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August 5

Quantum vacuum plasma thruster

There has been some discussion of the quantum vacuum plasma thruster, where it was reported at a NASA conference that it worked - producing thrust without propellant. However, according to the article it is not a reactionless drive because it acts against virtual particles in the vacuum. So.... virtual particles are short-lived "errors in bookkeeping", not a permanent receptacle for energy or momentum, right? So far as I know, anything that can be written as virtual particles can be written as some other exercise in uncertainty, can't it? So my assumption is that the energy and momentum of the photon somehow manages to pass out of the chamber, but ... it still has to end up as a photon on the far end, doesn't it? Which gets to the basic hard fact I don't know, namely how much energy it takes to get how much thrust with a QVPT, and whether that is any better than simply powering a laser and relying on the light pressure. That little NASA blurb I saw doesn't give the power used. On the other hand, I suppose a virtual antiparticle could, in concept, go out through the chamber, find a real matter particle to annihilate with somewhere... it'd be like the transactional interpretation with the advanced wave from the outer air or whatever pushing on matter in the chamber. Temporal mechanics... But then again, I don't understand what kind of virtual vacuum particle can make it through a resonant microwave chamber. If the virtual particles stay trapped in the chamber until they self annihilate how can they carry away any momentum?

I know, this concept is probably bogus and my stream of consciousness is running particularly muddy today... throw me a clue? Thanks. :) Wnt (talk) 01:30, 5 August 2014 (UTC)[reply]

My impression is that regardless of what the article says, a QVPT would violate the conservation of momentum, and as such is just as complete and utter nonsense as the perpetual motion devices that are intended to violate the conservation of energy. I made an edit to the article last Thursday. The version of the article right before I made that edit[1] was so badly biased that there wasn't a single sentence in it that would detract a reader from getting the impression that a QVPT is based on unquestioned physical theory, that's now been experimentally verified, and we just need to wait a couple years for the technology to be scaled up and start being used in spacecraft. I tried to help move the article at least a little closer to giving a balanced perspective, and I see that William M. Connolley has also made a half dozen edits today to try to help push the article toward a more balanced perspective, but it's still rather badly biased. For an article on basically the same exact thing that isn't quite as biased, see EmDrive. Red Act (talk) 02:54, 5 August 2014 (UTC)[reply]

A few comments:

Foremost, I have a pdf copy of the paper, anyone can feel free to contact me via the 'email me' link on my user page and I will send it off. I know a lot of us don't have access to it, and this will surely aid in discussion.
Said paper clearly states in the abstract "This paper will not address the physics of the quantum vacuum plasma thruster (QVPT), but instead will describe the recent test campaign."(bolding is mine) -- so even the authors are not confident enough in their understanding of the physical mechanisms at play to attempt to publish an explanation at present.
As for light pressure, the calculations I've seen seem to indicate that this small force is indeed greater than would be accomplished through using photovoltaic panels to power lasers on satellites. Turn it around: if solar powered lasers were a good way to accomplish positional thrusters on satellites, wouldn't we be doing that already? I won't link to the source because it's not reliable, but there are extended discussions of this work on metafilter and reddit that you might want to peruse.
Many people seem to think this must be observational error, because thrust was also recorded on the 'null test' engine, which was not thought to provide thrust. However, the true control was an RF load with no mechanism, and there was a true zero thrust for that test. I've pasted a key table of results below, if anyone wants to prettify it I'm sure it would be appreciated.
(Many of the key figures are screen grabs and I am personally disappointed that none of the authors could be bothered to put out a nice vector graphic to illustrate the potentially valuable findings.)
some people say it would violate conservation of momentum, but it's not quite so clear to me that that must be the case. I imagine there's all kinds of physics of virtual particles and different relativistic frames that might explain the apparent paradox. (These are mentioned and referenced in the Emdrive article.)
This paper is a record of conference proceedings I am not a physicist, but in most of the sciences that I am aware of, this kind of thing goes through minimal if any peer review, compared to a 'real' journal article.
I don't think anyone is really an expert on this stuff yet, so most of us will have to be content to wait for a bit and see what shakes out...

