Thrust from Microwaves - The Relativity Drive 567
dfenstrate writes "The latest New Scientist has an article about an engine that exploits relativity and microwaves to generate thrust. There is a working prototype." From the article: "Roger Shawyer has developed an engine with no moving parts that he believes can replace rockets and make trains, planes and automobiles obsolete ... The device that has sparked their interest is an engine that generates thrust purely from electromagnetic radiation — microwaves to be precise — by exploiting the strange properties of relativity. It has no moving parts, and releases no exhaust or noxious emissions. Potentially, it could pack the punch of a rocket in a box the size of a suitcase. It could one day replace the engines on almost any spacecraft. More advanced versions might allow cars to lift from the ground and hover."
Save New Scientist! (Score:5, Informative)
From the aRocket list... (Score:1, Informative)
The maths paper please (Score:2, Informative)
Re:Erm... I don't get it. (Score:5, Informative)
The aRocket post with paragraphs... (Score:5, Informative)
Tue Sep 19 17:56:42 PDT 2006
Russell McMahon wrote:
I don't know that a reactionless drive can't work - although I don't know how to build one
Without having gone into it in detail, his math seems okay up to eq 6 (when he is quoting well-known math), but thereafter he veers into the realms of error and fantasy.
Eqation 7 is incorrect in so far as it purports to describe the total forces on the waveguide - while it does correctly describe the sum of the forces on the ends of the waveguide, it does not take into account the forces produced on the sides of the tapered waveguide.*
All by itself that is enough to blow the conclusions of the paper completely out of the water. It is simply wrong. It doesn't work. You can stop reading here.
Now we get into the rather more dubious portion of the paper.
Eq. 8 is also in error - it is based on the incorrect statement "...as the two forces Fg1 and Fg2 are dependent upon the velocities vg1 and vg2, the thrust T should be calculated according to Einsteins law of addition of velocities." - but the conclusion does not follow, and use of Einstein's equation is inappropriate. There is no real-world summing of velocities, it is a mathematical trick (and there is an error int the math too). The ends of the waveguide are stationary relative to each other.
That is an elementary schoolboy (or snake-oil salesman's) mistake.
There are several other obvious mistakes in the paper, and he frequently states as fact things that are unjustified and on occasion untrue. There are also parts of it which seem to be meaningless.
For example, this is also incorrect: "The second effect is that as the beam velocities are not directly dependent on any velocity of the waveguide, the beam and waveguide form an open system."
The conclusion does not follow.
This is actually very confused - I don't think he even knows what he is saying. Relativity theory does not (directly) come into it at all.
I stopped looking for more errors about here.
Snake oil or error?
There was some mention of licencing the technology, but as it is in the UK patenting it here would be impossible - it is, after all, a perpetual motion machine (or it would be if Q approached infinity, which there seems no theoretical reason to suppose impossible), and you cannot patent a perpetual motion machine in the UK.
Even if it worked.
The question of how he got a grant is still
To the DTI, NASA etc: Please can I have half his grant for pointing out his mistakes? I promise I will use it do space r+d.
*Of course if you want to consider the waveguide as two pieces, forces on the tapered walls do not affect the result - but the math in eq7 would be wrong if you are looking at it that way, eg the lambda-g1 and lambda_g2 figures are for the ends of the waveguide, not the middle.
I think he first went wrong in his mind here - in fig 2.4 there is a vertical line in the middle of the diagram, implying that he was looking at the waveguide as two pieces, rather than as two ends and a tapered middle. You can of course look at it in either way, but in his analysis (even before we get into the error-full "relativity" stuff) he is trying to do both at once, and that will and has lead to error.
--
Peter Fairbrother
--a different AC
This is complete bollocks (Score:5, Informative)
In other words, reader beware. Crackpots abound.
Re:The maths paper please (Score:3, Informative)
http://www.newscientist.com/data/images/ns/av/sha
the link is provided by the article linked. It sounds interesting to me, though referring to the special "relativity" is a bit too much; basically one end of the tubes experience more normal force than the other (narrow end) would result in a net forward force, which drives the system.
Of course the key is the generation of the cavity and its material, and the magentron design.
Nontheless, it sounds interesting to me. Not an expert on these systems, though.
