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.
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?
As physicist Carlo Rovelli recently said about quantum mechanics and general relativity (the prevailing theory of gravitation), "When you try to put the two theories together, they appear to result in all sorts of contradictions and paradoxes."
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.
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.)
I have a series of posts discussing similar cases:
When Dreams or Visions Foretell a Death
More Dreams or Visions That Seemed to Foretell a Death
Still More Dreams or Visions That Seemed to Foretell a Death
Still More Dreams, Visions or Voices That Seemed to Foretell a Death
Some More Dreams or Visions That Seemed to Foretell a Death or Disaster
When the Future Whispers to the Present
When Dreams or Premonitions Seem to Act Prophetically
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. And quantum gravity may be centuries away from being ready for prime time.
But despite having no predictive successes and no grounding in evidence, quantity gravity recently got a reverent treatment by the frequently scientist-fawning Quanta Magazine. When I go to the site on the day I wrote this post, I see my entire screen filled up by a huge photo of the face of quantum gravity theorist Carlo Rovelli. We read of a 12-hour conversation with him. We hear him making groundless speculative claims such as this: "Basically, loop quantum gravity implies that space is not infinitely divisible — it’s made of elementary chunks, which are linked together into loops." We do not hear of any evidence backing up such claims. It isn't quite the usual "nothing but softball questions" interview we tend to get when scientists are questioned, because a few tough questions are asked. But the interviewer fails to ask the "where's the evidence?" type of questions that should be directed to a theorist spinning out cobwebs of speculation not backed up by evidence.
We have this confession from Rovelli that there's no experimental evidence for quantum gravity, mixed with not a report on actual observational results, but mere wild fantasizing about observational results that might occur:
"The main shortcoming is the lack of experiments supporting it. However, there’s hope on the horizon. There are some proposals to use loop quantum gravity to make sense of signatures in the cosmic microwave background radiation that’s left over from the Big Bang. And there’s another new idea I’m very excited about: If loop quantum gravity is right, there should exist tiny black holes weighing around 10 micrograms that are long-living and that interact only gravitationally. We’re thinking about ways to detect a background 'wind' of these particles. And perhaps these tiny black holes are actually what we call dark matter, a mysterious widespread astronomical phenomenon that we have not yet understood."
These cobwebs of speculation sounds as substantive as someone fantasizing that he may one day meet Bigfoot, and also that he may encounter a flying saucer that he can walk inside and inspect.
We are told this about Rovelli: "A decade later, he proposed a new 'relational' interpretation of quantum mechanics, which goes so far as to suggest that there is no objective reality whatsoever, only perspectives on reality — be they a physicist’s or a pigeon’s." In the same interview, we read this quote by Rovelli: "We must not confuse the knowledge we have with the reality of the world." Elsewhere Rovelli said, "At the foundations of physics there is today confusion and incoherence," a statement which may apply to his own assertions.
The Google Ngram page (which you can use at the link here) can be used to get a feel for the popularity of references of a topic in Google Books, over a period of time. The image below shows the relative frequency of references to three types of physics theory: string theory, supersymmetry and quantum gravity. The graph suggests that supersymmetry and string theory were failing efforts. Quantum gravity seems no more popular than string theory and supersymmetry after both suffered a sharp decline.
Using the same tool, I get the graph below, which suggests that people are getting much more evidence for apparitions than for quantum gravity. But that's to be expected. People see apparitions, but don't see quantum gravity.
But a recent article in Quanta Magazine suggests that the quantum gravity guys may be trying to catch up by injecting a little "ghost glamor" into their speculations:
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