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. J. Rodal says:

    Not a well-researched piece:

    Quote from: Sean Carroll
    “And what do we have for our propellantless space drive? Hmm — not quite that. No refereed publications — indeed, no publications at all.”

    Factually incorrect: Prof. Yang’s papers (on her theoretical analysis and experimental measurements of the EM Drive) have been published in the peer-reviewed journal Acta Physica Sinica -Chinese Edition- (ACTA PHYS SIN-CH ED)

    Quote from: Sean Carroll
    “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.”

    Incorrect. None of the EM Drive researchers in the US, UK and China have proposed that conservation of momentum is being violated. They all claim it does not (with different explanations). So the proper critique should be, instead, that their (different from each other) theoretical explanations are non-viable, and showing why they are non-viable. For example, if somebody claims as an explanation that they are using the Quantum Vacuum as something to push on, the critique should be that the Quantum Vacuum is frame-less, immutable and non-degradable, instead of writing that the authors are proposing that conservation of momentum is violated.

    As to whether space propulsion without on-board propellants is possible, the obvious scientific answer is: yes. Besides the scientifically obvious answers of Solar Sails and electrodynamic tethers (based on external fields) we have photon rockets for example, as perfectly valid means of space propulsion that require no on-board propellant and yet do not violate the law of conservation of momentum. Even just releasing thermal radiation (as in the Pioneer anomaly) is s valid (albeit extremely low thrust) means of space propulsion that requires no on-board propellant and yet does not violate the law of conservation of momentum.

    So, again, there is a (self-admitted) failure to examine what is being criticized, it is criticized on the wrong premise (that the authors claim that they don’t care about conservation of momentum, instead of criticizing their different conjectures to satisfy conservation of momentum), it advances a wrong, broadly-stated premise (that there cannot be propellant-less propulsion, which is false: Solar Sails, ElectroDynamic Tethers, Photon Rockets, Thermal Radiation, etc.) and it claims that none of this research has been published in peer-reviewed journals, thus ignoring the Chinese authors publications (which instead should be criticized based on their theoretical and experimental results).

  2. Keegan Moore says:

    As a dreamer the idea entices me, but the engineer in me struggles to even begin to believe what these scientists are suggesting. I fully agree that if they want any credibility they must publish in peer reviewed journals and scrutinize their results.

    Finally, I have to say “well said.”

  3. Buck Field says:

    On one hand, photon pressure does seem a possible, if minute, force that can be harnessed as a mass-equivalent, so given an immense amount of time, one would think a flashlight could have some force equal and opposite to the photon pressure of what it emits.

    On the other hand, physicists know the standard model is in need of reformation. Dr. Carroll writes about the basic understandings we lack for fundamental object concepts within physics, like time, mass, space, etc.

    From a project management perspective, historians and philosophers of scientific revolutions are the experts who are in the best position to inform us where to look for solutions, based on what seems most critical for the success of past reformations.

    For planning a successful research program, the work of Nancy Nersessian, Paul Hoyningen-Huene, and others in Kuhnian philosophy of science would seem to offer some of the best guidance on effective focus and resources to apply.

    This is not what Sonny White’s doing, although I wish him nothing but the best of luck.

    Another critical issue being missed is the opportunity these fanciful, science-fiction visions offer: alignment. The entire field of modern chemistry arose almost completely out of efforts to create gold from lead; Astronomy came from astrology, and so forth. Organizing efforts well requires an inspiring vision so everyone from the basement crackpot to the most anal-retentive financial auditor from Accenture knows the clear end goal vision without incurring overhead for communicating individually, explaining, training, reminding, etc.

    As they say that every cloud has a silver lining, the unique power of warp drive to bias our thinking and capture the imagination of the world can be used productively – and I would claim: it offers unique advantages – if harnessed properly.

  4. Jeremiah says:

    Wow, even after I flat out told you how momentum is being conserved….guess you didn’t buy it. The cool thing about it is, if you’re right..no biggie; Sean Carroll is the king of schooling Plebs like me after all (love your debates on YouTube). But if I’m right, I got to school Sean Carroll.

    I think folks would have had a better reaction to your blog post of it would have contained at least an iota of academic content. Maybe next time?

  5. darrelle says:

    Well, I do really like the picture they came up with. When can I get my Mark I Starship?

