Some thoughts about warp drive

A recent article posted by Gizmodo, titled “NASA Starts Work on Real Life Star Trek Warp Drive”, has stirred up a bit of excitement, and rightly so. Unfortunately, as with so many science-related articles, it’s big on hype and over-promises with both its headline and article text.

First, this isn’t new. In 1994, Miguel Alcubierre published a paper¹² detailing how we might construct a warp drive within the framework of general relativity. White (the NASA scientist mentioned in the Gizmodo article) is building on Alcubierre’s work, as others have (Van den Broeck, Krasnikov, et. al.).

The trick with a warp drive, or any space-time distortion within the framework of general relativity, is that it requires negative energy. What the hell is that? Just as matter has energy equivalence (E=mc²), “negative matter” has negative energy equivalence (-E=-mc²). We have circumstantially observed negative energy effects³, but only at tiny, tiny quantum scales.

When Alcubierre first formulated his warp drive (which, to be more precise, was a specific set of parameters fed into the field equations of General Relativity to attain the desired spacetime “metric” that would create a warp field), he realized that the (negative) energy cost of the drive would be greater than the mass of the entire universe⁴. Many scientists, inspired by his work, have come along since then and proposed improvements that have brought this requirement down⁵. This is what White is talking about with his “500 kg” figure.

The mass-energy equivalence of 500 kg of matter works out to 4.49×10¹⁹ Joules, or the equivalent of 10,740 megatons (10.7 gigatons) of TNT. In 2008, the world’s energy consumption was 4.74×10²⁰ Joules, so it represents about 10% of the global energy production. Not an unattainable number, but still enormous–especially when it’s meant to be concentrated in one spot (a spaceship).

Assuming we had the ability to harness and channel that much energy, and assuming we could “flip” it so that it was negative energy rather than positive⁶, there are still some unresolved questions. The “great” thing about a warp field is that you’re moving spacetime, not an object through spacetime, so you don’t have to “obey” the speed of light. Light within your pocket of spacetime moves as fast as light always has: way faster than you. Light outside your pocket of spacetime moves as fast as it always has, too: way faster than you (outside of that pocket). The pocket itself is what moves faster than light.

But…that still means it’s possible for an observer to “see” you arrive before you leave, thereby introducing all sorts of time travel/causality issues. Causality is fundamental to, basically, everything we know about anything. Thing one happens, causing thing two to happens. Exceeding the speed of light — the speed at which information travels — breaks this. Thing two happens, then thing one happens and causes thing two. What? It doesn’t make sense; it’s not how the universe works. There is mounting evidence that, in fact, the universe must work with causality intact⁷⁸, based on some really niche quantum mechanics stuff. This, alone, isn’t necessarily reason to lose hope, though. There are a number of scenarios and/or limitations wherein it’s possible that the nature of the warp field might distort space in such a way as to prevent “light cones” (descriptions of space and time relative to an observer) from inverting. TBD.

Finally, when one travels through space, one collides with all the little particles of dust out there. Space is unfathomably empty, but there are still a lot of them. Think about riding a motorcycle and bugs splattering on your helmet. Now, imagine that your motorcycle is traveling ten times as fast; those bugs just turned into bullets, and you’re dead if you hit one. Now, imagine you’re traveling faster than light and, rather than the bugs splatting against your windshield, they’re getting “caught” in the warp field’s leading edge as you travel through space. This particulate matter converts into energetic photons and other, more-exotic particles, at once, building up a doomsday radiation wave in front of your ship. When you get wherever you’re going, and shut off your warp field, this radiation wave is set loose. Everything in front of you, at the very least, is wiped out by what amounts to a supernova. It’s entirely possible that it isn’t directional, and instead just detonates — taking you with it.

So, yes, hurray! for serious reserach into FTL, and for not being satisfied with the speed of light being the absolute maximum speed at which one can traverse the universe. But just because NASA’s working on it, don’t kid yourself: this is a puzzle with a lot of really huge hurdles to overcome, some of which may be literally insurmountable.

And if they do build it, it might look like this.

1. Alcubierre’s original paper on arXiv [↩]
2. The more digestible wikipedia article on the “Alcubierre drive” [↩]
3. The Casimir effect [↩]
4. The paper detailing the issue with needing ridiculous amounts of negative energy [↩]
5. Chris Van Den Broeck, Serguei Krasnikov, et. al. [↩]
6. We have no idea where to even begin with this one; we don’t even know enough to say if it’s possible or not! [↩]
7. Novikov’s self-consistency principle [↩]
8. Hawking’s chronology protection conjecture [↩]