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Quantum Tunnel Shows Particles Can Break the Speed of Light

128 points by jonbaer 6y ago

david927 6y ago
> quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility It's only an impossibility in space-time. But the quantum world, until the wave function collapses, exists outside of space-time. Entanglement is under the same effect -- space-time simply isn't involved. It's the collapsing of the wave function that brings it into space-time. At least that's how I understood it.
- drdeca 6y ago
  That doesn't seem to fit with my understanding all that well? Alternatively, I'm just not sure what you mean by saying that it ( wait, what is the "it"? "the quantum world"? But the world is the quantum world?) "exists outside of space time" . If one said "the number 3 exists outside of spacetime", that would make sense to me, as saying that it isn't a thing with a location in spacetime, but just a thing. But I don't think I know what you mean.
  - vez- 6y ago
    The thing I think the original commenter was trying to get at is that spacetime (the mathematical model) and quantum mechanics (the mathematical model) for the most part are in no way unified. Things that happen in quantum mechanics are not necessarily explainable using spacetime. Although in general physicists believe a lot of things from spacetime for QM, like that information can't travel faster than light, it isn't actually baked into QM. That's my understanding at least.
    - drdeca 6y ago
      By spacetime do you mean like, in a general relativity sense? Because aiui quantum field theory is quite consistent with special relativity.
    - raattgift 6y ago
      The https://en.wikipedia.org/wiki/Dirac_equation https://en.wikipedia.org/wiki/Dirac_equation is now more than ninety years old and put quantum mechanics into the Minkowski spacetime of special relativity. There are several textbooks[1] covering extensions of this approach to curved spacetimes generally, and one will encounter the https://en.wikipedia.org/wiki/Klein%E2%80%93Gordon_equation#Gravitational_interaction https://en.wikipedia.org/wiki/Klein%E2%80%93Gordon_equation#... in graduate school settings. Quantum mechanics on some specific curved spacetimes are exactly Klein-Gordon solvable. The Standard Model of Particle Physics incorporates the Poincaré Group, which is the group of Minkowski spacetime ("flat spacetime") isometries. This means that the Standard Model particles are the same independent of where and when they are in empty flat spacetime (3 spacelike and 1 timelike translation invariances), their orientation (rotational invariance about the three spacelike axes), and they transform reliably under boosts (differences in constant velocity along the spacelike axes). So the Standard Model is defined by flat spacetime: and the causal structure of flat spacetime (in which the constant c plays a key role) is very much baked in. Studying modern quantum mechanics mostly means looking at patches of effectively flat spacetime in which there are particles of the Standard Model occupying different locations in the patch, boosted relative to each other, and interacting via the other mechanisms captured in the Standard Model's formulation. We inhabit a type of curved spacetime in which General Relativity guarantees at least an infinitesimal patch of exactly flat spacetime around every point in the universe. In regions with only gentle curvature -- like laboratories on the surface of the Earth, or in space probes in the solar system -- any curvature corrections to the assumed exactly flat spacetime of the Standard Model are tiny, because the region of effective (rather than exact) flatness is very large compared systems of particles under experimental study of their quantum behaviour. "Pretend it's flat" works exceedingly well in practice. When a researcher has to consider curvature (e.g. in relativistic massive objects like neutron stars) she or he can continue to work perturbatively against a flat-by-definition spacetime. The problems arise in the difference between a guarantee of a microscopic region of flat spacetime and an infinitesimal region of flat spacetime: if the radius of curvature is small compared to the spatial extent of a particle, things get ugly quickly, especially as we take the wavelength of the particle smaller (which means the energy of the particle climbs, and that is the sort of energy which creates spacetime curvature, so we get a nonlinear feedback, and classical General Relativity and Quantum Field Theory in Curved Spacetime make annoyingly different and incompatible predictions about what happens as one takes the limits of high particle energy and high curvature. Fortunately this incompatibility seems likely to occur only hidden inside event horizons, so it is a problem for the theories rather than a practical problem for all of us who are not actually rapidly approaching death within a black hole -- there also may be consequences for as-yet-undiscovered ancient tiny black holes, or stellar black holes many trillions of trillions of years in our future, so again we can take our time to understand the theoretical conflict). A bit more technically, the problem arises in perturbative approaches to QFT on curved spacetime: at low energies and low curvatures we have a fairly small number of correction terms which can be written out as a https://en.wikipedia.org/wiki/Taylor_series https://en.wikipedia.org/wiki/Taylor_series which we can truncate because the higher-order terms are demonstrably irrelevant. As we increase energies and curvature, irrelevant terms become marginal, then relevant; additionally, we start having to add more non-irrelevant terms. https://en.wikipedia.org/wiki/Renormalization https://en.wikipedia.org/wiki/Renormalization allows us to squash some of these terms together, but eventually we get an overwhelming growth of non-ignorable corrective terms and lose the ability to make predictions using this approach. This breakdown in perturbative renormalization ("perturbative quantum gravity" [2][3]) gives a useful qualitative definition of "strong gravity": it's where the perturbative approach breaks down. In terms of Feynman diagrams, it's where loops of gravitons enter into the picture; a bit more colloquially, it's where "gravitation's self-gravitation starts becoming non-ignorable". Although not a dead area of research, looking for ways to make renormalization work for strong gravity is less fashionable than looking for non-perturbative quantum gravity that (a) matches perturbative quantum gravity right up to the weak-side boundary of strong gravity, including classical General Relativity in weak gravity, (b) is calculable in practice, and (c) solves other non-gravitational problems that plague high energy particle physics that are amenable to testing, since we probably can't extract observational or experimental data from regions of strong gravity. Additionally, classical General Relativity fairly generically predicts the presence of gravitational singularities in spacetimes with significant amounts of matter[4]. Such singularities destroy the total predictability of the entire spacetime from a total sample of all the variables on an arbitrary slice across the whole space at a given time coordinate. In other words, there's an incompatibility between classical General Relativity and traditional initial values surfaces approaches to solving physics problems. This problem worsens in the presence of Quantum Fields because of Hawking Radiation: instead of a literal singularity there is instead a trapping structure that evaporates (or mostly evaporates, in "remnant" proposals) in the far future of most black hole systems. But the matter that is released in the evaporation cannot be predicted from the matter that was thrown into the black hole before evaporation, and at late times we lose the ability to account for quantum entanglements that existed when the black hole was growing. Unfortunately there are numerous black hole candidates in our universe, which we also know is filled by matter representable by quantum fields. Although this is not strictly an incompatibility of General Relativity and Quantum Field Theory, quantum physicists are very keen on preserving https://en.wikipedia.org/wiki/Unitarity_(physics) https://en.wikipedia.org/wiki/Unitarity_(physics) which is lost in black hole evaporation as is understood today. Preserving unitarity in the presence of strong gravity is a fourth goal of modern research into quantum gravity. - -- [1] e.g. Parker & Toms, QFT in Curved Spacetime (2009), https://books.google.co.uk/books?id=5nNuGMBBTjMC&dq=L.