On Interstellar Travel: Can we Break the Light Barrier?

On Interstellar Travel
Part 2 of 3: Can we Break the Light Barrier?

Zefram Cochrane, Is That You?
Protoss Corsair

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).

*   *   *

Warning: Physics Ahead!

* * *

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.

“Engage.”

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!”

“Arrhrhrhhrhrh!”

This paradox is illustrated below:

Trekparadox


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.

* * *

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!)

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On Interstellar Travel: Can We Reach For The Stars?

On Interstellar Travel
Part 1 of 3: Can we Reach for the Stars?

“You feel so lost, so cut off, so alone. Only you’re not.”
Contact

45 years ago, Neil Armstrong took one small step for (a) man, one giant leap for mankind. Over the intervening decades, it’s interesting to see the progress that mankind has made in outer space. As a species we have continued to leap forward, placing thousands of satellites into Earth orbit and sending probes all over our Solar System. Yet we have not taken any more small steps for man, or woman. There are no more bootprints on the Moon or any other celestial body than there were in 1972.

Manned space travel is difficult and perilous, and at the moment low-reward. Earth is the only Earth-like world in the Solar System; any colony we put on the Moon or Mars would require supplies from Earth just to survive. If you’re looking for a comfortable extraterrestrial world to live on, you’ll have to go interstellar. There’s a lot of ideas for how mankind could one day walk on an exoplanet – some realistic, some less so.

As part of the celebration of the 45th anniversary of the Moon landing, I’ve written up a series of three articles on interstellar travel. Today’s article will stick to (mostly) realistic slower-than-light travel options, while the next two pieces will delve into increasingly (but not infinitely) improbable modes of propulsion.


When I’ve Been There Ten Thousand Years
Traveling Much Slower than Light

Einsteinian space-time has three space-like dimensions, one time-like dimension, and an absolute speed limit of c, approximately 300,000 kilometers per second (kps). Nothing can move faster than c without also traveling backward in time. And since arbitrary time travel causes all sorts of logic-destroying stupidity, most scientists assume that time travel is impossible. Therefore, nothing can go faster than the speed of light.

In a realistic universe, it takes an awfully long time to get anywhere. The Apollo moon missions maxed out at around 11 kps relative to the Earth. Traveling to the nearest star would take 115,000 years at this pace. Actually, you’d never get that far. Starting from the Earth, the escape velocity of the Solar System is ~42kps. You’d need a considerably faster craft to ever exit the Solar System.

In the 1960s, the Orion nuclear pulse-rocket was “designed” as a deep space exploration concept. This starship would have used repeated thermonuclear explosions to push it at extremely high velocities (compared to conventional rockets). Such a craft could accelerate up to velocities of around 3%c. This would get you to Proxima Centauri in 142 years.

With much-slower-than-light travel, a journey between the stars will either require many lifetimes, or prolonged cryogenic freezing. Either way, all of your friends at home will be long dead by the time you reach your destination. And if people live on a starship for too many generations, they may eventually forget that they are on a starship.

*   *   *

Very-slow interstellar travel faces one major problem: Resource consumption. Where do you get fuel, water, and other materials while spending centuries between the stars? Every ecosystem requires light and heat, which means you have to generate energy, and energy is in short supply in interstellar space. Even nuclear reactors will run out of fuel during a thousand-year journey.

A Bussard ramscoop could gather interstellar gas for fusion power, but there’s not a lot of gas out there and it would be plain H-1. This is a much dirtier fusion fuel than He-3, and over the years would cause radiation damage to your fusion drive. You’ll burn most of the hydrogen that you collect just to create enough thrust to offset the ramscoop’s drag. And the ramscoop won’t collect any metals – if anything on your starship breaks, you can only hope that your ancestors brought a spare.

Some slow-starship designs completely bypass the energy problem by relying on laser energy beamed from Earth. This energy could be used both to propel the ship and to power its ecosystem. It’s certainly an elegant solution, as you could rely on an extremely large energy-producing infrastructure that doesn’t have to travel with your starship. But what happens when your benefactors run out of funding, are killed in a war, are destroyed by climate change or natural disasters?

