Two of the biggest theories of modern science are quantum mechanics (which deals with the subatomic world) and general relativity (a theory of gravity that works on a large scale, dealing with large massive objects). For decades, scientists have had the hope of uniting the two into a single theory. Einstein spent the last years of his life working on such a project, but came up empty-handed.
In the past few decades, some physicists have continued to work on theories that attempt to unify quantum mechanics and general relativity. Such theories are called quantum gravity theories. One class of these theories is called loop quantum gravity.
One occasionally sees news stories based on the predictions of loop quantum gravity. An example is this recent one, suggesting that black holes eventually become white holes that gush out matter.
But whenever you hear the phrase “quantum gravity” you should also think to yourself: not even half-baked. Or perhaps it might be better to think: not even tenth-baked. This is because it is perhaps centuries too early to be advancing a theory that tries to unite quantum mechanics and gravitation. One reason is that there are too many mysteries involved in gravitation and quantum mechanics. Uniting quantum gravity and gravitation might have to wait until we solve such mysteries.
The following might be a logical plan:
- We solve the basic mystery of what causes gravitation, something we don't understand. We know that gravitation is proportional to density of matter, but as it is easy to imagine a universe with no gravitation, we don't really understand why gravitation exists.
- We solve the mystery of why gravity is a trillion trillion trillion times weaker than all of the three other fundamental forces of the universe.
- We solve the basic problem of the nature of the collapse of the wave function, something which is still furiously debated by quantum mechanics theorists.
- We solve the incredibly perplexing problem of quantum entanglement, and how this spooky mysterious “action at a distance” can be occurring.
- We solve the mysterious “observer effect” in quantum mechanics, the bizarre fact that matter can behave very differently depending only on the way we observe matter.
- We clarify the mysterious “double slit” experiment, which suggests that both electrons and energy photons can switch back and forth between wave behavior and particle behavior.
- Then, after gaining a vastly clearer understanding of both quantum mechanics and gravitation, we attempt to create a single theory uniting both of them.
But some of our physicists have jumped straight to item 7 in this list before understanding the first six. This seems to make no sense. How can you unite quantum mechanics and gravitation into a single theory, when there are so many unsolved mysteries involved in both of them?
Quantum gravity is a nice little niche for some physicists. If you are a quantum gravity theorist, you can spend your year working on some theory that no one will expect to work, piling on one far-out speculation after another. If anyone complains about a lack of verification or predictions, you can say: come on, this is quantum gravity, what do you expect? I'm reminded of that Broadway song with the lyric: nice work if you can get it.
Quantum gravity theorists are good at speculations, but are not very good at justifying their work. One quantum gravity theorist admits that there is no evidence for quantum gravity, but she tries to justify her funding by saying this:
The irony is that quantum gravity phenomenology is as safe an investment as it gets in science. We know the theory must exist. We know that the only way it can be scientific is to make contact to observation. Quantum gravity phenomenology will become reality as surely as volcanic ash will drift over Central Europe again.
This is very unpersuasive reasoning. We are not sure that any workable theory of quantum gravity will ever be discovered, and it is very unclear whether such a workable theory will be developed anytime in the next 500 years. Far from “as safe an investment as it gets in science,” investing in quantum gravity theoretical research seems no more safe than betting on a horse race or buying a lottery ticket. A safe investment, on the other hand, is one that has a high likelihood of giving you a good return within the next decade (such as a mutual fund with a 50% mix of stocks and bonds).
Perhaps the main type of quantum gravity theory is what is called loop quantum gravity theory. Such a theory is based on the idea that time is quantized. You can get kind of an idea of quantized time by imagining that each second is a stack of time-slices, and that there are a limited number of these time-slices in each second.
I think this idea is misguided. The idea of quantized time reinforces the assumption of a strict segregation between this instant and the next instant. But rather than thinking in such a way, we should perhaps be moving in the opposite direction. Although it may shock our expectations, experiments on precognition suggest that there may well rarely be some kind of partial intermingling or information exchange between the future and the present. The same thing is suggested by many human experiences very well described in the book The Science of Premonitions by Larry Dossey MD. A particularly striking example is given on page 41 of the book. On May 2, 1812 an Englishman named John Williams had a dream of the assassination of the British prime minister Spencer Perceval. Williams had the dream three times on the same night, and the dream included very specific details. Nine days later Perceval was assassinated. As Dossey puts it, “The details of the assassination were identical to those of the dream, including the colors of the clothing, the buttons on the assassin's jacket, and the location of the bloodstain on Perceval's white waistcoast.” (See here for another author's discussion of this incident.)
It is hard enough to explain such experiments and experiences with our normal assumptions about time, and it seems even harder to explain them under some assumption of quantized time. If physicists wish to create some exotic new theory of time, they would do better to create one that can help explain experiments on precognition and human experiences of premonitions that came true. Rather than imagining a rigid “one-way street” leading between the past and the future, such a theory might allow for the possibility of a limited degree of mingling or communication between the past and the future, possibly in both directions. Such a theory might describe a separation between the past and the future that is more fuzzy and blurred than we normally imagine.
But such a theory may be a long way off. For the present, I simply suggest: when you hear that something is suggested by quantum gravity, remember that quantum gravity may be centuries away from being ready for prime time.