+Parker,+D.+Toms+(2009).+Quantum+Field+Theory+in+Curved+Spacetime:+Quantized+Fields+and+Gravity https://books.google.co.uk/books?id=5nNuGMBBTjMC&dq=L.+Parke... [2] http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... [3] https://en.wikipedia.org/wiki/Canonical_quantum_gravity https://en.wikipedia.org/wiki/Canonical_quantum_gravity [4] https://en.wikipedia.org/wiki/Raychaudhuri_equation https://en.wikipedia.org/wiki/Raychaudhuri_equation is a good place to start
- gizmo686 6y ago
  Quatom mechanics doesn't really have a concept of space-time. The notion of space-time is entirely a construction of relativity; and the two theories have yet to be unified. Collapsing the wave function doesn't bring a system into the domain of relativity. That happens at a fuzzy point where a system become massive enough for relativistic effects to become significant. In thoery, we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity. There is semiclassical gravity; which essentially says that the quantum fields described quantum mechanics occurs in the curved spacetime of relativity. This is (by design) just an approximation of the still unknown theory of quantum gravity; and we don't have good experimental evidence telling us how accurate of an approximation it is.
  - c1ccccc1 6y ago
    Quantum mechanics doesn't work well with general relativity (gravity), but it works just fine with special relativity. Special relativity still has a concept of spacetime, it's just that the spacetime is flat, not curved like in general relativity. (In fact, you can even do quantum mechanics in curved spacetime, as long as the spacetime has a predefined fixed curvature. But in GR, the curvature depends on how the matter and energy inhabiting the spacetime are distributed. That is what gives quantum gravity theorists trouble.)
  - tsimionescu 6y ago
    Quantum mechanics is fully unified with special relativity, and has a clear concept of spacetime. The Dirac equation was discovered in 1928 and was the key to solving this problem. This means that quantum mechanics is fully compatible with time dilation at large speeds, that it respects the limits of the speed of light (current article notwithstanding) etc. The only incompatibility is between quantum mechanics and general relativity - quantum mechanics can't work with the general relativityequations for a curved spacetime or for the gravitational effects of particles being in multiple places at once. These are the things that a theory of quantum gravity must solve.
  - raattgift 6y ago
    > Quatom mechanics doesn't really have a concept of space-time The Standard Model of Particle Physics incorporates the Poincaré group, which is the isometry group of the flat spacetime of Special Relativity, so it has an exact concept of spacetime built right into the formulation. Quantum mechanics and Special Relativity were unified in the 1920s by Dirac. Solutions for some curved spacetimes came not much later, and are now taught in several textbooks. The problem is that we don't have a general solution for quantum mechanics on arbitrary curved spacetimes (and some curved spacetimes are known to make quantum field theories insoluble with current methods). I refer you to my sibling comment https://news.ycombinator.com/item?id=24877746 https://news.ycombinator.com/item?id=24877746 for more details. > (by design) just an approximation of the still unknown theory of quantum gravity Kind-of. It is manifestly an approximative approach because the equation includes "mean" angle-brackets around the stress-energy tensor, which, speaking roughly, indicate that the curvature is generated by the averaged value of the stress-energy generated by the quantum field(s) are used, rather than an exact value at each and every infinitesimal point in the solution. This "classicalizes" the source term of the classical curvature, erasing quantum effects. It is not really in the strictest sense an approximation to a theory of quantum gravity, however any candidate quantum gravity theory had better explain results from semiclassical gravity that find some support in astronomical observations. Contrasting approaches -- like canonical quantum gravity -- "quantumize" the metric, leaving the source term fully quantum mechanical and non-averaged, and results from such approaches also find some support in astronomical observations. These are arguably approximations to a full theory of quantum gravity since one can cast them as a series of terms similar to a Taylor series; the problem is that the set of such terms appears to grow without bounds as one takes energy-momentum into the "ultraviolet" regime (UV ~ extreeeeeemely high energy-momentum densities). Worse, the relative weight of the contribution of some of the far-from-leading-order terms grow stronger in the UV limit, so not only can't we just ignore them, we also can't apply perturbative renormalization. Fortunately this probably only happens inside black hole event horizons and near the big bang, one of which is probably not observable, and the other of which is at high enough individual energies that the Standard Model doesn't predict results even if their collective mutual gravitation (and the contribution of dark matter and other energy-densities) could be factored out. > we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity We can calculate the effects of a boost, even an ultraboost, on particles with mutually-entangled properties just fine thanks to the Poincaré invariance baked into post-Dirac relativistic quantum mechanics. If you were flying past from an entanglement experiment in deep space at ultrarelativistic speeds, one could predict your observed redshift (etc) of the entangled properties exactly with no worries. (If you were flying radially to or from an Earthbound experiment at ultrarelativistic speeds you might want to incorporate some corrections from the https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab... for greater accuracy. The post-Newtonian expansion leading-order corrections attributable to Earth's gravity will not be large; linearized gravity is enough. Errors from the next-to-leading-order corrections will not be measurable with current technology, and we can brute force calculate at least next-to-next-to-leading order corrections, which is good enough for accelerating down towards the surface of a neutron star, like the material that is blown onto the Hulse-Taylor binary members.
- tsimionescu 6y ago
