Time travel worry

[Brightonian]

I know there's nothing new in SF but I haven't seen this issue addressed anywhere.

When you travel in time, how do you know you'll end up in the right place? HG Wells assumed that his Time Machine would remain at the same spot on the Earth's surface, disregarding the fact that this planet and all other heavenly bodies are in constant motion. Tracking all this would take some pretty advanced celestial mechanics, which would have been quite a challenge even if his Time Traveller could have taken Babbage's Difference Engine with him.

I think Arthur C Clarke once wrote that we would never have routine interstellar, let alone intergalactic travel, even at FTL speeds, because of the intractable complexity of finding your way back home. If you factor in the temporal dimension as well then the problem surely becomes uncomputable.

[admin]

Another time travel problem is that, if you travel in time, there is already matter waiting at your destination. It is the same problem you have with teleportation -- you arrive half human and half fly.

[slaven41]

I have some problems with time travel into the past. If you travel into the past, then it seems to me that you are forced into one of two scenarios. Either the timeline "branches" from the point at which you arrive in the past and you have effectively started a whole new universe. I'm rather skeptical of man having the power to create a whole new universe.

Or, the timeline doesn't branch, in which case you end up in the actual past that leads to the moment you took off from. In which case, everything you do when you get there is fixed, since it's already happened. This would seem to mean that there's no such thing as free will. If free will is an illusion, it's one that I hate to give up.

Time travel into the future, on the other hand, can be achieved simply by traveling extremely close to the speed of light. (Did I really just say "simply"?) One can imagine a time machine that effectively sends you into the future, simply by jiggling at near light speed. I think this gets around some of the other problems mentioned, since the machine would stay put relative to the surface of the earth in the same way that Jell-O (do they call it Jell-O in England? It's a sweet, fruit-flavored, see-through gelatin) stays on the plate as it jiggles. And this also does away with the "what's already there?" problem, I think. (Or maybe it doesn't. As it jiggles, it collides with the surrounding air at ultrarelativistic speeds.)

Anyway, there's my two seconds' worth.

--Dave

[Brightonian]

Chris Beckett's short story The Gates of Troy at least acknowledges a couple of these issues. The scenario is that a billionaire dad is treating his son and the son's bets friend to a trip in a time machine:

- I've never quite gathered why people use these at sea?
- Because when you travel back you take a few hundred tonnes of the surrounding matter with you, Dad said. Not too awkward if it's just water, but rather difficult on land. And on land you'd run a risk of materialising slap bang in the middle of a building or something.
- But isn't the planet in a different position anyway? I mean what with rotation and going round the sun and the sun itself, you know, going round the galaxy ...
Dad shrugged vaguely and looked away, as he did when irritated by pettifogging details.

SO there you have it - just shrug vaguely and look away ...

[slaven41]

Yeah, I guess we were stupid to ask, huh? :-)

[Brightonian]

When students ask me something I can't answer, I usually say "don't worry, that won't be in the exam". If they persist I say we'll be covering it later in the term, and hope they forget

[admin]

When students ask me something I can't answer, I say, "That's a very good question. I'll look it up and get back to you." Then I look it up on Wikipedia.

[Brightonian]

You too can be a time-traveller if you take your next New Year break on the island of Samoa - they're planning to jump forward 24 hours on December 29th.

http://www.bbc.co.uk/news/business-13330592

[Third Foundationer]

Regarding the original post,

There is a very humorous take on the idea of time travel/space travel in the old English science-fiction/comedy show Red Dwarf.

The crew is stranded in deep, deep space millions of years in the future. They discover a time travel device and test it, programming it to send them to the 15th century. Once the device is activated, nothing appears to change. The robot in charge of the device announces happily, "It worked! We are now in the 15th century in deep space!"

I can't find this on YouTube, but I always liked the idea that these poor guys have a device of unimaginable power but can't use it in any practical way.

--3rdF

[Brightonian]

I know there's nothing new in SF but I haven't seen this issue addressed anywhere.

At the time I hadn't read Asimov's story "Robot Visions". He did address the problem that a time machine would have travel in space as well as time, but the narrator performs a scientific equivalent of shrugging and looking vague:

... one could travel with the space-time motion of the Earth not in a literal, but in a "virtual" way that would enable a time-traveller to remain with his base on Earth wherever he went in time. It would be useless for me to try to explain that mathematically if you have not had Temporalist training. Just accept the matter.

Incidentally in the same story, the narrator states that time travel into the past is provably impossible, yet goes on to describe how a robot is sent into the future and brought back to the present. What is that if not time travel into the past?