Guiding principle: quantum gravity

One of the aims of fundamental physics is to obtain a theory that can combine gravity with quantum physics. As I mentioned before, theory space is vast. A successful venture into theory space needs a reliable guiding principle. Without any experimental result pointing out the direction we need to take, the selection of such a guiding principle for the formulation of a quantum theory of gravity is difficult.

Some people believe that quantum gravity is the domain of the Planck scale where quantum and gravitational effects coincide. It requires extremely high (experimentally unattainable) energy densities. It also assumes that such high energy densities allow things like black holes and worm holes to pop in and out of existence. That is however an unscientific notion. Things don’t just pop in and out of existence, least of them black holes, regardless of the energy density.

Moreover, there are no such things as worm holes. I don’t care that Einstein thought they may exist. The idea represents one of those cases where the mathematics is over extended to produce a spurious solution that, although allowed mathematically, has no physical meaning. So they cannot pop in and out of existence anyway.

Hence, it is unlikely that there is anything interesting happening at the energy scale represented by the Planck scale, or more accurately called the hypothetical Planck scale. Therefore, I would not recommend any statements about what happens at this hypothetical Planck scale as a reliable guiding principle for quantum gravity.

As a more reliable guiding principle, we need to address the question, what happens to the gravitational field produced by a quantum state? What I mean by a quantum state is a state of matter in which quantum effects are manifest. An example of such a quantum effect is entanglement. So the question in this case is, does the gravitation field become entangled with the quantum state, or is the gravitational field uniquely produced by some combination of the elements in the superposition that represents the entangled state?

We can address the question with our current theory of general relativity. In Einstein’s field equation for general relativity, the curvature tensor of spacetime is equated to the stress-energy tensor of the matter distribution. In the context of quantum theory, the latter becomes an observable – an operator that can be traced with the quantum state to produce the observed stress-energy tensor of the quantum state. Obviously, the observed stress-energy tensor does not represent the entanglement anymore. Therefore, the curvature of spacetime produced by such an entangled state is affected by a combination of the elements in the superposition and does not become entangled with the state.

What does this say about the guiding principle for quantum gravity? What it seems to say is that there is no need for quantum gravity. The spacetime that we live in is a background in which the intricacies of quantum physics play out without becoming involved. The only effect that the quantum state of matter has on the gravitational field is through a unique stress-energy distribution for the entire state.

This conclusion is based on the assumption that Einstein’s field equation is valid on the small scale of quantum physics. It has been tested at larger scale and so far no deviations have been found. Without any observed deviations, there is not strong motivation for expecting that it would not be valid at the scales of quantum physics.

However, there is one aspect that Einstein’s field equation does not explain. It shows the connection between the curvature of spacetime and the distribution of matter, but it does not explain how mass-energy curves spacetime. It does not give a mechanism for this process. Such a mechanism may be hiding in the quantum description of matter. If such a mechanism can be uncovered, it would lead to a more comprehensive theory that would “explain” the Einstein’s field equation.

The search for this mechanism may be somewhat different from a search for a theory of quantum gravity. However, it can be seen as a more focussed attempt at formulating a theory of quantum gravity. To find this mechanism, we can perhaps focus of fermions. I think there are still some mysteries associated with fermions that need to be uncovered. Perhaps that can lead us to an understand of the mechanism for the way that mass-energy curves spacetime.

“Post-empirical science”

The informed reader will know that the title represents an oxymoron. Without its empirical character science would not be science. It is very much what defines the cultural activity that we call “science” to be what it is.

Why then this glaring contradictory notion? It has popped up in the literature related to a recent “publicity stunt” where a simulation of a wormhole in a toy model was blown out of all proportions by being deemed to have created an actual wormhole. The simulation was done on a puny quantum computer incorporating merely 9 qubits.

Although this story has been hyped by various sources (and I am not going to give any links because I don’t want to mislead more people), many people have strongly criticized the story, including John Horgan, Scott Aaronson, Ethan Siegel, and Peter Woit. I can go on to try and clarify, but these posts are doing a much better job than I can.

Of course it is nonsense. A simulation is a numerical calculation of the physical process under study. It is not the real thing. And it does not matter whether the simulation is done with a classical digital computer or with a quantum computer. It is still just a simulation. Moreover, the amount of information that one can extract from 9 qubits is 9 bits, which is barely enough to specify one single ASCII character in a text document. So, no wormholes were created.

