Today’s column is dedicated to English theoretical physicist, cosmologist, researcher and author, Professor Stephen Hawking, who left this earth this past week on March 14, 2018.
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First of all, I want to thank all of you who sent questions and/or comments for me to consider. These are all great questions, and very representative of the kinds of questions I get all the time from people with active minds and loads of curiosity, but without formal training in science. I have also found that it is very difficult for the curious layperson to get reasonable and authoritative answers to questions on current lines of scientific inquiry — to separate the likely from the purely hyperbolic. I will try to get to all of your questions, although it may take me a while.
Most of you have probably heard of the death of Stephen Hawking. I thought it would be apt to spend a few minutes celebrating his life by talking about a bit of his work. ‘Celebrity scientist’ is a term which sounds almost like an oxymoron in today’s world. Especially when one considers that, unlike many others, Hawking was truly a scientific leader, not just a public figure who happened to be a scientist. There are other scientists of Hawking’s generation whom many would consider equally brilliant or accomplished scientifically, but Hawking of course captured the public imagination by being great despite a terrible disability. Overcoming incredible obstacles is a theme which resonates with many of us. And for good reason.
In addition to Hawking’s public appeal, he maintained a high standing among scientists. This was not easy. He had to maintain a clear line between what he can say, based on science, and his thoughts on other compelling, but non-scientific, topics. The public wants to hear what a brilliant mind has to say about many things. He had to make it clear that there are things he could say, based on science and his expertise, and some things which made sense to him, but were not based on science. An example:
Combustus reader Charles Beedy, who quotes Hawking as asking, posed: “Why does the universe exist? I don’t know an operational way to give the question or the answer, if there is one, a meaning. But it bothers me.“
Roughly translated, this says science is probably not capable of addressing this question. Hawking was willing to explore the limits of science with the public, but he was not willing to pretend he could provide science-based insights beyond these limits. Thank you Prof. Hawking. Lesser (celebrity) scientists have not been able to resist this kind of overreach.
My scientific connection with Hawking is largely via his work related to black holes. Hawking worked out many of the fundamental properties of black holes by considering the full implications of Einstein’s theory of general relativity (GR). For my part, I am an observer of gravitational waves (using the LIGO detectors). In 2015, we first observed gravitational waves, and the source of the waves was the merger of a pair of black holes. This was also the first direct observation of black holes. (More on that in a future post.)
The most amazing of Hawking’s black hole findings is now called “Hawking radiation.” It represents the first known connection between gravity and quantum physics. Some background: There are four known fundamental forces which we believe govern the workings of the universe and its contents. Two of these control our everyday experiences: Electromagnetism (electric and magnetic forces) binds atoms and molecules, and hence material objects, like us. And gravity, which keeps us bound to the earth, governs the motions of planets and stars, and in its most extreme form, is what makes black holes.
Gravity is very different from the other three forces: It is very, very much weaker (surprising? more on this in a future post) and it works by distorting space and time. (The figure below is a typical illustration of this.)
Combustus reader Mike Lottridge asked about visualizations of spacetime distortions in three dimensions — rather than two-dimensions put into a three-dimension visualization: “I have a tough time visualizing what space/time curvature looks like in 3d. Can you help?“
I’m afraid the best we can do is to illustrate via the distortions gravity makes on our sightings of objects, like distant stars. See the figure below of gravitationally lensed galaxies ~ a favorite of mine. The movie Interstellar includes lots of realistic space-time distortion due to strong gravity.
Another distinction of gravity: It is the only force for which our description does not include quantum theory. Physicists have worked for decades to come up with one, only to find the equations blowing up in their faces. String theory is fundamentally an attempt to construct such a theory of quantum gravity. But Hawking came up with a connection. The uncertainty principle of quantum mechanics posits that energy can be non-conserved for (very) short times. An implication of this is that a particle-antiparticle pair can spring from the vacuum for a short time, then recombine. This effect was first measured around 1950, and is now well-studied and characterized. Hawking postulated that if such a pair were to spring forth near the horizon of a black hole, then it would be possible for the pair to be separated — one falling into the black hole, forever gone, and the other promoted into existence as a “real” particle which could, in principle, be detected by an external observer. In this way, black holes would actually lose energy and eventually “evaporate.” The accompanying illustration shows a black hole evolving in time, vertically up, so that its horizon becomes cylindrical. Some “virtual” pairs form near it, with most recombining. But a few become separated when one falls past the black hole horizon.
While few doubt the existence of Hawking radiation, it has not yet been observed. The time it takes for a “normal” black hole to evaporate is very, very long, much longer than the age of the universe. It is possible, however, in some theories, for microscopic black holes to be formed in high-energy accelerator collisions, like those at the LHC at CERN lab near Geneva. These black holes would evaporate in a tiny instant of time. In any case, this connection made by Hawking between gravity and quantum physics is the best one we have, so far.
Another Hawking connection with my work is his postulate that there might be “primordial” black holes, formed just after the big bang, and possibly still around. This has become something of a hot topic again, and the subject of a future post!