How small can a black hole be?
April 1, 2008 | 5:55 pm
Yesterday at the American Astronomical Society’s conference in Los Angeles, evidence for the smallest black hole ever observed was presented. It comes in at a mere 3.8 times the Sun’s mass and is only 24 kilometers across. New Scientist has a story about it.
There has been a lot of discussion in recent days about microscopic black holes that could theoretically be created at the Large Hadron Collider. The black holes observed in the universe are entirely different creatures. A stellar black hole, made from a collapsing star, accretes nearby surrounding matter, growing larger over time. In contrast, the microscopic black holes at a collider would “evaporate” in a fraction of a second, unable to suck in any matter. So leaving aside the microscopic variety, are there limits to how small a black hole can be?
For a stellar black hole to form, two things need to happen: There must be a high enough mass and it must be in a small enough volume. So to look for the mass limits, we have to think about the densest stars we know: white dwarfs and neutron stars. These compact stars remain stable up to a certain point at which the mass gets enough to cause collapse to a black hole.
That mass limit for neutron stars is called the Tolman-Oppenheimer-Volkoff limit after the physicists who came up with it. Modern estimates of this limit are 1.5-3.0 times the mass of the Sun.
What this means is that the black hole just discovered is really pushing down to the lowest limits of stellar black hole mass and we are unlikely to find many smaller.
David Harris
Posted in Uncategorized |
April 1st, 2008 at 8:14 pm
Hawking Radiation (evaporation) is credibly disputed (http://arxiv.org/abs/gr-qc/0304042) and contradicts Einsteins relativity theory. We have not observed the results of quantum effects on micro black holes, so we really do not know what will happen, it could cause growth (http://arxiv.org/abs/0803.0659). Either way, we are scheduled to find out this summer.
April 1st, 2008 at 10:59 pm
Although evidence for Hawking Radiation is not conclusive, it is also not necessary for the argument about what will happen to microscopic black holes created in a collider experiment. Independent arguments that only require quantum mechanics indicate that microscopic black holes would decay rapidly.
Given this that this is a post about stellar black holes, it’s not the place to get into a detailed discussion of the physics, but I’d simply note that if black holes can be created in a collider experiment, it happens through some interaction pathway. Quantum mechanics says that that pathway can flow in reverse so the black holes must also decay even if the decay rate is not identical. The arguments for growth are not accepted by the physics community as far as my reading of it goes. (Your citation for growth is not to a paper about the topic.)
April 2nd, 2008 at 8:17 am
“theoretically created” - can’t say they will, can’t say they won’t. As an ordinary person, I am excitedly waiting to see what happens. But the scary movie “Event Horizon,” makes we a little nervous about the whole thing. Tingling with anticipation is the best way to describe it - let the experiments begin!
April 12th, 2008 at 10:16 am
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