Today I was browsing the How Stuff Works web site, and came across this article on “How Electromagnetic Propulsion Will Work.” The article is summed up by:
This sounds to me like this Goodwin guy is off his rocker. How can this thing provide propulsion with nothing to push against? I thought for a while that maybe this is just a impulse to get them suddenly to a high speed, but that can’t be it, because they’re talking about reaching hundreds of thousands of km/hour. So is this guy completely nuts, or could there be something to this?
I know there was a thread that would help with this a while back on how propulsion works in a vacuum, but I don’t have the time to look for it right now. I skimmed the article, and at the end, he said that it might not work at all, but the basic idea of propulsion against a vacuum is that you send momentum one way, and since momentum is conserved, you go the other direction.
As the example that the other thread used, find a relatively frictionless surface (maybe an ice rink or something), toss a medicine ball in one direction, and you’ll find yourself moving in the other. The force that moves you is equal and opposite to the force that you exert on the ball. This is basically how chemical propulsion works. In other forms of propulsion, the idea is similar. Exert a force in one direction, and a force is exerted back against you, even in a vacuum, because forces always come in pairs.
First of all, yes, Goodwin is off his rocker if he thinks that he can get a magnet to vibrate “only in one direction”. But it’s not due to the lack of anything to “push against”, as you say, but due to the requirement that momentum is always conserved, as g8rguy notes.
It’s the same thing. Momentum is conserved in a closed system, i.e. a system which has nothing outside to push against.
Electromagnetic pulses (i.e. bursts of radio waves) do carry momentum, so it’s not theoretically impossible to make such a drive. But I don’t see why it would be more efficient than feeding the same energy to a regular antenna.
If you had a laser pointed out the back of your ship and turned it on, you would eventually get somewhere, because you’re throwing photons in one direction, therefore you must move in the other. Unless you had one hell of a laser, however, it would be extremely hard to overcome the inertia of your spacecraft. Much more effective is spending more energy to toss particles with mass out the rear end, like burnt chemical propellant or water heated with an atomic micropile (oblique Asimov reference thattaway <- :)).
Conservation of momentum is a pain with respect to space propulsion. Having to throw stuff out the back to get anywhere means you have to carry a significant amount of mass around with the intention of throwing it away. There’s a few approaches to dealing with this:-
very high exhaust velocity, which gives you more push per pound you sling out, but which needs A LOT of energy per pound of reaction mass. (For Niven fans, this is the Kzinti Lesson restated.)
interaction with the insubstantial medium of space (solar sails, or using very large magnetic fields to gather up protons and sling them out the back. If you fuse them for energy first, you have a Bussard ramjet.)
using drives which disobey conservation of momentum, or REACTIONLESS drives. This isn’t new - quite a few people have claimed to have developed such a thing over the years. The story of the Dean Drive is documented on Jerry Pournelle’s site: http://www.jerrypournelle.com/science/dean.html
A reputable British scientist Eric Laithwaite gave a Faraday lecture at the Royal institution based on gyroscopes and their supposed ability to disobey conservation of momentum: http://www.ee.ic.ac.uk/news/obituary.html
which caused a fair amount of controversy.
And now there’s Goodwin’s work, which sounds distinctly dodgy to me. Can conservation of momentum be broken? After all, it isn’t a provable law but an assumed law which has stood the test of time. If (and that’s a BIG if) it doesn’t always apply, the exceptions must be rare or else we’d have noticed them. But people working in this area are really on the fringes - success would mean not just a revolution in space propulsion but a revolution in our lives and the laws of physics.
I don’t see any difference between Goodwin’s gizmo and those widgets which make stuffed animals jiggle and vibrate. They’d still jiggle in free fall, but their centre of mass wouldn’t move. He gives a “one chance in ten” of his system working, which means he considers the possibility that his vibrating magnets are actually vibrating their centre of mass and breaking conservation of momentum as one in ten. When people make claims like that, I get suspicious - it’s like watching a magic show and saying “I reckon there’s a one in ten chance that he’s actually cutting the lady in half and sticking her back together again.”
Rule of thumb: Whenever you read any webpage or print article talking about using magnets to do something really nifty, it’s probably bogus. To be sure, there’s plenty of nifty things you can do with magnets (yes, it really is possible to magnetically levitate a frog), but the legitimate uses are snowed under by the vast proliferation of scams and crackpots.
There are genuine propulsion systems which could be described as electromagnetic, but they don’t bear any real resemblance to the device in the OP. In an ion engine such as that used by the Deep Space One spacecraft, you use electromagnetic forces to throw a stream of ions out the tail end of your engine. The basic principle is the same as for any other rocket, there, the only differeces being what you’re throwing out the back and how.
A Bussard ramjet, by the way, wouldn’t work unless you can convert matter to energy at close to 100% (as opposed to the ~1% you get from fusion), or if you’re travelling at very low speeds. At any significant speed, the drag from your scoop would be greater than the thrust you get from “burning” the material.
The physics of drives which interact with the medium of space is no different from the physics of props on aircraft or boats - accelerate the medium backwards with respect to you - accelerate yourself forwards. There are no fundamental objections to this.
There are plenty of practical objections - the “medium of space” is so thin it makes high vaccuum labs jealous, the enormous magenetic fields required to manipulate it will tear their own generators apart, and regarding the Bussard ramjet; - we don’t know of any way even in theory to fuse single-proton hydrogen at a sensible rate.
However, just because we can’t do it doesn’t mean that it’s an overall dumb idea. There are variations on the theme - e.g. generate your own power onboard and just use the magnetic fields to gather and accelerate hydrogen, increasing your effective specific impulse. And of course, using your ramscoop as a brake still saves you a lot of reaction mass!