Perhaps the result they obtained from their simulation agreed well with what they expected to see, but that does not mean that it qualifies as being an experiment. Simulations and experiments are different things. Usually simulations are used when the direct calculations are too difficult. However, there is almost no limit on what one can simulate. It does not have to be something that can actually exist. If I have a set of equations that describe some weird imagined process that cannot exist in our universe, I can still program those equations into a computer and simulate it. For this reason, the results of a simulation can never take the place of an actual experiment.

What does this have to do with the notion of post-empirical stuff? Well, the problem lies in fundamental physics where it becomes progressively more difficult to perform experiments to learn about how things work. As a result, people are trying to motive that we start to learn about these things without having to do the experiments. That would have been great if it could work. Unfortunately, it has been tried before and found not to work. That was what the philosophers did before the advent of the scientific method. The nonsense they came up with still bounces around in the cultures of the world.

No! the day we cannot perform experiments to learn how this universe works is the day we stop learning more about our universe. A lot of people may go on coming up with stuff, but for sure, that stuff is worth nothing if it cannot be shown to work that way in our universe.

Unfortunately, there is already a lot of this going on, as this hyped wormhole nonsense demonstrates. It is related to several such non-scientific ideas that people work on and call physics, even though they don’t have much or any hope ever to show that it actually works that way through a scientific process.

The annoying thing is that there are prominent people in the physics community that are driving the hype. They’ve been doing this with other similar stories. Apparently, the reason for this hype is to induce funding agencies to give them more funding. Well, I think that if funding agencies can be led by their noses so easily, then the situation is more hopeless than I thought. These prominent people are not prominent for having done any solid scientific work. There are also other ways to become prominent. Well, I’ve ranted enough about people being prominent for the wrong reasons and don’t want to do it again.

In defense of particle physics experiments

As a theorist, I may have misled some people into thinking that I don’t care much for experimental work. In particle physics, there tend to be a clear separation between theorists and experimentalists, with the phenomenologists sitting in between. Other fields in physics don’t have such sharp separations. However, most physicists lean toward one of the two.

Physics is a science. As such, it follows the scientific method. That implies that both theory and experiment are important. In fact, they are absolutely essential!

There are people that advocate, not only the suspension of experimental work in particle physics, but even that the methodology in particle physics be changed. What methodology in particle physics needs to be changed? Hopefully not anything related to the scientific method! To maintain the scientific method in particle physics, people need to keep on doing particle physics experiments.

CMS detector at LHC

There was a time when I also thought that the extreme expense in doing particle physics experiments was not justified by the results obtained from the Large Hadron Collider (LHC). However, as somebody explained, the results of the LHC are not so insignificant. If you think about it, the “lack of results” is a fallout of the bad theories that the theorists came up with. So by stopping the experimental work due to the “lack of results,” you would be punishing the experimentalists for the bad work of the theorists. More importantly, the experimentalists are just doing precisely what they should be doing in support of the scientific method: ruling out the nonsense theories that the theorists came up with. I think they’ve done more than just that. Hopefully, the theorists will do better in future, so that the experimentalists can have more positive results in future.

I should also mention the experimental work that is currently being done on neutrinos. It is a part of particle physics that we still do not understand well. These results may open the door for significant improvements in our theoretical understanding of particle physics.

So, please keep on doing experimental work in particle physics. If there is an methodological changes needed in particle physics, then that is limited to the way theorists are doing their work.

In memoriam: string theory

Somebody once explained that when a theory is shown to be wrong, its proponents will keep on believing in it. It is only when they pass away that the younger generation can move on.

None of this applies to string theory. To be shown to be wrong there must be something to present. The mathematical construct that is currently associated with string theory is not in any form that can be subjected to any scientific testing.

What was shown to be wrong is supersymmetry, which is a prerequisite for the currently favored version of string theory – super string theory. (The non-supersymmetric version of string theory fell into disfavor decades ago.) The Large Hadron Collider did not see the expected particles predicted by supersymmetry. Well, to be honest, there is a small change that it will see something in the third run which has just started, but I get the feeling that people are not exactly holding their breath. I’m willing to say supersymmetry is dead and therefore so is super string theory.

