My guess is that even at MIT photons (the quantum description of
electromagnetic waves) travel at the same speed as all other
electromagnetic waves, namely the speed of light.
When two photons collide, they can annihilate and produce an
electron-positron pair, or, indeed, other stuff, always in
matter-antimatter pairs. These processes have been observed at the
Stanford Linear Accelerator Center (SLAC) in California, and also at
the European Organization for Nuclear Research (CERN), located in
Geneva, Switzerland. MIT's researchers collaborate on projects carried
out at SLAC and at CERN.
Energy and momentum are precisely conserved in these processes, as
in all other known processes. Energy and momentum are the only sources
of gravitational effects, so gravity doesn't immediately change in this
process. In the long run, some collection of particles generated in
this way will behave like any other collection of particles, including
ones obtained in more conventional ways.
There is a way that gravity can affect a similar kind of process.
Quantum fluctuations in the space near the event horizon of a black
hole allow electron-positron pairs to form (and photon pairs too), and
disappear again very rapidly. One of the pair can "fall into the black
hole", while the other one flies free. So a black hole sitting all by
itself in empty space with no ambient radiation actually emits more
stuff (photons, electrons, positrons) than falls into it, a process
called "Hawking radiation", after Stephen Hawking who first noticed
that it must happen.
It turns out that the smaller the black hole is, the faster the
rate of radiation is! Small black holes will spontaneously "evaporate",
losing energy and mass through this process. In the final second, their
energy output is roughly that of a good-sized star, but is mostly in
high-energy gamma rays. Big black holes don't evaporate so fast, and in
fact in the steady state, absorb material and radiation at a much
faster rate than they emit it (there's always the 3 degrees Kelvin
cosmic microwave background to depend on, and usually more stuff than
that near a black hole, such as a galaxy to supply material).
On the way in, though, stuff gets accelerated and heated and
twisted out of shape, also emitting x-rays. It is a very nasty
environment near a black hole -- they are probably the last things you
want nearby when you want protection from anything.
If you make a teeny-tiny black hole artificially by colliding
particles (and you need a *huge* amount of energy per particle to do
this, way beyond what's possible with equipment we can build), it would
evaporate instantly anyhow.
You can read some of our other answers about how gravity affects
how time runs. In short, you don't notice the difference if you just
look at your watch, as your time and your watch's time are the same,
but if you carry a precise clock into and out of a gravitational field
and compare it with another one that wasn't, in they will read
different total elapsed times.
Mike W. and Tom J.
(republished on 07/23/06)