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. BobC says:

    If we take the reported force measurements at face value (as correct observations), and discount all explanations that violate known physics, are there more conventional explanations that do, indeed, account for the observed forces?

    The test situation and the copper construction of the test apparatus reminds me of something I saw 40 years ago while in the military: A test load was improperly connected to the output of a targeting radar (or an incorrect load was correctly connected, I don’t recall which). Targeting radars back then used CW, not pulses.

    When energized, the radar energy was dissipated in the copper waveguide connected to the test load, instead of in the test load itself. The copper soon glowed and started to melt. The emission of gaseous copper was readily evident. And there undoubtedly was some small net force exerted as a result. (There certainly was among the test’s operators!)

    Similarly, the descriptions of the large amount of RF energy fed to the copper truncated cone of the mystery thruster did apparently cause significant surface currents and heating. No surprises there. And I would not be surprised if it was enough to locally cook off some metal. The extremely small observed forces are consistent with this.

    So, if a net force was measured by this mystery drive, I’d very much like to see the before and after mass measurements of the test apparatus, accurate to at least a single microgram, preferably to tens of nanograms.

    At this level of resolution, getting a stable initial mass measurement can be difficult, especially if solvents were used to clean the apparatus. It can take several heating cycles to drive off adsorbed solvents. Just ask the NASA and JPL folks responsible for preparing Mars landers.

    I’d also like to see the experiment repeated with the truncated cone made from an alloy with a very high melting point, or if that is not practical, to at least electroplate (chrome) or anodize the material, inside and out, to limit high-temperature surface losses and prevent surface oxidation.

    This kerfuffle reminds me of the FTL neutrinos “observed” in Italy. Not until the experiment was repeated with updated equipment (replaced cable) was the issue put to rest. In that case, however, no FTL claims were made, just that the observations appeared to imply them. The same may apply to this “EM Thruster”.

  2. Jeremiah says:

    Wow this blog post really bombed hard. You might as well have pasted “dadgummit!” five hundred times.

    Read the reaction…


  3. rtkufner says:

    That ship looks stupid. Yes, this is all I have to say on the matter.

  4. MBenson says:

    What would happen to a space ship traveling at warp speed that hits a rock or small asteroid? It seems the collision would destroy the ship, doesn’t it? After all, Kirk always warps within a galaxy, where there’s lots of such material around. Just wondering about that.

  5. Josh says:


    Sure the public media reaction to this has been… quite poor, but let’s not throw the baby out with the bath water. If you’re implying life’s too short to contemplate the matter for all, why be so overzealous? For you, specifically, it might not, but don’t you think we do need people working on the more outlandish ideas? And isn’t also true that sometimes the flashy, futuristic ideas (if properly reported in context) can stir up a bit more public excitement over science than current reality? [Note that I’m not defending what happened, just cautioning over generalizations]

    For instance, I think psi phenomena (telekinesis, clairvoyance, etc.) are incredibly unlikely and not worth tons of public engagement and research money. BUT I certainly don’t think they deserve no attention, money, or time. Fund and publicize the sure-bets in majority of course, but I would still want there to be some research on things like psi phenomena just on the off-chance.

    We need these kind of things out there to second-guess our cherished dogmas, and I don’t think it’s wise to dismiss them out of hand. Certainly we should be immensely skeptical until they provide impressive evidence, but there is some value in them (be it in self-scrutiny or an occasional stir of the emotions).

  6. Robert cattle says:

    Never mind given between 20 and say 80 years, you will know the answers.
    This naturally depends on if you believe in an afterlife.
    If you don’t, it will be a great experiment . You may die of shock!