Another reason why things are different with string theory is because the proponents found a way to extend the postmortem activity in string theory beyond their own careers. They get a younger generation of physicists addicted to it, so that this new generation of string theorist would go on working in it and popularizing it. What a horrible thing to do!

Why would the current string theorists mislead a younger generation of physicists to work on a failed idea? Legacy! Most of these current string theorists have spent their entire careers working on this topic. Some of them got very famous for it. Now they want to ensure that they are remembered for something that worked and not for something that failed. So it all comes down to vanity, which I’ve written about before.

String theory was already around when I was still a student several decades ago. I could have decided to pursue it as a field of study at that point. What would I have had to show for it now? Nothing! No accomplishments! A wasted career!

There was a time when you couldn’t get a position in a physics department unless you were a string theorist. As a result, there is a vast population of string theorists sitting in faculty positions. It is no wonder that they still maintain such a strong influence in physics even though the theory they work on is dead.

Postulates or principles?

Sometimes an idea runs away from us. It may start in a certain direction, perhaps to achieve a certain goal, but then at some point down the line it becomes something else. It may be an undesirable situation, or it may be a new opportunity. Often, only time will tell.

Quantum mechanics is such an idea. It is ostensibly a subfield of physics, but when we take a hard look at quantum mechanics, it looks more and more like mathematics. It has taken on a life of its own, which often seems to have very little to do with physics.

To be sure, physics would not get far without mathematics. However, mathematics has a very specific role to play in physics. We use mathematics to model the physical world. It allows us to calculate what we expect to see when we make observations of the phenomena associated with that model.

Quantum mechanics is different from other physical theories. While other physical theories tend to describe very specific sets of phenomena associated with a specific physical context, quantum mechanics is more general in that is describes a large variety of phenomena in different contexts. For example, all electric and magnetic phenomena provide the context for Maxwell’s theory of electromagnetism. On the other hand, the context of quantum mechanics is any phenomenon that can be found in the micro world. As such quantum mechanics is much more abstract.

We can say that quantum mechanics is not a theory, but instead a formalism in terms of which theories about the micro world can be formulated. It is therefore not strange that quantum mechanics looks more like mathematics. It even has a set of postulates from which the formalism of quantum mechanics can be derived.

But quantum mechanics still needs to be associated with the physical world. Even if it exits as a mathematical formalism, it must make some connection to the physical world. Otherwise, how would we know that it is doing a good job? Comparisons between predictions of theories formulated in terms quantum mechanics and experimental results of the physical phenomena associated with those theories show that quantum mechanics is very successful. However, in the pursuit of understanding the overlap between quantum physics and gravity in fundamental physics, the role of quantum mechanics needs to be understood not as a mere mathematical formalism, but as a fundamental mechanism in the physical world.

It is therefore not sufficient to provide mathematical postulates for the derivation of quantum mechanics as a mathematical formalism. What we need are the physical principles of nature at the fundamental level that leads to quantum mechanics as seen in quantum physics.

Principles differ from postulates. They are not expressed in terms of mathematical concepts, but rather in terms of physical concepts. In other words, instead of talking about non-commuting operators and Hilbert spaces, we would instead be talking about interactions, particle or fields, velocities, trajectories and things like that.

Another important difference is the notion of what is more fundamental than what. In mathematics, the postulates can be combined into sets of axioms from which theorems are derived. It would mean that the postulates are more fundamental. However, they may not be unique in the sense that different sets of axioms could be shown to be equivalent. In physics on the other hand, the principles are considered to be more fundamental than the theories in terms of which physical scenarios are modeled. There may be a cascade of different theories formulated in terms of more fundamental theories. Since, these theories are formulated in terms of mathematics, it can now happen that the axioms for the mathematics in terms of which some of these theories are formulated, are not fundamental from a physics point of view, but a consequence of more fundamental physical aspects.

An example is the non-commutation of operators in quantum mechanics. It is often considered as a fundamental aspect of quantum mechanics. However, it is only fundamental from a purely mathematical point of view. From a physical point of view, the non-commutation follows as a consequence of more fundamental aspects of quantum physics. Ultimately, the fundamental property of nature that leads to this non-commutation is the Planck relationship between energy (or momentum) and frequency (or the propagation vector).