Table of results

Configuration	Test Article	Thrust Direction	Thrust Range (μN)	Mean Thrust (μN)	Number of Test Runs
1A	Slotted	Forward	31.7 – 45.3	40.0	5
1B	Slotted	Reverse	48.5	48.5	1
2A	Unslotted	Forward	35.3 – 50.1	40.7	4
2B	Unslotted	Reverse	22.5	22.5	1
RF Load	50Ω Load	N/A	0.0	0.0	2

So, no real answers, but interesting stuff! SemanticMantis (talk) 03:08, 5 August 2014 (UTC) (ETA: table now displays properly)[reply]

My understanding of this thing is that it's just a photon drive which has been known to work (but produce pitiful thrust) for a long time now. 1 ScienceApe (talk) 04:51, 5 August 2014 (UTC)[reply]

No, photon-based systems are different. Instead, White believes he's going to get thrust by pushing quantum vacuum fluctuations. In White's words, "How does a Q-thruster work? A Q-thruster uses the same principles and equations of motion that a conventional plasma thruster would use, namely Magnetohydrodynamics (MHD), to predict propellant behavior. The virtual plasma is exposed to a crossed E and B-field which induces a plasma drift of the entire plasma in the ExB direction which is orthogonal to the applied fields. The difference arises in the fact that a Q-thruster uses quantum vacuum fluctuations as the fuel source eliminating the need to carry propellant."[2] Red Act (talk) 06:57, 5 August 2014 (UTC)[reply]

I think they still consume energy. Wouldn't that make it similar to an engine that expels massless photons but different from a plasma drive that expels mass particles? The measure of newtons per kilowatt leads me to believe they are creating a nonrecoverable energy outflow that would look similar to photon propulsion. --DHeyward (talk) 08:17, 5 August 2014 (UTC)[reply]

No, it's different. Although a photon has a zero rest mass and hence according to Newtonian mechanics it should have zero momentum, relativistically it does carry a tiny bit of momentum; see Photon#Physical properties. So a spacecraft accelerating itself forward a bit by emitting a laser beam out the back is still obeying conservation of momentum. But a spacecraft that somehow pushes on quantum fluctuations isn't leaving a beam of particles behind it carrying momentum in the opposite direction (virtual particles don't exist for macroscopic periods of time), so if the spacecraft is accelerating, momentum isn't being conserved. Quantum fluctuations can indeed carry momentum, as per the uncertainty principle, but only for very short distances (momentum and position are conjugate variables). A vacuum containing no (nonvirtual) particles or macroscopic fields doesn't carry any momentum macroscopically. Red Act (talk) 11:07, 5 August 2014 (UTC)[reply]

So where is the energy they express in N/kW going? --DHeyward (talk) 11:40, 5 August 2014 (UTC)[reply]

The energy they put into the test device during the testing process got turned into heat.

Here is a critique of the experiment, that was posted by John Baez, a somewhat famous physicist and mathematician. To me, the most damning thing about the experimental results is that the device that was designed to be similar to the "working" device, but was specifically designed not to work, produced just as much measured thrust as the device that was intended to work. If the "working" device doesn't produce any more force than the "dummy" device that just exists for the express purpose of measuring how much measured force you get from just experimental error, then what should be the obvious conclusion is that the "working" device doesn't produce any force at all that can't be attributed to experimental error.

Here is another post by John Baez, which talks about how the "quantum vacuum virtual plasma" that's the crucial ingredient that enables the device to function, isn't even something that actually exists![3] Quantum vacuum fluctuations exist, but they don't form a plasma, so you can't push them like you can push a plasma. Red Act (talk) 14:50, 5 August 2014 (UTC)[reply]

I'm not going to debate the physics, and I too remain very skeptical of this device. Still, your "most damning thing" does not make sense to me. The paper discusses two control-type items. The fact that the 'unslotted' engine still produced thrust doesn't necessarily mean that the measurement was experimental error. Logically, it could also be the case that they simply didn't understand the function well enough to disable it. To my reading they just removed one piece. refrained from carving out slots at the end of the gizmo. The much better control item, the real null test, is referred to as "RF load" which did indeed come out to zero thrust with their measurement system. It is possible that something went wrong in the measurements between the RF load tests and the 'unslotted' tests, but the fact that the RF load came out to zero does give some evidence against measurement error. SemanticMantis (talk) 15:21, 5 August 2014 (UTC)[reply]

