Warp Drives and Scientific Reasoning

A bit ago, the news streams were once again abuzz with claims that NASA was investigating amazing space drives that violate the laws of physics. And it’s true! If we grant that “NASA” includes “any person employed by NASA,” and “investigating” is defined as “wasting time and money thinking about.”

I say “again” because it was only a few years ago that news spread about a NASA effort aimed at a warp drive, a way to truly break the speed-of-light limit. Of course there are no realistic scenarios along those lines, so the investigators didn’t have any tangible results to present. Instead, they did the next best thing, releasing an artist’s conception of what a space ship powered by their (wholly imaginary) warp drive would look like. (What remains unclear is how the warpiness of the drive affected the design of their fantasy vessel.)


The more recent “news” is not actually about warp drive at all. It’s about propellantless space drives — which are, if anything, even less believable than the warp drives. (There is a whole zoo of nomenclature devoted to categorizing all of the non-existent technologies of this general ilk, which I won’t bother to keep straight.) Warp drives at least inspired by some respectable science — Miguel Alcubierre’s energy-condition-violating spacetime. The “propellantless” stuff, on the other hand, just says “Laws of physics? Screw em.”

You may have heard of a little thing called Newton’s Third Law of Motion — for every action there is an equal and opposite reaction. If you want to go forward, you have to push on something or propel something backwards. The plucky NASA engineers in question aren’t hampered by such musty old ideas. As others have pointed out, what they’re proposing is very much like saying that you can sit in your car and start it moving by pushing on the steering wheel.

I’m not going to go through the various claims and attempt to sort out why they’re wrong. I’m not even an engineer! My point is a higher-level one: there is no reason whatsoever why these claims should be given the slightest bit of credence, even by complete non-experts. The fact that so many media outlets (with some happy exceptions) have credulously reported on it is extraordinarily depressing.

Now, this might sound like a shockingly anti-scientific attitude. After all, I certainly haven’t gone through the experimental results carefully. And it’s a bedrock principle of science that all of our theories are fundamentally up for grabs if we collect reliable evidence against them — even one so well-established as conservation of momentum. So isn’t the proper scientific attitude to take a careful look at the data, and wait until more conclusive experiments have been done before passing judgment? (And in the meantime make some artist’s impressions of what our eventual spaceships might look like?)

No. That is not the proper scientific attitude. For a very scientific reason: life is too short.

There is a more important lesson here than any fever dreams about warp drives: how we evaluate scientific claims, especially ones we encounter in the popular media. Not all claims are created equal. This is elementary Bayesian reasoning about beliefs. The probability you should ascribe to a claim is not determined only by the chance that certain evidence would be gathered if that claim were true; it depends also on your prior, the probability you would have attached to the claim before you got the evidence. (I don’t think I’ve ever written a specific explanation of Bayesian reasoning, but it’s being discussed quite a bit in the comments to Don Page’s guest post.)

Think of it this way. A friend says, “I saw a woman riding a bicycle earlier today.” No reason to disbelieve them — probably they did see that. Now imagine the same friend instead had said, “I saw a real live Tyrannosaurus Rex riding a bicycle today.” Are you equally likely to believe them? After all, the evidence you’ve been given in either case is pretty equivalent. But in reality, you’re much more skeptical in the second case, and for good reason — the prior probability you would attach to a T-Rex riding a bicycle in your town is much lower than that for an ordinary human woman riding a bicycle.

The same thing is true for claims about new technology. If someone says, “NASA scientists are planning on sending a mission to Jupiter’s moon Europa,” you would have no reason to disbelieve them — that’s just the kind of thing NASA does. If, on the other hand, someone says “NASA scientists are building a space drive that violates Newton’s laws of motion” — you should be rather more skeptical.