Re:Isn't that every engine? (Score:2, Informative)
As a visceral example go ride a bicycle through air. Doubling your thrust will not double your speed, but you will experience directly that you have, indeed, doubled your thrust.
In the best case scenario, i.e. if this guy can solve the little problems such as pressure on the chamber walls, his engine, by his own calculations, does not simply run with nonlinear acceleration with a given thrust, but actually "runs out of juice."
In the colloquial, it stops working.
KFG
Re:Is anyone else reminded... (Score:5, Informative)
It is also possible to accelerate a rocket by shining a beam of light off it...
While in both cases there are much better ways to achieve same result, these will certainly work.
Re:Erm... I don't get it. (Score:4, Informative)
Oh, there have been any number of people who have put forward various intertialess drives for independant review. You are right, there is a difference between the honest crackpot and the snake oil salesman (thank god, or I might be in real trouble myself), but sometimes tests actually just waste time and resources when the theoretical failures can be defined without actual test.
And my point was that he hasn't actually built anything legitimately testable in a lab yet. The forces are so small that we'll need to fly the puppy to judge it at all. This is different from the solar sail which already know could work by theory and ground based test.
I can build you three or four mechanical variations on the theme that will even stand up to review in the sense that they seem to work perfectly well in the lab, much better than this one does because they'll actually scoot across the airtable, but the reason why they won't work in space are well enough understood that no one is going to waste a bird to send one up.
It's perfectly possible to become an honest crackpot by simply getting a bit of the equations wrong and have that failure perfectly obvious to other people.
KFG
Wikipedia article (Score:3, Informative)
http://en.wikipedia.org/wiki/EmDrive [wikipedia.org]
It's a fairly interesting read, and even though it's still rough in spots it's certainly better-informed than the scientifically-confused New Scientist piece linked in the submission. I particularly suggest reading through the analysis of Shawyer's claims.
Re:Save New Scientist! (Score:5, Informative)
The trick would be to join a narrow and wide end using walls that don't point more towards the wide end. But alas, that's an impossibility of geometry.
Re:Not possible (Score:2, Informative)
Re:journalist, at least, is totally clueless (Score:3, Informative)
Close. The name is Casimir [wikipedia.org]
Total Bullshit (Score:3, Informative)
But even if this is the reporter's goof, confusing acceleration and velocity, the inventor claims that the device would work better for hovering (presumably in Earth's gravity) instead of accelerating. This shows that the inventor does not understand relativity or basic physics. If his device could make a car hover then it could also accelerate the car at 1 G.
According to the physics fact book [hypertextbook.com], a 2001 Jaguar KX8 and a 2000 Mitsubishi Eclipse can each accelerate at 3.8 m/s^2 which is less than 1/2 G.
Since the inventor does not understand one of the simplest applications of relativity (gravity is the same as acceleration) I do not trust his calculations that claim some relativistic effect is giving him a force that will violate the conservation of momentum and energy.
Re:Forgetting some things? (Score:4, Informative)
The radiation pressure [wikipedia.org] does exist, but it has nothing to do with Lorentz force [wikipedia.org]. And you can, actually, propel yourself by shining a flashlight away from you. The matter annihilation engines work on this principle, for some decades [wikipedia.org] by now.
The only problem with this propulsion method is that you need an awful number of photons, and you wouldn't like to be in a spot that they hit. Some writers theorized that the Solar system would need an energy shield before it can launch a photon-driven starship from anywhere close to it.
Re:'bout damn time I get my flying cars (Score:3, Informative)
Yes, and what was the energy to force ratio quoted in TFA? 700 watts (just a little less than 1 hp) to get 83 millinewtons of force? That force accelerates 83 grams at 1 metre per second squared. That's about 3 ounces. So we'd need 5 hp per pound to get 1 m/s2 acceleration. Take a 1000 lb car, add 350 lbs for two passengers, and we need over 6,500 hp to get minimal acceleration. (1 m/s2 gets you from 0 to 60 mph in about 30 seconds.. most cars do much better than that.)
They're going to need enormous improvements in efficiency before this concept could be practical, assuming that the device actually works.
