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The 'Impossible' EM Drive Being Tested By NASA May Finally Be Explained (technologyreview.com)
MarkWhittington writes: The EmDrive, the so-called "impossible" space drive that uses no propellant, has roiled the aerospace world for the past several years ever since it was proposed by British aerospace engineer Robert Shawyer. In essence, the claim advanced by Shawyer and others is that if you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea, noting correctly that such a thing would violate physical laws. However, organizations as prestigious as NASA have replicated the same results, that prototypes of the EmDrive produces thrust. How does one reconcile the experimental results with the apparent scientific impossibility? MIT Technology Review suggested a reason why.
We'll eventually find out we really live in a simulation...
Great job lets not even try to attempt to summarize the article, instead lets post this like its a trailer for the 11 o'clock news!
Really appreciate the complete lack of even a whiff of the explanation in the summary.
Don't disappoint your bird dog. Go to the range.
https://en.wikipedia.org/wiki/...
Only the State obtains its revenue by coercion. - Murray Rothbard
I was honestly expecting to find an explanation of some subtle source of experimental error that covered it, not a possible theory explaining why it (maybe) works. I'm really looking forward to experimental testing of the improvements predicted by the theory. Who knows? With a decent explanatory theory, it might even be possible to turn it into a practical thruster. That would be awesome.
If you have multiple emitters into the chamber, angled toward a reflector, each emitter has a vector of momentum parallel to the axis of the motor, and another perpendicular to it. If the emitters are spaced properly, the perpendicular vectors will cancel, and the parallel components, summed, will be less than the momentum of the photons leaving the chamber through the "nozzle", giving a net forward thrust.
If they miniaturize it and connect it to a battery and a computer it could behave as the back to the future hoverboard. It could also mean drones without without a wind propeller.
Have they tried analyzing this thing with an E-meter?
What should be most telling was that this "engine" produced more "thrust" when it was turned off than when it was turned on. That should tell you how stupid this thing is but everyone is so caught up in the "I want to believe" moment that they ignore all the warning signs.
The "thrust" measured by every study is so tiny that it's equivalent to the gravitational attraction of the contraption to your body. There are thermal effects that could explain this tiny thrust. So when the contraption was cooling off, the thermal effects were enough to push the air in a non-symmetric way that it produced a tiny push on the sensors. You could have measured more thrust just blowing on the stupid thing.
Most scientists have snorted at the idea, noting correctly that such a thing would violate observed physical laws.
The EM drive was discussed at length on other sites, and few posts were able to shine any light on the issue. Some items of note:
First, if your understanding of physics does *not* predict the Casimir effect, then you probably shouldn't be blithely dismissing the theory. The EM drive is based on a theory of physics that's more sophisticated than simple "momentum is conserved". It supposes an hypothesis that's different than what is currently accepted, but in a subtle way that is difficult to detect.
It's similar to relativity: most of our tests validate Newtonian physics, but you find relativity when you go looking for it.
Second, if you want to appeal to Noether's theorem, note that the theorem refers to smooth manifolds. If space is quantized, then Noether's theorem doesn't apply (despite being true). It's possible that Noether's theorem will break down at small scales. (If space is smooth, ie *not* quantized, then the true location of any particle is a [mathematical] real number with infinite entropy and it's action is non-computable. Not that having a non-computable universe is a problem, but...)
All in all, I get the impression that everyone commenting on the EM drive should probably keep quiet and let the experts sort it out.
I don't have any comment on either the theory or the experiment, but it's an interesting proposal.
From the Wikipedia page:
This is analyzed by Rothman and Boughn[32] who point out that the standard theory of radiation pressure is more complicated than the simplified analysis suggests.
I'm of the firm belief that if humans can think of it, we can eventually find a way to do it, even if it's by proxy.
Go back 100 years and the differences in thinking about what is "impossible" would be marked.
The ship stays where it is and the engines move the universe around it
The engine appears to work by utilizing a Maxwell's demon.
Contribute to civilization: ari.aynrand.org/donate
That's how.
If Photons have mass (i.e. not relativistic mass, actual mass) the world becomes a damn site simpler, but a lot of the fluffier Physics theory breaks (QM + Standard model + bits space time).
