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:Interesting idea...

[drudd]

Not quite. A photon's momentum is related to its wavelength by the following relation:

p = h / lambda

so (p^2 * c^2) - E^2 = -m^2 * c^4 =>

((h^2 * c^2 ) / lambda^2) - E^2 = -m^2 * c^4

c / lambda = frequency, so:

h^2 * frequency ^2 = -m^2 * c^4

and h * frequency = energy (for photon)

so E^2 - E^2 = -m^2 * c^4, and c != 0,
so m must equal 0.

Unless you can demonstrate that either
h*frequency != Energy for photon
or h / lambda != momentum, both of which
have been established experimentally, you
cannot say a photon has mass (also see the logic in my previous reply to another poster).

Doug

Interesting idea...

[NOC_Monkey]

At first, this looks like a silly idea, using directed photons to counteract gravity. But, as shown by Einstein's mass-energy equivalence equation (E=mc^2), but expanded to include an object's momentum (p):

(p^2 * c^2)-E^2=-m^2 * c^4

Then we solve it for mass:

-(SQRT((p^2 * c^2)-E^2)/c^4)=m

Shows that a highly energetic photon _can_ have some mass. The problem is that the energy need is so high that any really significant change in the local measure of (g) would take enormous amounts of energy, most likely needing a direct-conversion reactor to genereate any usable mass. To give some scale to this, the fission of a single U-235 nucleus gives off enough energy to set off a mousetrap. And remember, a fissioned nucleus is not completely consumed. Only a small fraction of the mass is converted into energy. For a device like this to work, we would need to generate more energy than has been produced throughout all of human civilisation.

Nice use of Bryce, tho...

This doesn't make any sense.

[anatoli]

I mean, energy-wise. Let's consider this picture.

BS (Fig.1)

where B is a gravitating body and S is a grav-shielded spacecraft.

As the spacecraft has zero mass, it has zero potential energy with respect to the body. Now let's switch our shields off.

What happens? The spacecraft starts falling.

B<---S (Fig. 2)

This is because the spacecraft instantly acquires potential energy. This energy can be utilized when the spacecraft falls onto the body and emits lots of heat upon impact.

B*S (Fig.3)

(* signifies the great thermal explosion.) Let's switch our shields on again (presuming we've survived the impact). The craft starts to float away from the body.

Switch them off, we fall and generate more heat. On again, float away. Off again, falling, generating heat. Cycle through Fig. 1--3 ad libitum.

What we have is a perpetuum mobile of the first kind. Unless the energy required to switch the shields on and off is more than the energy we can generate. In this latter case, the shield is useless (what potential energy do you have with respect to the Andromeda galaxy?)

Note how we don't care about the exact means of establishing the gravitational shield. Use any method. Energy conservation means you cannot win.

Of course there's a smallish possibility that energy conservation does not hold. In which case we're better off fitting our cars and homes with perpetuum mobile. This has much more sense than exploring distant worlds.
--

Its a big....hmmm

[TheCarp]

Ok...basically he has 2 coils next to eachother.

Look at the diagrams...thats all it is. A big
metal tube with a couple of wire loops inside...
looks like 2 or 3 turns each.

Now....he claims to be throwing 300 A into
this sucker...depending on voltage...thats a
shitload of juice. Perhaps he is producing a
magnetic feild strong enough, and properly
oriented so that the coil is actually being
suspended above the electronics in the scale
below it?

I dunno...it all seems really bogus. AFAIK the
generally accepted theories of Gravity state that
Mass warps space in 3 dimensions. So light is
effected by gravity only because the space it
is taveling thorough is bent (ie it follows
a straight line in a curved space).

Personally...I am very quick to call this pure
bunk. Its just too pretty. I notice he claims it
has been tested...yet there are no pictures of
the actual device...just diagrams.

It should also be mentioned that just because the
math works, doesn't mean it physically works.
You can play with math and make a good case for
"white holes" (ie the opposite of a black hole..
it spews out matter and never takes any in)
however...its just because the math works both
ways...there is no evidence that a white hole
would actually exist.

Perhaps this guy is just a crackpot...I would be
interested to see more evidence myself. Another
criticizm is...where are the photons comming
from? He is talking about radiation etc...yet
all he is really doing is dumping ALOT of current
through a couple of coils that are near eachother.

I am definitly skeptical.

Re:A Question on Details

[spiralx]

... but if two particles come near each other and recoil from each other as a result of a graviton exchange, wouldn't that be a bit backwards from what actually happens? Namely, gravity is attractive (I think we all can agree on that!) and so it would seem that the absorption of a graviton would cause a particle to accelerate towards whence the graviton came.

The analogy of a particle emitting a force particle such as a gravition or photon, recoiling and then another particle absorbing the force particle and also recoiling works in the case of a repulsive force but falls down in the case of an attractive one.

What you need to remember is that in quantum field theory force carrying particles (bosons) are just excitations of the underlying field - a photon is just an excitation of the underlying EM field. As such, when considering interactions like this you really have to consider it in the full sense of quantum field theory rather than simply treating them as particles, and seeing as how in this case the graviton would be "virtual" I doubt little things like the Conservation of Momentum apply :)

Of course this is all in theory, since we don't have a quantum field theory of gravity. From recent developments it appears as though it will require a lot of unusual features, but IMHO it will at low energies approximate to a similar process as the other field theories.

SKAM!

[BWS]

Is it me or is it rather courious that these so called scientists have an AOL web page? I mean if they are reputible at all, won't they have a web page URL that either:

1] belongs to a commerical company
2] a university

but AOL? camon, no smart people comes out of AOL. no self respecting scientist would publish stuff on AOL.

Any crack pot [with some physics courses] can generate some mumble jumble drawing and formula and stuff. I won't belive it till its published in any reputible Science/Physics Journal [no, not the Playboy Science Section. The reputible journals have people that will analysis it and only publish the ones that are acutally true or promising, not any mumbe jumble. With the web, you can publish anything.

BOTTOM LINE: wait till its published in a scientific journal, that way its gone through some peer review and we can be sure its not all crap!

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.