The Black Box Around Black Holes
November 6, 2014
In many ways, theoretical physics is the field closest to the old-school nature of science. Every day, astrophysicists are building more and more elegant theories of things we don’t fully understand while clashing with each other over the true nature of the Universe.
Theoretical physics today is where evolutionary biology was when Darwin and Mendel were publishing or where chemistry was when Mendeleev was piecing together the periodic table. Physicists are holding a black box, shaking it and feeling it and trying to figure out what’s inside. Some theories turn out to be right, many more are wrong and slowly the box opens, but the opening is a long and volatile process.
Take black holes for example. They are very difficult to study because we can’t directly see them and they are so far away. Some physicists hold that nothing that goes in can ever come out, others say tiny bits of radiation fly out, and UNC physicist Laura Mersini-Houghton says they may not even exist. At least not how we think.
According to Mersini-Houghton’s newest paper, published online at the non-peer reviewed arXiv.org, “quantum effects” predicted by one theory of how the universe works overpower the gravity from another theory to the point where black holes cannot physically form the way scientists think they do.
You may have seen some headlines about this story claiming that the paper proves black holes do not exist or that this work calls the Big Bang into question, and while these headlines might be overreaching, this study could potentially throw a wrench into an already volatile area of physics. To understand why, however, we need a little background in black holes, general relativity and quantum mechanics.
Crushing Stars and Birthing Black Holes
Astrophysicists tend to agree on a select few theories of how black holes form, the most common being that black holes are born in dying stars.
Stars are huge gas balls. They are so huge that their gravity keeps planets in orbit and crushes atoms in their cores to make the light we see and lots of heat. That heat is what keeps the star from crushing itself with its own gravity.
At the end of a star’s life it smashes fewer and fewer atoms together, creating less and less heat until finally it starts to collapse in on itself. Gravity pulls all of the gas that once made up the star into a tinier and tinier space. Sometimes this causes the star to explode, but even when it does it leaves a tiny ball with enough gravity to suck in anything around it. That is a black hole, a supermassive point with so much gravity that even light cannot escape.
A Tale of Two Theories
Most astrophysicists agree on the theory of black holes up until that last point, whether anything can escape. This is where an almost undetectable amount of radiation pits general relativity and quantum mechanics against each other.
General relativity is Einstein’s theory of gravity and it does a great job describing the Universe on a large scale. According to Einstein, objects with high mass bend space-time — a combination of 3 space dimensions and one time dimension. Everything moves through space-time, so if you bend it you change the way something moves. Think of space-time like mini-golf. As your ball rolls around, it is more likely to roll down slopes and into dips than to stay at a higher elevation. Einstein predicted that massive objects create similar slopes and divots in space-time, thus attracting nearby objects.
General relativity holds that black holes cram so much mass into so small a space that space-time actually bends in on itself and that once you get close enough, past a boundary called the event horizon, you can’t ever get out again. The event horizon is like the 18th hole at mini-golf that takes your ball back to the guy behind the counter.
Quantum mechanics, on the other hand, is the theory of all things small: covering the behavior of molecules, atoms and subatomic particles. One of the major rules of quantum mechanics is that information is never lost, but if light and matter fall into a black hole and can’t come out, you lose information. That is a big no-no.
Stephen Hawking has made several attempts to reconcile this “Information Loss Paradox." The most popular one is a concept called Hawking radiation. There's not a great physical analogy for how it works but the closest approximation is that “quantum forces” inside the black hole cause the formation of matter and antimatter. Normally matter and antimatter annihilate each other, but the thinking is that if a matter/antimatter pair was made on the event horizon, conceivably one particle could wind up outside the black hole and escape. No one has ever observed Hawking radiation, but many quantum theorists accept this idea.
You might be thinking that if two theories don’t agree on the explanation of something, one or both of them is wrong. You would be half right. Things either escape from black holes, or they don’t. That doesn’t mean, however, that whichever theory is “wrong” is “wrong” about everything and does not have any value.
