... but not without a massive (no pun intended) assist from a fellow HuffPost commenter!
I'm getting ahead of myself (not too unusual when the topic is time travel). Let me drop back to the beginning: This all started when I saw Jillian Scharr's HuffPost article "Wormhole Time Machine Called Best Bet for Back-In-Time Travel," and could not refrain from posting a comment on how to use time-travel to realize the age-old dream of traveling way faster than light.
(This whole topic is of no little professional interest to me, seeing as how I've just started outlining the plot for my third science-thriller, Triploidy, which will deal tangentially with the Search for Extraterrestrial Intelligence, and an FTL drive has, for some reason, been anathema to SETI for virtually its entire half-century of existence.)
Anyway, to put you in the picture, here's that original post:
Time travel would be really, really useful. Consider the following scenario:
- take your time machine and wrap it around the hull of a spaceship;
- now set out for, say, the Andromeda galaxy (you'll want to move at some significant percentage of the speed of light to keep from aging too quickly to pull this off);
- travel, say, a light year Andromeda-ward; assume that takes you a year and a day;
- now turn on your time machine and go back a year and a day.
Voila! You've traveled a light year in no time (it's still the same time as it was when you first left Earth). Continue on in this fashion until you've reached Andromeda -- it's 2.5 million light years away, but as far as the universe is concerned you've reached it in no time at all!
So if time travel is possible, you can forget about Einstein's old speed-of-light limit -- you've leveraged your time machine to create an infinite-velocity drive!"
Little did I know that this innocent (albeit admittedly somewhat tongue-in-cheek) posting would rub some of my fellow Science Section commenters the wrong way. In fact, at one point there had been so many replies to replies and replies to replies to replies, each further indented toward the comment display's unmovable right margin, that I was compelled to stop on grounds that --
the Lorenz contraction of these comments has rendered me too skinny to tap a keyboard!
Well, it seemed pretty funny at the time.
Most noteworthy among my critics was one who styles himself "JBS -- part time misanthrope and full time curmudgeon." In the grand tradition of HuffPost commenters, JBS took me to task for any number of things, most trenchantly, my having grossly overestimated the maximum achievable Einsteinian time-dilation factor.
Let me unpack that objection a bit: Even though the trip to Andromeda described above would take no time as far as the larger universe was concerned, the same could not be said of me. I'd actually have to endure those two and a half million years (somehow), before flipping the switch that would cancel them all out. And, again, going back in time would only cancel out those eons for the outside universe -- I, on the other hand, would still have aged two and a half million years!
Well, I'm in relatively good shape, but nowhere near that good. Hence, my above proviso about moving "at some significant percentage of the speed of light." What that does is to invoke the effect predicted by Einstein's special theory of relativity, whereby time slows down as one approaches (albeit never passes!) light speed (or "c" as the cognoscenti call it). Admittedly, I'd have to travel at about 99.9999999999999 percent of c to bring the transit-time down to a survivable ~40 years, but what the hey -- it's a thought-experiment, right?
Unfortunately, Uncle Albert also predicted (and experiment has since confirmed) other, less beneficial effects of skimming in that close to lightspeed -- as JBS was quick to pounce upon: My scenario, he pointed out --
... doesn't allow for how much fuel you're going to need to accelerate to any significant portion of light speed. Your mass increases in proportion to the speed of light at the same rate time dilation increases -- that is as your time dilation increases, so does your relativistic mass
-- and the greater your mass becomes the more fuel that's required to accelerate that mass.
There, I thought I had him: A Bussard ramjet would solve the fuel problem handily, or so I believed. Proposed in 1960 by physicist Robert Bussard, the ramjet doesn't trouble to lug its fuel along, instead it scoops up the ambient hydrogen found floating in interstellar space and dumps it into a fusion drive.
No fuel on board, no excess mass problem -- right?
As JBS rejoined,
The faster the Bussard goes, the more efficiency it loses to Bremsstrahlung radiation so that while drag is increasing, thrust is decreasing.
Without going into the physics of Bremsstrahlung radiation, let me just admit it turns out that a ramjet may well max out at half the speed of light -- pretty respectable in terms of present-day technology, but nowhere near enough to see me through to the Magellanic Clouds as other than a dust of desiccated organic molecules scattered over the floor of my spacecraft.
To do myself justice, I'm not the only, or even the first, one to make this particular mistake. Check out Poul Anderson's classic science fiction novel Tau Zero for an entire book built around the premise that a ramjet drive is capable of accelerating at a rate equivalent to one earth gravity indefinitely.
And, as it turns out, Poul is not alone: Dr. Michael Minovitch's keynote to this past month's first Starship Congress was entitled "On the Possibility of Achieving Interstellar Space Travel at Near Optical Velocities." In it, Minovitch reckons that a ramjet might achieve 0.7 gravities of acceleration indefinitely, irrespective of drag. And he's a bad man to bet against: Back in the sixties, he invented the gravity-assist trajectories that later enabled the two Voyager probes to build up enough speed by their fly-bys of Jupiter and Saturn to leave the solar system entirely.
Be that as it may, though, I had to concede that a Bussard ramjet probably wasn't going to achieve the hair-off lightspeed needed to see me safely to our nearest-neighbor galaxy.
So, a cool, but ultimately unrealizable idea. Bummer!
But wait! (as they say in the 2 a.m. infomercials): there might be something salvageable from this commentary debacle. Sure, physical, biological me might not last out the time-machine-assisted trip, but a robotic starship might well.
What good would that do? Think about it this way --
It'd give you something almost as cool as instantaneous travel: namely, instantaneous communication! To achieve it, return to the original scenario, only minus the man! Now we're no longer trying to send our physical selves to Andromeda, or wherever -- we're just trying to send messages.
And that, it would seem, we can accomplish in zero time (allowing, of course, we've got a time machine). But, making that one small allowance, we can foresee a universe interconnected in real-time -- the equivalent of an intergalactic Internet, whose packets are time-traveling starships, messages-in-a-bottle that deliver their contents anywhere, in no time at all.
It's Star Trek's sub-space radio, only in normal space!
Never would have gotten there, though, without JBS's constructive criticism!
Now, all's we need is that time machine! ☺