* (Warning: long post)
** (Krishno series will continue. No, really it will. I just needed to get a physics post out of my system)
In an earlier post, I'd talked about entanglement, where a photon could affect the behavior of another (its entangled counterpart) even when separated in space. Bear with me while I dial up the geek factor now.
In our macro world, every event we can conceive can be traced back to its origin by simply following a trail. As an example, consider flipping channels on a TV using a remote control. Seems pretty fancy - two bodies, a distance apart, affecting each other without an apparent connection between them. But if you consider that the remote emits an infra-red light that propagates through the air and is then sensed by the TV, the resultant changed channel becomes an anticlimax. It reduces to a simple chain of events that occurred across connected media (remember determinism?). No entanglement here.
In the quantum world though, entanglement is a lot like that J. Chan movie about a set of twins (Twin Dragons - youtube it), where when one of them gets hit, both get hurt. But in reality, this sort of thing has only been observed at the scale of quantum particles. It's a leap, but imagine if we could extrapolate this observed effect from the quantum scale to our macro one. It hasn't been done yet, but no physicist will admit it's improbable - that's just going to cut her research grants.
Before I get to the good parts (about the time travel), here's a quick swipe at Einstein. The Einstein-Podolsky-Rosen argument glosses over this phenomenon of non-locality - where a particle influences another particle in some different locality through no quantifiable connection - when claiming that the quantum-mechanical description of physical reality is incomplete. It gets interesting when we bring Einstein's special relativity (STR) into the picture. The essence of STR is that no particle with mass can be accelerated to the speed of light. This limits every conceivable action (save human imagination) to below the speed of light. This includes the transmission of information. If information were to travel faster than light, it'd have to bend space.
If a bit/byte were to achieve such a speed, STR stipulates that the object carrying this information would become simultaneously infinitely massive and contracted in length. As a kid, I wondered why Einstein didn't just say "nope, not possible, not happening". Why these disclaimers about mass and distance? I guess that's just how these physicists roll - Minimizing culpability at every step, aka covering your behind. At that speed-of-light point, the problem would become cyclical, where the energy needed to accelerate this infinitely massive object to c would itself be infinite, i.e. more than all the energy in the universe.
Anyhow, with entanglement, information transmission takes on a new shape entirely. A bit can be transmitted non-locally and instantaneously by manipulating a particle at point A and reading the effect of this manipulation on an entangled particle at point B. For any non-zero distance AB, the speed of this transmission is higher than c (speed of light). Einstein is on such a high pedestal though that physicists are creating new frameworks that'll allow for STR and the phenomenon of entanglement to co-exist. It'll be abstract, possibly math-intensive, and won't really make much sense, but atleast the house of Einstein will remain undisturbed. See Bell's inequality (technical).
Ok, now, about the time travel. I lied. I still don't see how it's possible. What is possible though is just as interesting, so stay with me here.
So far, we've talked about the non-local interaction of two particles as it applies across the first three dimensions (length, breadth, height). As observers, we humans are limited to just these three dimensions, and to an extent, the fourth (time) that we exist in. However, I haven't seen any research showing evidence that these entangled particles can't straddle even this fourth dimension while influencing each others' behaviour. After all, time is nothing but another dimension…just the way you can separate particles by a distance (length) and then by a distance along another axis (width – creating a diagonal) and a third – height, it's a reasonable extension to separate two entangled particles by time, the next higher dimension.
Consider then a situation where we know that a particle under our control has an entangled counterpart at a different point, not in space, but in time.
Even if we can’t physically travel back or forward in time, we can have this entangled particle exist in the future or past, by introducing a lag into the process that creates the entangled particle. Particle 1 is spit out at time t, and Particle 2 at time t+t1 (future).
If we can now make that second particle interact with its surroundings by messing with the original - we have a way to change history or to chart the future. I won't bother animating the possibilities.
*Update (Sep 30, 2009): Quantum entanglement visible to the naked eye
*Update (May 23, 2010): Teleportation over 10 miles