Black holes by their nature are difficult to observe, so the evidence for their existence is by necessity indirect. After all, a black hole is an object whose gravitational influence is so strong that anything coming too close cannot escape. From an observational point of view, we don’t ever expect to “see” a black hole directly, but because the gravitational pull accelerates matter, it can emit intense amounts of light — including radio waves, X-rays, and gamma rays.
The image on the right is Cygnus X-1, the earliest observed bright X-ray source that convinced many astronomers that black holes really exist — they aren’t just mathematical entities from general relativity, but objects that can be observed and characterized. Since the discovery of Cygnus X-1 in 1964, many other objects have been identified as black hole candidates using these basic criteria: the mass as observed from the gravitational pull on companion objects is very high, yet the object itself is very small in size.
A prime example of this is the bright X-ray source at the center of our Milky Way galaxy, known euphoniously as Sagittarius A* (“A star” when spoken aloud). Using the motion of stars orbiting around Sagittarius A*, astronomers determined its mass to be 4 million times the mass of our Sun; I always have my astronomy 101 students calculate this mass from real astronomical data using Kepler’s laws of motion, since it’s a great illustration of how well the laws of physics work, even for unknown objects. The size of Sagittarius A* is no larger than the orbit of Uranus — about 20 times the distance from Earth to the Sun. In other words, whatever is at the center of our galaxy is far more massive than any star (which top out around 200-300 times the mass of the Sun), and is too small to be a cluster of stars. The scientific consensus is that Sagittarius A* is a supermassive black hole, similar to objects observed at the centers of other galaxies.
Careful readers will notice that I am being cautious: I am not saying these objects are definitely black holes, only saying the consensus within the community says they are. I don’t advocate slavish following of conventional wisdom, but although I’m not an astronomer, based on the evidence I see and understand, I think they’re right. The most important questions one can ask in science are “how do you know?” and “how can we distinguish between different solutions to the same problem?” There is no other object that has been proposed that fits the observed data as well as a black hole.
So I admit I was biased when I began to read today’s Guest Blog at Scientific American. (This is the same blog I contributed to earlier, so I have more than a bit of fondness for it.) The post is entitled “Maybe Black Holes Don’t Really Exist“, which is link bait if I’ve ever seen it — but it’s a fair point of view, so it’s worth reading and considering. After all, the evidence for black holes is necessarily indirect, so if someone comes up with a legitimate alternative explanation for the same physical phenomena, it’s something that should be considered — especially if the alternative solves some of the problems with the black hole model.
This idea just doesn’t work for me. The author, George Chapline, brings up two major objections to the black hole model, one theoretical and one observational. The observational objection may have some merit, based on my limited astrophysics knowledge (and I ask anyone who studies astrophysical black holes to comment): the kinds of jets of high-velocity matter seen shooting from black hole candidates are not fully understood, although partial explanations involve the discs of gas that swirl around the suspected black hole. It’s a challenging problem involving high temperature plasmas and strong magnetic fields, so (again speaking as an outsider) I’m not shocked that mysteries still exist. Discarding the entire black hole model based on that seems premature to me.
The theoretical objection Chapline raises is that any object with an event horizon — a boundary that marks where nothing can escape the pull of the black hole’s gravity — is incompatible with quantum mechanics. His reason is that there isn’t a universal time associated with an event horizon, which is a true statement: the passage of time measured by an observer depends on their motion relative to the black hole. That’s an inevitable consequence of relativity, but it doesn’t just apply to black holes: the measurement of time on Earth is slightly different than the measurement of time by a satellite in orbit (a correction factor GPS and other communication satellites have to make). In fact, time is always measured relative to an observer, and two observers moving quickly relative to each other will not agree on how much time has passed. That’s Einstein’s relativity, and it is not controversial. Event horizons are also not controversial from a basic understanding of general relativity (and in fact the 18th century physicist Laplace predicted something very similar to them!); whether they exist in astrophysical objects is of course another question, since of course observing an event horizon directly is not an easy task.
Chapline is also correct that ordinary, non-relativistic quantum mechanics has a universal time: every particle described using the regular, low-energy version of quantum mechanics (the type most people have heard about, and the type I’ve written about most) uses the same time. However, it’s important to remember that the non-relativistic version of quantum mechanics is a useful approximation to the fully-relativistic version when energies are small, much as Newtonian physics is a useful approximation to relativistic mechanics when velocities are small compared to the speed of light. Quantum field theory, the relativistic version of quantum physics, doesn’t require a universal time — each particle carries its own time, so it is puzzling to insist on a universal time for a black hole. It’s also telling that there are a huge number of physicists who work in general relativity and quantum physics, and none of them I know have raised this particular objection — despite the fact that quantum field theory and gravity notoriously don’t play well together. (That’s definitely a subject for another day!)
All of these objections could be swept away if the alternative model to black holes was a convincing one: after all, emotional attachment and conventional wisdom make for poor science. However, the proposed replacement — called a “crystal star”, a “frozen star”, or a “dark energy star” — seems to be based on some rather speculative ideas. Some aspects of the crystal star idea isn’t too far-fetched, really: we already have such an object in astrophysics, known as a neutron star. A neutron star is the end result of a star much more massive than our Sun, whose core collapses under its own weight until it compresses into a nucleus-like object with the mass of a star but the size of a city. (I am not doing justice to the awesome weirdness of neutron stars! They certainly merit their own post at some point.) The outer surface of the neutron star is believed to be a solid crust of atomic nuclei left over from the original star’s core, so you could technically stand on it — if you could withstand the intense temperatures and crushing gravity.
Neutron stars can only grow to be so large before gravity makes them collapse even more; the end-point usually considered for this collapse is a black hole. The crystal star idea seems to be an extension of the neutron star concept, where matter is even more highly compressed and dark energy — the negative-pressure entity that causes the universe to accelerate — prevents total collapse into a black hole. Chapline evokes quantum-gravitational effects to keep things from complete collapse, which is a bit dodgy as there is no complete quantum theory of gravity. (I don’t wish to get into the entropy of black holes here, though I probably should to do full justice to Chapline’s point of view.) However, in the understanding of most cosmologists, dark energy doesn’t get trapped in the same way matter does: the more it is confined, the less pressure it exerts, and it only comes into its own when it has a lot of space.
I have no doubt that any true quantum theory of gravity will have something to say about black holes, both about whether information is truly lost when particles cross an event horizon and about the singularity at the heart of a black hole. However, based on our current understanding of general relativity — a very successful theory! — and astrophysics, it doesn’t seem to me that the crystal star idea has sufficient motivation to throw out everything we’ve learned. To its credit, the crystal star idea is a testable alternative to black holes, so though my money is against it, I’ll be interested to see how it may shape up.
(Thanks to Arthur Kosowsky for helping me clarify some of my arguments.)