Practical Gravity Shielding for Spacecraft?

Anonymous Coward writes, "I saw this site today: 'The Gravitational Spacecraft from Fran De Aquino Warping to the deep space...' A researcher in Brazil says that it can be done. What do you think? Anyone working in this area care to comment? The site can be found here." Slashdot has no official editorial position on the feasibility of gravitationally shielded spacecraft, but if anyone wants to send us a review unit, we'll gladly put it through its paces and do a writeup about it.

Comments

Re:Mass of proton = 0?

[drudd]

Not proton, PHOTON.

Photons are massless, and do not exert gravitational force. Light does respond to gravity, however, since it is just following the shortest path of a deformed fabric of space.

Photons have to be massless, otherwise they would have infinite energy:

E = mc^2 * gamma

gamma = 1 / sqrt( 1 - (v^2 / c ^ 2))

insert c for v, and you get a zero in the denominator, thus any particle which travels at c must be massless.

Doug

Re:Mass of proton = 0?

[Anonymous Coward]

Almost correct. Photons have zero rest mass, but they also have energy. The deformation of spacetime in general relativity is measured by the stress-energy tensor, to which photons may contribute. So photons do exert a gravitational force. -- just another astrophysicist reading Slashdot instead of reducing his Keck data

Not quite.

[zunger]

OK, some physics perspective on this...

One-line summary: These guys are better at graphics than they are at physics. No, it doesn't work.

Since the discussion below is a bit more technical than I usually send to Slashdot, here's a brief summary:

* No, it doesn't work. There are in fact some very fundamental reasons why it can't be made to work, either.

* It can be shown (through some fairly messy math) that it is not possible to cancel gravity by introducing other particles, unless you somehow had negative-mass particles to strap to your spaceship.

* Even if you could do all of this, how would you hold a sphere of photons in place?!

The rest of this is a bit heavy, so you may just want to skim. If you want to get a fairly easy-to-manage and good intro to this topic, you may want to check out Rindler's _Introduction to Relativity_.

Okay, on with the show. There are three basic problems with this suggestion.

Problem 1: "Photons have a null gravitational mass." It is true that the rest mass of a photon is zero. (The rest mass of a particle is the mass of that particle as measured by an observer at rest with respect to it. Special relativity tells us that objects with nonzero rest mass must travel slower than light, and objects with zero rest mass travel exactly at the speed of light.) However, the "effective mass" of an object for gravity purposes is <i>not</i> its rest mass. The quantity which generates gravitational fields is a more complicated quantity called the stress tensor, which is nonzero any time there is energy or momentum in a system. Photons definitely have a nonzero stress tensor and as a result do produce gravity.

In fact, because of the way the stress tensor is defined, it is impossible to entirely cancel it with <i>any</i> configuration of masses, unless you somehow had something with a negative mass. If you find something like this please let the rest of us know; there are several well-known, mathematically valid, ways to design superluminal drives based on them. However there are also several results that quantum mechanics prevents the formation of more than infinitesimal quantities of such matter.

So that's problem one; photons don't have "null mass" (whatever that means) and so don't automatically cancel gravity. On to

Problem 2: If we had a shell of photons around our ship, they could exactly cancel the gravitons. Not quite. First of all, I should say that gravitons are the smallest quanta of gravity and are in many ways analogous to photons. (In the language of general relativity, where gravity is a stretching of spacetime, a graviton represents a localized deformation of spacetime. Similarly, a photon represents a local deformation of the electromagnetic field, which is the particle physics way of stating the fact that light is an electromagnetic wave.)

Now, the way gravity works (in graviton language) is that gravitons can be emitted or absorbed by anything with a nonvanishing stress tensor. (The bigger the stress tensor, the more easily this happens) So say two massive objects are moving along; Alice (having a nonzero stress) emits a graviton, and recoils by conservation of momentum. Bob (also having a nonzero stress) absorbs this graviton, and also recoils. So effectively both Alice and Bob have changed their momentum, i.e. exerted a force on one another.

Now, say I wanted to cancel this out with some shell of Mystery Particles which would give the exactly opposite forces. These particles would have to do a couple of things:

* First of all, they had better travel at the speed of light so that they arrive at the same time as the gravitons in order to cancel them out. So these had better be massless. (See note above)

* Second, they should be able to interact with any particle that has mass, and they should interact equally strongly with any two particles of the same mass. (e.g., if our mystery particle hit electrons twice as hard per unit mass as they did protons, they wouldn't just cancel the gravity, they would also knock about everybody's particles quite a bit.)

Now it turns out (and this would require some lengthy tangents to explain in detail) that these two conditions are enough to <i>uniquely</i> specify the properties of this particle! In particular, the particle must have zero mass, and (for those of you with some physics background) have spin 2 and be symmetric tensor fields. These are exactly the same as the properties of the graviton, which isn't surprising since these things have to cancel them. But...

* It's fairly straightforward to show (using a bit of field theory) that forces mediated by spin-2 symmetric particles are always attractive. (Like gravity, and as opposed to electromagnetism. EM is mediated by photons, which have spin 1 and can be either attractive [opposite charges] or repulsive. [like charges]) So unfortunately, no Mystery Particles can cancel out gravity.

OK, and just in case this isn't enough, one other point:

Problem 3: Even if you had some magic way of getting around this, e.g. introducing multiple species of graviton and giving them some extra interactions which let them exactly cancel gravity while somehow not affecting the ordinary operation of gravity in <i>any way</i>, (Which, incidentally, doesn't work; if these particles are just like gravitons, they should be produced in nature just like gravitons and so would be everywhere) How the hell are you going to keep a bunch of massless particles sitting in a perfect shell? They're <i>massless</i>. They tend to fly away.

(OK, and now a more technical note for those of you who bothered to read this far: There is actually one other possible way to satisfy the constraints for a mystery particle, allowing it to couple to matter identically to gravitons without also being a graviton. Namely, the particles can be part of a multiplet of particles which are related to gravitons by some continuous symmetry [discrete symmetries would just give multiple graviton species] and so all matter would have to couple universally to them as well. This could be done even for particles that did not have spin 2.

But some math prevents this from working. If we want our mystery particle not to have spin 2, the symmetry must relate particles of different spin. By a key result known as the Coleman-Mandula Theorem, the only way to achieve this is with a type of symmetry known as supersymmetry. If you do this, you find that it is possible to construct a complete suite of particles called a "gravity multiplet," consisting of the graviton and several particles with spins ranging from zero to 3/2, all of which couple universally to matter and do everything we wanted.

So does this solve it? Well, not quite. The condition for these supermultiplets to cancel gravity is very well-known; it's called the BPS condition, and it says that the objects exerting the forces on one another have to obey very specific relationships between their charges (electric and other) and their masses, as well as some requirements about their relative shapes and orientations. When you work it all out, there are a few very specific configurations where this happens, but (1) It doesn't happen for arbitrarily shaped objects like spacecraft, and (2) All of this would require that supersymmetry be present and unbroken in nature. This would have some <i>really</i> visible experimental consequences, e.g. atoms looking pretty much nothing like they actually do. Such a symmetry is quite definitely ruled out.

So I'm afraid that there's nothing of this sort that can cancel out gravity. Sorry...

Yonatan

This is utter nonscience!

[morris57]

Rob, Jeff, Roblimo, Andover, VA:

You guys have lots of money now. Hire a freaking science consultant already. A responsible site that claims to be "News" does not put this story under the category of "Space" (thankfully it was not posted under "Science"). Put it under "Humor" or "Crackpot Theories".

It's time to realize that there is a lot of bogus stuff out there. Please don't be fooled. You are smarter than that.