I want to touch on this experiment.  It’s very cool.

The article is well written and should explain everything to you.  What I want to talk about is the general concept and the specifics of the science.  I’m going to ramble a bit since this is an interesting notion and covers a great deal of territory.  Prepare for a headache.

Put simply, the researcher is trying to send a message back in time utilizing an aspect of quantum mechanics called “retrocausality.”  Essentially, that’s the idea of having an effect precede its own cause.  An example would be winning the bet on a horse race before you arrive at the track.  Or, as the article puts it, something like “arriving at the train station before you’ve left home, [or] winning the lottery before you’ve bought the ticket, [or] graduating from high school before you’ve been born — or something like that.”  Something like that indeed.

While the experiment in question will be scientifically fruitful even if it fails, the premise is intriguing and wrought with all manner of fantastic—and terrible—consequences.  It would not be an immediate key to unlock communicating with the past.  On the contrary, this is a special setup where the receiving equipment is specially designed for the sending equipment, and both have to be functional for even the remotest hope of success.  Still, it’s more than fascinating.

Before anyone makes claims that this is pure science fiction (which would indicate you’d not read the article since it does a great job of explaining the science), let’s consider the basics.

Einstein’s relativity already makes clear time travel to the future is not only possible, it’s implicit in the laws of the universe.  Even if his entire theory was overturned and replaced by something else, the new explanation for the macroscopic cosmos would have to include the same temporal aspect.  Why?  We already have proof it’s true.  The best example is this: Assume a person is standing at the foot of Mount Everest and another person is standing on its peak.  For those two people, time moves at measurably different rates.  Specifically, time moves slower at the mountain’s base than it does at its top.  The difference for a mountain is insignificant due to the scale, but it is still measurable.  The same is even true for something like a water tower but is very much smaller.

The reason for this is gravity’s effect on time.  Relativity defines the relationship between gravity and time as, in simple terms, the closer you are to a gravitational force, the slower time goes; and, the more powerful a gravitational force, the more it manipulates time.  This is why a black hole could theoretically stop time altogether in its local neighborhood (the same would be true for any other significantly gravitational mass).  It’s also why someone at a higher altitude is living in a faster time than a person at sea level.  Their distance from the Earth’s center of gravity defines the speed of time they observe.  Gravity warps space and time, and space and time are the same fabric from which our universe is sewn, and that means gravity’s effect on time is measurable.

Relativity also shows us that speed impacts time.  The faster you go, the slower time moves for you.  That’s why the classic example is identical twins: One twin stays on Earth while another travels at near light speed in space for approximately a year from his/her perspective.  When the twin in space returns to Earth, the other twin will have aged at a dramatically faster rate and they will no longer be temporally identical (of equal age).  Again, this is known to be true given experiments that have proved this aspect of Einstein’s theory.

Eventually, as we learn more and advance our technology, I suspect it will be possible to design a system that would take advantage of this inherent time travel mechanism (via either route) in order to move ahead into the future.  Whether that be by manipulating gravity or utilizing unimaginable speed is anyone’s guess.  I suppose it could also work by other means that we’ve not yet discovered.

Now, what all that means is time travel to the future is possible in more than one way.  The quandary comes from time travel to the past.  The physics behind that premise appear to make it impossible—although not quite.  Beyond Newtonian and Einsteinian physics that make time travel into history quite difficult, we have quantum mechanics.

Quantum phenomena essentially break all the rules of the macro universe.  For instance, they do not follow the laws of gravity as we know them.  What makes a planet orbit a sun or a moon orbit a planet simply does not apply at the quantum level.  That’s true of pretty much all relativistic physics (anything above the quantum level).  Therein lies the joy inherent in the search for a theory of everything, and therein lies the greatest challenge: Defining via science a universe that is explainable at both the Einsteinian and quantum levels has thus far proved impossible despite the safe assumption that they are both aspects of the same laws (they do, after all, both occur in the same universe, right?).  The point being that quantum mechanics is a whole new way of seeing the universe that apparently bends all the rules—or, more accurately, breaks them with ease.

So what does that have to do with this article?

Relativity and current theoretical physics (excluding quantum theory) preclude all but the most remote possibility of time travel to the past.  To the future?  Sure, Einstein proved that and you can see it for yourself right here on this small blue planet.  But to the past?  That’s a horse of a different color, and it’s one that so far has calculated to an almost absolute impossibility.  Not quite impossible since we’re still learning, but close enough for what we presently know.  So in steps quantum mechanics.

Since quantum phenomena seem to break every other rule of the known universe, why not that one?

