The spousal overunit found this listverse list posted to imgur and brought it to my attention. Given that I am both a scientist who’s taken some of these classes (long ago, prior to beginning my dissertation work) and a science fiction fan prone to pondering time travel, I feel semi-qualified to talk about what’s in there.
First off: She’s right, it’s badly written. The listverse author has accidentally made complicated things even more complicated.
Second: There are a number of either conceptual or presentation errors in here.
The current record holder for time-traveling is Sergei Krikalev. He has traveled about 337 million miles in orbit at some 17,450 mph – reaching a grand total of 0.02 seconds into the future. This means that from now on, he takes a step two hundredths of a second before you see him take it.
That’s not what it means at all. Krikalev has simply aged 0.02 seconds less than he would have, had he not done all that space travel. Alternately, he’s travelled 0.02 seconds into his own future. But when he takes a step, he takes a step. To be snarky about it (it’s my blog), if you see him 0.02 seconds later, you would have to be seeing him from 3720 miles away (light travel delay). Minus human perception lag, where EVERYTHING you experience is a few milliseconds after it happens.
Anyway, most of the ten points made in the posting amount to two:
1.) Paradoxes. The grandfather paradox, bootstrap paradox, and predestination paradoxes.
The Grandfather paradox (entirely separate from the situation in “I am my own Grandpa” from Mark Twain, novelty songs, and who knows what else) is when you go back in time and kill your own grandfather (or father, etc) – anything that would stop you from existing and therefore you would be unable to go back in time and do it. It extends to any case that would be self-defeating… going back in time to prevent a terrorist attack, except that now that it didn’t happen, why would you go back in time to prevent it? That’s what is known as an open time loop, which isn’t really much of a loop.
The Bootstrap paradox (named after Heinlein’s “By His Bootstraps”, and prominently featured in Heinlein’s “All You Zombies” and its excellent movie adaptation “Predestination”) is one of impossible objects – Your future self uses a time machine to return to the past (time A), and hands you the blueprints for the time machine. Then you build the machine, go back in time (from time B to time A), and hand your past self the blueprints for the time machine. Who wrote the blueprints? They exist because they exist. Your future self didn’t write them, he got them from his future self an earlier time through the loop. The time machine creates its own existence.
Say you misplace your Oscar acceptance speech, so you get in your time machine and travel back 30 minutes to a time when you still know where the sheet of paper is, retrieve it, and return to the future in the nick of time to deliver it for your “Lincoln” performance. This is #3 on this list.
But to the point: any object traveling through time, for the duration of the trip, no longer impresses itself on history. After 100 million years, that sheet of paper has worn to dust, as has the traveler. But the show must go on, and the Oscar goes to the same person, who now accepts it without the speech, because it no longer exists outside history to return to him in the future.
This is poorly written: as described, the reason you can’t find the speech is that future-you came back and grabbed it and brought it 30 minutes into the future. The sheet of paper just skips over those 30 minutes with (maybe) no harm done, while you re-live an extra 30 minutes. However, there IS an actual concern there. Let’s return to the time machine blueprints: Paper (or digital data, or even stone tablets) have a limited lifespan. However long it might be, if there’s a time loop that could go infinitely long, they’ll decay. At some point, future-you will hand present-you a barely-held-together illegible rag or worn, crumbling stone tablet. Option 1 is that you re-copy the blueprints onto something else (which means the infinite loop won’t be identical every time, unless that’s part of every loop), Option 2 is that these blueprints aren’t normal matter anyway because they were never created, Option 3 is that this is impossible.
The scientific term for these are the Novikov Self-Consistency Principle, which basically states that open time-like curves (i.e. a path through time that does not lead to itself, i.e. a grandfather paradox) cannot exist. It’s still an open question of whether or not a closed time-like curve (predestination paradox) can exist. They probably can’t, because a closed path could still lead to the bootstrap paradox.
To handle this, there’s the many worlds hypothesis, which posits that every possible action can and does happen, in parallel universes… but this does not constitute a theory, or explain why every possible consequence would happen.
2.) Practical concerns with the laws of physics.
These relate to things like the continued lack of proof that Closed Time-like Curves can exist, and how the physics of black holes and wormholes don’t really seem allow these things to happen.
For starters, one way to travel backwards through time is to travel faster than the speed of light. Let’s say you see a hideously large explosion on the Planet Zog and race to give aid. Relativity tells us that each observer in an inertial reference frame (not accelerating) will see their own self-consistent view of history but the same speed of light. If you’re travelling at or less than the speed of light, the most any observer will see is you arriving at Planet Zog at the same time you left. If you travel faster than the speed of light, there is some observer who would be able to see you arrive at Planet Zog before you left Space Base Alpha. (interestingly, this is the Picard Manoeuvre from Star Trek: TNG’s “The Battle”). Cause and Effect would be reversed, which can’t happen. Faster than light communications would theoretically allow you to send a message to that other ship and have their reply arrive before you sent the initial question.
As for black holes and wormholes, theoretically our equations state that things should move faster than the speed of light within the event horizon, but that’s more of a sign that our physics breaks down than anything realistic. What’s often ignored is that as you approach the event horizon of a black hole, your apparent time slows down – as seen from an outside observer, it will take an infinite amount of time to actually cross the threshold.
What about just the wormhole? It’s not going to happen. The Cosmic Censorship Hypothesis states that you cannot have a singularity (the impossible point that would form a wormhole) outside of an event horizon, and that’s what you would need.
There are a couple of other concerns the imgur post DIDN’T touch on, including:
We have (as far as I know) no real proof that the past “still” actually exists. All actions and reactions we know of happen now. While they are certainly related to what came before, is there a “before” to go back to?
Likewise, we don’t actually have any indications that the many-worlds hypothesis is correct. It’s an enormous amount of additional energy to add to the universe, to have infinite alternatives. There are suggestions that the inflationary model of the universe is correct, which implies regions far outside the observable universe still exist, and a sufficiently large number of permutations of those would produce every concievable parallel universe. We can only see the regions from which light would have reached us, and that puts an outer boundary at 13.7 billion light years… we don’t really know what’s beyond that, although the theory that predicts them seems to be correct.
And then… the laws of thermodynamics state that matter cannot be created or destroyed, but what happens when you travel back in time (to grab the speech, for example)? There are two of you (one going from Time A to Time B, one going from Time B to Time A). Actually, there should be a third, going at least from Time A to Time B with the speech. Perhaps when you’re going backwards in time you have negative mass, and therefore negative energy, and that balances out the second and third versions of you. The problem is, there doesn’t seem to be any such thing as negative mass.
It could make for a nifty story though…