Indeed, the more damning thing would appear to be Baez's second point, that while they conducted the experiment in a vacuum chamber with the door closed, they left it at ambient atmospheric pressure because their RF amplifier contained electrolytic capacitors which can't withstand a vacuum. -- ToE 23:12, 5 August 2014 (UTC)[reply]

This is certainly outside my training, but I've done some study in the history of science in the 18th and 19th century. When some surprising effect has come up in science in the last couple of centuries (and failure to conserve momentum is certainly "surprising," just like the recent "faster than light particles" which were debunked) even before someone finds a good mathematical and theoretical basis for it, good researchers will try not just to duplicate the effect but to actually eliminate it by controlling for possible confounds or measurement errors. Faraday was well known to find some small surprising electromagnetic effect, and then to work diligently until he was able to eliminate it by controlling some minor flaw in the (then very crude but ingenious) equipment. It is encouraging that he effect was replicated to some extent in a NASA lab and not just at the lab of the first discoverers, or even at the lab in China. A robust effect should show up when tested with apparatus built by different parties and used in a different lab by different researchers, One looks for control experiments to work, so that the effect is there when it should be and gone in a well selected control experiment, but apparently it did not go away in the control experiment. Extremely sensitive apparatus was used to measure the thrust; how far is the thrust above the limit of detectabiity or the noise level of the instrument? Were the NASA researchers "true believers" or skeptics? Edison (talk) 18:23, 5 August 2014 (UTC)[reply]

I appreciate your comments, but this experiment was well controlled. Twice now I've posted above about how there were two different types of "control" tests reported on. The simplest control with no mechanism was measured as providing zero thrust. The apparatus used was a "Scientech SA 210 precision weighing balance (resolution to one micronewton)." The highest thrust recorded was 48.5 micronewtons. The "zero" thrust should I suppose be reported as "0+/- 0.5 micronewtons". The table above now displays properly, and has all the results. SemanticMantis (talk)

Tey pump 28 watts into a metal enclosure in a test chamber, it heats up, there are certainly convection currents, and they get a resulting unexplained force of micronewtons, like the weight of a few milligrams? If they'd put a propellor over it it would have spun from the convection currents. It could be rising like a hot air balloon, or from the air streaming around it due to convective heating. They need to do the high vacuum testing and report back. Edison (talk) 23:49, 5 August 2014 (UTC)[reply]

Focusing solely on the (reported, purported) precision of the balance is rather missing the point. When the two options to explain a miniscule effect are "entirely novel physics" and "experimental error", it takes more than what we've got here to favor the former over the latter as a credible conclusion. Given that we're still at the science-by-press-release stage (there hasn't been a peer-reviewed paper with these results), and given that this was a relatively brief experiment (my understanding is it was forty person-days of setup, experiment, and takedown; so four people working for just two weeks, who between them only managed to test two control conditions) I can't reasonably conclude that they have exhaustively considered all possible sources of error. Honestly, today's XKCD pretty much hits the nail on the head. TenOfAllTrades(talk) 14:59, 6 August 2014 (UTC)[reply]

It's worth noting that there actually is a full report online ^{[4] (search with the title for copies...)} - the original link I followed appeared to be merely a conference abstract. Some statements on the power from the paper are:

During the first (Cannae) portion of the campaign, approximately 40 micronewtons of thrust were observed in an RF resonant cavity test article excited at approximately 935 megahertz and 28 watts.
During the subsequent (tapered cavity) portion of the campaign, approximately 91 micronewtons of thrust were observed in an RF resonant cavity test article excited at approximately 1933 megahertz and 17 watts.

And from the paper that Red Act posted:

"The near term focus of the laboratory work is focused on gathering performance data to support development of a Q-thruster engineering prototype targeting Reaction Control System (RCS) applications with force range of 0.1-1 N with corresponding input power range of 0.3-3 kW."
"Up first will be testing of a refurbished test article to duplicate historical performance on the high fidelity torsion pendulum (1-4 mN at 10-40 W)."