  Relativistic quantum mechanics exists and is a complete theory - the Dirac equation shows how particles move taking into account the speed of light limit, and in the limit of small speeds reduces to the Schrodinger equation. Moreover, one of the most important results that confirmed entanglement is possible was the no-communication theorem, that proved that it is impossible to use entanglement to communicate faster than light. This is also suggests that wave-function collapse of an entangled system can't really be a physical phenomenon.
  - tashi 6y ago
    > can't really be a physical phenomenon Can you say more about that? What are the alternatives to being a physical phenomenon?
    - sdenton4 6y ago
      Being a mathematical formalism which happens to nicely describe the reality, perhaps?
    - tsimionescu 6y ago
      If you want to learn more about this, it is usually referred to as "measurement problem". Basically, quantum mechanics is extremely accurate at predicting experimental results by using 2 completely separate rules: the Schrodinger equation on one hand, and the Born rule on the other. The Born rule is what is also referred to as wave function collapse: if you want to accurately predict what happens to a particle after it interacts with a detector (but NOT after an interaction with another particle), then you have to use the Schrodinger equation as a probability. Now the physical meaning of these 2 different rules are debated. In the standard (Copenhagen) interpretation of QM, it doesn't make sense to talk about the properties of particles until you measure them, so neither the wave function nor the Born rule can be given a physical meaning. In the MWI interpretation, all results of the Schrodinger equation exist in different worlds, so there is no collapse, the detector just finds out "in which world you live". In superdeterministic theories, the Schrodinger equation is just an approximation of a currently unkown fully deterministic theory of motion. In pilot-wave theory, the Schrodinger equation describes an actual wave that particles generate themselves (the motion of the wave and the motion of the particle influence each other); entangled particles are just particles that live on the same wave. There is no collapse here, but the wave from each particle exists in the whole universe and can change non-locally.
hilbert42 6y ago
As I mentioned several days ago here: https://news.ycombinator.com/item?id=24837545 https://news.ycombinator.com/item?id=24837545 I would like to know mode
hilbert42 6y ago
As I mentioned here https://news.ycombinator.com/item?id=24837545 https://news.ycombinator.com/item?id=24837545 a few days ago in an earlier post on the same topic, I raised the matter of how this finding gels with other related physics. There, I asked physics experts to explain the issues to this dummy but so far none have been prepared to take the bait. If, for instance, particles can exceed c0 (the speed of light in a vacuum) during tunnelling then how does this fit with the fact that vacuum permittivity and vacuum permeability 'also sets' the speed of c through the relationship: c = 1/(μ0 ε0)^0.5 This means that either a particle has to completely 'vanish' from the physical world during tunnelling or that μ and ε don't apply during this time, or that they change value. It seems to me the ramifications of this are very significant. The corollary of this is that if the wave function collapses outside spacetime then it's meaningless to discuss the particle's speed. That said, some months ago I read a report that some group actually measured (or estimated) the time of an election's quantum jump from one shell to another. If this is actually measurable time then it seems that the wave function is not outside or independent of spacetime (perhaps at these super-fast speeds/super-short times we're seeing the granularity of matter and or the intersection of the quantum and relativistic worlds). That would be fun wouldn't it. That's a super rough outline of my point (it's too involved here to go into other possibilities that arise from QED etc., and I don't claim sufficient fluency to put them cogently). There's a bit more in that earlier post (but sorry I didn't explain what I was driving at very well).
- hashtagmarkup 6y ago
  If the "particles" don't have any mass, the "issues" you're assuming are likely irrelevant, as they would result in division by 0.
  - SpaceRaccoon 6y ago
    But massless particles still have relativistic mass due to their velocity. That's why photons still have momentum and can exert pressure.
    - rrobukef 6y ago
gog-ma-gog 6y ago
Articles like this on the internet are usually completely garbage, but Natalie is a really fantastic journalist. If you’re worried about woo because of the title, there isn’t any here (and quanta magazine is generally great for coverage of science) tldr: there’s a calculation you can perform in quantum mechanics that suggests particles can tunnel through barriers faster than they could have traveled through free space at the speed of light. Scientists have now measured this more precisely, and this effect seems to hold up, but is still very small, so it’s hard to understand the ramifications of this for things like causality.
- hilbert42 6y ago
  Right, articles like this are often short on crucial details, it's speculation until we see the research papers. Re causality etc., as I mentioned above, if the duration of wavefunction collapse can actually be measured then this puts a whole new spin on the matter (sorry, puns aren't my forte but it seemed appropriate here). ;-)
  - hilbert42 6y ago
    Update, FYI, I've just found some links about the matter and posted them above. These describe an experiment where an electron was seen transiting smoothly between two energy states whilst being in two places at once.
- 6gvONxR4sf7o 6y ago
  I wish more science journalists were like this. Able to not get all the core facts wrong.
SomewhatLikely 6y ago
> Steinberg, who agrees with the statistical view of the situation, argues that a single tunneled particle can’t convey information. A signal requires detail and structure, and any attempt to send a detailed signal will always be faster sent through the air than through an unreliable barrier. This argument doesn't make a lot of sense to me. One particle arriving can signal information that some event has occurred at the source. Re: structure/complexity I imagine you could use different elements (and maybe spin too?) to form several bits.
- deleted 6y ago
  [deleted]
- lisper 6y ago
  The devil is in the details. I haven't done the math (I'm probably not capable of actually doing the math) but my bet is that it is not possible to set up a tunneling barrier that does not also effect the spin component of the wave function. My further bet is that it will turn out that the uncertainty introduced into the spin component by the tunneling barrier is just enough to produce a no-communications theorem for tunneling just as there is one for entanglement. I'll even place a third bet that there's a Ph.D. thesis (if not a Nobel prize) at the bottom of that rabbit hole.
- h0l0cube 6y ago
  I think it's a matter of a low signal-to-noise ratio. So if you send one bit, but there's a low probability the other side will receive it, you'll need so much redundancy and error correction to achieve the same bandwidth as a regular signal, that it ends up being at least the same transmission rate.
- lacker 6y ago