Well, don’t hold your breath.  Even quantum mechanics does not as yet appear able to breach some of the accepted laws of the universe, and that is especially true when it comes to time.  But we don’t know everything and are still learning, so there’s always a possibility.  We’ve already seen some quantum phenomena that appear to move backward in time.  Let me caveat that by pointing out this little quantum truth: It’s impossible to directly observe quantum phenomena at our present stage of scientific and technological advancement.  The nature of quantum mechanics means directly observing such a phenomenon actually changes its outcome and state, so we can never truly see what’s happening—at least not directly (this is the property that makes quantum encryption 100% secure and impossible to break, since intercepting or even third-party observation of such a message would by its nature change the content of the message itself).  Look into Schrödinger’s cat if you want an example of this (among other quantum properties).

And if the experiment works?  That leads us to the philosophical idea of temporal paradoxes.  If an effect can precede its own cause, imagine what could happen.  Again to use the best example, we go to the grandfather paradox.  The idea is this: A grandson travels back in time and kills his own grandfather before his father was conceived.  What happens?  The grandfather, the “cause” in this case, has died before the grandson, the “effect” in this case, so how can the grandson have ever traveled back in time and killed his own grandfather if he wasn’t born?  Therein rests the headache of time travel: What really happens when you tinker with the past?  Does the present unravel and correct itself to coincide with its new history?  Does some “break” occur in time and a new universe follows the new history while our present remains as is?  Or does the universe simply implode or explode?  Or something else?

Considered in the scope of the experiment, what if someone were to send a message back to Hitler giving him the information he needed about his enemies’ plans so he could outmaneuver them?  Or what if the Babylonian Empire was sent a message about its eventual downfall and how to avoid it?  Or what if it’s something as simple as a message sent back to an individual who died telling them not to board that particular airplane on that particular day?  Can you beat fate?  If you do, what happens to the rest of us?  How does making one very small change long ago eventually define what we call the present?

Chaos theory shows in no uncertain terms that the smallest variable in an overwhelmingly large equation can have devastating repercussions on the outcome if it’s changed in even the tiniest of ways (one need only look at atmospheric physics to understand that).  This is called the butterfly effect (more accurately, sensitive dependence on initial conditions).  It’s easy to demonstrate (even if one hasn’t seen Jurassic Park).  Put a small droplet of water on your hand and see where it goes.  Now, dry your hand and put another drop of water in exactly the same spot.  Does it follow the same path every single time?  No.  That’s the butterfly effect: the idea that a butterfly can flap it’s wings in Brazil and thereby be specifically responsible for producing or preventing a tornado in Japan (or anywhere else).  The smallest change in a dynamical system can have a drastic impact on that system’s later state.

So it is with the idea of time travel to the past.  A paradox is only one example.  Even the smallest change one hundred years ago could mean a very different present.  The further back you go, the more likely it is to dramatically change the equation as it moves forward (since a small change would have more time to interact with more variables as the system progresses).  Going back to yesterday and killing a mouse would have less of an impact on today than would going back one million years ago and killing a mouse.  That mouse and its role in history would be more dramatically represented over a million years than would the same mouse over one day.  Here’s a way to look at it: You go back one million years and kill that mouse, but that mouse was the food source for one of humanity’s ancestors who will now go hungry and have to search elsewhere for dinner; how does that change impact the future of our own species?  There’s no way to know, but it’s easy to conclude the change is greater as you move further into the past, and that regardless of how small the impact in history (think of an amplifier; the more time that passes, the more a tiny little change gets amplified).

Ultimately, this experiment is so wonderfully enticing and frighteningly worrisome.  It’s the same feeling Einstein had when he unleashed the atom for the American military.  It’s the natural progression of knowledge coupled with the concern over how that knowledge could be used.  We can’t stop progress out of fear, though, and I would argue that with anyone.  Stagnation is the enemy.  “Fear is the mind killer.”

Okay, do you have a headache yet?  I hope not.  Instead, I hope you’ve picked up a tidbit or two, but more importantly, I hope you’ve seen how far we’ve come in science and how much we know and how much more we don’t know.  Finally, I hope you see why I’m so excited about this attempt to send a message into the past.  If it doesn’t work, we learn a lot of very important stuff.  If it does work, we learn a lot of very important stuff—and we open a huge door to possibilities we’d never considered before.

[via Phil; this was originally going to be part of an open thread but it rapidly grew far too complicated for just a cursory mention; sometimes my brain wants to run through an idea no matter how much of a mess it creates in the process]