MadSci Network: Astronomy |
Maybe, but not as you describe. To understand this, we first have to understand the photons are NEVER at rest. This is sort of a hard thing to explain if you don't have some background in relativity, so I'll do my best here and if you have further questions, feel free to email me. So, Special Relativity tells us that all inertial observers (those that are not accelerating) measure the speed of light to be the same, regardless of their relative velocities. In other words, there is no way that you can put yourself in a reference frame in which you will measure the speed of a photon to be anything other than ~300,000,000 meters per second, not to mention zero. So what does this have to do with a black hole? General Relativity, the theory of gravity which explains how spacetime is warped by the presence of matter (more precisely, any kind of energy), is really an extension of Special Relativity. One way to think of this is as follows: even though the spacetime around a black hole is curved, one can (except at the singularity in the center) always look at some very small region which is very nearly flat. An analogy is magnifying a circle. If you zoom in on the edge of a circle, at some point it begins to look more and more like a straight line. "Approximately flat" spacetime means that within that small region, the symmetries and transformations of Special Relativity, as well as their implications, hold true. Let's then consider an intrepid astronaut, Rob, who has volunteered to fall into a black hole. We'll make the black hole very large, so that at any given time (except near the singularity), the spacetime Rob occupies will be approximately flat. At the exact instant Rob crosses the event horizon, he shoots a laser radially outward (i.e., perpendicular to the event horizon surface). We know from Special Relativity that the photons will move away from him at the speed of light, and not simply stick on the event horizon. So that's a long way of giving a "no" answer. The question then arises as to what does happen to the photons from Rob's laser. That's even harder to answer. You may be familiar with the concept of "redshift", which is the stretching of the wavelength of light. Suppose Rob fires his laser at regular (to him) intervals before he crosses the event horizon. His partner Fab, hovering at some distance away, will see the wavelength of this light increasingly redshifted the closer Rob gets to the event horizon. In fact, though by Rob's watch the laser is being shot at regular intervals, Fab will see the intervals get longer and longer. Now, Fab will never see the light fired exactly at the event horizon, but by extrapolating his data from previous laser shots, his explanation of this is that the light was redshifted to an infinitely long wavelength. This may seem strange, but part of the reason for this is that "time" is wrapped up in "spacetime", such that it's difficult to separate it from the space part in a consistent way. It's one of those seemingly bizarre situations that pop up a lot in relativity, precisely because measurements like this will change depending on what reference frame you are in. Now it is in principle possible to make a "sphere of light" surrounding the black hole. To do this, though, you would shoot the photons precisely *tangent* the the event horizon surface. Then the photons would orbit the black hole forever. Of course this is an unstable situation, since moving the photon an infinitessimal amount would cause it to either fly away or fall into the singularity. This in turn brings up the funny question of how quantum mechanics factors into all of this, since it seems to tell us that the position of the photon is never definite. Exactly how this folds back in to General Relativity, which regards all positions as definite, is an area of active research.
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