The Search For Real Black Holes

Virtual Particles

Black holes are, when described through classical physics, singularities of infinite mass from which no matter or energy (light) can escape. In 1974, however, Steven Hawking demonstrated that black holes display very different properties when viewed through quantum mechanics - they in fact emit light and matter.

According to the fundamental ideas of quantum field theory, the creation of "virtual particles," that is, a pair of particles where one is an "anti-particle" of the other, is something which happens everywhere in space, all of the time. Normally, these two particles will destroy each other, but if this occurrence were to happen next to a black hole, it is quite possible that one of the two particles would fall into the black hole, and the other would not.

This second particle would be radiated away from the black hole. To make this particle "real," energy would be taken from the black hole, thus reducing its mass and energy! Hawking showed that this type of particle radiation has all of the properties of thermal radiation, and the temperature of the radiation is linked directly to the mass of the black hole. It can be calculated that the thermal radiation emission's temperature is inversely proportional to the mass of the black hole.

The larger the black hole, the less radiation one could observe, and the smaller the black hole, the more radiation. Unfortunately, we have said that black holes are objects of great mass, so the likelihood of the existence of black holes that emit large amounts of radiation is small. Even if there were black holes with low mass, they would be "used up" quickly by these emissions.

Using Astronomical Instruments to "See" Black Holes

The idea of "seeing" a black hole is not an acceptable idea, since black holes are defined as objects which emit no radiation. A few methods have been proposed to show the effects of black holes, however.

Distant Binary System:

A binary system with a normal star and a black hole, both circling each other at a great distance (compared to the star's size) would show us a star being affected by Doppler shifts in the star's spectral lines. (The perceived color of the light changes depending on the relative direction of the star.) A possible example of this is EPSILON-AURIGAE, a binary star system with an "invisible" companion (although this companion may just be a few times dimmer, not a black hole).

Close Binary System:

In a close binary system containing a normal star and a black hole, the black hole is close enough to "suck" gas off of the star (shown to the left in orange). This gas will be superheated by this, and begin emitting X-rays (shown in blue). In 1970, the X-ray-observing satellite Uhuru* discovered Hercules X1, an X-ray source whose signal has the characteristics of a pulsar. The signal had an additional periodicity -- after almost two days of pulsing once every ~1.2 seconds, it would "stop" for a few hours.

This fact, along with a slight Doppler shift found in the normal signals, implied a binary system. This led to the discovery of the visible star which completed the system... HZ HERCULIS...

Hot Gas:

Finally, one can predict that matter that is drawn into a black hole will be heated before falling "all the way in." (This emission is not a contradiction to the characterization of black holes provided above!) In 1972/1973, an object believed to be a black hole, as defined by this property, was found: CYGNUS X1.