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Q & A: fast photons

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Most recent answer: 10/22/2007
Q:
I heard that they did this at M.I.T, I don’t know if it’s true, but they fired photons at eachother at an extremely high speed, that some of them hit eachother and formed positrons. Now supposing that if we can do that in space, with enough light and energy, that we make a gravitational pull so big, that it creates a space warp field. Would it be possible that the warp field could be like a protective net, or a way that it could alter time, and freeze it?
- Socrates (age 12)
SFS (Seoul Foreign school), South Korea
A:
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.

(published on 10/22/2007)

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