  7. Pentcho Valev says:

    The Eagleworks group seems to have made laser beams travel faster than the speed of light, in violation of Einstein’s relativity:
    “On 29 April, Nasa scientists wrote an article on Nasa Spaceflight that they had tested British scientist Roger Shawyer’s controversial electromagnetic space propulsion technology called EmDrive and were unable to disprove their results, indicating that the technology worked. However forum users looking at the experiment results also found that when lasers were fired into the EmDrive’s resonance chamber, some of the laser beams had travelled faster than the speed of light…”
    “Thrust measurements of the EmDrive defy classical physics’ expectations”, said the researchers, who have been testing the highly controversial electromagnetic space propulsion technology. They posted on the Nasa Spaceflight forum that when lasers were fired into the EmDrive’s resonance chamber, some of the laser beams had travelled faster than the speed of light…”
    So did laser beams travel faster than the speed of light? Are NASA officials going to reply? A few months ago Scottish scientists managed to slow down light inside vacuum but the result, published in Science, is now long forgotten:
    “Physicists manage to slow down light inside vacuum (…) …even now the light is no longer in the mask, it’s just the propagating in free space – the speed is still slow. (…) “This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum,” co-author Romero explains in the University of Glasgow press release.”
    “The speed of light is a limit, not a constant – that’s what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently.”

  8. RonS says:

    If I understand correctly, the velocity c is also the limit at which the electromagnetic force can propagate in the EM field, and thus allow an atom to remain coherent. Above that an atom or molecule would simply deconstruct.

    @Pentcho Valev
    Science writers are notoriously bad at explaining the actual science behind research findings (Jennifer Ouellette excluded). Science news releases are often so poorly described and greatly sensationalized that they become hype opportunities for the expounders of pseudoscience.

    I think you might find the explanation for the “slow” group of photons in that Scottish experiment by looking up something called group velocity.

  9. Pentcho Valev says:

    RonS: “I think you might find the explanation for the “slow” group of photons in that Scottish experiment by looking up something called group velocity.”
    You saw “group velocity” repeated in Padgett’s paper and now hope that the term could somehow save Einstein’s relativity?

  10. Daniel Kerr says:

    From the abstract of the Padgett paper: “Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.”

    So the phase velocity is the same, and thus there’s no violation of relativity. Relativity only restricts the phase velocity of light be constant, it makes no such restriction on the group velocity. If they were able to reduce the phase velocity, that would be a violation.

  11. Pentcho Valev says:

    Daniel Kerr: “So the phase velocity is the same, and thus there’s no violation of relativity.”

    They didn’t measure the phase velocity. They measured the speed of a SINGLE photon IN A VACUUM, and found it lower than c:

    Glasgow researchers slow the speed of light

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  13. Richard Kriske says:

    There is an incredible glaring error in both Relativistic theory and in Quantum Mechanics, hopefully it will be resolved with the Neutrino Oscillation experiments– I hope that they find that the Neutrino travels faster than light and that it also has mass, as that would finally put an end to what appears to be an obvious mistake in Special Relativity, that seems to have come about by limiting the dimensions of time to one dimension, as if you have a spatial surface of three dimensions, shouldn’t it have a time “vector” pointing to both sides, why only one side (the two times would behave radically different, even if you argue that they are really the same vector with a positive and negative direction)? Our imaginations seem to have gotten in our way on the whole fabric of space-time, and the positive and negative times have been used by Feynman in QED, yet no one has given them a second thought when the talk about Relativity.

  14. Daniel Kerr says:


    Exactly, they didn’t measure the phase velocity, so what’s the problem? They measured a single photon, which doesn’t necessarily have to be an excitation of an underlying plane wave but some other solution to Maxwell’s equations. It’s still a solution, so the phase velocity is still c. In order for the single photon to break relativity, it would have to be an excitation of a wave which isn’t a solution to Maxwell’s equations. Maxwell’s equations permit solutions with group velocities less than c, so there’s no violation here.

  15. Pentcho Valev says:


    Two photons were released simultaneously across identical distances towards a defined finish line. One arrived first, the other arrived later. I am afraid no reasonable interpretation would provide the conclusion that both travelled at c.

  16. Daniel Kerr says:


    From their own conclusion, the point of the paper is that photons propagate at the group velocity even in vacuum, not the phase velocity. They even cite a 1992 paper that yielded this result in matter: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.68.2421

    Again, I don’t see how this is a problem, it is entirely what you expect if a photon is indeed a quantized EM radiation field. This is what relativity and Maxwell’s equations predict. Perhaps it’s surprising that individual particles have properties of the ensemble, but this would be a problem with the interpretation of quantum mechanics, not relativity.