  I think your intuition is correct and that Steinberg is agreeing with you here. Steinberg is just saying that a single particle alone cannot generate a causality paradox. But it still isn't obvious why you couldn't use this effect with multiple bits and some error correct to send signals faster than light. Looking at the paper being discussed here - https://iopscience.iop.org/article/10.1088/1367-2630/abb515/meta https://iopscience.iop.org/article/10.1088/1367-2630/abb515/... - the authors also don't seem to have an explanation. One final caveat worth noting is that our results do not definitively prove that an ensemble of free particles will always be a preferable method of transmitting a signal to an ensemble of tunneled particles. It is our intention to include a discussion of this in a follow-up paper. This is basically the scenario you're talking about, forming several bits out of different particles and using them in conjunction to send information faster than light. The researchers aren't sure why this wouldn't happen in their model.
aaron695 6y ago
Flat Earthers and people who think you can break the Speed of Light are the same, let's not kid. You can word play if you chose, but it's not clever. You can't send information faster than the speed of light.
- vixen99 6y ago
  Your comment is entirely word play.
- absolutelyrad 6y ago
  This comment won't age well.
  - hilbert42 6y ago
    An aside, Lord Kelvin made a similar sweeping statement about the 'completeness' of physics knowledge at the end of the 19th Century just before QM saw the light of day!
- deleted 6y ago
  [deleted]
dvh 6y ago
> The researchers reported that the rubidium atoms spent, on average, 0.61 milliseconds inside the barrier, in line with Larmor clock times theoretically predicted in the 1980s. That’s less time than the atoms would have taken to travel through free space. Light travels 182km in 0.61ms so it would be significantly slower than light.
- crispyporkbites 6y ago
  So if the barrier was 182km in thickness, the particles would arrive faster than the speed of light
- heavenlyblue 6y ago
  Particles rarely travel in straight lines but light does
szundi 6y ago
I suppose we won’t get the notification about apologizing and retracting the results.
kbwt 6y ago
I am not a physicist, but the experiment does not seem convincing to me. The Larmor clock doesn’t measure a proper time as in spacetime distance between two events. Instead it measures the rotation of a dipole in an electromagnetic field. The experimenters apply a combination of an electrostatic field (the barrier) and a magnetic field (the timer driving Larmor precession). In the frame of reference of the spinning particle, this is exhibited as a sum of two electrostatic fields. The barrier is a locally uniform repulsive electrostatic field, while the the timer is a radially varying axial electrostatic field. The particle is a dipole, tilted from the timer axis by the precession angle. When the particle tunnels through the barrier, it also tunnels through the timer field, without precessing over the tunneled distance just as the particle is not exhibiting repulsion from the barrier over the same distance. This is experimentally verifiable as the amount of missed precession has a lower bound proportional to the sine of the angle between the magnetic field and the barrier.
- i_cannot_hack 6y ago
  I'm curious, what field are you in? For a non-physicist you seem quite knowledgeable and confident about physics and quantum phenomena
  - kbwt 6y ago
    I work on software for augmented reality and distributed systems. That background is not directly applicable to quantum physics, but I like to think that I am highly trained at spotting logical errors.
  - theshadowknows 6y ago
    Right?!?
- amai 6y ago
  Tunneling doesn't mean jumping. The probability wave of the particle does exist inside the barrier, see e.g. https://commons.wikimedia.org/wiki/File:TunnelEffektKling1.png https://commons.wikimedia.org/wiki/File:TunnelEffektKling1.p... So yes, it "tunnels" through the timer field also, but the probability of interacting with the timer field is not zero. I would be interested if the Lamor clock also shows a shift in the reflected particles. Because that would mean, also the reflected particles spend some time inside the barrier.
deleted 6y ago
[deleted]
elgfare 6y ago
Somehow this reminds me of myelin sheaths, which IIUC are insulating barriers that increase the rate of transmission in neurons.
GnarfGnarf 6y ago
The barrier's space is warped and compressed, so the particles are traveling a shorter distance, and thus at a lower speed than c.
- edna314 6y ago
  Yes.
- GOONIMMUNE 6y ago
  What is warping the barrier's space? The barrier's mass?
johndoe42377 6y ago
Shows - how so? In a simulation based on some model? Okay.
newsbinator 6y ago
I'm not a physicist and this sounds like a fun finding. What are the minimum properties that constitute an effective "barrier" to jump? Would a single atom serve as a barrier to jump? Could we set up a particle accelerator that is just a circle of single atoms to jump, at faster than the speed of light?
amai 6y ago
The preprint with full text is available here: https://arxiv.org/abs/1907.13523 https://arxiv.org/abs/1907.13523
- amai 6y ago
  And here is another article with more details: https://www.scientificamerican.com/article/quantum-tunneling-is-not-instantaneous-physicists-show/ https://www.scientificamerican.com/article/quantum-tunneling... It was even discussed on HN: https://news.ycombinator.com/item?id=23917908 https://news.ycombinator.com/item?id=23917908
  - amai 6y ago
    "Steinberg admits that his team’s interpretation will be questioned by some quantum physicists, particularly those who think weak measurements are themselves suspect." Here is an article about some physicists who think weak measurements are suspect: https://physicsworld.com/a/are-weak-values-quantum-after-all/ https://physicsworld.com/a/are-weak-values-quantum-after-all...
daxfohl 6y ago
If the math of this has been known for a while, has anyone done the e2e math of what a FTL communication through this protocol would look like? Seems like it should be a straightforward calculation. My guess it it is a straightforward calculation, and that it is real. We'll have a hard time dealing with it existentially, but the math will be right and the physics will eventually be shown to correspond. We'll have something where a particle can be entangled with its past/future self, the future self being affected by the past self and vice versa. And that should be okay. It'll reignite the debate of the possibility of changing the past vs the lack of free will, effect preceding cause, but in a way it will push the "shut up and calculate" forward because it's all math. At least, that's what I somewhat hope will happen. It kind of reminds me of the relativity revolution. There was an inconsistency in E&M theory due to the constant speed of light, and we invented all kinds of ways to work around that, when it turned out that if you just assume constant speed of light then it worked out. Same thing here, we're philosophically rejecting FTL communication but perhaps the right thing is to accept it, follow the math QM gives us, and recognize that it's possible, that it's all just math, and while it's mind-bending, it's not actually all that spooky. Relativity is just the limit case of zero tunneling. QM is tricky, as soon as you think you have it cornered with a contradiction, it slips away at the last second. My guess, we'll discover that yes it is possible in a closed system for a cat to send the signal back in time that triggers the gun that shoots its former self. But it's not possible for that contradiction to escape the system, so upon opening the box, you'll always see a live cat. Or something like that.
- daxfohl 6y ago
  The nice thing about this would be more clarity around FTL stuff. The current thing where wave function collapse correlations can go FTL but communication cannot has always felt to be a bit of a cop out. Would love to see this defined more precisely.