To clarify (Score:3, Informative)
This apparatus is a Radiometer. And it's not really working by the expansion of gas on 'hotter black side' -- the pressure throughout is essentially constant. The effect is caused by the movement of the rareified gas at the edges of the vein due to the temperature gradient.
Better explanation (and historical context): http://math.ucr.edu/home/baez/physics/General/Lig
When the apparatus is refined, by using a much better vacuum, suspending the 'blades' in a way with less resistance, and coating them in inert material the light pressure can be observed directly -- it will spin with the dark side leading. The link above says this was first achieved in 1901.
Re:That actually works - kinda... (Score:3, Informative)
The fan generates a force F as you've said, but as the parent specified (and I think is what you missed), that force is acting to propel the boat backwards since the fan is blowing air toward the "front" of the boat. Thus, if the fan yields a force F + g forwards, the net force is (F + g forwards) + (F backwards) == (F + g - F forwards) == g forwards. Or, given a rough force vector diagram:
<--F-- . --F-->-g->
Thus, the boat is propelled forward with a net force of g, which is less than simply turning the fan around to use a force of F to propel the boat.
Such was my understanding, anyway. Like I said, I could be wrong.
Re:Erm... I don't get it. (Score:2, Informative)
The idea that the internal forces will balance is Newtonian. If we have a ping-pong ball bouncing back and forth in a horn-shaped chamber, we have two cases to consider: when the ball is close to the center, it bounces from one flat end to the other and the force of each bounce balances the force of the bounce at the opposite end. When the ball is close to the edge, it bounces from the large flat side and then hits a wall. The force of the impact with the wall transfers some energy from the ball to the chamber, and slows the ball down slightly. The energy transferred to the chamber has one component perpendicular to the chamber's axis, and that component moves the chamber sideways a little bit. There's also a component that's parallel to the chamber's axis, and that component precisely matches the loss of force when the ball hits the small flat side moving at a slightly lower speed.
Problem is, we're talking about photons. They don't have the option of slowing down.
When a photon loses energy by reflecting off the chamber wall, it can't lose speed, so it loses mass. The energy gained by the chamber wall is translated into heat, not linear motion. When the photon gets to the small end of the chamber, it's still moving at the speed of light, but has less effective mass, so it imparts less energy to the chamber with that reflection.
The energy is conserved properly, even if the traditional notion of Newtonian momentum isn't, and it's well established that relativistic mechanics don't preserve the Newtonian concepts of mass or momentum.
It's also worth noting that this isn't a perpetual motion machine. As soon as the whole system moves, the photons inside the chamber lose their energy and have to be re-excited.
Old Article with pictures. (Score:2, Informative)
Re:Forgetting some things? (Score:3, Informative)
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* For quantum pedants: I'm assuming the gas is not in a coherent state, OK? Ergodicity applies. Very reasonable when the apparatus is at room temperature.
Re:Forgetting some things? (Score:2, Informative)
Re:That actually works - kinda... (Score:3, Informative)
The idea is that if you position the fan right and trim the sail correctly the sail will act like a u-tube [classictube.com] (tube as in tube not as in webpage) put at the end of a jet-engine (look at the wikipedia article on thrust-vectoring or the Harrier).
The air is pushed forward by the fan and then turned around 180 by the sail therefore it leaves the system fan+sail backwards and therefore pushes it forward. As the whole system is far from perfect thrust is less than if you just pointed the fan backwards.
Re:Forgetting some things? (Score:3, Informative)
Nothing funny, just a little omitted detail: the force light exerts on the side walls.
Sure, the guy is taking into account the force exerted on side walls perpendicular to the direction this thing is supposed to travel to. However, the side walls are at an angle to that direction, and therefore also experience a force against the direction this thing is supposed to travel to. This latter force is missing from the diagram in the PDF file. Unfortunately I can't do mathemathics well enough to check if it's included in the mathemathics part - can someone else confirm that it's missing ?
Or to put it another way: the side walls are slanted so that the force exerted on them by light has a component that pushes against (and, I suspect, exactly matches) the net force generated by the end plates.
This still raises the question on where the observed force comes. Maybe the power cord experiences heat expansion and pushes it a little, or maybe turning on power generates a magnetic field that's attracted to something ?