The other night on Nova there was a repeat of an episode discussing time travel. A lot of time was spent discussing the so-called grandfather paradox wherein a time traveller goes back in time and kills his grandfather thus preventing the time traveller from ever having been born etc, etc. One possible resolution was to invoke parallel universes, including one where the traveller was, in fact, never born.
But time travel would seem to have another type of inevitable paradox. One that’s seldom discussed (in my opinion at least) that involves information and knowledge. For example, could a time traveller go back in time to, say, a very young Einstein and teach him about relativity. If so, where did the knowledge of relativity come from? Who would have thought it up, and how? I don’t see how parallel universes avoid this type of scenario.
Well if you use the parallel universe explanation again, you still have infinite worlds in which Einstein did postulate relativity…along with infinite worlds in which it was never theorized at all…along with infinite worlds in which I discovered it, etc…
Heh. This paradox was addressed in, of all places, “Back to the Future”. Remember Michael J. Fox playing “Johnny B. Goode” while Chuck Barry’s cousin had him on the phone?
There is a good article in the current issue of DISCOVER Vol. 22 No. 9 (September 2001) all about this. Really interesting. It explains in detail, and in laymens terms for people like me about how this works (supposedly). Go buy it. It’s like 4 bucks.
I did not see the show and I do not believe in time travel and I do believe 99% of the the discussions of time time travel are either to sell a book or get a research grant.
Given that, there is no difference between a material object (me going to kill my grandfather) traveling through time and information (you going to help Albert) traveling through time. I state this based on the premise that everything ultimately is information. If you can resolve the grandfather paradox by postulating parallel universes, then you can resolve your problem the same way.
If you could tell me exactly what time is and the mechanics of how it functions, I will happily provide you with a more detailed answer.
Perhaps viewing all time travel as merely multivariant information replication in 11-D phased space with noise limitations…oops, gotta go write up a research proposal.
In universe A, Albert comes up with relativity. You get this information from him (indirectly of course) and travel back and time, splitting into Universe B. You can never go back to Universe A. You tell Einstein-B about the relativity Einstein-A invented and he publishes it.
The information came from Einstein-A, who is in a universe you have cut yourself off from with time travel.
I wouldn’t be so hasty. Kip Thorne, one of the best (maybe the best) theorist on relativity alive today, set about disproving the notion of time travel sometime back in the 80’s (I think…maybe the 90’s). Apparently all of the speculation about time travel was annoying him so he was going to set it to rest once and for all. Much to his dismay, however, he found absolutley nothing in the math of relativity that prohibited time travel.
Mind you, that does not automatically mean that time travel is possible since relativity is a theory and parts of it may prove to be wrong in the future and the ‘correct’ version of things may prohibit time travel. Still, as far as our best physicists can determine today, time travel is theoretically possible even if it may be practically impossible.
Just because we can’t imagine how such a thing could reasonably work (due to paradoxes) doesn’t make it go away. Common sense tells me that an object can only be in one place at one time but quantum mechanics tells me particles can be in more than one place simultaneously (ala the double-slit experiment). I don’t like it (in that it doesn’t fit well with my sense of how things should be) but there it is anyway.
Excellent book: “Black Holes and Timewarps” which I recommend highly to anyone interested in either of those tangled subjects.
It is important to note that quantum vacuum effects are likely to cause a wormhole to collapse no matter how hard we try to create one, as-- due to the grandfather paradox-like effects-- the wormhole acts like a giant fluctuation amplifier and causes its own instability.
However, this is only speculation; until GR is mized successfully with QM we just don’t know for sure. AFAIK, no theory forbids time travel, either GR or QM. Does it?
When I was studying special relativity, the heavy emphasis on frame of reference stuck with me. I’ve often wondered if you could apply the same concept to time travel. The basic concept works like this. Lets say you’re 25 years old, travel back in time to the middle ages. One year later you have experienced 26 years of life while moving backwards in earth reference time a few hundred years. The net result of this is that while you can make changes to the past of the earth you left, you cannot make changes to YOUR past. I suppose a simpler way to put it is no matter how far back in time you go, you’re always in the present.
On a related note…I wonder what happens with the conservation of mass in time travel…
IANATT (I am not a Time Traveler) But wouldn’t a closed loop set-up help alleviate the problems with both paradox and conservation? What I mean is what if time takes time travel into consideration? If you travel back in time and try and kill your grandfather you will fail, something will happen that will stop you from killing him. We know it will fail because he’s alive now. If you think about it, that wacky story he keeps telling about a mysterious man trying to kill him as a child starts to make perfect sense. A few sci-fi writers have tried to tackle this.
I also think that a closed time loop would take care of the conservation laws since your not creating anything your just moving it from one point in the loop to another. As long as the net total of the loop stays the same wouldn’t that take care of that?
Of course I am COMPLETELY out of my league right now.
Well I’m not a physicist, but Whack-A-Mole mentioned a
big wig physicist who couldn’t show time travel
contradicted relativity. Well what I thought this thread
was going to ask seems to me to give a contradiction
between time travel being possible and relativity.
Here is what I thought the thread would start as:
Relativity postualtes that information cannot travel faster
than light. But with time travel I can start at point
A with some “information” (whatever that is) travel one
light year to point B, then travel back in time to
half a year after I left point A and release this
“information”. The “information” has now travelled a
light year in half a year. Or, it has travelled twice the
speed of light. Since 1 does not equal 2 (as I think was
debated in another thread ;)…) we have a contradiction.
Yue Han’s comment that the notion of parallel universes rescues us from paradox here, just as with the ‘grandfather paradox’
GWF Hefel reminded me that there really may not be a fundamental difference between ‘knowledge information’ and the information that encodes, say, a person. Indeed, our DNA sequences and the letters of the words in a relativity text are basically the same thing (I think!?)
Just like changing direction requires an acceleration, an application of force in ordinary mechanics, time travel–if you’re not following a spacetime geodesic–requires an expenditure of tremendous concerted energy. The only way we can experience this directly is when we sneeze.
I would also agree with Free Refill in so far as IMHO time travel might be possible, but it’s impossible to chnage the past. The past is over, history is already written, and since your start for the time voyage will happen after Einstein’s discovery of relativity (or whatever event you want to modify), you cannot change it because if you could, the change would already be in effect before your departure, so it wouldn’t be a real change. It will be like it has always been.
I remember reading in a coverstory of Der Spiegel about time travel paradoxes that said some scientist had published a mathematical proof that time travel can not change the past.
:goes downstairs and digs out the issue:
That guy’s name is Igor Novikov, the proof was published in 1996.
That article also claims “all” the scientists agree in the assumption that no time machine, even if it can be realized, can carry its passenger back to a point of time prior to its construction.
Yes, time travel would allow for information to travel faster than light, and vice versa. Special relativity, however, does not anywhere state that it’s impossible for information to be transmitted faster than light. In fact, the reason that SR is often considered to prohibit information being sent faster than light is that that would imply that time travel is impossible, and everyone just knows that time travel is impossible.
The most plausible ideas thus far suggested (such as wormholes) for time travel do, indeed, limit you to the time when the Machine was first built, but that’s not an absolute result. A “warp bubble”-type starship (yes, there’s real papers in the peer-reviewed scientific literature that discuss this) might not have that same limitation.
In fiction, the only author I’ve ever found to do a good job with time-travel paradoces is Heinlein, and then only in his earlier works, most notably his short story “By his Own Bootstraps”.