It seems unlikely that this current theory (about Unruh radiation) is correct for a very simple reason:
@ "At very low accelerations, the wavelength of this radiation is so large that it does not fit within the universe (ie it is greater than the Hubble distance). As the acceleration increases, the wavelength drops to less than the Hubble distance and the spacecraft appears to receive a kick."
Why would there be one and only one universe? Any event that could create one could create two or 3 or N universes. Indeed it would have to have something very special to make one and then stop. Something that does not exist in other branches of physics.
If there's more than one, then space is continuous and there's no limit on the wavelength of Unruh radiation. So no kick when it fits into the universe.
But the universe won't notice the maintenance downtime to support development of universe 2.0.
Me.
We apologize for the inconvenience.
God.
Send one of these suckers into space and see if it works? AFAICT, if it does you have a spaceship with no fuel requirement, if it doesn't you just have some space debris. As it has no fuel, it doesn't have to be a big craft, so it could go up with something else.
Donte Alistair Anderson Roberts - hi son!
Karma: Chameleon
The truncated cone may be allowing group motion of free electrons to propagate with less resistance causing an imbalance of force.
You might find this interesting.
"Most scientists have snorted". So their parents let them commit the mistake so they can ground them in their rat labs until find a solution. And I wasn't suposed to write that either! That's beCAUSE I can do better. Now returning to the REAL subject:
--> I hope it doen't make weapons of _colossal_ desctruction.
The amount of thrust they're seeing, even at microNewtons, is far higher than could be produced by the radiation pressure of simply emitting photons at those energy levels. If it wasn't, there wouldn't be all this fuss.
NASA measured an average of 91 microN with 17 W, or 5.3 microN/W. The Chinese measured 720 milliN at 2500 W - about 300 microN/W. By contrast, expected radiation pressure would be closer to 0.003 microN/W.
Why would anyone engrave "Elbereth"?
If this was just about photons having mass, well then simple flash lights would be showing similar thrust. But they don't as such measurements show flashlights demonstrate much much much less thrust by directly emitting photons.
This thing is warping space slightly within the cavity, and whatever net inertia due to the microwave photons slinging back and forth inside the cavity is not balanced - resulting in a much larger thrust than emitted photons from a flash light.
Somewhere somebody suggest attaching some these thrusters to the ISS to keeps its altitude trimmed without needing to refuel - couldn't hurt to try?
What does this statement mean?
Reading the reference http://arxivblog.com/?p=207 didn't help me understand.
It looks like nonsense because it treats photons as if they were Newtonian particles and with ignorance of Maxwell's equations and relativity.
Start with section 2. It treats photons as particles with some momentum m*v. I mean, what? That's just wrong. Photons are relativistic p = E/c and quantum mechanical, E = 2\pi hbar f.
I mean take a look at this:
"Normally, of course, photons are not supposed to have inertial mass in this way,
but here this is assumed. It is not clear what the size of this mass is, but it is
clear for example that light inside a mirrored box produces a kind of inertial mass
for the box. "
So in orthodox physics, photons are not supposed to have inertial mass, but also in orthodox physics light makes inertial mass and it's clear that it's so.
The second statement, about light inside a mirrored box, is so because of relativity and the assertion of the equivalence principle. Electromagnetic fields are part of the stress energy tensor (following Maxwell) which feeds into the source term of general relativity. So yes, there is some sort of inertial contribution, but in fact it can be computed pretty exactly, and it's extraordinarily tiny, and really mostly related to the energy density of the EM field.
So relativity sometimes, but not other times? WTF?
And if the non-standard theory that inertia comes from matter interacting with Unruh radiation, how exactly does that work with photons? Photons don't interact with photons. Zero cross section until the point that they are so energetic they can pop out electron/positron pairs from the vacuum, which is so far not an experimentally accessible regime.
Presumably the idea is that the Unruh radiation inside the cavity is quantized in a particular way different from free space, but wouldn't that mean that inertia of (presumably charged) particles inside that cavity would be altered? But he was talking about the non-sensical 'inertial mass' of the photons themselves. WTF?
I don't mind non-standard theories and their exploration at all, but it's necessary to be clear which standard axioms are being rejected and which others are preserved, and follow that consistently. I just saw very unclear physics.
