What happens if you throw a big chunk of antimatter at a big chunk
of matter? The result depends on the impact velocity, and can be
summarized in the following simple table:
Velocity Result
Zero There is a tiny bang and the two chunks bounce away
from each other essentially intact.
Relativistic The two chunks interpenetrate, all the particles
interact, and there is a whopping big explosion.
In between I dunno, but I'm willing to make a guess...
If a chunk of matter contacts a lump of antimatter, there WILL be
an explosion. Even if the impact velocity is zero, the atoms at the
contact surface will be attracted by Van der Walls forces. The electrons
and positrons will interact and be destroyed. The resulting storm of
photons will probably not be enough to overcome the electromagnetic
forces that will suck the protons and anti-protons to their doom. Shlurp.
Bang.
But this interaction will only occur at the contact surface between
the two chunks. If the impact velocity of the two chunks falls below some
critical value, the explosion at the contact surface will push the two
chunks apart before much damage gets done.
What, one wonders, is this critical velocity?
In general, this is a complex question, and depends on the mass and
shape of the chunks. The answer is, in fact, in the province of bomb
design, which deals specifically with the question of how to shape masses
of explosive material so that they hold together long enough to explode.
Fortunately, we are not really interested in the general case. We
only want to know what happens when a teeny tiny impactor made from anti-
matter strikes a large, spacecraft-sized, object. I suspect that in such
a case the critical velocity is the speed of sound within the impactor.
If the impact velocity is higher than this, the shock wave generated at
the contact surface will not have time to propage and blow the impactor
apart.
What, you ask, is the speed of sound in a typical solid? I don't
have the slightest idea. Nor is it relevant, since the impactor will
be heated to plasma temperatures. I would guess that the critical
velocity is somewhere around 100 km/sec. This is a perfectly reasonable
number for a railgun projectile, and is about the kind of velocity one
would need in any case if one hopes to hit a manuvering target with an
unguided projectile.
As for the mass of antimatter necessary to wreck a spacecraft,
note that the interaction of 4 micrograms of antimatter with 4
micrograms of matter will produce approximately 1.0E+16 ergs, which is
the energy of 200 grams of high explosive. This is roughly comparable
to a 30mm cannon shell. A series of dramatic and realistic experiments,
conducted over Germany 45 years ago, demonstrated that this was
sufficient remove a wing from an armoured 20 tonne aircraft. Four
milligrams of antimatter is roughly the equivalent of a 500 pound bomb.
Recent experiments, conducted in the South Atlantic, have demonstrated
that this is sufficient to destroy a 4000 tonne seagoing warship or a
20,000 tonne seagoing merchant ship. (It is not always sufficient to
destroy a 500,000 tonne supertanker. Make of this what you will.)
A final interesting question is: 'How much antimatter does it take
to break a planet into little bits a la the DEATH STAR?' The answer
is... LOTS.
The gravitational binding energy of the Earth is approximately
1.0E+33 ergs, give or take an order of magnitude because I'm working
this out on a piece of scratch paper while I wait for my lousy stinking
solar wind spectral simulation routine to run. This (the binding energy,
not the simulation routine), is the equivalent of one million tonnes of
antimatter. Where, I wonder, did Governer Tarkin get the stuff? Where
did he keep it? In his closet perhaps? His cleaning service must have
been surprised.
Paul R. Gazis
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