128 points by jonbaer 6y ago

> quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility It's only an impossibility in space-time. But the quantum world, until the wave function collapses, exists outside of space-time. Entanglement is under the same effect -- space-time simply isn't involved. It's the collapsing of the wave function that brings it into space-time. At least that's how I understood it.

drdeca 6y ago
That doesn't seem to fit with my understanding all that well? Alternatively, I'm just not sure what you mean by saying that it ( wait, what is the "it"? "the quantum world"? But the world is the quantum world?) "exists outside of space time" . If one said "the number 3 exists outside of spacetime", that would make sense to me, as saying that it isn't a thing with a location in spacetime, but just a thing. But I don't think I know what you mean.
- vez- 6y ago
  The thing I think the original commenter was trying to get at is that spacetime (the mathematical model) and quantum mechanics (the mathematical model) for the most part are in no way unified. Things that happen in quantum mechanics are not necessarily explainable using spacetime. Although in general physicists believe a lot of things from spacetime for QM, like that information can't travel faster than light, it isn't actually baked into QM. That's my understanding at least.
  - drdeca 6y ago
    By spacetime do you mean like, in a general relativity sense? Because aiui quantum field theory is quite consistent with special relativity.
  - raattgift 6y ago
    The https://en.wikipedia.org/wiki/Dirac_equation https://en.wikipedia.org/wiki/Dirac_equation is now more than ninety years old and put quantum mechanics into the Minkowski spacetime of special relativity. There are several textbooks[1] covering extensions of this approach to curved spacetimes generally, and one will encounter the https://en.wikipedia.org/wiki/Klein%E2%80%93Gordon_equation#Gravitational_interaction https://en.wikipedia.org/wiki/Klein%E2%80%93Gordon_equation#... in graduate school settings. Quantum mechanics on some specific curved spacetimes are exactly Klein-Gordon solvable. The Standard Model of Particle Physics incorporates the Poincaré Group, which is the group of Minkowski spacetime ("flat spacetime") isometries. This means that the Standard Model particles are the same independent of where and when they are in empty flat spacetime (3 spacelike and 1 timelike translation invariances), their orientation (rotational invariance about the three spacelike axes), and they transform reliably under boosts (differences in constant velocity along the spacelike axes). So the Standard Model is defined by flat spacetime: and the causal structure of flat spacetime (in which the constant c plays a key role) is very much baked in. Studying modern quantum mechanics mostly means looking at patches of effectively flat spacetime in which there are particles of the Standard Model occupying different locations in the patch, boosted relative to each other, and interacting via the other mechanisms captured in the Standard Model's formulation. We inhabit a type of curved spacetime in which General Relativity guarantees at least an infinitesimal patch of exactly flat spacetime around every point in the universe. In regions with only gentle curvature -- like laboratories on the surface of the Earth, or in space probes in the solar system -- any curvature corrections to the assumed exactly flat spacetime of the Standard Model are tiny, because the region of effective (rather than exact) flatness is very large compared systems of particles under experimental study of their quantum behaviour. "Pretend it's flat" works exceedingly well in practice. When a researcher has to consider curvature (e.g. in relativistic massive objects like neutron stars) she or he can continue to work perturbatively against a flat-by-definition spacetime. The problems arise in the difference between a guarantee of a microscopic region of flat spacetime and an infinitesimal region of flat spacetime: if the radius of curvature is small compared to the spatial extent of a particle, things get ugly quickly, especially as we take the wavelength of the particle smaller (which means the energy of the particle climbs, and that is the sort of energy which creates spacetime curvature, so we get a nonlinear feedback, and classical General Relativity and Quantum Field Theory in Curved Spacetime make annoyingly different and incompatible predictions about what happens as one takes the limits of high particle energy and high curvature. Fortunately this incompatibility seems likely to occur only hidden inside event horizons, so it is a problem for the theories rather than a practical problem for all of us who are not actually rapidly approaching death within a black hole -- there also may be consequences for as-yet-undiscovered ancient tiny black holes, or stellar black holes many trillions of trillions of years in our future, so again we can take our time to understand the theoretical conflict). A bit more technically, the problem arises in perturbative approaches to QFT on curved spacetime: at low energies and low curvatures we have a fairly small number of correction terms which can be written out as a https://en.wikipedia.org/wiki/Taylor_series https://en.wikipedia.org/wiki/Taylor_series which we can truncate because the higher-order terms are demonstrably irrelevant. As we increase energies and curvature, irrelevant terms become marginal, then relevant; additionally, we start having to add more non-irrelevant terms. https://en.wikipedia.org/wiki/Renormalization https://en.wikipedia.org/wiki/Renormalization allows us to squash some of these terms together, but eventually we get an overwhelming growth of non-ignorable corrective terms and lose the ability to make predictions using this approach. This breakdown in perturbative renormalization ("perturbative quantum gravity" [2][3]) gives a useful qualitative definition of "strong gravity": it's where the perturbative approach breaks down. In terms of Feynman diagrams, it's where loops of gravitons enter into the picture; a bit more colloquially, it's where "gravitation's self-gravitation starts becoming non-ignorable". Although not a dead area of research, looking for ways to make renormalization work for strong gravity is less fashionable than looking for non-perturbative quantum gravity that (a) matches perturbative quantum gravity right up to the weak-side boundary of strong gravity, including classical General Relativity in weak gravity, (b) is calculable in practice, and (c) solves other non-gravitational problems that plague high energy particle physics that are amenable to testing, since we probably can't extract observational or experimental data from regions of strong gravity. Additionally, classical General Relativity fairly generically predicts the presence of gravitational singularities in spacetimes with significant amounts of matter[4]. Such singularities destroy the total predictability of the entire spacetime from a total sample of all the variables on an arbitrary slice across the whole space at a given time coordinate. In other words, there's an incompatibility between classical General Relativity and traditional initial values surfaces approaches to solving physics problems. This problem worsens in the presence of Quantum Fields because of Hawking Radiation: instead of a literal singularity there is instead a trapping structure that evaporates (or mostly evaporates, in "remnant" proposals) in the far future of most black hole systems. But the matter that is released in the evaporation cannot be predicted from the matter that was thrown into the black hole before evaporation, and at late times we lose the ability to account for quantum entanglements that existed when the black hole was growing. Unfortunately there are numerous black hole candidates in our universe, which we also know is filled by matter representable by quantum fields. Although this is not strictly an incompatibility of General Relativity and Quantum Field Theory, quantum physicists are very keen on preserving https://en.wikipedia.org/wiki/Unitarity_(physics) https://en.wikipedia.org/wiki/Unitarity_(physics) which is lost in black hole evaporation as is understood today. Preserving unitarity in the presence of strong gravity is a fourth goal of modern research into quantum gravity. - -- [1] e.g. Parker & Toms, QFT in Curved Spacetime (2009), https://books.google.co.uk/books?id=5nNuGMBBTjMC&dq=L.+Parker,+D.+Toms+(2009).+Quantum+Field+Theory+in+Curved+Spacetime:+Quantized+Fields+and+Gravity https://books.google.co.uk/books?id=5nNuGMBBTjMC&dq=L.+Parke... [2] http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... [3] https://en.wikipedia.org/wiki/Canonical_quantum_gravity https://en.wikipedia.org/wiki/Canonical_quantum_gravity [4] https://en.wikipedia.org/wiki/Raychaudhuri_equation https://en.wikipedia.org/wiki/Raychaudhuri_equation is a good place to start