Ego is destroying science and unfortunately it makes terrible scientists. Worse, it appears we have terrible scientists as the majority of scientists.
Science advances one funeral at a time - Max Planck
Why would you be able to detect the thrust when applied to the ISS but not on a bench test in the lab?
I present
Clarke's first law
When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
Clarke's second law
The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
Clarke's third law
Any sufficiently advanced technology is indistinguishable from magic.
https://en.wikipedia.org/wiki/Clarke%27s_three_laws
It seems it was conceived by a single man, who obviously could not test it alone (it takes expensive Nasa facilities, it seems). And yet it seems to work, with much surprise from all the world's most expert scientists. One might wonder if he's an alien, or a man from the future... It 's a perfect science-fiction plot, does anyone know this movie: https://en.wikipedia.org/wiki/...
I'm no physicist, but this new explanation sounds just as wacky as the inventor's original explanation of why it (allegedly) works.
Table-ized A.I.
The main trouble with all this talk is it doesn't mention amounts. One may be able to have all kinds of funny effects like Casimir and radiation behind the horizon and what not and it's interesting to figure out how momentum is conserved but to go from a measurable effect to 'let's use it for propulsion' is outright silly. The effect will be in the wrong ballpark and you'll be much better off by just using removing the back and using the actual microwaves for propulsion.
Laws of physics.... Laws of physics are created by men to explain certain things, they aren't set in stone and aren't absolute..
Tried to post earlier, but nothing after typing in captcha...
Anyways here goes again
Clarke's first law
When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
Clarke's second law
The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
Clarke's third law
Any sufficiently advanced technology is indistinguishable from magic.
I strongly believe that this drive does not work but still needs to be investigated. First we find new things that we must consider when testing physical phenomenom. Second we might actually find some new mechanishm that translate force to earth, air or somewhere else.
Short version: photons seem to have inertial mass after all.
You cannot just rewrite fundamental physics to fix one issue without also looking at the implications of your theory for other predictions which is it likely to change. Worse it seems that nobody has tested these drives for the emission of charged particles. A far, far simpler explanation is that this drive works by electron emission. There are a variety of way this can work which all work in a vacuum but whic would unfortunately not work in space where you are electrically isolated and would eventually build up a counter charge and cause the thrust to reduce to zero over time. This all uses established fundamental physics so it would be nice to see this ruled out BEFORE coming up with crazy new physics. It might be less exciting but it is better science.
The lopsided nature of the cone causes the Ether Turtle's shell to become warmer than its belly. This difference is uncomfortable to reptiles and makes it shift around a bit, causing the turtle underneath to adjust to compensate, in turn triggering a similar re-shuffling of the turtle below it, and so on all the way down, causing the universe to shift position relative to the probe.
Table-ized A.I.
Its a puzzle to me why its a puzzle to them.
When an object gains speed it gains kinetic energy. If there was no inertia - ie objects would instantaniously changed speed from X to Y given a force with no measured speed inbetween then this would mean the energy transfer between the pusher and the object would, for an instant, be of infinite power (power = energy / secs where secs = 0). Which clearly isn't allowed under any known physics.
I've always wondered about a photon sometimes creating virtual positron-electron pairs and then anihilating back into the photon, doesnt that imply that a photon as it travels goes a bit slower than the speed of light?
Because we CAN detect thrust on the bench... Did you not read anything at all about this emDrive?
Just put the thing in a small satellite, launch it as a secondary payload somewhere and run the thing for a few years and see if its orbit changes (in way consistent with thrust not solar pressure or atmospheric drag). Its the only way we'll know for sure one way or another.
All you need is a simple flux capacitor, a conductor to capture a lightning that can deliver 88 Gigawatts of electricity. That is all.
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
But it is well known that the momentum of a photon depends on its frequency. If the device spits out microwaves in two directions, the same number of photons per second, but the microwaves come out in one direction are of a higher frequency than the other, then more momentum will be emitted in photons in one direction than the other. I would expect the quantities involved to be miniscule, but that's the first guess I had in mind when I read about this.
John_Chalisque
...says fuck jPL, fuck NASA, fuck EmDrive, fuck space travel, fuck time travel, fuck slashdot, and FUCK YOU!!!!!!!!!!!!!!!!!
