Is Time Travel (going backwards) Possible?

[Umbran]

Why trust any elephant that much?

The Elephant never forgets, of course.

Especially a FICTIONAL one?

If being fictional is the worst thing he's done, well, then he's a better entity than most of us, and therefore far more trustworthy.

 

[Nellisir]

They say they are only certain to a 1 in 10 tredecillion (1043) or 14-sigma level of certainty, but they're getting their info from BaBar. Why trust any elephant that much? Especially a FICTIONAL one?

Exactly.

 

[Umbran]

Perhaps very pertinent?

To answer this question without reference to elephants...

Um, maybe?

Here's the thing - this is quantum mechanical result. The only real entrances we have into time travel are through General Relativity.

Note how no theory to unite gravity and quantum mechanics has gotten very far? We don't know how much that result bears on our only current path to an answer for the question.

 

[freyar]

Perhaps very pertinent? http://www.sciencedaily.com/releases/2012/11/121119094627.htm

To answer this question without reference to elephants...

Um, maybe?

Here's the thing - this is quantum mechanical result. The only real entrances we have into time travel are through General Relativity.

Note how no theory to unite gravity and quantum mechanics has gotten very far? We don't know how much that result bears on our only current path to an answer for the question.

I'll agree with Umbran as far as he goes on this, but I'd also add that there's probably nothing to do even with quantum time travel in this result. Specifically, this experiment looks at a type of particle known as a "B0," which has the unusual (but not unheard of) ability to change into its own antiparticle. Now I'm going to oversimplify grossly. What BaBar did more or less is measure the speed at which B0 turns into its antiparticle compared to the speed at which the antiparticle turns back. There are really a lot of technical details that make that not quite right, but the gist is that the two speeds are different. So the one reaction is not the same speed if you run it forwards vs backwards in time. This is called time-reversal violation, and it's been expected for a long time. In fact, it's probably been measured before with other types of particles, but those earlier measurements have some more errors. So there's really nothing about going back in time; it's about whether two reverse processes happen at the same speed going forward in time.

Another point to make is that there is actually always (mathematically proven) a way to reverse a process and get the same speed. This is called CPT because it involves reversing the time (T) (ie, the order of the process), parity (P) (which is reversing all the spatial motion), and charge (C) (turning all particles to their antiparticles). If you do that, you get two equal speeds; if not, well, you'll (1) have to subject your results to incredibly intense scrutiny and (2) win fame, fortune, and a Nobel prize if you're right (but you're probably not ).

There's a reasonably nice not-too-technical explanation here.

 

[KarinsDad]

So there's really nothing about going back in time; it's about whether two reverse processes happen at the same speed going forward in time.

And that's where "the public perception" of physics can sometimes get out of sync with what can really be concluded from experiments.

 

[Dannyalcatraz]

To answer this question without reference to elephants...

Um, maybe?

Here's the thing - this is quantum mechanical result. The only real entrances we have into time travel are through General Relativity.

Note how no theory to unite gravity and quantum mechanics has gotten very far? We don't know how much that result bears on our only current path to an answer for the question.

So...you're saying that article might be irrelephant?

 

[Nellisir]

To answer this question without reference to elephants...

Um, maybe?

Here's the thing - this is quantum mechanical result. The only real entrances we have into time travel are through General Relativity.

Note how no theory to unite gravity and quantum mechanics has gotten very far? We don't know how much that result bears on our only current path to an answer for the question.

And thus the "perhaps"

 

[freyar]

And that's where "the public perception" of physics can sometimes get out of sync with what can really be concluded from experiments.

True enough, and the media doesn't usually help. In this case, even the press release from SLAC, the lab that hosts the BaBar experiment, makes a big error just for the sake of getting a splashier headline (see the link I posted above).

 

[Umbran]

Another point to make is that there is actually always (mathematically proven) a way to reverse a process and get the same speed.

Yah. This is a case where natural language and physics language differ. You gave a nice link, but maybe I can take a really short stab at it...

What he's talking about is not a physical process you can enact in the real world. When you say "reverse a process" in natural language, it means actually reversing it - Bruce Banner turns into the Hulk, the Hulk turns into Bruce Banner. Reversible.

When we talk about this in natural language with respect to time, and the arrow of time, we are usually talking about things on the line of "we stir sugar into water, and it dissolves". This is not reversible - we cannot stir the other way and have all that sugar come out of solution and reform crystals.

Freyar is talking about a form of mathematical reversal - kind of like saying that if you flip *all* the numbers to be their negatives, the thing runs the same way. This is not equivalent to stirring the water the other way - it includes reversals that we cannot enact in the real world (yet?).

 

[freyar]

Yah. This is a case where natural language and physics language differ. You gave a nice link, but maybe I can take a really short stab at it...

What he's talking about is not a physical process you can enact in the real world. When you say "reverse a process" in natural language, it means actually reversing it - Bruce Banner turns into the Hulk, the Hulk turns into Bruce Banner. Reversible.

When we talk about this in natural language with respect to time, and the arrow of time, we are usually talking about things on the line of "we stir sugar into water, and it dissolves". This is not reversible - we cannot stir the other way and have all that sugar come out of solution and reform crystals.

Freyar is talking about a form of mathematical reversal - kind of like saying that if you flip *all* the numbers to be their negatives, the thing runs the same way. This is not equivalent to stirring the water the other way - it includes reversals that we cannot enact in the real world (yet?).

That's a pretty good stab at it!