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New Test Supports NASA's Controversial EM Drive
An anonymous reader writes: Last year, NASA's advanced propulsion research wing made headlines by announcing the successful test of a physics-defying electromagnetic drive, or EM drive. Now, this futuristic engine, which could in theory propel objects to near-relativistic speeds, has been shown to work inside a space-like vacuum. NASA Eagleworks made the announcement quite unassumingly via NASASpaceFlight.com. The EM drive is controversial in that it appears to violate conventional physics and the law of conservation of momentum; the engine, invented by British scientist Roger Sawyer, converts electric power to thrust without the need for any propellant by bouncing microwaves within a closed container. So, with no expulsion of propellant, there’s nothing to balance the change in the spacecraft’s momentum during acceleration.
A lovely summary from the Physics StackExchange which sums up my thoughts:
"The initial tests were at atmospheric pressure. To test the fan hypothesis, an easy way is to vary the pressure, another easy way is to put dust in the air to see the air-currents. The experimenters didn't do any of this (or at least didn't publish it if they did), instead, they ran the device inside a vacuum chamber but at ambient pressure after putting it through a vacuum cycle to simulate space. This is not a vacuum test, but it can mislead one on a first read.
In response to criticism of this faux-vacuum test, they did a second test in a real vacuum. This time, they used a torsion pendulum to find a teeny-tiny thrust of no relation to the first purported thrust. The second run in vacuum has completely different effects, possibly due to interactions between charge building up on the device and metallic components of the torsion pendulum, possibly due to deliberate misreporting by these folks, who didn't bother to explain what was going on in the first experiments they hyped up. Since they didn't bother to do a any systematic analysis of the effect on the first run, to vary air-pressure, look at air flows with dust, whatever, or if they did this they didn't bother to admit their initial error, this is not particularly honest experimental work, and there's not much point in talking about it any more. These folks are simply wasting people's time."
http://physics.stackexchange.com/questions/23725/is-the-emdrive-or-relativity-drive-possible
In conclusion, they did a really really bad experiment and got a bad result. Wow!
"After consistent reports of thrust measurements from EM Drive experiments in the US, UK, and China – at thrust levels several thousand times in excess of a photon rocket, and now under hard vacuum conditions – the question of where the thrust is coming from deserves serious inquiry."
It produces much more thrust than it should if it were just the effect of shooting microwaves.
There's nothing currently practical about it. It's in the experimental stage. If we had a spacecraft flying around on one of these, I'd be much more confident. The last time an observation violated the laws of physics like this, it turned out to be a loose cable connection.
By Noether's theorem, if we're violating the law of conservation of momentum, the laws of physics must vary from place to place in the Universe. (Unfortunately, I don't understand general relativity well enough to generalize this.) If this actually works, we're going to need a rewriting of physics comparable to Special Relativity. We definitely should push ahead with testing this thing, although I still think it's going to turn out not to be a reactionless space drive.
"When you have eliminated the unacceptable, whatever is left, however improbable, must be the truthiness" - Holmes
If I were to peer-review a paper on this, I would insist on a plausible physical explanation for the claimed measurement. The burden of proof is on them: they are making a truly extraordinary claim, one that, if true, would entail revising all of physics from its very foundation.
When somebody sounds like a total fucking crackpot, they almost always are.
You might have missed high temp super conductivity entirely then. The phenomenon was measured and replicated in many labs - but it was at least a few years before any plausible theory came out - and 20 years on we do not have firm agreement on the cause.
No, it is a violation of physical law, just not the ones you're thinking of.
We're talking about the law of conservation of momentum here. It isn't the microwaves. We know the energy-to-momentum ratio of photons, and the reason using photons for thrust is impractical is that the momentum is far too small for given energy. TFA says this looks far more powerful than a light drive.
A pity that you made no effort to understand what laws of physics it's appearing to break.
"When you have eliminated the unacceptable, whatever is left, however improbable, must be the truthiness" - Holmes
The expulsion (should that not be expulsion or something?) are micro waves ...
hence the name: EM drive.