So these figures are 1.4 uN/W, 5.4 uN/W, 1 mN/W (wished-for), 100 uN/W. Which seems a little diverse, but at least it gives us an idea. Now for comparison, 1 W = 1 J/s = 11.1265 femtograms per second. Multiply that by 299792458 m/s and get 3.3356 microgram m / s² = 3.3356 ~~micro~~nanonewtons. So I'm calculating out that 28 watts x 3.3356 = 93 ~~micro~~nanonewtons and 17 watts x 3.3356 = 57 ~~micro~~nanonewtons - figures that are nearly equivalent to their results, but the second is not quite as big. Nonetheless, their input power of 17 watts is calculated from the power emitted - reflected, leaving me with some room for suspicion.

As cool as this is, I'd like to see some more evidence that this really beats a photon drive. On the other hand, per my discussion of refraction below, I'm thinking you might be able to soup up a photon drive by a small factor by that sort of effect... so I wouldn't immediately assume a factor under 2 is 1. Wnt (talk) 15:57, 6 August 2014 (UTC)[reply]

I'm all for E = pc, but 3.3356 microgram m / s² = 3.3356 nanokilogram m / s² = 3.3356 nanonewtons. MKS, y'all. -- ToE 21:12, 6 August 2014 (UTC)[reply]

Awww shiiii... -- no, wait a minute, that means this idea is still on, which is good news! Thank you for catching the error! Wnt (talk) 21:27, 6 August 2014 (UTC)[reply]

When they used a resistor as a control, did they put it in the same metal enclosure used with microwave input? And just feed it 28 watts of DC electricity/ Thats one of the controls Faraday might have tried if he found an AMAZING effect like this, rather than rushing to publish it in the world's popular press. Edison (talk) 04:09, 7 August 2014 (UTC)[reply]

They did run the resistive load inside the chamber - see Figure 13. Nonetheless, there do seem to be some differences in the wiring as far as I can tell - with so many conductors, and a potentially conductive vacuum chamber, and resonant effects at certain microwave frequencies, I am suspicious that some unexpected induction could be going on. But.... they're NASA engineers, and I have only the vaguest notion of the workings of a microwave oven, so I think I'd better consider they might not be dumber than me. :) Wnt (talk) 10:43, 7 August 2014 (UTC)[reply]

If a test in space is too expensive, they could test it in vacuum chambers of varying sizes to get an idea where the resonant effects are going when the chamber size approaches infinity.

Aaaaannnd... it's [on http://xkcd.com/1404/ xkcd]. - ¡Ouch! (^{hurt me} / _{more pain}) 15:11, 7 August 2014 (UTC)[reply]

Cooking with car exhaust

Let's say you hook up your exhaust pipe on your car to a contraption that allows you to heat up food. Would the contaminants in the car exhaust, also contaminate your food and make it unfit for consumption? ScienceApe (talk) 04:47, 5 August 2014 (UTC)[reply]

Not if the system is well designed. The thermal conductivity of a metal barrier is high, but the mass diffusivity is low; see the mention of metals in Permeation#Description.

Similar to your idea, in the "Food fables" episode of Mythbusters they cooked a Thanksgiving dinner using the heat produced by a car engine. However, they placed the food within the engine compartment, instead of using a contraption attached to the exhaust system. Red Act (talk) 05:29, 5 August 2014 (UTC)[reply]

Top Gear did that years before Mythbusters. 131.251.254.110 (talk) 12:14, 5 August 2014 (UTC)[reply]

Manifold Destiny beat them by a decade or so. See, we used to read these things called "books" printed on some sort of tree product before the interwebs were as popular... DMacks (talk) 15:29, 5 August 2014 (UTC)[reply]

During World War 2 and other conflicts, didn't the troops sometimes cook their rations by placing them atop a tank's engine compartment? 24.5.122.13 (talk) 05:50, 5 August 2014 (UTC)[reply]

You just wrap it in aluminium foil - see: Engine Cooking and How to cook food on you car engine. Richerman (talk) 06:42, 5 August 2014 (UTC)[reply]

Ah, there's nothing quite like the taste of incomplete-combustion-smoked barbecue. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:15, 5 August 2014 (UTC)[reply]