gizmo686 6y ago
Quatom mechanics doesn't really have a concept of space-time. The notion of space-time is entirely a construction of relativity; and the two theories have yet to be unified. Collapsing the wave function doesn't bring a system into the domain of relativity. That happens at a fuzzy point where a system become massive enough for relativistic effects to become significant. In thoery, we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity. There is semiclassical gravity; which essentially says that the quantum fields described quantum mechanics occurs in the curved spacetime of relativity. This is (by design) just an approximation of the still unknown theory of quantum gravity; and we don't have good experimental evidence telling us how accurate of an approximation it is.
- c1ccccc1 6y ago
  Quantum mechanics doesn't work well with general relativity (gravity), but it works just fine with special relativity. Special relativity still has a concept of spacetime, it's just that the spacetime is flat, not curved like in general relativity. (In fact, you can even do quantum mechanics in curved spacetime, as long as the spacetime has a predefined fixed curvature. But in GR, the curvature depends on how the matter and energy inhabiting the spacetime are distributed. That is what gives quantum gravity theorists trouble.)
- tsimionescu 6y ago
  Quantum mechanics is fully unified with special relativity, and has a clear concept of spacetime. The Dirac equation was discovered in 1928 and was the key to solving this problem. This means that quantum mechanics is fully compatible with time dilation at large speeds, that it respects the limits of the speed of light (current article notwithstanding) etc. The only incompatibility is between quantum mechanics and general relativity - quantum mechanics can't work with the general relativityequations for a curved spacetime or for the gravitational effects of particles being in multiple places at once. These are the things that a theory of quantum gravity must solve.
- raattgift 6y ago
  > Quatom mechanics doesn't really have a concept of space-time The Standard Model of Particle Physics incorporates the Poincaré group, which is the isometry group of the flat spacetime of Special Relativity, so it has an exact concept of spacetime built right into the formulation. Quantum mechanics and Special Relativity were unified in the 1920s by Dirac. Solutions for some curved spacetimes came not much later, and are now taught in several textbooks. The problem is that we don't have a general solution for quantum mechanics on arbitrary curved spacetimes (and some curved spacetimes are known to make quantum field theories insoluble with current methods). I refer you to my sibling comment https://news.ycombinator.com/item?id=24877746 https://news.ycombinator.com/item?id=24877746 for more details. > (by design) just an approximation of the still unknown theory of quantum gravity Kind-of. It is manifestly an approximative approach because the equation includes "mean" angle-brackets around the stress-energy tensor, which, speaking roughly, indicate that the curvature is generated by the averaged value of the stress-energy generated by the quantum field(s) are used, rather than an exact value at each and every infinitesimal point in the solution. This "classicalizes" the source term of the classical curvature, erasing quantum effects. It is not really in the strictest sense an approximation to a theory of quantum gravity, however any candidate quantum gravity theory had better explain results from semiclassical gravity that find some support in astronomical observations. Contrasting approaches -- like canonical quantum gravity -- "quantumize" the metric, leaving the source term fully quantum mechanical and non-averaged, and results from such approaches also find some support in astronomical observations. These are arguably approximations to a full theory of quantum gravity since one can cast them as a series of terms similar to a Taylor series; the problem is that the set of such terms appears to grow without bounds as one takes energy-momentum into the "ultraviolet" regime (UV ~ extreeeeeemely high energy-momentum densities). Worse, the relative weight of the contribution of some of the far-from-leading-order terms grow stronger in the UV limit, so not only can't we just ignore them, we also can't apply perturbative renormalization. Fortunately this probably only happens inside black hole event horizons and near the big bang, one of which is probably not observable, and the other of which is at high enough individual energies that the Standard Model doesn't predict results even if their collective mutual gravitation (and the contribution of dark matter and other energy-densities) could be factored out. > we would probably expect to see some relativistic effects in entangled systems; but predicting what those effects are requires a theory of quantum gravity We can calculate the effects of a boost, even an ultraboost, on particles with mutually-entangled properties just fine thanks to the Poincaré invariance baked into post-Dirac relativistic quantum mechanics. If you were flying past from an entanglement experiment in deep space at ultrarelativistic speeds, one could predict your observed redshift (etc) of the entangled properties exactly with no worries. (If you were flying radially to or from an Earthbound experiment at ultrarelativistic speeds you might want to incorporate some corrections from the https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab... for greater accuracy. The post-Newtonian expansion leading-order corrections attributable to Earth's gravity will not be large; linearized gravity is enough. Errors from the next-to-leading-order corrections will not be measurable with current technology, and we can brute force calculate at least next-to-next-to-leading order corrections, which is good enough for accelerating down towards the surface of a neutron star, like the material that is blown onto the Hulse-Taylor binary members.
tsimionescu 6y ago
Relativistic quantum mechanics exists and is a complete theory - the Dirac equation shows how particles move taking into account the speed of light limit, and in the limit of small speeds reduces to the Schrodinger equation. Moreover, one of the most important results that confirmed entanglement is possible was the no-communication theorem, that proved that it is impossible to use entanglement to communicate faster than light. This is also suggests that wave-function collapse of an entangled system can't really be a physical phenomenon.
- tashi 6y ago
  > can't really be a physical phenomenon Can you say more about that? What are the alternatives to being a physical phenomenon?
  - sdenton4 6y ago
    Being a mathematical formalism which happens to nicely describe the reality, perhaps?
  - tsimionescu 6y ago
    If you want to learn more about this, it is usually referred to as "measurement problem". Basically, quantum mechanics is extremely accurate at predicting experimental results by using 2 completely separate rules: the Schrodinger equation on one hand, and the Born rule on the other. The Born rule is what is also referred to as wave function collapse: if you want to accurately predict what happens to a particle after it interacts with a detector (but NOT after an interaction with another particle), then you have to use the Schrodinger equation as a probability. Now the physical meaning of these 2 different rules are debated. In the standard (Copenhagen) interpretation of QM, it doesn't make sense to talk about the properties of particles until you measure them, so neither the wave function nor the Born rule can be given a physical meaning. In the MWI interpretation, all results of the Schrodinger equation exist in different worlds, so there is no collapse, the detector just finds out "in which world you live". In superdeterministic theories, the Schrodinger equation is just an approximation of a currently unkown fully deterministic theory of motion. In pilot-wave theory, the Schrodinger equation describes an actual wave that particles generate themselves (the motion of the wave and the motion of the particle influence each other); entangled particles are just particles that live on the same wave. There is no collapse here, but the wave from each particle exists in the whole universe and can change non-locally.