  17. Pentcho Valev says:


    If, in a two-photon race, one photon reached the finish line first and the other arrived later, then at least one of the photons was not travelling at c. This conclusion is unavoidable and totally independent of any theoretical argument.

  18. Daniel Kerr says:


    Okay, but it doesn’t violate relativity for the reasons discussed in the paper and that I summarized, so there’s no issue here. It’s in fact predicted by relativity, yet you previously claimed relativity needed to be saved. If we agree now then what are we arguing over?

  19. Pentcho Valev says:


    Once we admit that photons can be emitted at a speed lower than c, things get disastrous, although relativity does not seem to be directly threatened (actually it IS threatened). Just an example: we measure the redshift of light coming from a distant object, but this redshift could be due to slowing down of the speed of the light by something analogous to the mask in Padgett’s experiment (Halton Arp’s intrinsic redshift???). Next, if the mask can slow down the speed of light, then it is reasonable to assume that gravity can do so too, and the gravitational redshift is also due to slowing down of the speed of light.

  20. Jeremiah says:

    Sure would be nice if some of you smart people would come over to the EmDrive developments thread where all this hoopla began and help out. Lots of math and science happening there. We’re trying to figure this thing out!


  21. Daniel Kerr says:


    That’s more of a problem for cosmology, not relativity. I can’t say how this would affect things as I’m at best a novice in cosmology, but considering the effect required very particular lens geometry and persisted only over 1 meter of a potentially much longer beam path, I don’t think it would be very significant.

    In terms of redshift though, it’s the phase wave that’s red shifted in the Doppler effect and that’s what’s measured, not the group envelope. I don’t see why redshift measurements would be affected then.

  22. Pentcho Valev says:

    Daniel Kerr: “In terms of redshift though, it’s the phase wave that’s red shifted in the Doppler effect and that’s what’s measured”

    Yes, and the Doppler effect disproves relativity. Consider this:

    Albert Einstein Institute: “The frequency of a wave-like signal – such as sound or light – depends on the movement of the sender and of the receiver. This is known as the Doppler effect. (…) Here is an animation of the receiver moving towards the source:

    http://www.einstein-online.info/images/spotlights/doppler/doppler_static.gif (stationary receiver)

    http://www.einstein-online.info/images/spotlights/doppler/doppler_detector_blue.gif (moving receiver)

    By observing the two indicator lights, you can see for yourself that, once more, there is a blue-shift – the pulse frequency measured at the receiver is somewhat higher than the frequency with which the pulses are sent out. This time, the distances between subsequent pulses are not affected, but still there is a frequency shift: As the receiver moves towards each pulse, the time until pulse and receiver meet up is shortened. In this particular animation, which has the receiver moving towards the source at one third the speed of the pulses themselves, four pulses are received in the time it takes the source to emit three pulses.” [end of quotation]

    That is, the speed of the pulses relative to the stationary receiver is c=3d/t, but relative to the moving receiver is c’=4d/t=(4/3)c, where d is the distance between subsequent pulses and t is “the time it takes the source to emit three pulses”.

    Clearly the speed of light (relative to the observer) varies with the speed of the observer, in violation of Einstein’s relativity.

  23. Daniel Kerr says:


    I think you would benefit from a more rigorous treatment of the relativistic Doppler effect which has a different transformation law between flames than the classical one for sound you’re referring to: http://en.wikipedia.org/wiki/Relativistic_Doppler_effect

    To understand it simply, realize that for the relativistic Doppler effect, the wavelength and frequency change in proportion to each other relative to the velocity of the frame they’re measured in, such that their division is always c. There is no violation.

  24. Pentcho Valev says:

    Daniel Kerr: “relativistic Doppler effect which has a different transformation law between flames than the classical one for sound you’re referring to”

    Please be correct, Daniel. The Albert Einstein Institute is teaching Doppler effect in LIGHT, and acccordingly I am referring to LIGHT, not sound.

  25. Daniel Kerr says:


    I’m going to drop this exchange now, the wikipedia page has the explanation you seek while the calculation you did for the wave’s velocity is not provided by the webpage you shared. You supplied the calculation yourself and made an error. The wikipedia page can clarify that for you if you’re willing to engage with it. My continued participation here is not going to make you budge one way or the other in trying to understand it.