In the 1980s I spent a lot of time touring Australia on motorbikes, and I met a bloke on the side of the road, who had a little water boiler, which slipped over the header pipes. It held just enough water to make a cup of coffee and was a lot more convenient than getting the Primus stove out. My grandfather once told me when the family were out picnicking on the motorbikes in the 1950s, he used to put the teapot on one of the cylinders of his flat-twin to keep it warm. I never quite believed him... --TrogWoolley (talk) 12:42, 5 August 2014 (UTC)[reply]

It's a common way to cook in cold, snowy climates. Wrap it in foil, tuck it into your snowmobile's exhaust headers, and when you get to where you're going, the food is done. Justin15w (talk) 15:16, 5 August 2014 (UTC)[reply]

If you shove something in an exhaust pipe, it will clog it, and result in carbon monoxide spilling into the passenger compartment, killing everyone inside. Not a very good way to cook stuff. 108.170.113.22 (talk) 16:11, 5 August 2014 (UTC)[reply]

Only if it can withstand the pressure of the engine exhaust stroke and the exhaust system itself is weak/leaky.[5] DMacks (talk) 16:18, 5 August 2014 (UTC)[reply]

Sorry, I don't see where I ever implied that it would be smart to stick the food inside the exhaust pipe. Justin15w (talk) 14:55, 6 August 2014 (UTC)[reply]

The main thing here is to separate the food from the actual exhaust fumes...foil should suffice for that. The temperatures you'll get would be higher up by the engine's exhaust manifold - and I think that's where most engine cooking is done. I've known a few real experts at it - and they use the water outlet to get temperatures around 100 degC and the exhaust manifold for higher temps. I've had eggs boiled in the top of the radiator too. I'm told that it's possible to fry bacon on disk brakes after a long downhill section...but that seems like an "advanced" technique! SteveBaker (talk) 19:50, 5 August 2014 (UTC)[reply]

It's curious whether science ape is still alive? (Not so much that I actually want an answer.) μηδείς (talk) 22:32, 5 August 2014 (UTC)[reply]

Energy and momentum of refracted light

This is actually based on the question about quantum vacuum plasma thrusters above, but here let's stick to known physics of plain old refraction. As I understand it, light travels both more slowly and in a different direction when it enters an object with a high refractive index. I assume this means that it has both a different momentum, and different energy. Otherwise, for example, an object in the water struck by a photon would be propelled according to the original heading of the photon before it was refracted, and therefore not directly opposite to the photon at the point of impact!

What is the momentum and energy of a refracted photon?

Is the momentum delivered at the surface of a refracting object ~~parallel~~ ~~perpendicular~~ nay, parallel to the surface?

What is the momentum delivered to, precisely? In terms of which particle?

Now suppose light is produced within the high refractive index material, speeding up to full c when it reaches the surface and changing direction. Does it extract energy and momentum from the boundary of the material?

Does the ratio of momentum / energy vary depending on the refractive index of the material in which the light is produced?

Does this allow for a potential interpretation that the quantum vacuum plasma thruster is equivalent to siting your microwave emitter within material of some unusual refractive index, and if so what? Wnt (talk) 16:31, 5 August 2014 (UTC)[reply]

Refraction is a special case of conservation of momentum. Snell's law is a special case of refraction. Deriving Snell's law from conservation of momentum is a common homework problem in an advanced physics class. The math is on our article, and it's really easy algebra ... but suffice to say, if you're not already pretty quick with the mathematical physics, then "it's going to take a while to explain..." Do you want help tracking down reference material on this topic? Nimur (talk) 18:26, 5 August 2014 (UTC)[reply]

Ach! You're right... somehow I misremembered and had the arrow pointing 90 degrees off here. The momentum is indeed conserved in the directions parallel to the surface. But surely the slower-moving light doesn't retain its full momentum in the perpendicular direction? And... oh phooey, I'm trying to recall how it can be that light can keep its full transverse momentum, yet be moving both more slowly and at less of an angle relative to the surface while doing so. Wnt (talk) 19:03, 5 August 2014 (UTC)[reply]

Oh, are you kidding me? I went searching to figure out how momentum was conserved, arriving at stackexchange which highlights the Abraham–Minkowski controversy, i.e. that there has actually been disagreement on what the momentum is in the dielectric medium. The rival forms were

The Minkowski version: $p={\frac {nh\nu }{c}}$
The Abraham version: $p={\frac {h\nu }{nc}}$