hilbert42 6y ago

As I mentioned several days ago here: https://news.ycombinator.com/item?id=24837545 https://news.ycombinator.com/item?id=24837545 I would like to know mode

hilbert42 6y ago

As I mentioned here https://news.ycombinator.com/item?id=24837545 https://news.ycombinator.com/item?id=24837545 a few days ago in an earlier post on the same topic, I raised the matter of how this finding gels with other related physics. There, I asked physics experts to explain the issues to this dummy but so far none have been prepared to take the bait. If, for instance, particles can exceed c0 (the speed of light in a vacuum) during tunnelling then how does this fit with the fact that vacuum permittivity and vacuum permeability 'also sets' the speed of c through the relationship: c = 1/(μ0 ε0)^0.5 This means that either a particle has to completely 'vanish' from the physical world during tunnelling or that μ and ε don't apply during this time, or that they change value. It seems to me the ramifications of this are very significant. The corollary of this is that if the wave function collapses outside spacetime then it's meaningless to discuss the particle's speed. That said, some months ago I read a report that some group actually measured (or estimated) the time of an election's quantum jump from one shell to another. If this is actually measurable time then it seems that the wave function is not outside or independent of spacetime (perhaps at these super-fast speeds/super-short times we're seeing the granularity of matter and or the intersection of the quantum and relativistic worlds). That would be fun wouldn't it. That's a super rough outline of my point (it's too involved here to go into other possibilities that arise from QED etc., and I don't claim sufficient fluency to put them cogently). There's a bit more in that earlier post (but sorry I didn't explain what I was driving at very well).

hashtagmarkup 6y ago
If the "particles" don't have any mass, the "issues" you're assuming are likely irrelevant, as they would result in division by 0.
- SpaceRaccoon 6y ago
  But massless particles still have relativistic mass due to their velocity. That's why photons still have momentum and can exert pressure.
  - rrobukef 6y ago

gog-ma-gog 6y ago

Articles like this on the internet are usually completely garbage, but Natalie is a really fantastic journalist. If you’re worried about woo because of the title, there isn’t any here (and quanta magazine is generally great for coverage of science) tldr: there’s a calculation you can perform in quantum mechanics that suggests particles can tunnel through barriers faster than they could have traveled through free space at the speed of light. Scientists have now measured this more precisely, and this effect seems to hold up, but is still very small, so it’s hard to understand the ramifications of this for things like causality.

hilbert42 6y ago
Right, articles like this are often short on crucial details, it's speculation until we see the research papers. Re causality etc., as I mentioned above, if the duration of wavefunction collapse can actually be measured then this puts a whole new spin on the matter (sorry, puns aren't my forte but it seemed appropriate here). ;-)
- hilbert42 6y ago
  Update, FYI, I've just found some links about the matter and posted them above. These describe an experiment where an electron was seen transiting smoothly between two energy states whilst being in two places at once.
6gvONxR4sf7o 6y ago
I wish more science journalists were like this. Able to not get all the core facts wrong.

SomewhatLikely 6y ago

> Steinberg, who agrees with the statistical view of the situation, argues that a single tunneled particle can’t convey information. A signal requires detail and structure, and any attempt to send a detailed signal will always be faster sent through the air than through an unreliable barrier. This argument doesn't make a lot of sense to me. One particle arriving can signal information that some event has occurred at the source. Re: structure/complexity I imagine you could use different elements (and maybe spin too?) to form several bits.

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lisper 6y ago
The devil is in the details. I haven't done the math (I'm probably not capable of actually doing the math) but my bet is that it is not possible to set up a tunneling barrier that does not also effect the spin component of the wave function. My further bet is that it will turn out that the uncertainty introduced into the spin component by the tunneling barrier is just enough to produce a no-communications theorem for tunneling just as there is one for entanglement. I'll even place a third bet that there's a Ph.D. thesis (if not a Nobel prize) at the bottom of that rabbit hole.
h0l0cube 6y ago
I think it's a matter of a low signal-to-noise ratio. So if you send one bit, but there's a low probability the other side will receive it, you'll need so much redundancy and error correction to achieve the same bandwidth as a regular signal, that it ends up being at least the same transmission rate.
lacker 6y ago
I think your intuition is correct and that Steinberg is agreeing with you here. Steinberg is just saying that a single particle alone cannot generate a causality paradox. But it still isn't obvious why you couldn't use this effect with multiple bits and some error correct to send signals faster than light. Looking at the paper being discussed here - https://iopscience.iop.org/article/10.1088/1367-2630/abb515/meta https://iopscience.iop.org/article/10.1088/1367-2630/abb515/... - the authors also don't seem to have an explanation. One final caveat worth noting is that our results do not definitively prove that an ensemble of free particles will always be a preferable method of transmitting a signal to an ensemble of tunneled particles. It is our intention to include a discussion of this in a follow-up paper. This is basically the scenario you're talking about, forming several bits out of different particles and using them in conjunction to send information faster than light. The researchers aren't sure why this wouldn't happen in their model.

