welcome to part three of a little screed originally provoked by this admittedly tongue-in-cheek new york times article blaming snafus plaguing our expensive new large hadron collider on gremlins from the future. all cheekiness aside, stories like this serve as fodder for the public's mindless love of the three great fictions of science fiction. part one raided the star wars' intergalactic cantina. part two pulled the plug on star trek's warp drive. today we take a time-out on time-honored time travel.
actually, "trek wars" fans get a bit of good news this time: time travel is possible. but that's good news only if you're looking for a one-way ticket into the future, because there ain't nothing else on the itinerary.
as pointed out in part two, gravity warps space, and since space and time are initmately bound (whence the term "space-time"), gravity also warps time. this has been demonstrated with atomic clocks, which run slower under gravity's influence. take this idea to its conclusion and you can "time-travel" by simply parking a spaceship next to any gravitationally intense object. neutron stars, white dwarfs and black holes are perfect. as you bask in the glow of the gamma ray death scream of interstellar matter spiraling past you into oblivion, events on earth will appear to flit by, but to those on earth monitoring your ship, you'll have entered a state of essentially suspended animation.
but wait — what if there are no black holes in the neighborhood? don't worry, einstein's theory of special relativity demonstrates that the effects of high-speed motion (acceleration) can simulate the effects of gravity for the traveler. we all experience this anytime we ride in a vehicle. when we speed up, we're pulled into our seat; when we cruise, we feel nothing (other than the normal pull of gravity, which we typically ignore), as we do when standing still; when we slow down, we're pulled out of our seat. so in lieu of finding a black hole, you can "time-travel" by simply stepping on the gas and not letting up. as you eventually approach the speed of light, you'll seem suspended in time to those left behind as your existence is extended thanks to relativity.
still, neither of these scenarios represent the sexy type of time-travel that "trek wars" fans love dreaming about: where they get to undo or avoid some remorseful event in the past, play the ultimate stock tip or become a b-movie actor:
sorry to disappoint again, but there ain't no going home. (and obviously not in 2004!) not only are there just as many practical theories about time-travel as there are about warp drives — that is, exactly bupkis — there's no evidence that time can travel backwards or that we can travel into the past. time's arrows fly in only one direction.
consider the definition of time: the interval between two events. if correct, in order to reverse time, we must reverse the events. consider a glass bottle, tossed from your hand to the trash bin. it hits the rim and shatters on the floor, sending pieces big and small everywhere. reversing time is therefore a humpty dumpty act; a time machine would have to retrace the trajectory of every shard as well as the trajectory of the intact bottle as well as the trajectory of your hand — that's a bit of a workout, isn't it?
actually, more than you can imagine, considering that your hand, the glass, the trash bin, the floor and the air surrounding them are all composed of a ginormous (you just knew that word was coming, didn't you?) menagerie of subatomic particles, all in constant motion and interaction. suddenly, the workload on our time machine just went up exponentially: it has to retrace the changes in the spin, vector, orbit, charge, vibration, etc., etc. — that is, every characteristic we can name, including those we have yet to discover — of every particle in that ginormous cloud of particles making up every object we're sending back in time. the machine would also need to isolate the cloud of particles comprising you the operator from its effects (it wouldn't do you any good to go back to 1976 and not remember that leisure suits actually sucked!), no small task if we have to account for the air moving in and out of your lungs, the hair and dandruff falling from your scalp, and the dust mites in your eyelashes!
now that's a workout! imagine trying to send a city or the entire planet back in time. and i suppose it would be just rubbing it in to point out that you wouldn't be able to send anything back before the date you first turned on your machine. (you wouldn't have any data!)
needless to say, no one has a clue how to reverse the order of events in our universe. relativistic physics allows us to play games with our perception of events, because our perception changes as we change our frame of reference, which affects when the light that originally captured the events finally reaches us:
changing our frame of reference changes the apparent distance (time) between the dots (events) on the timeline without affecting their order.
but just changing our frame of reference is like playing games with a film projector. speeding the film up, slowing it down or trying to run it backwards just ain't the same as manipulating the actual events it portrays — no more than photoshopping a image of yourself will get you back your hairline. that's voodoo. the two have nothing to do with each other. we might be able to play with the projector, but time's arrows fly on, unperturbed.