Comment by tehwalrus | Hacker News Reader

Comment by tehwalrus | Hacker News Reader

> "Now that you’ve changed things, time travel wasn’t invented in your lifetime, so either you vanish and the whole thing is undone, or your time machine does."

Can we please stop telling people that time travel will/can cause the object in your hand to change in front of your eyes?

Objects in spacetime follow the path defined by the laws of physics. If you brought an object from your timeline[1] it (and indeed you) will not change to suit the new reality you're in. It will just be an object, like you, stranded from another place, even if the return journey closes because of one of your actions.

The first housecat in Australia just hangs out and eats everything, it doesn't start acting like a koala. (traveling in space and time are the same thing, even though one is more restrictive in which way you can go.)

EDIT: There is another alternative, which is that there is only one causal universe, and it is impossible to change things that happened in the past (e.g. every time you try to kill Hitler you fail, and maybe make him tighten up his security making it impossible for a local-time assassin to do their job, all of which was reported in local documents that you already knew about, or similar.)

[1] or universe, if you believe in multiple possible worlds.

There is another alternative, which is that there is only one causal universe, and it is impossible to change things that happened in the past (e.g. every time you try to kill Hitler you fail, and maybe make him tighten up his security making it impossible for a local-time assassin to do their job, all of which was reported in local documents that you already knew about, or similar.)

Clearly if changing the past is impossible, then traveling to the past it's completely impossible, since your simple presence in a place is a change. It makes no sense that the laws of physics would prevent you from killing Hitler but not from changing the position of the air molecules around you,

not at all. it just means you were always there in the past, and any action you took has already affected your own future.

Time travel is entirely outside of our theory of physics.

If you want to insist on rules, you have to say that we can't do it, not argue about how it would work.

(I mean, it is entirely possible that the universe is a-causal in the presence of certain types of events that we do not yet understand)

sorry to contradict you, but all the laws of physics are totally reversible, so in theory you could turn back physical time. The problem is that you need to do that for each atom, molecule from your body in order to reverse all the interactions. However it won't be time travel proper, except for the atoms/molecules in question.

So if I had specifically said 'the popular notion of time travel', would that have avoided this digression?

Because to me it looks like you were itching to contradict me.

(I'll totally own the fact that I might have made an imprecise overstatement or whatever, I'm lazy and this is just a message board, the context is clearly enough popping in and out of time, not magic level reversibility)

Well, point taken, indeed the popular notion of time travel is quite far fetched. However, at a basic level it is not outside the current scientific consensus. But things can change, we don't know a lot of what can, or can not be possible. And yes, this is a message board, we're here to contradict each other :))

I think it is lots of fun to pick at things, but I think it is more fun to pick at what someone is trying to say, rather than picking at how it was said.

(I'm being cranky here, I didn't get worked up over any of this or anything)

Physics is reversible only at microscopic level. The arrow of time is defined by statistical mechanics, and second law pretty much means that processes tend to go only in one direction.

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