There is probably no issue more vexing to sci-fi authors that FTL travel. That is, to the extent we care about the ‘sci’ and not just the ‘fi’. After all, talking-head aliens who speak perfect English might be unlikely, but they don’t contravene any fundamental laws of physics. FTL travel does.
So do we have resign ourselves to a future trapped with a senile star that’s going to bake our planet to the ‘cooked-through’ stage in maybe a little as 100 million years (give or take, and depending on which model of stellar evolution you believe in)?
Well . . . maybe.
Let’s start by saying the theory of general relativity is a little goofy. It’s maybe a bit too—well, relative. We all know that time goes forward, but general relativity doesn’t: it can’t tell which way time goes, nor does it care. This creates problems with simultaneity, with causes problems with causality. “It’s not my fault! Really! I didn’t cause anything cuz, well . . .” And then there’s the Twin Paradox. (Look it up.)
Then there’s the whole quantum gravity issue: quantum mechanics and general relativity don’t mix. There have been some bold attempts and they are on-going, but they tend to have this problem of telling us what we already know in new, strange, and incomprehensible ways. For example, String Theory predicts the mass of the electron. I can predict the mass of the electron too—I get on-line and look it up before I go measure it. The point isn’t to diss people much smarter than we are who are working on String Theory. The point is that a theory that ‘predicts’ what is already well known is not nearly as useful as one that predicts things that are unknown. Just about any theory can do the former, if you give it the right inputs (which is to say, it’s easier to find the answer if you know it beforehand).
So String Theory may get us there someday, but we are not there yet. There does seem to be, however, a fairly general feeling that the answer to the question about the ‘arrow of time’ and such are buried deep down in the quantum end of the business. Could FTL travel be there too?
We don’t know but every once in awhile along comes something about sneaky neutrinos or quantum entanglement. Now sneaky neutrinos teach us mainly to be careful in accounting for the latency in GPS and the speed of data transmission of our cabling, which matters when stalking sneaky neutrinos. But quantum entanglement is different—and don’t expect us to explain it well (or much at all).
The gist is something like this: quantum states can apply to a system of particles, not just the individual particles within the system. That is to say, the fact of being a system of particles implies those particles are entangled. Thus, if the state of one particle changes, so must the state of another, because the state of the system is what matters. And this change must occur instantaneously.
No speed-of-light nonsense here: no waiting while Particle A yells across the room (or the galaxy) to Particle B: “Hey Dummy! It’s your turn to be Up!” Nope, it just happens.
What to make of this? Nothing, is the usual answer. It’s information that can’t exceed the speed of light, and this state change arguably conveys none. Quantum states change all the time. So if I’m here, ‘staring’ at my quark, how do I know if the quark in Andromeda that it is entangled with caused the state change I’m seeing? (I say ‘staring’ because we all know observing something affects it, so maybe it’s my staring that’s mucking things up.)
Normally, all I could do is beam a message to the guy—or the thrint—in Andromeda, asking “Hey Joe! Did you diddle your quark at such-and-such a time?” and wait a couple million years for the answer. (Which is likely going to be yes, because quark-diddling is big fun for thrints.)
This is not useful.
But think of this: suppose you could ‘freeze’ a particle to the point where its state did not change very often and suppose you could watch it in such a way so as to not muck things up too much. And let’s imagine Joe is a lot closer—say a light-hour or two—so we chat if we want. And say Joe, sitting there with his particle that is entangled with yours, flips it really fast; much faster than the frequency at which we expect random fluctuations to occur with our ‘frozen’ particles.
So here I am, watching my particle and it goes:
– – – — — — – – –
And I go:
– — – — – – — – — —
Hey! We did it!
But can we? General relativity says nothing about freezing particles, and we have no idea what quantum mechanics says—or if it’s right. It is silly to say we know all about that—we don’t even have a quantum theory of gravity yet!
But sending Morse code to thrints isn’t likely to do us much good. So what about wormholes? They are a staple of sci-fi (including ours). Do they offer any hope? In theory, no—or mostly no. If you drop information (hence energy) in wormhole, two things happen: 1) the info is destroyed; 2) the wormhole collapses. They are not, as the physicists say, traversable, so they don’t break the Law because info can’t be sent through one. You see, Nature abhors a vacuum and wormholes, everything else.
Except not really. Nature is of course pretty much all vacuum and there is something wormholes like: negative energy. What the hell do you mean negative energy? we hear you cry.
Well, there’s such a thing as vacuum energy, and anything less than that is negative energy. (Look up the Casimir effect, if you dare.) And it appears (in the view of some) that negative energy can stabilize a wormhole and make it traversable.
But there are other sneaky tricks out there. Consider space-time. It is expanding. We know this. How fast? That’s is at first glance a silly question—there’s nothing ‘outside’ space-time to measure the expansion by. But, in principle, it can be any arbitrary rate (if there was something to measure it against). During the inflation phase that followed the Big Bang, the universe may have expanded at a rate that was essentially infinite (whatever that means).
But the point is that this expansion carries stuff along with it, just as an ocean current carries you along with it. With respect to the water you’re sitting in, you are not moving: you can only detect your motion by measuring it with respect to a ‘fixed’ point, like the shore.
Now, the Law says info can’t travel faster than the speed of light through space-time. It doesn’t say anything about how fast space-time itself can move. So just sitting there, in your comfy inner tube, you aren’t breaking any laws! “Really, officer—I wasn’t speeding! In fact, I wasn’t even moving!”
But can you move just part of space-time (the part with you and said comfy inner tube)?
Dr. Miguel Alcubierre, a theoretical physicist and expert on general relativity, thinks maybe so. He has postulated as much, saying it may be possible to expand space-time behind you and compress it in front of you while you sit in a ‘warp bubble’ (your inner tube). At what do you need to do this? Yep—negative energy!
Okay, so a bunch smart people say this will never work: Hawking radiation will fry you; the bubble will destroy anything in front of it when it ‘decelerates’, and so on. But these are engineering problems. Maybe you just need SPF 10100 sunscreen so you just get a nice tan and to be damn careful about where you hit the brakes. “Terribly sorry about your planet, ma’am. Won’t let it happen again!”
But we are out of the realm of violating fundamental laws and into the realm of solving problems. That’s a big, big step (except that—oh, by the way—you might go backwards in time. But probably not. Quantum mechanics may spank you—and throw your ass in a black hole; then the rest of you.)
Well, okay. But are ya gonna just sit there and be QM’s bitch? Nah, didn’t think so. Me neither.
What does this all mean? In a nutshell, wiggle room. For the species and especially for us poor ol’ sci-fi writers. And we’ll take all we can get.
One last thing—antigravity. We all love antigravity (that’s another thing general relativity seems to have dropped the ball on). Gravity is solely an attractive force; gravitational energy is positive. This means antigravitational energy must be negative. If we solve the whole GUT problem (no, not the one caused by an overindulgence of pizza), there’s gotta be antigravity in it, and thus oodles of negative energy!
So there you have it—wiggle room! See ya on the Other Side. (If you get there first, say Hi to the thrints—assuming you can get them to come out of hiding.)
I want to offer one final-final note, outside the snark-fest of the foregoing. The theory of general relativity is undoubtedly right, just as Newtonian physics is undoubtedly right. Newtonian physics works fine within its regime; outside the regime, things happen that cannot be explained, and are indeed inconceivable, under Newtonian physics.
The same can be said for general relativity. We know general relativity holds within its regime, and we know it doesn’t explain everything. There is a lager regime; one that (at a minimum) marries general relativity with quantum physics. Within this larger regime, what ‘inconceivable’ things might be possible?