On Interstellar Travel
Part 2 of 3: Can we Break the Light Barrier?
This is Part 2 of the three-part series “On Interstellar Travel”, written to celebrate the 45th anniversary of the Moon landing. In the previous installation I discussed slower-than-light interstellar flight. Today we make the faster-than-light (FTL) plunge!
Under Einsteinian physics, nothing can move faster than light with respect to spacetime. However, spacetime itself can move as fast as it wants to. Shortly after the Big Bang, the universe expanded much faster than light. Therefore, even with “realistic” physics, FTL travel is at least somewhat plausible.
The Star Trek style “warp bubble” is one of the most enduring faster-than-light concepts in science fiction. A starship doesn’t move inside its warp bubble, so it doesn’t need to worry about time dilation or other relativistic effects. The warp bubble itself moves at speeds much faster than the speed of light.
In the 1990s, Miguel Alcubierre developed a mathematical theory that supports faster-than-light bubbles in Einsteinian space-time. Dr. Alcubierre’s academic paper refers to “the warp drive of science fiction” as inspiration. In fact, it’s directly based on Star Trek. Interestingly enough, ever since Alcubierre’s rise to fame, many modern sci-fi authors have equipped their starships with “Alcubierre drives”. This places the Alcubierre drive in the same hallowed position as cyberspace, a science fiction concept that inspires a real-world concept that inspires more science fiction. This image may be the ultimate circular reference: NASA’s concept art of a “USS Enterprise” powered by Alcubierre drives based on warp drives based on Star Trek.
Now, Alcubierre’s original theory was explicitly impossible. Generating the warp field required obscene quantities of “exotic matter” and “negative energy”, and there was no way to steer the warp field. However, since the Alcubierre drive is purely theroetical, it’s possible that tweaks to the math could greatly decrease its energy requiements.
Alcubierre and Star Trek disagree in one major respect: what happens to matter (or light) entering and exiting the warp field? Trekkie ships routinely engage in warp-speed combat, slinging phasers, disrupters, and photon torpedoes without dropping out of warp. That wouldn’t work with a “realistic” Alcubierre field – the edge of the warp bubble is an area of severely distorted space, much like the event horizon of a black hole. Any energy or matter passing through the edge would be severely distorted if not destroyed. This should affect communications as well, unless your communications signals exist in a parallel dimension (ie subspace communicators).
The characteristics of a warp-based interstellar civilization would depend on just how fast their ships, and their communications signals, could travel. In pre-JJ Abrams Star Trek, ships took days to weeks to travel around Federation or Klingon space, and much longer than that to cross the galaxy. However, you could have a real-time conversation with a Starfleet admiral from very far away. This allowed the major Trek powers (Feds, Klingons, Romulans, Cardassians etc) to build well-coordinated interstellar empires, while still preserving a sense of distance. Isolated backwater worlds could exist in pre-Abrams Trek because distance was actually meaningful. Unfortunately, in the post-Abrams universe the Moon appears to be in low Earth orbit, and Qo’noS is just a few miles further. Most illogical.
Does a Warp Drill Pierce the Heavens?
When does the fantasy stop making sense?
“Hard science fiction sticklers” are very intelligent people who rank higher on the evolutionary tree than the rest of us. You know this is true because they have highly sophisticated brainstem reflexes. After all, they roll their eyes as soon as they hear “faster than light travel”.
So why is FTL such nonsense? Well it contradicts our current understanding of science, but that shouldn’t be a game-breaker. After all, the whole point of sci-fi is to show speculative technologies. However, one of the persistent complaints about FTL travel is that it seems to require the existence of time travel. Hard sci-fi fans recoil in horror at the thought of time travel, as it inevitably leads to silly logical inconsistencies.
Actually, indiscriminate use of FTL travel could cause logical problems even worse than time travel. Let’s try out some science-fanfiction: a thought experiment in the setting of Star Trek (The Original Series).
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Warning: Physics Ahead!
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Captain Bob of the USS Paradox leaves the Earth at 8:00AM traveling on maximum impulse power, a speed of 99.5%c. At 8:20AM, Captain Bob suddenly realizes that he forgot to lock his space-car door. So he orders Commander Spock to turn around and head back to Earth at Warp 9.
Traveling at 99.5%c causes 10-fold time dilation, so when Captain Bob turns the starship around at 8:20AM, only 2 minutes have passed by on Earth. Since it takes time for Earth’s light to reach Bob, if he looks at an Earth clock it will show “8:01AM”.
“We have re-oriented and are ready to engage warp drive,” says Spock.
Warp 9 is around a thousand times lightspeed so it takes just over a second for Captain Bob to get back to Earth. The Earth clock now reads “8:02AM”.