  6. phayes says:

    Heh! I remember the Emdrive from the New Scientist farce back in 2006. Strictly speaking, J. Rodal is right about one thing: Roger Shawyer (Emdrive’s chief crackpot) did make the absurd claim that there was no violation of conservation of momentum, and ‘backed it up’ with some ‘explanation’. I remember finding the [potential consequences of the] Emdrive’s frame-dependent thrust particularly amusing.

  7. Ce says:

    Agreed… we all want to get out into space! There was also some flap on Space.com:

    “Test results indicate that the RF [radio frequency] resonant cavity thruster design, which is unique as an electric propulsion device, is producing a force that is not attributable to any classical electromagnetic phenomenon and, therefore, is potentially demonstrating an interaction with the quantum vacuum virtual plasma,” the NASA team wrote in their study (http://www.space.com/26713-impossible-space-engine-nasa-test.html)

    Is this another case of nonsense masquerading as fact? The whole problem with youtube and the intertnet is trying to identify legitimate information. I try to research the sources, but that is not always as straight forward… and how do you evaluate metaphysical discussions that intersect with physics and cognitive discussions?

  8. Eric Kraemer says:

    It is preposterous to imagine there is some magical land on the other side of the vast sea. Clearly one would sail right off the edge of the earth an die a horrible death.

    There are much better things to spend our time on than this fantastical imagining.

    Easily said, if one was willing to dismiss any and all evidence wholesale due to the preposterous nature of the proposal. T-rex riding a bike – indeed…I smell a hint of scientific fundamentalism in the proposed thought process.

  9. explorador says:

    There is one thing that has impressed me far more than the warp drive. It is the capability of Sean Carroll for reading all the comments in his blog and resisting the urge of replying! Seriously, what a ninja of the self control.

  10. Andrew Palfreyman says:

    All propellantless drives suffer from not only non-conservation of momentum but hard choices when it comes to their energy performance. When operating in field-free flat spacetime at constant input power P, either:
    a) the thrust remains constant, in which case you eventually exceed breakeven and thereafter (speed > 2/k, with k in Newtons/Watt) continually produce free energy
    b) the thrust varies as P/v, just like an automobile on a road. But this implies that somehow the drive “knows” about the velocity v, which implies a preferred reference frame, which in turn violates Einstein’s SR core principle of equivalent physics in all inertial frames.

    So for any propellantless drive, one has to choose between trashing Einstein or trashing Noether in respect of the energy performance.

  11. Doug Little says:

    It would be nice if Dr. Carroll could look at the claims and assess them on their merits rather than hand waving them away. People have been working on these ideas for a while now and have produced some papers explaining the effect and several experiments that supposedly show the effect, whether or not they hold up under peer review or not is another matter, but it would be nice to have someone who is capable of assessing the claims from a scientific perspective and then point out why those claims are incorrect to the layman. I have been reading about this for a while now but don’t have the expertise to be able to pick through the papers and determine where they have gone wrong, if indeed they are wrong at all.

  12. John says:

    I believe the word you are looking for is “scathing”

    Well said Sean!

  13. Richard Kriske says:

    If time has more than one dimension it is possible to move faster than light. There is a great contradiction not only in General Relativity, but in Special Relativity and in Electromagnetic Theory. A magnetic field is an imaginary, since you have to generate a photon to test it, and that is Electromagnetic— Same is true of an Electric field. Faraday did not realize this, as it was before Quantum Mechanics. If an Electric field is generated, even though the exchange particle is Electromagnetic, it is different than a Magnetic field, in that it acts differently. This can be put into the QM bracket and you can see that the difference between the two is the non-commuting part of the QM bracket. So the two fields are the same field, but it depends on which dimension of time is measured first—there are two dimensions, one harbors the E field, one the H field. These two dimensions once again show up in Special Relativity, and are particularly noticeable in the Twins paradox. You also see their effect in the Oscillation theory of the Neutrino. So how do you step into one dimension and get around the Conservation laws that connect the two? That I don’t know.

  14. Doug Little says:

    Here is a good overview of the current state of this research, if you want to call it that. It has plenty of links, some to papers that try to explain the effect. Hey we all can dream right, you never know.


  15. Gary Godfrey says:

    @Chakat Firepaw :

    Another fun fact, an acceleration of 1 g happens to give you a Lorentz Boost Parameter (lambda) of 1.0 per year. Unlike velocities, Lorentz Boost Parameters are additive so after 23 years, lambda=23. According to Special Relativity, an observer back on Earth sees your velocity = c * tanh(lamda) = very close to but less than c (not 23c !!). He also sees your clock going cosh(lamda)=5E9 slower. So, if you stop accelerating at this point and coast for 1 of your years, the Earth observer will see you travel 5E9 lyrs. In total you actually travelled farther than this because of the distance traveled in the 23 years you spent accelerating. Not bad for getting out there with conventional physics !

