A Brief History of Quantum Gravity

459px-Einstein_patentofficeProblems cannot be solved by thinking the way we thought when we created them. – Albert Einstein

The incompatibility of quantum mechanics and gravity is seen as a big issue by all physicists. The Planck length

contains the gravitational constant G and Planck’s quantum h, and therefore it is the scale at which “quantum effects of gravity” are supposed to become important. But that is all what physicists have found out about quantum gravity. No theory exists, let alone any evidence of an observable effect. You’ll find the topic somewhat more elaborated in Hawking’s book A Briefer History of Time. The chapter about quantum gravity is comprised of 21 pages, of which almost 20 pages are devoted to repeating gravitation and quantum theory. Andrzej Staruszkiewicz, editor of a renowned physics journal, commented on this topic:

It is tempting to assume that when so many people write about “quantum gravity”, they must know what they are writing about. Nevertheless, everyone will agree that there is not a single physical phenomenon whose explanation would call for “quantum gravity”.

The fact that no theory for quantum gravity exists does not preclude the existence of numerous experts of quantum gravity. According to the science historian Federico di Trocchio, such a „second category of experts“ consists of those whose knowledge will become immediately obsolete once the riddles scientists are studying have been understood. They make their living on the problems that are being tackled unsuccessfully, or, like some string theorists, make ridicuous claims of having explained „the existence of gravity“.

There is simply no theory that combines general relativity with quantum theory. All theoretical recipes cooked up until now have failed, as for instance the so-called ADM formalism, a reformulation of Einstein’s equations of general relativity. It has gotten nowhere, but it is nevertheless considered a bible leading the way. Another great couturier of theories is Abhay Ashtekar, who regularly summarizes the accomplishments of the Loop Quantum Gravity he fathered. His résumé:“it is interesting”.Actually, it is interesting for 30 years now. Not exactly news anymore. But I fear that we will have a long wait for such a Theory of Everything that eventually unifies quantum gravity and general relativity. Success just comes in different flavors.

Theoretical physicists seem to have deployed a number of assumptions that block deeper reflection. For instance, there is the belief in an unalterable gravitational constant G. Solely for this reason, all theoretical attempts trudge through the Planck length’s eye of a needle and come across as trying to push a door which is labeled “pull”.

The essence of the problem – and I think the only thing worth worrying about – is just this: The ratio of the electric and gravitational force of a proton and an electron that form a hydrogen atom is a huge number and nobody knows where it comes from. Period. The only idea with respect to this riddle came from Paul Dirac, the Large Number Hypothesis. – more about that later. Where does that number, 1039 , come from? Beware wannabe unifiers: either you explain it or you had better shut up.

(with quotes from „Bankrupting Physics“ and „The Higgs Fake“)

Mach’s Principle – Relating Gravity to the Universe

432px-Ernst_Mach_01What’s the origin of the inertia of masses? This is one of the most profound questions in physics. Let’s talk about this aspect of Mach’s principle, named after the Viennese physicist and philosopher Ernst Mach (1838-1916). Mach suggested that the weakness of gravity was due to the universe’s enormous size. Of course, the first estimates for the size and mass of the universe were not available before 1930, after Hubble’s discovery of the expanding universe. Sir Arthur Eddington first noticed that the gravitational constant may be numerically interrelated with the properties of the universe. By dividing c2by G, he obtained roughly the same value as dividing the universe’s mass by its radius:

c2/G ≈Mu/Ru. This is quite extraordinary.

Astonishingly, Erwin Schrödinger had already considered this possibility in 1925 (Ann. Phys. 382, p. 325 ff.), though he couldn’t know about the size of the universe!

Mach, around 1887, could not even have dreamed of such measurements, but his aspiration to formulate all the laws of dynamics by means of relative movements (with respect to all other masses!) turned out to be visionary. Einstein was guided to general relativity by that very idea. Although Einstein gave Mach due credit, he didn’t ultimately incorporate Mach’s principle in his theory. Mach’s central idea, that inertia is related to distant objects in the universe, does not appear in general relativity.

One possibility to realize Mach’s principle is to make up a formula where the gravitational constant is related to the mass and size of the universe. Because the universe is so huge, G would be very tiny. This radical idea was advocated by the British-Egyptian cosmologist Dennis Sciama in 1953ii:

c2/G Σ mi/ri,

Sciama2the sum taken over all masses i in the universe. Or, in other words, the gravitational potential of all masses in the universe amounts just to the square of the speed of light – intriguing!

