Would time travel to the past be possible if ?

[Sasquatch544]

Would time travel to the past be possible if...?

We didn't focus on reversing time itself (which is obviously impossible), but focused on accessing a past parallel universe. And creating some sort of portal to these past parallel universes, like a "quantum" wormhole (i know it sounds like science fiction). Obviously (if this was possible) we are centuries, probably even millenniums away from this technology, but theoretically, could this be done?

 

[StevieTNZ]

I would assume there would exist no 'currently in the past' parallel universe. I'm under the impression when new universes are created (according to Many-Worlds), they exist - on a time scale - at the same point on that scale.

 

[Drakkith]

There is currently no known way to access a parallel universe, if one even exists, nor do I believe there is such a thing as a "quantum wormhole", so your question has no answer since it doesn't deal with current physics.

 

[Chronos]

Parallel universes are so comforting. They explain everything and are unfalsifiable.

 

[SpecialKM]

Parallel universes are so comforting. They explain everything and are unfalsifiable.

Just like God!111

 

[HarryRool]

Parallel universes are so comforting. They explain everything and are unfalsifiable.

Parallel universes are in fact falsifiable. If we every develop a self consistent and paradox-free fundamental theory of physics that (unlike quantum theory) somehow selects out a particular reality from all of the possible realities, the theory of parallel universes would be rendered false.

 

[HarryRool]

There is currently no known way to access a parallel universe...

A wormhole operating as a time machine (or any spacetime containing closed timelike curves) leads to inconsistencies unless the wormhole connects parallel universes. [See David Deutsch, Physical Review D, 44, 3197 (1991)] As it is quite easy for any wormholes that exist to become time machines, the existence of wormhole implies the existence of connections between parallel universes.

... nor do I believe there is such a thing as a "quantum wormhole"...

Actually, the existence of quantum wormholes, unlike classical wormholes, is not controversial. It has long been assumed that any reasonable theory of quantum gravity would have to describe spacetime at the submicroscopic level as a "quantum foam" containing every possible geometry -- including wormholes.

A good popular explanation of these points is The Physics of Stargates: Parallel Universes, Time Travel, and the Enigma of Wormhole Physics by Enrico Rodrigo. You might also want to check out the wormhole chapter of Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip Thorne.

 

[HarryRool]

We didn't focus on reversing time itself (which is obviously impossible), but focused on accessing a past parallel universe. And creating some sort of portal to these past parallel universes, like a "quantum" wormhole (i know it sounds like science fiction). Obviously (if this was possible) we are centuries, probably even millenniums away from this technology, but theoretically, could this be done?

In the most conservative approaches to quantum gravity, it is not possible to create a wormhole. One would instead enlarge a preexisting submicroscopic quantum wormhole (large quantities of which are presumed to exist in every cubic meter of space). This would require some sort of ultra-high frequency negative energy source. Although no one knows how to create such a source, the existence of such a thing has not, despite a lot of effort, been shown to be inconsistent with the laws of physics.

Short answer: Theoretically, it could be done.
[Keep in mind, however, that it is also theoretically possible to produce enough antimatter to annihilate all matter in the galaxy.]

 

[Drakkith]

I won't disagree with you Harry, but since they are 100% hypothetical at the moment and are thought to need negative energy or mass or whatever pretty much shuts the door on them for me personally. Until we have good reason to think otherwise of course.

 

[HarryRool]

I won't disagree with you Harry, but since they are 100% hypothetical at the moment and are thought to need negative energy or mass or whatever pretty much shuts the door on them for me personally. Until we have good reason to think otherwise of course.

Drakkith,

You're not thinking like a theoretical physicist. Once you believe the theory (because it's passed a battery of experimental tests), you're supposed to believe its predictions. Remember, black holes were purely theoretical objects for most of the 20th century. Yet during that time lots of theorists firmly (and justifiably) believed in their existence.

So far, despite considerable effort to prove otherwise, quantum theory permits scenarios involving arbitrarily negative energy (especially if scalar fields exists). Our best theories (QM & GR) tell us, therefore, that wormholes could be in principle be constructed.

 

[Drakkith]

The problem is that our theories are NOT perfect. General Relativity predicts a singularity at the middle of a black hole, yet to my knowledge most scientists do not actually believe there is a "real singularity", only that it's a product of an incomplete theory. One cannot simply accept all predictions, especially when they contradict other theories or laws. There must be good reason to accept it. So currently there is little reason to believe that we can construct or use wormholes.