The supposed thrust is many orders of magnitude too small to be of any use to keeping the ISS in orbit. If it were that strong of a thrust, everyone would believe it's real.
This space intentionally left blank
Now to find a planet with giant worms.
The guy who said the election was rigged won the presidency with the second-most votes.
The theories are different (?), but this reminds me of seeing a light mill for the first time and immediately thinking a light engine should be possible one day.
You were right the first time. The people testing the EmDrive were clowns, and Mike McCulloch should never have received a PhD. This reddit thread, which he participated in, should be sufficient to destroy his credibility.
I am incensed this made it to the front page. This is worse science than most climate deniers manage; they are usually a little less blatantly unphysical. The firehose needs a way to tag things as complete bullshit, preferably with the ability to submit a rebuttal link. There is no need for Slashdot to run hoax stories, terrible science, or anything else blatantly untrue.
Those who advocate genocide deserve every protection afforded by law, and none afforded by common human decency.
http://www.wired.com/2015/05/n... Here's a snippet: So who are these guys? Despite the fact that the group works out of Johnson, under the auspices of NASA, Eagleworks still only runs on $50,000 a year in funding. “That’s not enough to conduct a high-quality experimental research program,” says Davis. “They’d need $1.5 million, $2 million for five, six, seven years.” Research into breakthrough propulsion physics—even when it had its own lab at Glenn, under Millis—has never been particularly well-funded. So “the way that this really happens is people dabble in addition to their day job,” says Millis. According to him, Eagleworks started with White working on concepts in his free time, not officially supported or sanctioned by NASA, and then eventually got a little money to run his lab out of Johnson. But the NASA banner doesn’t legitimize the work—if anything, NASA seems to want to keep the project under the radar.
In tomorrow's news, it will be unveiled that Robert Shawyer is a fake name.
> If you bounced microwaves in a truncated cone, thrust would be produced out the open end. Most scientists have snorted at the idea
I can see why porcine scientists would snort with approval. After all, if you bounce refrigerators around in a truncated cone (nozzle), they'll produce thrust as they exit the narrow end. If you bounce toasters around, they'll produce thrust out the nozzle. Same with coffee makers. Therefore, if you bounce microwaves around a nozzle ...
In the first case, you're letting the universe add up the exact amounts over a few days and measuring a large sum once with a fuzzy ruler.
In the latter case, you're adding up millions of tiny amounts with fuzzy rulers and your answer is almost entirely fuzz.
More practically;
External influences and process mistakes on the bench in the lab mean that your results are overly optimistic, and it probably won't work in space.
External influences and process mistakes on the ISS aren't bugs but features - regardless of what they were, it got the station to move, so figure out what you did, keep doing it and you've got a propulsion system.
What's that you say, an actual News for Nerds story? Bravo, Slashdot!
Protect your browser with the Force Safe Search add-on
I try, but as a non-physicist/non-mathematician, all I can really get out of this saga is:
1. Some guy builds and runs a funky apparatus in his lab/garage, and gets some strange results. He reports these excitedly to the world at large.
2. He's obviously smart but possibly deranged, since he claims that the apparatus violates the conservation of momentum, which is a classic crackpot move.
3. Any reputable scientists who have these results brought to their attention uniformly and immediately dismiss them as obvious crackpottery.
4. One night, while drunk, a small group of reputable scientists build the apparatus in their own lab, as a joke, and observe the same strange results.
5. Repeat steps 3-4 a bunch of times.
6. ???
7. Space probe to Alpha Centauri in my lifetime?
So the complaint with this drive is that no matter is sent out the back(law of conservation of mass). but since we know that mass and energy are transferable states and that we can in fact split an atom eg. destroy matter in the classical meaning of the law and remove energy from the action, the notion that this drive is not going to produce force is obsolete.
This is just like the theory of warp, when vibrating so fast one can travel through one dimension into another
The idea that there is something wrong with the current "laws", more accurately, theories, warrants study. What we have with data on electromagnetism and gravity are theories which have been tested in particular and limited experimental conditions. It has never been proven that the EM and gravity behaves in the way which has been tested in all contexts because there are such a large number of other contexts which have not been investigated. That is, it is not out of the question that EM And Gravity may be more complex than current theory and may behave in a different way than now understood under certain field arrangements and conditions which have not been tested. The assumptions of EM and gravity behave the same way even in settings on which there is no data is an unfounded extrapolation that since these forces act one way under certain settings the behave in the same way under all other settings, without any data on those other settings.