And... the article says that this has been proposed as the basis of a reactionless drive! So I may not remember half of the optics I learned in physics class, but maybe I'm on the right track anyway. :) It all comes down to a paper said to reconcile the differences: [6] Now maybe if I learn what a polariton is I can figure out if you can have virtual polaritons... :) Wnt (talk) 19:17, 5 August 2014 (UTC)[reply]

Lots of things have been "proposed as the basis of a reactionless drive". For example, you could put a strong guy in a sealed chamber and have him repeatedly punch one of the walls really hard. That idea has now been proposed at least once. That should be enough (evidently) to get funding from NASA for an experimental test. If I conduct the test on Earth instead of in deep space, and use a sufficiently imprecise accelerometer, I'm sure I'll detect some effect.

With that off my chest, I'll try to answer your refraction questions. Just considering the 1-dimensional case, you can handwavingly think of the slower speed in a medium as being due to the light being repeatedly absorbed by the medium and re-emitted after a delay. The absorption transfers momentum to the medium, giving it a kick forward, and the re-emission gives it an equal kick backward. The net effect is that the medium returns to its initial velocity, but in the mean time it moved somewhat in the direction of the light.

If you put a light source in the medium, it will be pushed backward when it emits the light, and the light will give an additional net backward kick to the medium as it leaves. This leaves you with light moving in one direction, and the light source and medium moving in the opposite direction, with equal and opposite momenta. This is not a reactionless drive but a photon rocket.

I don't think you can explain Snell's law in this handwavy particle picture, but I'm not sure it's relevant anyway. -- BenRG (talk) 20:20, 5 August 2014 (UTC)[reply]

@BenRG: I keep reading that the idea of light being absorbed and reemitted in a medium is wrong, but I can't actually claim to know quantum electrodynamics. I'm not disputing that a photon rocket would work in a refractive medium, indeed, that's my goal. The question is, what is the efficiency of a photon rocket in a refractive medium? I am thinking that the ratio of the thrust (i.e. the amount of momentum produced) to the fuel consumption (i.e. the energy needed to create the photon) could be different than in a vacuum. And when using the Minkowski version of the momentum, the canonical momentum in the 2010 paper, this is pretty clearly true. And if the photon, once created in the medium, has more momentum, then it seemingly has more mass - apparently it is associated with what I'd think is a sort of "quantum fluctuation", the polariton, in the medium. And the thing is, I don't know that it is impossible for a virtual polariton, associated with a photon, to exist in vacuum, yet pass through solid dielectric in which we know polaritons can exist, and then exit the far side before releasing its photon, thereby allowing the drive to emit photons only in a situation where it is as if it were in a refractive medium??

@Nimur: Thinking about it, I can't help but think my arrows is the diagram have to be right. When a photon strikes a big heterogeneous lump of something, the entire object has to recoil directly away, no matter what parts are transparent or how it refracts on the way to its final absorption. And when a photon has been refracted and strikes the little lump inside, that has to recoil directly away. This means there needs to be some kind of recoil from the photon's approach to the refractive surface of the object that is parallel to the surface, which means that momentum is not being conserved when the photon hits it. I know the article says otherwise but I don't see how this could be wrong. The 2010 paper somewhat agrees with this, saying that while photons pass through a refractive object (assuming they escape) it ought to be moved in the same direction, but admittedly it does say that is not experimentally proven, which surprises me. Now because an object can't be moved without energy, this means that the photon must give up some of its energy somehow - presumably some sort of "binding energy" - when entering the object. I don't know if any real drive can actually get the discount price for the photon by producing it in the reactive material though... I'm afraid I'm still a little confused. :) Wnt (talk) 11:41, 6 August 2014 (UTC)[reply]

Wnt, I think you mean to say that radiation pressure exists; to which I emphatically agree. To the rest of your questions, let me simply reiterate: momentum will be conserved. If the surface is perfectly ideal and planar, then there will be exactly zero momentum transferred parallel to the surface. Perfect surfaces don't exist; but they are useful mathematical idealizations that allow us to correctly formulate more complicated mathematical models of non-ideal surfaces. Nimur (talk) 16:04, 6 August 2014 (UTC)[reply]

