What the Em? "Reactionless Drive" Demonstration Replicated

"Reactionless drives" have long been a more fantastical staple of science fiction. Propulsion requires a reaction mass -- you have to accelerate atoms of some propellant and throw them out of the back of your spaceship. For every action there is an equal and opposite reaction, and without that opposite and equal reaction, there is no action -- you have to throw stuff out of the back of your ship if you want your ship to go ahead.

But science-fiction writes postulated a drive that could accelerate a ship, probably because they didn't want to deal with the very large tanks of stuff on their imaginary spaceships.

But "reactionless drives" and "inertia engines" and the like have been thought to be flat out impossible.

Ten years ago, a British aerospace engineer named Roger Shawyer announced that he had taken a small cone and rigged it to bounce microwaves inside the cone. This caused the cone to thrust forward -- slightly -- without any reaction mass shooting out the back of it.

Most physicists thought this was inconsequential. When you're observing very small effects, like the tiny amount of thrust acting on this cone, there is usually a small experimental error that easily explains it. I mean -- not that this is what happened -- but when you're down to tiny effects you have to consider your building's ventilation system and how many cars were passing on the street outside, setting up a wave that vibrated your building. Things like that. Small external things that completely explain the tiny effect you observed in a completely boring and deflating way.

But several people say they've replicated Shawyer's cone and its thrust without reaction mass.

Michael McCulloch, a physicist at Plymouth University with a blog called Physics on the Edge tries to explain the reactionless drive using his own pet theory (for which his blog is named).

His theory is kind of enormous with big implications.

If it's true, which we need to assume it's not. But it's interesting.

His theory concerns inertia and why inertia exists at all.

What is inertia? It is the tendency of an unmoving body to remain unmoving and a moving body to remain moving. Simple enough thing we observe every day -- it takes a lot of effort to make a bowling ball fly through the air. It doesn't want to fly through the air. But once you throw the thing, it will continue moving until it's acted upon by some other force to slow it down and stop it. On earth, that's gravity pulling it to the ground and then friction in the ground bleeding off its momentum until it stops moving.

But in space, in vacuum without air resistance and without gravity, it would keep moving forever, until it fell into a star or something.

So we all know what inertia is. We feel it every time we get up from a chair or try to slow down from a sprint.

But why is it? We accept, usually, that inertia is just an inherent consequence of mass. If you have mass, you have inertia, and if you have more mass, you have more inertia.

But physics is sometimes about asking why something has an "inherent" property, and asking what is the exact mechanism by which that property manifests itself.

">So here's his theory. Bear in mind this is his ur-theory he's trying to get people to take seriously, and he's predisposed to accepting the EmDrive's thrust as real (as it fits his theory).

[W]hy inertia exists at all has puzzled scientists for centuries.

McCulloch's idea is that inertia arises from an effect predicted by general relativity called Unruh radiation. This is the notion that an accelerating object experiences black body radiation. In other words, the universe warms up when you accelerate.
According to McCulloch, inertia is simply the pressure the Unruh radiation exerts on an accelerating body....

At very small accelerations, the wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia can take only certain whole-wavelength values and so jumps from one value to the next. In other words, inertia must quantized at small accelerations....

The cone allows Unruh radiation of a certain size at the large end but only a smaller wavelength at the other end. So the inertia of photons inside the cavity must change as they bounce back and forth. And to conserve momentum, this must generate a thrust.

While the Unruh effect may have already been observed (there is argument about that), the specific mechanism by which it might work is not known; the "Unruh radiation" this McCulloch speaks of remains an unproven speculation.

Okay: So here's what Unruh radiation is (or might be, if it exists).

In quantum mechanics, pure vacuum is not actually empty. The thing is, though we talk about bodies being made up of particles, in fact particles don't exist really. Particles are simply localized excitations of fields that permeate the universe; particles are a high-energy point on these fields.

That's not the speculative part; that's standard physics at this point.

Vacuum, while containing no particles, still nevertheless contains fields. Those fields simply haven't interacted with any other fields to produce the local excitations we know as particles (protons, electrons -- everything).

This guy is proposing that something accelerates into a vacuum, it interacts with the fields present, and it causes those fields to push back against the acceleration.

It's that push-back that we know as inertia. Now I understand that as far as a body resisting an acceleration; you push into a net of fields, you excite some to become particles, they push back.

Apparently the part about there being observable particles when you accelerate in a vacuum isn't isn't controversial, but is predicted by the theory of relativity -- a standing observer would see a vacuum, an accelerating one would see a vacuum containing particles. The controversial, speculative part is that these particles are responsible for inertia.

What I don't really understand is how this effect also causes something in motion to remain in motion, that is, why once you're moving, these same fields start pushing you forward. It seems to have something to do with something... heady. A Rindler horizon and wavelengths too long to fit into the Hubble sphere (the sphere of all detectable light; the observable universe itself).

Of course, if it's ever established that inertia is just caused by some radiation, it raises the question pretty quickly as to whether you can then manipulate a ship's inertia -- tune it down -- and make it easier for a propulsion system to push around.

Now that's definitely out there.

However, none of that is even close to accepted, and most people do not accept it.

One thing that has to be kept in mind is that these cones only produce a tiny thrust. No one has made them produce a big thrust that is beyond explanation by mundane means. And even if this effect does exist, it might be confined to very light things, and very small accelerations.

And it still just might not exist at all, seeing that it violates the law of conservation of momentum:

In short, this model buries Newton's third law in quantum speculation. Through this model the EmDrive can defy Newton’s law by thrusting against the vacuum of space rather than violating the law directly.

[It[ isn't something to get too excited about. This new work doesn’t prove the effect is real. It does not even agree with claimed results that strongly. The model also raises further theoretical problems, such as requiring that the speed of light changes within the cavity, which would violate the central property of special relativity. Even if the EmDrive works as this model claims, it still violates well tested laws of physics.

In all honesty I'd love to see a device like this actually work, but I'm not convinced the effect is real. While this new work is interesting, it doesn't make the effect more convincing.

I'd love to see it work too. Actually, there are two things of enormous sci-fi consequence here, the reactionless thrust and tuning down your ship's inertia, and I think the latter one is the big one. If you can tune down your ship's inertia it really doesn't matter what kind of a drive you stick on the back of it.

But most of these small-and-hard-to-detect new "superdrives" turn out to be nothing but experimental error that get people excited and even to invest in companies and which accomplish nothing but wasting time, money, and interest.

Thanks to RD Brewer.

Speaking of a Huge Body Spitting Hot Gas Out Of Its Rear: Neil DeGrasse Tyson, Lover of Science (TM), has chimed in on Twitter. Correction: No he didn't -- this is a parody quote that Kari made up. Ah well. Sounds like NDT!

"Many refer to this supposed "em drive" as a potential leap forward. Ignoring the fact that before leaping, humanity needs to do some serious looking. #earthday

Update: remember, that's a parody by Ghost of Kari, not NDT.

posted by Ace at 12:51 PM

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