“That’s funny,” says Captain Bob, “If only two minutes have elapsed, then I can’t possibly have traveled further than 2 light-minutes.”
Commander Spock points at something behind Captain Bob’s back. “Look behind you.”
Captain Bob looks back in the direction he came from. Sure enough, the USS Paradox appears just one light-minute away. “That’s odd. If my ship is out there, and I’m also right here, that means we’ve duplicated ourselves.”
Spock nods. He knows the feeling.
“But if that’s so, then at some point in warp flight we must have gone straight through a past version of ourselves.”
Spock slowly raises a single eyebrow. “That is most illogical. Two objects cannot occupy the same space at the same time.”
“Oh, you’re rig…” The Captain faces the camera with the wide eyes of a cartoon coyote who’s just realized he’s standing on thin air. Then his entire starship explodes in a glorious blast of illogic.
Less than three millparsecs later, the Millennium Falcon sails gracefully out of the still-glowing fireball. “Whew, that was a close one Chewie!”
This paradox is illustrated below:
Solving the FTL Paradox
Cutting the Einsteinian knot
So from a naive perspective, FTL seems completely impossible. The existence of a warp drive would cause collisions throughout space and time, logic-eating paradoxes that could fundamentally alter the rules of the universe in crazy and unpredictable ways. For example, the travel time between Earth and Qo’noS could inexplicably decrease from several weeks to a few minutes. Oh wait, I already mentioned that one.
That said, there is one easy way to immediately banish all FTL paradoxes: Do away with Einsteinian relativity.
Relativistic paradoxes only occur because there is no “correct” (aka “absolute”) frame of reference. If an absolute frame of reference exists on some cosmic level, then you can easily prevent any time travel or paradoxes. Let’s go back to our previous example, using a cosmic background frame that is stationary with respect to the Earth.
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Since Captain Bob is moving with respect to the cosmic background, he experiences time dilation and the background does not. So when Bob’s clock reads 8:20 AM, the cosmic clock has advanced by 200 minutes and reads 11:20AM. Bob has traveled 199 light-minutes in the cosmic reference frame, but due to time and length contraction this is only 19.9 light-minutes in Bob’s reference frame. In Bob’s reference frame, the Earth clock only reads 8:01AM, the same as in the first example.
Bob slaps himself in the forehead. “Oh crap, I forgot to lock my space-car door.” He reaches for the space-fob on his space-keys. “Commander Spock, turn this thing around. Maximum warp, engage.”
When the USS Paradox engages warp drive, it travels 199 cosmic light-minutes in 12 cosmic seconds. Since Bob is still under 1:10 time dilation, his clock only advances 1.2 seconds. It reads 8:20:01 by the time he reaches Earth. However, the Earth clock says 11:20:12AM – 12 seconds later than when Bob entered warp.
“Look behind you,” says Spock.
Bob looks over his shoulder and sees an image of the USS Paradox 100 light-minutes away. “Wait a second,” he says. “I’m still seeing a duplicate image of our ship. I thought that meant we could collide with ourselves?”
Spock shakes his head. “No, Captain. 201 minutes have elapsed here on Earth, and our trip only took 200 Earth minutes. It will take another 198 minutes for our light to catch up to us, but it’s only light. There is no duplicate of our ship out there.”
Bob visibly relaxes. “So there’s no way that we could run into our past selves?”
“Of course not, Captain. That would just be ridiculous.” Spock keeps a straight Vulcan face, but his human half is laughing on the inside.
* * *
As long as faster-than-light travel exists within an absolute frame of reference, individual people and ships can experience all the time dilation they want, but the universe will never see two copies of the same object in the same place at the same time.
There’s one big obstacle to getting rid of relativity: if an absolute frame of reference exists, it should be fairly easy to observe. Whatever direction the Earth is moving during the spring, it’s moving the opposite direction in autumn. If there is a fundamental cosmic background frame, we should be able to detect our motion relative to this background. In fact, the absence of a seasonal difference in physics is exactly what drove Einstein to invent the theory of relativity in the first place.
This non-observation can be “solved” by assuming that the absolute frame of reference only applies to objects in warp space. After all, if Bob returned to Earth under impulse drive, he’d experience the “normal” time dilation effects described by Einstein.
Of course, if a FTL starship has to follow totally weird laws of physics just to exist, it may require a more fundamental change in space-time than an Alcubierran warp bubble. Instead of trying to create a bubble of exotic space in the ocean of realspace, it may make more sense to throw your entire starship into a different dimension.
This concept is best described as Hyperspace, and will be the subject of Part 3 of this article. (Thanks for reading!)