aaron695 6y ago

Flat Earthers and people who think you can break the Speed of Light are the same, let's not kid. You can word play if you chose, but it's not clever. You can't send information faster than the speed of light.

vixen99 6y ago
Your comment is entirely word play.
absolutelyrad 6y ago
This comment won't age well.
- hilbert42 6y ago
  An aside, Lord Kelvin made a similar sweeping statement about the 'completeness' of physics knowledge at the end of the 19th Century just before QM saw the light of day!
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dvh 6y ago

> The researchers reported that the rubidium atoms spent, on average, 0.61 milliseconds inside the barrier, in line with Larmor clock times theoretically predicted in the 1980s. That’s less time than the atoms would have taken to travel through free space. Light travels 182km in 0.61ms so it would be significantly slower than light.

crispyporkbites 6y ago
So if the barrier was 182km in thickness, the particles would arrive faster than the speed of light
heavenlyblue 6y ago
Particles rarely travel in straight lines but light does

szundi 6y ago

I suppose we won’t get the notification about apologizing and retracting the results.

kbwt 6y ago

I am not a physicist, but the experiment does not seem convincing to me. The Larmor clock doesn’t measure a proper time as in spacetime distance between two events. Instead it measures the rotation of a dipole in an electromagnetic field. The experimenters apply a combination of an electrostatic field (the barrier) and a magnetic field (the timer driving Larmor precession). In the frame of reference of the spinning particle, this is exhibited as a sum of two electrostatic fields. The barrier is a locally uniform repulsive electrostatic field, while the the timer is a radially varying axial electrostatic field. The particle is a dipole, tilted from the timer axis by the precession angle. When the particle tunnels through the barrier, it also tunnels through the timer field, without precessing over the tunneled distance just as the particle is not exhibiting repulsion from the barrier over the same distance. This is experimentally verifiable as the amount of missed precession has a lower bound proportional to the sine of the angle between the magnetic field and the barrier.

i_cannot_hack 6y ago
I'm curious, what field are you in? For a non-physicist you seem quite knowledgeable and confident about physics and quantum phenomena
- kbwt 6y ago
  I work on software for augmented reality and distributed systems. That background is not directly applicable to quantum physics, but I like to think that I am highly trained at spotting logical errors.
- theshadowknows 6y ago
  Right?!?
amai 6y ago
Tunneling doesn't mean jumping. The probability wave of the particle does exist inside the barrier, see e.g. https://commons.wikimedia.org/wiki/File:TunnelEffektKling1.png https://commons.wikimedia.org/wiki/File:TunnelEffektKling1.p... So yes, it "tunnels" through the timer field also, but the probability of interacting with the timer field is not zero. I would be interested if the Lamor clock also shows a shift in the reflected particles. Because that would mean, also the reflected particles spend some time inside the barrier.

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elgfare 6y ago

Somehow this reminds me of myelin sheaths, which IIUC are insulating barriers that increase the rate of transmission in neurons.

GnarfGnarf 6y ago

The barrier's space is warped and compressed, so the particles are traveling a shorter distance, and thus at a lower speed than c.

edna314 6y ago
Yes.
GOONIMMUNE 6y ago
What is warping the barrier's space? The barrier's mass?

johndoe42377 6y ago

Shows - how so? In a simulation based on some model? Okay.

newsbinator 6y ago

I'm not a physicist and this sounds like a fun finding. What are the minimum properties that constitute an effective "barrier" to jump? Would a single atom serve as a barrier to jump? Could we set up a particle accelerator that is just a circle of single atoms to jump, at faster than the speed of light?

amai 6y ago

The preprint with full text is available here: https://arxiv.org/abs/1907.13523 https://arxiv.org/abs/1907.13523

amai 6y ago
And here is another article with more details: https://www.scientificamerican.com/article/quantum-tunneling-is-not-instantaneous-physicists-show/ https://www.scientificamerican.com/article/quantum-tunneling... It was even discussed on HN: https://news.ycombinator.com/item?id=23917908 https://news.ycombinator.com/item?id=23917908
- amai 6y ago
  "Steinberg admits that his team’s interpretation will be questioned by some quantum physicists, particularly those who think weak measurements are themselves suspect." Here is an article about some physicists who think weak measurements are suspect: https://physicsworld.com/a/are-weak-values-quantum-after-all/ https://physicsworld.com/a/are-weak-values-quantum-after-all...

daxfohl 6y ago

If the math of this has been known for a while, has anyone done the e2e math of what a FTL communication through this protocol would look like? Seems like it should be a straightforward calculation. My guess it it is a straightforward calculation, and that it is real. We'll have a hard time dealing with it existentially, but the math will be right and the physics will eventually be shown to correspond. We'll have something where a particle can be entangled with its past/future self, the future self being affected by the past self and vice versa. And that should be okay. It'll reignite the debate of the possibility of changing the past vs the lack of free will, effect preceding cause, but in a way it will push the "shut up and calculate" forward because it's all math. At least, that's what I somewhat hope will happen. It kind of reminds me of the relativity revolution. There was an inconsistency in E&M theory due to the constant speed of light, and we invented all kinds of ways to work around that, when it turned out that if you just assume constant speed of light then it worked out. Same thing here, we're philosophically rejecting FTL communication but perhaps the right thing is to accept it, follow the math QM gives us, and recognize that it's possible, that it's all just math, and while it's mind-bending, it's not actually all that spooky. Relativity is just the limit case of zero tunneling. QM is tricky, as soon as you think you have it cornered with a contradiction, it slips away at the last second. My guess, we'll discover that yes it is possible in a closed system for a cat to send the signal back in time that triggers the gun that shoots its former self. But it's not possible for that contradiction to escape the system, so upon opening the box, you'll always see a live cat. Or something like that.

daxfohl 6y ago
The nice thing about this would be more clarity around FTL stuff. The current thing where wave function collapse correlations can go FTL but communication cannot has always felt to be a bit of a cop out. Would love to see this defined more precisely.