Which is not to say you should be absolutely skeptical. It’s worth spending five seconds asking about what kind of evidence for this outlandish claim we have actually been given. I could certainly imagine getting enough evidence to think that momentum wasn’t conserved after all. The kind of thing I would like to see is highly respected scientists, working under exquisitely controlled conditions, doing everything they can to be hard on their own work, subjecting their experiments to intensive peer review, published in refereed journals, and ideally replicated by competing groups that would love to prove them wrong. That’s the kind of thing we got, for example, when the Higgs boson was discovered.

And what do we have for our propellantless space drive? Hmm — not quite that. No refereed publications — indeed, no publications at all. What started the hoopla was an article on a web forum called NASAspaceflight.com. Which sounds kind of respectable, until you notice it isn’t affiliated with NASA in any way. And the evidence that the article points to is — wait for it — a comment on a post on a forum on that very same web site. Admittedly, the comment was written by someone who actually does work for NASA. But, not to put too fine a point on it, lots of people work for NASA. The folks in this particular “Eagleworks” group at Johnson Spaceflight Center are a group of enthusiasts who feel that gumption and a bit of elbow grease might possibly enable them to build spaceships that do things beyond what the laws of physics might naively let you do.

And good for them! Enthusiasm is a virtue. Less virtuous is taking people’s enthusiasm at face value, rather than evaluating claims soberly. The Eagleworks group has succeeded in producing, essentially, nothing at all. Their primary mode of communication seems to be on Facebook. NASA officials, when asked by journalists for comment on the claims they leave on websites, remain silent — they don’t want to have anything to do with the whole mess.

So what we have is a situation where there’s a claim being made that is as extraordinary as it gets — conservation of momentum is being violated. And the evidenced adduced for that claim is, how shall we put it, non-extraordinary. Utterly unconvincing. Not worth a minute’s thought. Let’s get on with our lives.

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102 Responses to Warp Drives and Scientific Reasoning

  1. Tom Andersen says:

    While I have about the same level of hope as Sean about the viability of these drives, the theory is quite simple. In the same way that a car moves forward – by gripping the incredibly heavy earth – these drives claim to get grip on the entire universe. So the drive looks reaction-less in the same way that driving would look reactionless if the earth and the tires were invisible or hidden (say in some of the extra dimensions that give so many theorists pay cheques these days).

  2. Bill Jefferys says:

    The only interesting thing about this is: What mistake did they make?

    But as Sean notes, there’s not enough time or interest to follow this up to the point where we’d find out. We have better things to do.

  3. WJ says:

    I know better but I must admit that my heart went pitter-patter when this hit the news last month. Therein lies the trap. I so wish this to be true it temporarily fogged my judgement.

  4. John Call says:

    Typo, no real comment.
    Third from last paragraph reads “And what to we have for our propellantless space drive” I believe you meant “And what do we have for our propellantless space drive.”
    Good post.

  5. Michael Tiberi says:

    Oh, there just having a go. Cut ’em a little slack. Most of us knew it was bogus within a few seconds.

    But you just wait. When my portable worm hole generator goes online I’ll be laughing all the way to the bank. I just have to wait until Elon Musk makes me a lithium ion battery the size of Jupiter and I’m good to go 😉

  6. Lord says:

    Not a solar sail then?

  7. Mark says:

    maybe there is something to the EMDrive. There is a website http://emdrive.com/principle.html. There is also video of the device working on that web. The principle of operation is shared with the now common Laser Gyroscope.

  8. Ray Gunn says:

    Hopefully NASA will get back to its main mission of covering up evidence of alien artifacts, as recently reported by The Science(???) Channel.

    Reminds me that I’ve always wanted to ask a conspiracist how NASA managed to both fake the Apollo moon landings AND build a secret military base on Mars. But maybe the fake moon landings were filmed at the secret Mars base … this would explain the U.S. flags blowing in the “martian” winds, the multiple shadows which are actually sunlight reflected by the dual Martian moons, and the bouncy moon-walking which look suspiciously like a sped-up version of astronauts walking in a 1/3-G environment.

