Traveling back in time could be possible (opens in new tab)

Note that under our current understanding of General Relativity, wormholes will rapidly collapse under their own gravity so quickly that not even a single photon would be able to traverse it because it closes off.

There's a theoretical way to stabilize it: put 'exotic matter' in the middle. Unlike all other matter, exotic matter has a negative energy density. This is the same sort of matter that would be needed to create an Alcubierre warp drive.

Unfortunately, we don't have any idea how to go about creating exotic matter. The closest we've come is the Casimir effect, when a region between two very close conductive plates will in some ways act as if there were negative energy.

[1] http://www.bibliotecapleyades.net/ciencia/negativeenergy/neg...

I broke my rule of not reading medium articles, but how exactly does this prevent the grandfather paradox? Once you're back in time you can still stop yourself in the future from entering the worm hole...

It's not clear what all the rules are and how this prevents the grandfather paradox.

What ever mechanism that prevents the paradoxical violations also makes this not very exciting (as far as time travel goes): you have to spend the "40 years" between you visiting one end of the wormhole and you entering the other at near-luminal velocity (hence you would only perceive it as 1 anyway). It's not so much "time travel" as an instant return ticket (where only your personal real time is spent, assuming your ship is exactly as fast as the one that carried the wormhole).

Which does actually have an interesting effect if you make them in pairs. It makes books like The Commonwealth Saga technically almost viable (e.g. where they build networks of portals and then just run trains through them non-stop to travel between worlds). Except that instead of using portals to explore new worlds, they would have to send a probe to those planets with a wormhole on board, at near the speed of light. And trips might cost you a bit of relative time to make a trip and return (e.g. the difference between the speed of light and the speed the probe delivered the wormhole from both portals added together would be how far in the relative future a trip would send you).

This all of course requires exotic matter which probably doesn't exist; which is why this whole thing probably doesn't work in the first place (e.g. why stable wormholes probably don't exist). Still might make it's own excellent sci-fi book if someone ran with the constrains of this, and it would technically be harder sci-fi than most.

I don’t see how time travel doesn’t violate the law of conservation of energy or conservation of mass. That seems pretty fundamental to me, so it would require converting an extremely large amount of energy into the mass that is going back in time.

Theoretically yes, but what they didn't tell you is that all those particles travelling through will completely lose all their information and order how they belong together. So please dear sci-fi authors stop with these ridiculous plots.

If I read that correctly, you are never able to travel back further than the creation of the wormhole.

The real question I have is how would you ever stabilize something as powerful as a black hole.

It seems we have laws of physics covering everything from Newtonian mechanics, relativity and quantum mechanics... Is there any law stating time travel must go one direction? Has it ever been proposed?

> If this negative mass/energy matter exists

Once

Travelling backwards in time is not possible, it's obviously not possible, and it's a wonder that the idea gets given such credence in educated circles, apart from it being a nice thought and a diverting intellectual exercise.