The fact is that based on a present-day understanding of physics and engineering, a slower-than-light “generation ship” is really not much more realistic than faster-than-light travel. If we ignore the difficulties of energy generation and resource collection in interstellar space, we might as well ignore the rest of physics.

And let’s say someone develops a technology that allows a civilization to live forever without an external energy source – why would you even want to live on a planet at that point? Just stay in interstellar space.

In a universe where human civilization is limited to much-slower-than-light travel, there would be no such thing as an interstellar civilization. Humanity might eventually spread out to a bunch of stars, but each solar system would have its own unique way of life. The human colonies might communicate with each other, but they really couldn’t trade effectively, and no one could travel back and forth between different stars. There could be countless alien civilizations in the galaxy, but we might never encounter them because they are too far away.


Oh my God, it’s full of stars!
Traveling at near the speed of light

According to Einstein, funny things happen when you get near the speed of light. Time slows down. Distances get shorter. Mass gets more massive. A traveller moving at 99.5%c will experience 10-fold time dilation, length contraction, and mass increase. That means he experiences time passing 10 times slower than someone at rest. Relativity may sound funny, but it isn’t just empty theory – our entire telecom and GPS system is programmed with relativity in mind. If Einstein was wrong, then none of the technology you’re using to read this blog article would work.

Science fiction authors have played with the concept of time dilation for many decades, because it’s fun. An interstellar traveller may live for a normal human lifespan but witness thousands of years of galactic civilization in fast-forwards.

There’s one massive problem with near-lightspeed travel, and it’s mass. Well, it’s really energy, but we all know that’s the same thing. If you’re using time dilation to age 10x slower, that means you are also 10x as massive as you were at rest. If you were to stop moving, you’d need to shed kinetic energy equal to 9x your rest mass, a truly absurd amount. In order to get moving again, you need to gain an equally ridiculous amount of kinetic energy.

How ridiculous is this? Well, the rest mass of a 70-kg (154#) human is 6.3 exajoules. That’s equivalent to 1,500 megatons of TNT, or 3 times the total energy of every nuclear bomb ever detonated. Now imagine spending nine times that energy just to accelerate a single person to near-lightspeed. We haven’t even considered the mass of the starship yet!

Even with antimatter or black holes, it is very difficult (and highly dangerous) to come up with this kind of energy. Science fiction writers have either ignored the energy problem, or circumvented it with handwaving pseudophysics. (“It’s an inertialess drive!”) In Speaker for the Dead, Ender Wiggin wondered if a star winked out every time a starship started moving. (since the ship picked up a vast amount of energy without spending any energy)

Astute readers might wonder, if a starship can pick up energy ex nihilo, could it harness that energy to some other cause? At the very least, with enough energy you could completely destroy any planet you crashed into. Of course, if you had the technology to generate “free” energy, you may already have much more efficient ways to destroy a planet.

In a universe where travel occurs at near-lightspeed, there could be something resembling interstellar trade and travel, it would just be very difficult. If faster-than-light communication exists, it’s plausible that far-flung human colonies would stay in touch with each other, sharing the same Internet and the same entertainment and a similar culture. However, travelling to see another star system for yourself would require a major time commitment. Anyone you left behind at home would be much older by the time you reached your destination, or dead if your journey was too long.

Unless, of course, your interstellar civilization managed to dramatically extend their lifespans. Simple anti-aging and regenerative medicine techniques could keep human-like bodies alive for many hundreds of years, long enough to reach nearby stars.

However, if you wanted to tour the hundreds of billions of stars in the Galaxy, at 4 years per star you’d have to live a trillion years. Neither medicine nor mechanical prowess could keep a physical body functioning for that long. You could repeatedly switch bodies, but it’s better to transsubstantiate into an energy being. An energy being might think and act on a totally different timescale compared to biologicals. If your consciousness was slow enough, or your memory long enough, you could hold a conversation with your friends across the galaxy despite a 20,000 year lightspeed delay. At that point, you would definitely not resemble a human in any meaningful way.

Oh, what was that sound? I guess it was the rumbling boom that happens when you break the plausibility barrier. I believe that brings today’s episode to a close!

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Come back later for Parts 2 and 3, where I will delve into interstellar propulsion ideas less constrained by reality.