'll need three coordinates to do it properly), where are you going to put your origin? Sure, you can put it at the centre of our galaxy, which the Sun rotates around, or at the centre of mass of the 'Local Group' of galaxies, which our galaxy rotates around. You can also put it as your location, wherever that might be at any particular time.

(Choosing the origin isn't the whole of the story. E.g. say you choose the centre of the Sun. Then where will you put your axes? You can put the x-axis going straight through the centre of the Earth if you like. Then travelling through time while holding your spatial coordinates constant wouldn't move you very far from the Earth's surface. Only a million or so miles, maximum).

Quiz question - who first came up with the idea of the Milky Way, understood as a flattish bunch of stars that the Sun belongs to, being one of many such bunches, and appearing to us as a streak in the sky?

Answer: Thomas Wright of Shugborough Shepherds Monument fame :)

Did you grok that CERN story about neutrinos travelling faster than light? And to Italy too! :)
I assume the time machine remains in the Earth's reference frame as it moves back through time. That is if it moves continuously. If it makes a series of small enough discrete jumps then conservation of momentum should keep it in roughly the same spot. If it makes too large a discrete jump then you would probably be dead, but not in eight seconds. A writer can always explain that Einstein showed that we cannot speak of separate space and separate time but only of spacetime. It doesn't explain anything but who can argue?
Primer is a tremendous time travel movie if you haven't seen it already Mr Cannon :)
And even if you have, it certainly rewards repeat viewing.
Try this. Nothing in the universe is standing still. Everything is moving in one direction or another. Where it's moving, or even if it or you are moving only depends on your view point.

That goes for the area of space something takes up and the time that elapses while it's taking up that space too. So, no two objects can occupy the same "space" at the same "time"...SHAZAM!! "Spacetime".

That would mean to travel back in time everything in the universe would have to re-occupy the same 'space' and 'time' it did at the moment you'd want to be there. You weren't there to begin with at that 'space' and 'time', so it ain't happenin'.

People are objects just like any other amalgam of atoms in the universe. Just taking up space at a particular time. We'd just like to re-schedule some appointments.
CERN recently reported their apparent finding that neutrinos whizzing from Switzerland to Italy travelled at 1.00002 times the speed of light. Once the light barrier gets broken, stuff gets weirder than it was.

In 1949, Gödel came up with new solutions to Einstein's field equations for general relativity which are consistent with time travel.
A time traveler is only traveling in one dimension, not four, so perhaps gravity keeps him/her anchored in the other three dimensions, moving with the Earth as it rotates (backwards) and travels (backward) around the Sun, etc, just as we do when we move forward in time, which we usually do at a fairly constant pace.

If you went far enough in time that the location changed (building burnt, earthquake, sea covers Atlantis, etc.), that might be a problem.

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