  16. Daniel Kerr says:

    Dismissing it on Bayesian claims is in a sense, giving it too much credit. From what I’ve read it seems like an intentional, alternative semantic interpretation to conventional physics. They identify the radiation force with the Lorentz force or identify the group velocity as being the phase velocity and etc… the quantum vacuum interactions with the engine being an interaction with a plasma. The problem is the theory is using an inconsistent physics dictionary, so it’s not surprising they claim their experiments have the desired results. You’d have to convert this theory/evidence into conventional physics language before evaluating it in a Bayesian framework. I think you would fine that the theory was not self-consistent and the evidence would not in fact be evidence before you even got that far though.

    The point still stands, it’s not worth evaluating the self-consistency of this idea with the correct formalism. The burden is on them to cast the theory in a conventional physics formalism that uses consistent semantics.

  17. Ce says:

    Dear Doug, It is an unfortunate fact that anyone can put something up on the Internet… with some knowledge and writing skill they can post something that seems plausible… but there is little information except for self references… and nothing to connect them to known, legitimate research organizations… really hope some of this stuff is legit… interstellar flight is something that excites many of us… I think Dr. Carroll’s point is that there is not much verifiable or accepted science fact behind many of these sensational claims… and the people are rarely affiliated with legitimate academic or other organizations… that is why an objection from a serious science source such as Dr. Carroll, gives pause.

  18. Richard says:

    Ed wins the Internet today. Or at least this small corner of it 🙂

  19. Andrew Palfreyman says:

    Sean Carroll inhabits and is a part of the universe.
    Sean Carroll by his own admission states that the universe is preposterous.
    Therefore, Sean Carroll is preposterous.


  20. Guilherme says:

    Thank you for this post! I’ve had to make these arguments so many times to engineers before—and even a couple physics students!—that it’s even depressing. In their view, “I’m no fun”. Better being “no fun” than plain wrong! Now when someone wants to claim superluminal speeds are possible, I’ll just send them here!


  21. Antonio (AKA "Un físico") says:

    Thus, if someone says “NASA scientists are building a space drive that violates Newton’s laws of motion” we should be rather more skeptical than if he says “NASA lads have proven that greenhouse gases cause climate change”: no scientific articles supporting the first claim, but many favouring the last. The bayesian reasoning for beliefs is something “amazing” that one can learn from “outstanding” scientific blogs like this one. BUT, it is a pitty that all this has nothing to do with science.

  22. Swami says:

    Belief in pseudoscience miracles is almost like believing in religious miracles. If you have enough faith, then it just has to be true.

  23. Apoc says:

    What’s worse, a handwaved theory to explain observed effects, or a handwaved cursory dismissal of those observed effects?

    “I certainly haven’t gone through the experimental results carefully” sums it up. This is an arrogant shameful article. Really disappointing.

  24. These EMDrive devices are reminiscent of Eugene Podkletnov’s claims from the 1990s, of small (.05%) reductions of the Earth’s field directly above his spinning YBCO superconductors. As with the EMDrive, there were claims of successful replications, from others. At the height of its flowering the saga of anomalous superconductor effects spawned a number of dedicated websites. One of the best of these, now defunct, was created by Pete Skeggs, with the very appropriate and creative title of “Quantum Cavorite”.

    I was so intrigued by these claims that I began playing with 1 inch YBCO superconductors, a few years back, to see if anything showed up. To my delight I did detect a small acceleration pulse of about one milli-g. But, I quickly discovered that it was a false signal, resulting from the acoustic ‘pop’ emanating from the sudden expansion of the liquid nitrogen as 1.2 megawatts was discharged through a coil immediately surrounding the YBCO chip. I’ve since eliminated this false triggering problem with timing circuits, and plan to continue the experiments. Even if nothing shows up I feel it will have been worth the effort as a guide for properly conducting experiments.

  25. DAVID PENNER says:

    When Captain Kirk went into warp drive and travelled to the other side of the Galaxy, saved the Universe and turned around and came back to earth to get the engines recharged, shouldn’t it have been thousands od years in the future?
    Dave Penner