The idea that you haev these laws called EM and Gravity which are just absolute perfection and we cannot question and cannot have any flaws with them, and we assume without data that they behave in the same way always and in all situations even ones where there is no data, and that when an anomaly is detected it is automatically assumed it cannot be due to a problem with the theory of EM and gravity, is just plain arrogance.
I've been following Professor McCullouch's works for a couple of years now. His theories make more sense than the alternatives. I suggest checking out is web-log at http://physicsfromtheedge.blogspot.com/ and getting his book.
"Uh oh–you've read all five of your free articles for this month."
Uh, no, I've yet to read a single one (and have fixed IP).
"Become an Insider for unlimited access to online stories for as low as $29.95/year."
Do you really think I'll pay $30 to folks who can't count to one!?
This "thing" shows the interesting interaction between the engineering community and the scientific community. This is why you should take scientist statements with a grain of salt.
Engineers: look, this works!
Scientists: that violates the laws of science and is impossible.
Engineers: who gives a sh*t what you think? Here's the data
Scientists: the data must be wrong
Engineers: you try it
Scientists: we have no f*cking idea what's happening, but it's happening
Engineers: f*cking pinheads
Scientists: oh, maybe this is what's happening
If you take scientists too seriously, you never get past step #1.
"According to McCulloch, inertia is simply the pressure the Unruh radiation exerts on an accelerating body"
If that were the case, shouldn't bodies with inertia feel increased pressure on one side relative to the pressure felt when at rest? Why is any force needed at all? Inertia is simply kinetic energy, or did I miss something. Can anyone explain this, the article isn't really making sense to me.
...is that the EM drive's thrust has been reproduced by several independent institutions.
So now where are all the pedantic Slashdot experts that just recently were absolutely sure that the EM drive was bunk?
Lots of things can be wrong. Tests can be badly designed. measurement devices can be badly calibrated, people can lie.
but if the result really is thrust, then there is thrust. actual test results are true, period.
now, it could be that the effect is coming from other source of energy or some other way, and that may in fact be consistent with the laws of nature as we know them. Or, very unlikely, we could just be wrong about some detail of the laws of nature. (but thats probably one of the last cases to consider)
but, the result is the result. it can't be "wrong"
Just call it an impulse drive and call it a day?
What we have here is a new version of the solar sail! Hear me out.
The device bounces photons back and forth within the cone, effectively either a) "slowing" their speed (the speed of light), or b) at a minimum, altering their direction. Surrounding environmental photons, however, continue to travel in the direction they already were. The cone shape provides a larger area, or "net", at one end to catch more photons moving in that direction, which then push the device in the direction they are traveling as they collide with the slowed/altered photons.
If this is correct, theoretically this device could reach the speed of light!
I might be way off, but I hope someone much smarter than me, with many more resources, looks into this further.
To make it even worse, the microwaves are contained in a sealed chamber, so they're not being emitted in any direction. AND the observed forces are ~1000x larger than you would get from a photon-rocket, which is what you get if you simply blasted them all out the rear in the most efficient manner possible. If it's not experimental error, then something very strange is going on.
--- Most topics have many sides worth arguing, allow me to take one opposite you.
My hunch - Same effect as you see in Crookes radiometer.
https://en.wikipedia.org/wiki/...
The problem is producing hard vacuum is - well really hard. Lots of people have fooled themselves here.
Bring one up on the shuttle - fire it up - will work until it is full de-gassed..
You will notice that there is no link to a paper - where one might be able to see their methods and figure if they know how to make a really hard vacuum..
So...
Gravitation is described by Einstein's non-linear field equations with singularities.
Quantum mechanics yields the best experimental predictions of any theory, but unfortunately is not consistent with relativity.
Inertial mass may be related to Unruh radiation.
Mass may come from the Higgs boson
Physicists can't even agree whether a simple metal cone with some radiation in it is actually producing thrust or not in a repeatable experiment.