  9. Robert A Dorrough says:

    Incredible claims require incredible evidence, or something like that. You say Eagleworks and Lockheed Martin’s Skunkworks claim of a portable fusion reactor which fits on the back of a truck in 5 years comes to mind. They’ve got 4 years and 4 months to make good and the clock is ticking. It may just be a publicity stunt, not as bad as them flying their crash prone F-35s over full football stadiums but still…
    Anything claiming to violate the laws of physics will not work. That said, there are still undiscovered physical laws.

  10. Sean Carroll says:

    Typo fixed, thanks.

    Fun to watch as some of the comments illustrate the need for the post.

  11. Solomon says:

    I feel like you may actually be understating the degree to which the basic claim here is incredible. Given that Nöther’s theorem demonstrates the equivalence between conservation of momentum and the translation symmetry of physical laws, the claim that a drive like this can work is basically equivalent to saying that a device using nothing beyond quite common technology can somehow access some sort of – heretofore unknown – new physics that violates translation invariance to a degree noticeable on laboratory scale.

  12. Ed says:

    If spacetime is created by quantum information, maybe it can be annihilated by ordinary misinformation, thus creating the reactionless misinformation drive, which though cool, is still not as cool as the infinite improbability drive.

  13. Patrice Ayme says:

    Nice essay, and I agree with most of it. However, to play the Devil’s Advocate:

    Are propellant-less drives less plausible than energy-less universes born from nothing? Just wondering.

    As far as curving things weirdly, given the Einstein gravitational equation, it all depends upon what is on the right hand side, the energy-momentum tensor. But surely we have thoroughly investigated all forms of imaginable matter outside of the 96% which completely elude us?

  14. Yay! I’ve been waiting for Carroll to write a blog post on this. As an aerospace engineer with a physics degree, it really made me upset that money and time are wasted on these worthless fantasy ideas, and it really makes me suspect the sanity of my fellow engineers who actually buy into this garbage. I even got angry enough to write a blog post myself, but it’s nowhere near as good as Carroll’s (as is expected).

  15. Ce says:

    Hi, I have read about an Alcubiere “Warp” drive and commented on it somewhere in Dr. Carroll’s blog… My understanding was that it used “exotic matter” or a casmir effect to make it run.. It’s good to confront the problems with these sorts of claims, altho there seemed to be a grain of truth (Alcubiere’s idea has been mention in some documentary or other). And Everetian multiverse is certainly still being debated… But while were at it, it sure would be nice to clear up some of the confusion over alternatives to the Copenhagen Interpretation of QM. I have listened to some videos on Bohemian Mechanics that seem eminently reasonable, but get the overall impression that the ideas are not well accepted by the mainstream. Why or why not, exactly? I also saw something on Quantum Field theory, and am unclear if that is a viable challenger to the Copenhagen Interpretation… (And Dr. Carroll has alluded to QFT). Max Tegmark polls his seminar audience on those who accept Copenhagen alternatives, like “Modified Dynamics.” Why don’t we hear more on these alternatives? Nobody really discusses them for the more amateur listeners … apparently Quantum Information theory is up and coming, sp how about more on that? And if people are working on Quantum Computers, which presumably run on QBits as defined as in superposition, what happens to Quantum Computers in these other theories where superposition is not needed? How ’bout it guys?

  16. Gaehazzi says:

    Sean, your views are passe. Note that we now live in the post-post-modern era, and the following principles substitute for Newton’s laws, that few [geeks and nerds] still remember or bother about:
    1. All creatures are entitled to opinions.
    2. Science is a [white, male, rich, old, Western] opinion.
    3. All opinions are equally correct [provided they are politically correct].
    4. To disrespect a cultural group’s science is politically incorrect in the extreme.

    Philosophers [from the other side of the campus] have long demonstrated that the laws of physics, such as Newton’s, are actually laws of men, a cultural construction, thus amenable to deconstuction and reconstruction. The natives that live deep in Papua’s jungles entertain different laws, which are equally correct, so why not NASA’s entertainers?

