# Re: Not quite magic physics [was Re: Quantum Computers]

hal@finney.org
Sun, 22 Aug 1999 18:26:21 -0700

> This is related to a question I've wondered about the maximum density
> of light. Say I have a number of really large lasers (powered by
> stars) arranged in a large sphere, perhaps like a globular cluster.
> Say I beam all of those lasers at a single point in space. What
> happens? Do I generate matter? Do the beams simply pass through
> each other?

I don't really know the answer to this, but I speculate below.

> A related question would appear to be -- since a mass density
> above a certain level will generate a black hole, will an
> energy density equivalent to that mass density (by e=mc^2)
> do the same thing?

Technically, gravitational curvature is not generated by mass density as such, but by what is called the stress-energy tensor, T. This can be thought of as a symmetric 4 by 4 matrix, which represents the local mass/energy density, the local momentum flow, and the internal stresses in the matter which occupies the local region of space. The mass/energy density is just the upper left term of the matrix, although this is the one which is most important for weak gravitation like we normally deal with.

The distinction becomes important when dealing with questions like, if you accelerate an object so that its "relativistic mass" becomes large enough, would it form a black hole? The answer obviously has to be no, otherwise you would have given meaning to absolute motion, which is contrary to the principle of relativity. In terms of T, the coordinates may be large in some particular coordinate system, but that is fundamentally a projection effect and does not indicate that the absolute magnitude of T is large.

Matters are different if you have a relativistic dust or fluid, which has many particles all moving in different directions and all at very high relative velocities. Now any given coordinate system is going to measure high values of some components of T, and its absolute magnitude truly is large.

The simplest case in between would be two masses heading towards each other at relativistic speed, such that in the c.o.m. frame their relativistic masses would sum to be large enough to form a black hole. I think in this case it does happen. What happens is that there is an analog of magnetism in the gravitational field, a component which depends on the velocity of the attracted particle. You could have two masses moving along near each other with the same large velocity, and almost no gravitational interaction between them even though their relativistic masses are large. This can be explained by saying that there is actually a large gravitational field, but because the test particle is moving rapidly there is a "magnetic" component which causes a repulsion between the two particles and that balances the gravitational attraction.

Now when you send in a test particle going in the opposite direction, the magnetic effect is reversed, and it sees a very strong gravitational field. Each particle going in the opposite direction sees the other as having a large gravitational field, and when its own mass is added the geometry is such that the black hole forms.

> If so, wouldn't this be a way of generating small black holes
> that science fiction writers and some physicists use for
> various purposes?

I think the energy densities are too high to be practical.

> And finally, if laser light to black hole conversion is feasible,
> does anyone have any idea how much we would have to juice the
> laser at Lawrence Livermore to create black holes (of say
> neutron mass or larger).

I don't think it works like this; the problem is that the necessary size to pack the photons into is too small. A neutron of mass 1 amu or 1E-27 kg corresponds to an energy of 1E-10 J or 1E9 eV, which unfortunately implies a black hole size of 1E-54 meters! So you need to pack your photons into an area that small in order to form a neutron-sized black hole, which is really impossible.

Hal