Look at the drawing at top. There are two gray arrows representing the recoil from the impact of the photon. Because the one on the right doesn't line up with the original incident ray, and I know the entire container of water as a whole has to be displaced directly opposite to that ray, there must be some other arrow on the left. So are you saying it would point straight to the left, or something else? Wnt (talk) 17:43, 6 August 2014 (UTC)[reply]

The absorption-and-reemission picture is handwavy, meaning that it's wrong but the conclusions I derived from it are correct. :-) The medium is "dragged along" with the light, then returns to its initial velocity after the light leaves. I think it's entirely possible that the photon rocket in a medium could be more efficient—that would depend on the detailed engineering properties of everything involved—but it won't be over-unity efficient, and it won't violate momentum conservation.

The Maxwell momentum seems to be related to phase velocity, and like phase velocity it can't be interpreted as the motion of a physical object (something you could use to send a signal).

I don't know anything about polaritons, but if they do make sense in a vacuum (even as virtual particles), they would just be another description of a photon, and not an additional thing accompanying photons. -- BenRG (talk) 23:24, 6 August 2014 (UTC)[reply]

On a related issue, does anyone have a references that derives the momentum of electromagnetic ration from Maxwell's equations? Graeme Bartlett (talk) 07:42, 6 August 2014 (UTC)[reply]

I pulled my copy of Griffiths from the shelf, and as it turns out, this is exactly where my bookmark was! Chapter 8, §8.2.1, Newton's Third Law in Electrodynamics, in which the author outlines the logic by combining the Biot-Savart law and noticing that (when a test charge moves), there appears to be a missing momentum term. This term exactly corresponds to momentum carried in the electric and magnetic fields themeselves. With some algebraic manipulation, the missing momentum can be expressed by writing the effects of the fields in tensor (i.e., matrix) form, therefore representing a stress tensor that corresponds to a radiation pressure and a shear force. When there is a force, it can be related to a momentum. Again, by algebraic manipulation, the momentum is decomposed into a mechanical component (any moving charge may also have a moving mass); and an electromagnetic component; the sum of these is the divergence of the stress tensor.

Nimur (talk) 16:04, 6 August 2014 (UTC)[reply]

Yawning pulling muscle

What is it called when you yawn really wide, and pull a muscle out of alignment in fleshy part of your jaw and it hurts tremendously until it becomes situated like it's supposed to be. Is that a hernia? 108.170.113.22 (talk) 16:43, 5 August 2014 (UTC)[reply]

That's a cramp. Here's a hernia. InedibleHulk (talk) 16:50, 5 August 2014 (UTC)[reply]

Also possibly a subluxation. InedibleHulk (talk) 16:56, 5 August 2014 (UTC)[reply]

It's also called yawning too hard and getting a cramp. μηδείς (talk) 22:29, 5 August 2014 (UTC)[reply]

Temporomandibular joint dysfunction may be an interesting read. --Jayron32 01:52, 6 August 2014 (UTC)[reply]

Lithops plant withering

I recently bought a Lithops plant which has just now become very withered (picture). Google turns up some conflicting advice on whether it requires more water or less. I should note that I live in Southern California, and we have recently had some extremely humid weather. So what should I do? 75.4.20.212 (talk) 17:29, 5 August 2014 (UTC)[reply]

(WP:OR ahead, my advice based on ~15 years experience, never lost a succulent) I wouldn't add water unless the dirt is bone dry, and it hadn't had water for a few weeks. Your specimen actually looks fine to me. Check the dirt by sticking a toothpick down in an inch or so, leave it for a minute, then check for any moisture. If you can detect any moisture, don't water yet. If it's very dry, just give it a small splash of water. If that's what it wanted, it should 'fill up' and lose some wrinkles within a day or two. But really, that plant can live off of a few drops of water a year. Generally, succulents are easier to kill by over watering than under watering. Even if badly dessicated, they can spring back once water is added. But, if water logged, they will completely die rather quickly. Make sense? SemanticMantis (talk) 18:38, 5 August 2014 (UTC)[reply]

The soil is actually a tiny bit damp near the surface, so I'll just monitor my plant and use the toothpick test. Thanks for the input. 75.4.20.212 (talk) 18:46, 5 August 2014 (UTC)[reply]