What you are describing is a photon drive where photons are the propellant. But the fine article explains:
After consistent reports of thrust measurements from EM Drive experiments in the US, UK, and China -- at thrust levels several thousand times in excess of a photon rocket, and now under hard vacuum conditions -- the question of where the thrust is coming from deserves serious inquiry.
The reason I don't believe it is real is the same reason I don't believe cold fusion is real. They put in metric ton-loads of energy and measure a very small effect. They say they will need to increase the efficiency by many orders of magnitude to create a practical device. I say they probably made a mistake somewhere and the tiny effect they measured is either noise or due to something else they haven't yet accounted for.
We don't see the world as it is, we see it as we are.
-- Anais Nin
You might want to look at this nice summary from Reddit of all the experiments performed in China and at NASA about these drives:
The FACTS as we currently know them about the EmDrive and Cannae Drive
How come Slashdot never gets Slashdotted?
Note that this is the fifth experiment so far that has reproduced the effect (and every new experiment tries to account for some explanation that could possibly invalidate the previous one; e.g. for the last one, they ran it in vacuum).
Wrong.
Conservation of linear momentum is most certainly NOT derived from Thermodynamics.
Conservation of linear momentum is a mathematical consequence of translational symmetry - in other words, momentum is conserved if the laws of physics are invariant in space. Similarly, angular momentum is conserved if the laws of physics are invariant by rotation.
100KW is the theoretical energy that you might be able to make a deep-space craft out of with this drive. The power it's been tested with so far is three orders of magnitude lower.
BS. That most certainly does not "stand to reason". Higher-energy photons have more momentum, not less, yet this thing uses microwaves (much lower energy than visible light) and gets orders of magnitude more thrust than could be explained by the quite-well-understood thrust from EM radiation. Besides, why would there be a net thrust in one direction? The microwaves should escape the cavity in all directions, not just out the back, if they're escaping at all. A light drive has to be open at the back, or the photons would bounce off the rear wall and counter the thrust they imparted to the ship by bouncing off the reflector around the emitter.
There's no place I could be, since I've found Serenity...
Leaving aside the fact that light has momentum and therefore is sufficiently "physical" a propellant for this example, and the fact that this thing produces orders of magnitudes more thrust than a few Watts worth of photons could impart, you're still missing a really key problem:
You can impart momentum on a mirror by shining a flashlight on it, but you can't impart momentum on a sealed box by having a lit flashlight *inside* it!
The EmDrive uses a sealed cavity. There's nowhere for any propellant to come out, even if there were any!
There's no place I could be, since I've found Serenity...
There are several medications doctors prescribe although they and even the researchers that invented them don't how or why they work.
Its not easy to measure 50 micro-newtons of force when you change a power level by 50 watts.
Currents cause magnetic forces. Things get hot and outgas producing thrust. RF power cables get hot and distort causing a force.
Think about it. The device weighs something like 5Kilos. That is 50 newtons gravitational force. So a 1 micro-radian tilt will cause a 50 micro-newton force. Walking across the lab floor could cause that amount of deflection. If the chamber is 1 meter across, a 0.1 degree temperature change on one side of the chamber (from a nearby power supply) could cause that much tilt.
There of course could be force just from photons - but that is a simple and well understood photon drive - known for at least 50 years now - basically a light-sail.
This is a very difficult experiment to do correctly, and they have not published in enough detail.
Meanwhile: conservation of momentum has been tested under conditions ranging from ultra-cold gas atoms to 100GeV particle collisions, to orbiting neutron stars. The RF fields they use are very modest. At SLAC we run hundreds of megawatts, not 50 watts. We have superconducting cavities where we easily see the deflection caused by the momentum in the microwave fields - operating at many thousands of times higher power than this experiment - we see nothing unexpected.
So: Difficult experiment. No unusual physical conditions. Apparent violation of one of the most carefully tested conservation laws in all of science.
It it literally more likely that the sun will not rise tomorrow (since that is also based on conservation of momentum) than that this experiment was correct.
Conservation of linear momentum is a mathematical consequence of translational symmetry - in other words, momentum is conserved if the laws of physics are invariant in space. Similarly, angular momentum is conserved if the laws of physics are invariant by rotation.
And energy is conserved if the system is invariant over a translation in time.
Hooray for Emmy Noether.
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