Later, very few researchers pursued the idea, among them the cosmology “outlaw” Julian Barbour.iii During the inauguration of the Dennis Sciama building at the University of Portsmouth in England in June 2009, everybody talked about all the work that Sciama had done… except for Sciama’s reflections on inertia. What a pity!

It is idle to speculate whether Einstein would have warmed up to the idea. In any case, the size of the universe could only be guessed fifteen years after the completion of general relativity. So today, Mach’s principle has a miserable reputation and is sometimes even dismissed as numerological hokum. To the arrogant type of theorists who consider the question of the origin of inertia as obsolete chatter from Old Europe, I recommend they to look up “Inertia and fathers”, by Richard Feynman, on YouTube. In this video, Feynman wonders about one big question. Where does inertia actually come from? A truly fundamental problem. For more, see chap. 5 of “Bankrupting Physics”.

iiSciama, MNRAS 113 (1953), p.34.

iiiBarbour, gr-qc/0211021.

Can Dimensionful Constants Have a Fundamental Meaning?

WhittOf Course!

As with many of the thoughts in this blog, this is contrary to common wisdom, but I think it particularly weird how the perceived wisdom that “only dimensionless constants can have fundamental meaning” has been established. Not only has this idea become representative of a methodology that has replaced thinking by calculating, but the full ignorance of the statement reveals itself only if we look at the history of physics, for example, with the book “A History of the Theories of Aether and Electricity” by Sir E. Whittaker.

First, let’s recall how the idea came to be. It doesn’t require exceptional intelligence to realize that our definitions of the meter and second, etc., are arbitrary, and it is just as evident that dimensionful constants, such as h, c, G, and others, are expressed with such arbitrary units. This discussion has attracted some attention in the debate over theories about the variable speed of light, and it has been claimed (e.g., by Ellis) that every dimensionful quantity can be set to unity using an appropriate reference. While this is a possible mathematical formulation, the question remains whether such a procedure makes sense physically. It doesn’t!

If we think of a temperature map, then according to the above logic, as temperature is a dimensionful quantity, it could be set to unity at every point – the analogy is one-to-one, but the number of people appreciating a forecast with unit temperature would appear limited.

This is probably why Einstein didn’t mind pondering the variation of a dimensionful quantity when he considered a variable speed of light in 1911. Are we to understand from Ellis’ critique that Einstein didn’t have a clue about the basics of his own theory? As one of the few reasonable people discussing the subject, John Duffield, has recently pointed out by reference to original quotes, that Einstein’s attempts around 1911 were a sound approach to describe the phenomenology of general relativity with a variable speed of light. In the meantime, other researchers have shown that even general relativity can be formulated in terms of a variable speed of light.

Today’s physicists are not only ignorant about these ideas, but they are actively distributing the ideology that “only dimensionless constants can have fundamental meaning”, just like three theorists summarizing their discussion in the CERN cafeteria. Oh, had Einstein had the opportunity to listen to their half-assed thoughts, while having a cappuccino there! Surely, he would suddenly have understood how misguided his 1911 attempts on a variable speed of light were. (cf. Whittaker II, p. 153). There is more to tell, but to see the full absurdity of the “only dimensionless constants are fundamental” argument, look at Whittaker’s treatise on the development of electrodynamics. (vol. I, p. 232). Kirchhoff, Weber, and Maxwell would never have discovered the epochal relation ε0 μ0 =1/c2, and had they not assigned a meaning to the above “dimensionful” constants – our civilization would never have been bothered by electromagnetic waves.

Why Physics Needs to Ask Questions

File:Schopenhauer.jpgThe German Philosopher Arthur Schopenhauer once said that thequestion “Why?” is the mother of all sciences. In our era of modernity, physics offers a lot of answers, almost every day. These answers however, refer to sophisticated questions that presuppose a lot of “established” concepts. What bothers me is that some more basic problems seem to have been forgotten in this process. I want to illustrate that with two examples from particle physics and astrophysics.

