Going for a blast into the real past (quantum retrocausality)

[TechBear_Seattle]

Going for a blast into the real past

If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum "retrocausality."

That's science talk for saying he hopes to find evidence of a photon going backward in time.

"It doesn't seem like it should work, but on the other hand, I can't see what would prevent it from working," Cramer said. "If it does work, you could receive the signal 50 microseconds before you send it."

Uh, huh ... what? Wait a minute. What is that supposed to mean?

Roughly put, Cramer is talking about the subatomic equivalent of arriving at the train station before you've left home, of winning the lottery before you've bought the ticket, of graduating from high school before you've been born -- or something like that.

The full article can be read at http://seattlepi.nwsource.com/local/292378_timeguy15.html

[meegbear]

1. Quantum mechanics is like cartoons ...

you really can't appreciate them unless you're stoned.

[TechBear_Seattle]

2. Which certainly would explain most of the theoretical physicists I've known.

:rofl:

[Warpy]

3. The ones I knew at MIT

actually did spend their lives stoned.

[Crayson]

4. How do you TEST this???

I mean, this sentence:
"If it does work, you could receive the signal 50 microseconds before you send it."

Now imagine a test situation:

You have a button to press, which sends the signal that you "will" receive 50 seconds earlier.

Chronologically:
- You receive the signal.
- You press the button.


This is the classical free will situation:
- You receive the signal
- You decide NOT to press the button
But like this you wouldn't have received the signal??
This is usually the point where the universe explodes
^_^


You can never be sure that the signal you received was caused by yourself in the future, or by something else, or totally random.

Good Luck !

[Occulus]

6. I suppose you could test it by aborting the sending

Say, 35 microseconds in. That would be enough time to have received the signal, but also to abort after having received it. Wouldn't that mean that the signal would have to have been coming from a different universe, one in which the button was pressed? And wouldn't that basically confirm by observation the "many-worlds interpretation"? <--- I thought that last was flatly impossible; I thought other universes (IF they exist) could not be reached :shrug:

If you aborted sending the signal every time, but still received it prior to when you would have sent it (and consistently prior, too, always aborting at the same point in the countdown), wouldn't that pretty much rule out anything random? We already know it's not the sender sending the signal- they aborted, or will abort (what's the syntax when talking about reversed events, anyway?)- and it's doing it at the same time, consistently, so it's not random or equipment failure.

What does that leave but for the signal to be coming from a universe in which it actually was sent? And, if that's the case, do the recipients in both universes receive the signal, or is one guy somewhere scratching his head, wondering where "his" signal "went"?

The scary part is, if they can figure out how far back in time the signal goes, and then abort in between, a causality violation becomes testable. Which, again, I thought was Impossible.

Wow. Really really cool stuff in any case :thumbsup:

edited to add: hang on, I just considered something. What if causality violations themselves are impossible because of other universes?

Suppose you were to actually try to answer the old "go back in time and kill your grandparents" question. Might you, in essence, actually be leaving this universe- one in which you have a past and exist as an adult- and going into another universe, landing at a point on the timeline prior to your birth? In such a scenario, it wouldn't be a causality violation for you to kill your grandparents and still be around, because in a real sense they're not "your" grandparents- and never are. They're the grandparents of your other-self, and killing them doesn't kill you. Observers in both universes see no change in events; in the one you left, your grandparents lived, and you grew up; in the one in which you arrived, you were never born; however, because the self that killed your grandparents (more properly, the grandparents, because they're not, strictly speaking, yours) came from a different universe, causality remains unviolated.

Could the existence of multiple universes solve the "problem" of violations of causality?

[DireStrike]

8. I imagine that you will not receive the signal if you would abort it.

This is retrocausality, not acausality.

You'd sit there, saying, "Ok... gonna run the test any second now and abort it immediately..."

This is similar to having a normal signal sending device, and saying "Ok, if I send the signal 5 seconds from now I'll get the result in 5.1 seconds..." and then you decide to abort, and 5.1 seconds later there is no signal.

It would be a revealing test, provided we can get the thing to send signals back in time in the first place.

[Speck Tater]

5. The Endochronic Properties of Resublimated Thiotimoline.

Read and and learn that you don't mess with time without drastic consequences.

http://homepage.mac.com/jhjenkins/Asimov/Stories/Story062.html
http://en.wikipedia.org/wiki/Thiotimoline

[AlienGirl]

7. Fiziwig!! I haven't seen you in a while!

:hi:

[drm604]

9. That article seems to me to be slightly confused.

Near the top it states

That's science talk for saying he hopes to find evidence of a photon going backward in time.

But, reading further down, the experiment they propose would not show that photons travel back in time. Instead it would show that one photon can somehow send information to another photon in the past. That's still fascinating (if true) but it does not equate to "evidence of a photon going backward in time".