Speculations on a Black Hole Experiment
Black holes are not unique in bending light; they just do it better. In fact, Stephen Hawking has said that light at the event horizon and tangent to it would circle the black hole, returning to its starting point. He left the method of generating the tangent light as an exercise for the student. Postponing solution of this problem for a while, let’s consider another possible application of the principle. Conceptually, an experiment could be designed to transmit a beam of energy towards a black hole in such a way that some of the energy could be received back at the transmitting site. The received energy would not be reflected by the black hole, but would be re-directed unmodified, except in amplitude.
Let’s consider how such an experiment might be set up. The first step would be to find a nearby candidate black hole (say, 10 light years distant) and establish a good track so that its position could be predicted accurately for 10 years ahead. The earthside transmitter would use a laser capable of generating a very intense series of pulses in as tight a beam as possible. The pulses would be coded so as to facilitate auto correlation and high signal to noise ratio.
During the test the transmitter would be aimed to cover the area where the black hole would be when the test pulses arrive. When the beam arrives at the black hole, even with a tight beam, it will have spread to a diameter much larger than the black hole. All of the laser light will undergo significant Einstein effect bending, but a small part of the beam will be bent in such a way as to approach the event horizon almost tangentially. These select quanta will undergo a deflection of almost 180 degrees. (A full 180-degree deflection is impossible by the nature of the event horizon.) The remainder of the original beam will be fanned out over a large solid angle, probably exceeding 90 degrees.
The conclusion of the experiment comes in twenty years (2 x the distance in light years) when the earth has moved ahead along its own path in space, and when the pulse-coded signal is received and correlated with the transmitted signal. It would have no Doppler shift. Receipt of the signal would verify the existence of the hypothesized black hole.
Whether or not such an experiment could ever be accomplished in real life depends on parameters such as transmitter power, distance to the black hole of interest, losses due to dust, process gain, and so on. In the limiting case, the signal strength approaches zero as the deflection approaches 180 degrees. The theoretical signal strength increases for lesser deflections. The best target would be a suspected nearby black hole on a line perpendicular to the earth’s proper motion. In any case, success will probably require counting quanta rather than measuring received signal strength.