  17. Graham says:

    hey, I always thought that the fundamental Laws were always there, well at least up to the point of the ‘Big Bang’. We never ‘created them’ all certain folks did was come up with the math to describe them.

  18. Jayarava says:

    As David Hume said in 1748:

    “…no testimony is sufficient to establish a miracle, unless the testimony be of a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish…”

    His essay “Of Miracles” is still very good on these kinds of issues.

  19. Pete says:

    great article and thanks to Ed, his comment made me laugh out loud !

  20. Gary Godfrey says:

    New but un-physical ideas aren’t really necessary. Just accelerate at a constant 1 g for about 23 years, and you can travel to anywhere in the universe (ie: billions of lyrs) in well less than your lifetime. Your ship would look like a big flashlight with a ball of anti-matter at the focus of a visible light parabolic reflector. You would throw matter into a hole in the ball causing it to heat up so it emits visible photons. Your rocket’s exhaust would be a parallel beam of photons. All that’s left are just a few engineering and social challenges !

  21. Steven Docker says:

    Nice posting Sean. Every time I read about this “reactionless drive”
    my heart sinks. Feynman had it right – the easiest person to fool is
    yourself, something the research teams that have apparently
    confirmed that this effect exists haven’t quite grasped.

    It is, I think, a sad reflection that so many people want this to be true
    even though there is zero evidence. The thing that makes me chuckle
    though is that when this was first announced and the “effect” investigated
    by different research teams, one of the teams found that the “thrust”
    from the “drive” was detected even when the power was turned off.
    Hey wait – not only is this drive reactionless it also requires zero power.

    PS – I’ll go out on a limb here and say that the “effect” is nothing more than
    the “engine” heating up by different amounts at different points and then
    causing out-gassing of the material of said “engine’s” external surfaces
    at different rates in different directions and producing a net thrust in the
    direction where the outgassing is greatest. It’s is even easy to prove via
    spectroscopic analysis of the vacuum chamber walls after the engine has
    been running for a while. Simple. No new physics needed.

    Sorry for the excessive use of quotation marks …

  22. Maybe a “warp drive” project, Quixotic it may be, could result in some unforeseen, practical technology spinoff.

  23. Chakat Firepaw says:

    @Gary Godfrey:
    1g for 23 years means a deltaV of about 23c [1]. For a photon drive that’s a mass ratio of over 9.7 _billion_.

    It might be easier to figure out “Gold Cross” style cloning and mind recording. Then you could send preserved samples and recorded copies of people and simply rebuild your crew after the long journey.

    [1] Fun fact: 1g is almost exactly 1 light year per year per year.

  24. Kasuha says:

    As Richard Feynman once famously said: It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.

    EmDrive has its homepage. It has its Wikipedia page with whole lot of further references. And three independent groups already confirmed that it works, this NASA associate is just one of them. You’re late to come in dismissing the project from the standpoint of theory, hundreds of others did it years ago. If you want to contribute at this stage, you have to figure out why the three independent experiments were positive.

    I’m not claiming it really works. I even believe all current theories explaining how it works are wrong. But I am watching the project for a few years already and instead of getting quietly swept under the rug, there’s piling evidence that the thing is real. It’s about time to take some serious care of it, whatever way it ends up.

  25. bob sykes says:

    NASA has become a clown show, staffed by charlatans and frauds. It’s not only reactionless drives (violation of conservation of momentum) or warp drives (violation of relativity), it’s arsenical DNA, temperature adjustments, etc, etc … The list goes on and on.

    By the way, a velocity of 23c violates relativity. You cannot travel or communicate at speeds greater than light. An, in fact, no object with mass can even attain the speed of light.

    Both relativity and conservation of momentum are foundational principles of modern physics, as is the quantum mechanics (the only true representation of physical reality).