Here is a silly example. I have a 7-month-old labradoodle named Ollie. If I were a scientist trying to write a theory of how my apartment works, I would note that when Ollie naps in the living room, he ALWAYS naps on my legs. But another apartment theorist could note that Ollie is not allowed on the couch, and he is ALWAYS kept off. Both theories accurately describe what goes on in my apartment.
There is a case when these two theories disagree. When I have my legs up on the couch, Ollie Legs Theory predicts he will nap on the couch, but Ollie Couch Theory predicts that he can’t do that. Both theories are largely good at describing the rules of the apartment, but in this specific case something needs to be modified to satisfy both or one of them is wrong.
Now in this example, an observer could easily see Ollie laying his head on my lap until I give into his guilt-trip, get off the couch and sit with him on the floor. Scientists studying black holes, however, do not have the same luxury as we can’t see what goes on in a black hole. This makes tweaking theories much more difficult.
The tool that theoretical physicists have to work on these theories is math, and Mersini-Houghton’s paper contains lots of it. This particular study is a mathematical simulation of a dying star’s collapse and a black hole’s formation.
Mersini-Houghton’s simulation begins with a dying star falling in on itself. Next, as the star collapses, Mersini-Houghton’s model predicts the star will let out huge amounts of Hawking radiation.
That radiation spells trouble for the falling star formation of black holes, ironically because of another of Einstein’s discoveries. Einstein predicted that mass and energy are interchangeable — you know it as E = mc2. As the star emits all this Hawking radiation it draws more energy/mass from the center of the star to replace it. The model predicts the loss of energy/mass to be so great that the star’s gravity can no longer keep the collapse going. The star would stop shrinking and bounce back outward, instead of shrinking into a black hole.
What Does that Mean?
If Mersini-Houghton’s calculations are correct and generalizable, this would mean that black holes don’t form when stars collapse in on themselves, but rather through other processes, some of which scientists may not have thought of yet. But before condemning star collapse theory to the realm of falsehood, there are a few considerations we need to think of.
- The study has not yet been peer-reviewed. Peer review is standard procedure for scientific papers as experts then offer critiques. The fact that a paper has not received peer review, however, does not mean that it has no value or that it will not be peer-reviewed later. Many physicists post their research to arXiv.org, where this paper is published, to get some critiques before formally submitting it to a peer-reviewed journal, and Mersini-Houghton’s first paper on this subject appeared at arXiv.org and was later published in a peer-reviewed journal. For now, Mersini-Houghton’s findings are good to discuss and debate, but they can be strengthened a lot by peer review.
- Some headlines about this research have said this paper proves black holes cannot exist. That is not actually what the paper says. The paper runs a simulation of a collapsing star and finds that in that simulation, other forces get in the way of making a black hole. But there are other ways that scientists have theorized the formation of black holes, such as two stars colliding. Also there is plenty of physical evidence for black holes. Scientists can’t see them directly but they can see how their gravity influences stars and planets around them.
- Mersini-Houghton’s simulation makes a few simplifying assumptions. For example, a collapsing star needs to crush down below a specific size before its gravity becomes densely packed enough to make a black hole. That size is called the star’s Swarzschild radius. Mersini-Houghton used a spherical Swarzschild radius in her simulation, and found the dying star couldn’t crush down that small. In real life the Swarzschild radius may not be spherically symmetrical, though. That might allow the dying star to reach it, and it might not, but the point is that Mersini-Houghton’s model works for a specific set of cases, but may not be generalizable to every black hole out there.
These considerations are not meant to devalue Mersini-Houghton’s work. In fact, this study could be a huge factor in opening the black box around black holes. It could add a new rule to the theory of how collapsing stars form black holes or even force scientists to develop a new theory of where black holes come from. This study may be disproven by black hole experts, but either way it prompts an investigation which can only serve to strengthen the field and open that black box just a little more.
- Daniel Lane
Daniel Lane covers science, engineering, medicine and the environment in North Carolina.