It's amazing that given this sorry state of physics, so many physicists have the boldness to make definitive statements about anything concerning gravitation, mass, or reactionless drives. Guys, get your house in order and get your theories cleaned up.
We don't like relying on unexplained phenomena anymore than scientists do.
Their observation was within their error bounds, and massively contradicts well-tested theories. That's called, "back-to-the-blackboard" for the testing lab. It's not necessary for them to invent a new theory to cover their results (and note that McCulloch's theory can be safely discarded), but there's no point in anyone attempting to reproduce a non-result. Similarly, no one should feel compelled to believe in or reproduce over-unity devices, and any such claims should be treated with the greatest skepticism -- ditto for claims about faster-than-light neutrinos.
George_Ou is entirely correct: the burden of proof is on the person claiming a new phenomenon. So far, the evidence is distinctly lacking, and there's no reason to believe this will even pass peer review. It would be nice if this device worked; I want to go to Alpha Centauri too. However, anyone willing to throw out centuries of empirical evidence based on a single admittedly flawed test is entirely too credulous.
Those who advocate genocide deserve every protection afforded by law, and none afforded by common human decency.
I wonder if that theory can predict any bounds on the acceleration. The effect measured so far is minuscule, barely measurable, and it's quite possible it will never be significant enough to be useful even under long space voyage conditions
That makes a hell of a lot of sense... sadly.
You tried playing expert w/ me & failed hypocrite - FACT: You tried throwing your weight around on me doing what I've done w/ hosts (yet you can't show shit for yourself that's better) saying you knew more than I do on them & security.
Shall I post that link for everyone to verify that?
Ask & ye shall recieve!
It's also where you said I was 30. I'm nearly double that age & certainly have provably MORE expertise in the art & science of computing vs. your lowly blowhard JUNKIE self the "wannabe expert" who speaks for 'everyone' in absolutes (like where you said the community shuns me? I'll post upmods by the 100's & especially on my hosts posts that will shut you up AGAIN, fast, bloward).
FACT: I don't see a DAMN THING you've ever done on security or code. Ever. Nobody has.
FACT: Nobody speaks well of your work (I suspect you have none) yet they do of mine in both code + security that I can easily directly prove (from tons of sources) & you tried to say "the community shuns me" you opiate addict hypocrite? LOL! New News/Clue - YOU don't speak for everyone. Care to debate that? Go for it! I'll eat you ALIVE easily as I have before you pompous delusional freak.
* Get off your HIGH "horse" (& I do mean horse), opiate addict!
(You're incredibly delusional & STUPID thinking the LOW junkie likes of YOU can "HOLIER THAN THOU" cut others down when the truth is, you are STUPID for being an opiate addict. Yes, I KNOW "your kind" - You're the worst ANIMAL there is and I do mean animal (lying thieving scum that will sell their mom for a hit)).
APK
P.S.=> You try cut me down & failed - Attack me w/ lies + YOUR mistakes noted above regarding ME? I'll destroy you w/ TRUTH for it, junkie. Once a junkie, always a junkie - "I'm in recovery" is bs. You've just got another dealer in a methodone/suboxone clinic is all. Funniest of all is your 50k for /. - I doubt you have it. I know how fast a 3 grand a week horse habit starts (weeks only). I've dealt w/ SO MANY of "your kind" is why. I can come up with that fast myself (more by far). The ONLY way you *might* have it is from inheritance OR an SSD back payment (a scam junkies pull saying, like you, "I'm disabled" which taxpayers like MYSELF pay for) - let's assume you do though - most of "your kind" falls back off the wagon, it'll be GONE, fast... apk
Assuming you're prepared to believe this guy had a good enough grasp on all the physics involved, this was debunked ~8 months ago: https://www.reddit.com/r/EmDri...
Thing is, the photons emitted are also absorbed, as the system is self-contained. This means that any imbalance in momentum of photons generated will be exactly reversed by photons absorbed. There's also the fact that the thrust claimed and reported is far higher than could be accounted for by photon momentum even if the photons were sent out of the system instead of being absorbed. Photons suck at transferring momentum.
"When you have eliminated the unacceptable, whatever is left, however improbable, must be the truthiness" - Holmes
playing a practical joke on the scientists.
p = mv