Today’s neutrino experiments deliver data about the so-called mixing angles, which represent the probabilities of the three neutrino flavors (elecron, muon, tau) being transformed into each other. This is an experimental answer on the `top level’, but the underlying question “Why do three types of neutrinos exist?” (once raised by Emilio Segre) is about to fade. Today’s physicists would be even less likely to bother with the question “Why do neutrinos exist at all?”, though one may justifiably wonder why Nature had created such peculiar and elusive objects. To address such questions properly, one has to study a little bit of history (in this case, Wolfgang Pauli’s ideas of around 1930). This would lead to the still more basic question of whether energy has to be conserved during the beta decay – as discussed at the time by Niels Bohr. Also, there is still another fundamental question which seems to have been forgotten by the overwhelming importance of radioactivity in physics (indeed, its discovery triggered its biggest revolution). Why did Nature invent radioactivity after all, and why in different types? Couldn’t one just think of a world with stable nuclei? Is the very existence of radioactivity just a superfluous whim of Nature? Or is there something we still fail to understand?

When one follows the latest news in cosmology, it appears that the acceleration rate of the universe is changing (“How much?” would be the question) – in the jargon, this is called the time dependence of the cosmological constant or of dark energy. However, we have barely digested the very existence of dark energy (2011′s Nobel prize), as it means asking: “Why is the expansion of the universe accelerating?” This is a worthwhile problem to ponder, however, but on a still more basic level we can ask: “Why is the universe expanding?” Of course, Edwin Hubble’s observation of 1930 is now interpreted as indicating such expansion and there is no reason to warm up the `steady state’ model which was popular in the 1960s. However, a good reason for why the universe must expand does not seem to exist (I shall come back later to an idea of Robert Dicke in 1957). Theorists would immediately argue that Einstein’s equations do allow only for contracting or expanding solutions, which is true. But this does not answer the question in terms of a logical necessity, as Einstein used to phrase it: “I want to know if he [the creator] had a choice”.

I do not think that going to ever more sophisticated levels of questions and answers, while leaving behind the important ones, will advance physics in the long run. And surely, the pragmatic approach “Well it is as it is, let’s continue to collect more data” cannot be said to be a wise one.

What Einstein Said About Fundamental Constants


Reality and Scientific Truth: Discussions with Einstein,von Laue and Planck is a collection of conversations between Einstein and Ilse Rosenthal-Schneider, a lady who took a PhD in philosophy in Berlin around 1920. After fleeing the Nazi regime in 1938 and settling in Australia, she continued her discussions with Einstein by letter. The book is a unique source of the views Einstein held about the fundamental constants of physics. It is worthwhile to compare them with some modern opinions on the subject.

In a letter dated May 11th, 1945, Einstein wrote that he believed that numbers “arbitrarily chosen by God” do not exist and that their “alleged existence relies on our incomplete understanding”. Similarly, he contended in a letter of March 24th 1950 that “dimensionless constants in the laws of nature, which, from a rational point of view, could have other values as well, shouldn’t exist”.

Such a statement obviously refers to numbers such as the inverse of the fine structure constant (which is about 137), which is reminiscent of Richard Feynman, who forty years later wrote in a very similar vein: “It’s one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say, ‘the hand of God’ wrote this number, and we don’t know how He pushed his pencil.” At the same time, Feynman advised his colleagues that: “All good theoretical physicists put this number up on their wall and worry about it.”

It is interesting that both Einstein and Feynman seemed to be convinced that this was a puzzle to be tackled while at the same time being aware that such a conviction was not testable in a strict sense. Einstein, in another letter from Oct 13th, 1945, wrote: “Obviously, I can’t prove that. But I cannot imagine a reasonable unified theory which contains a number that could have been chosen differently by a whim of the creator.”

Since then, the attitude of physicists toward fundamental constants seems to have changed. Modern cosmology, for example, has terrific data, and Michael Turner of the University of Chicago explained in 2010 in Munich how these observations are accommodated using a couple of parameters by the current ‘concordance model’ of cosmology. After the talks at the Siemens foundation, there is always an ample possibility for discussion (and a very nice buffet). Turner, as he admitted frankly to me, is not very interested in the initial conditions of the universe such as density, photon to baryon ratio or similar stuff. He would be happy to find the correct equations that would make the world go, rather than bothering with how it got started. No doubt, however, these initial conditions (although Einstein would have been delighted by the data) are numbers he would have sought to explain using the theory. All these numbers make the world a little more complicated than it should be in principle. “A theoretical construction has little chance to be successful, unless it is very simple” Einstein wrote on April 23rd, 1949, and here he was certainly in agreement with Isaac Newton’s credo: “Truth is ever to be found in simplicity, and not in the multiplicity and confusion of things.”