ADVERTISEMENT
  About the SA Blog Network













Guest Blog

Guest Blog


Commentary invited by editors of Scientific American
Guest Blog HomeAboutContact

Why Is Quantum Gravity So Hard? And Why Did Stalin Execute the Man Who Pioneered the Subject?


Email   PrintPrint



What is the hottest problem in fundamental physics today? Physics aficionados most probably would answer: quantum gravity. Of all the fundamental forces of nature, only gravity still stands outside the rubric of the quantum theory. The difficulty of quantizing gravity has led to radical theories such as string theory, with its bold predictions of higher space dimensions and parallel universes. It’s unclear if these theories are "crazy enough to have a chance of being correct," as Niels Bohr used to say. And too few people know the dramatic early history of this field.

In fact, the field of quantum gravity was born in 1916, even before physicists had properly explained the other fundamental forces, electromagnetism and the nuclear forces. Twenty years later, a young Russian physicist by the name of Matvei Bronstein realized that gravity would be the hardest force of all to quantize. But before he could do something about that, he was swept up in Stalin’s Great Terror and executed at the age of 30.

Right: Matvei Bronstein shortly before his arrest, courtesy of Gennady Gorelik

Our modern understanding of gravity is based on Einstein’s general theory of relativity, but Einstein himself realized that it was incomplete. Shortly after publishing his most famous theory, he remarked that gravitational effects would cause electrons to spiral in on atomic nuclei. To stop that would take a quantum revision of general relativity. In 1916 he wrote: "Due to the intra-atomic movement of electrons, atoms would have to radiate not only electromagnetic but also gravitational energy, if only in tiny amounts. As this is hardly true in nature, it appears that quantum theory would have to modify not only Maxwellian electrodynamics, but also the new theory of gravitation.” (To be sure, the implosion of atoms would take 1030 years to complete. At the time, Einstein assumed the universe was infinitely old. If he had known it was only just a dozen billion years young, his argument would be less compelling.)

For Einstein, the gravitational instability of atoms was one of many reasons to search for a unified field theory. Quite a few theorists felt Einstein’s unification urge, although most of them gave up and turned their attention to real problems of atomic physics and to developing quantum mechanics. One was Wolfgang Pauli. Among Pauli’s many accomplishments was a theory he and Werner Heisenberg published in 1929, which applied quantum principles to electrodynamics. Their paper proclaimed that the same approach could be taken to gravity: "Quantization of the gravitational field, which appears to be necessary for physical reasons [referring to the Einstein's remark cited above], may be carried out without any new difficulties by means of a formalism fully analogous to that applied here."

Meanwhile, a young Russian physicist, Lev Landau, undertook to probe into the foundations of the quantum theory of electrodynamics. What troubled Landau was the question of how Heisenberg’s famous uncertainty principle, when combined with relativity, applied to electromagnetic fields. Landau claimed that such a relativistic uncertainty made it impossible ever to measure the field at a point. And if you cannot measure the field, even in principle, did the concept of a field really make any sense? If it didn’t, Pauli and Heisenberg’s approach fell apart.

It was Bohr who came to rescue Pauli and quantum electrodynamics in an article he wrote with Leon Rosenfeld in 1933. Notoriously obscure, the 60-page article identified the weak point of Landau’s thought experiment – namely, its assumption of point-like particles. Bohr and Rosenfeld explained why one should measure an average field in an extended region of space, not at a single point. However, Landau, with his famous passion for clarity, was not persuaded.

It was at this point that a close friend of Landau, Matvei Bronstein, entered the scene and — comprehended Bohr’s idea better than Bohr did.

In 1934 he published a short 3-page paper elucidating why the act of measurement did not call the concept of a electromagnetic field into question. You can always suppose the measurement apparatus to be arbitrarily massive, allowing the field to be measured to arbitrary precision. Bronstein was equally at home with both gravity and atoms, and here came the moment when he grasped why Pauli was too optimistic about quantum gravity. The reason is that, when it comes to gravity, mass is the gravitational analog of electric charge. You do not have freedom to choose mass and (gravitational) charge separately, as you do in electromagnetism. The more massive the apparatus is, the more it affects the force of gravity you are trying to measure.

In Bronstein’s full-dress research on quantum gravity, published in 1936, the most fascinating part was the essential difference between quantum electrodynamics and the quantum theory of gravity. Bronstein showed that the quantum limit of measurability would become apparent for particles with a certain characteristic mass, which is now known as the Planck mass (which Max Planck had introduced in 1900 with no reference to quantum gravity). Since gravity in general relativity is described by geometry, Bronstein concluded that the difficulty of quantizing gravity brought the entire nature of space and time into question: "The elimination of the logical inconsistencies … requires a radical reconstruction of the theory, and in particular, the rejection of a Riemannian geometry dealing, as we have seen here, with quantities which are unobservable in principle, and perhaps also the rejection of our ordinary concepts of space and time, replacing them by some much deeper and nonevident concepts. Wer’s nicht glaubt, bezahlt einen Thaler."

The last phrase in German came from a Grimm’s fairy tale: "If you don’t believe, pay a Thaler." It connoted that Bronstein’s conclusion, however implausible it might seem, was unavoidable.

Bronstein might have gone on to look for those "nonevident concepts" instead of space and time, but first he had to deal with an existential problem in cosmology. Some physicists felt that the redshift in the light of distant galaxies, which Edwin Hubble took evidence for the expansion of the universe, might be due to the phenomenon of "otired light.” The idea was that as photons interacted with the quantum vacuum on their long journey across the cosmos, they might "oage" and redden. Bronstein disproved this hypothesis. If the photons were aging, the amount of redshift would differ in different regions of the spectrum, whereas Hubble had showed that the redshift was the same for photons of every color.

Bronstein also had a keen interest in education and published three books for teenagers on the discoveries of helium, x-rays, and on invention of radio. He was proving to be one of the most promising physicists of his generation.

Left: Lev Landau, Niels Bohr, Leon Rosenfeld, and Matvei Bronstein at the Kharkov conference of 1934, courtesy of Gennady Gorelik

But on the night of August 6, 1937, he was arrested. The police demanded that he turn over his weapons and his poisons – he gave a laugh as his response. He was executed in a Leningrad prison in February 1938, one of the hundreds of thousands of innocent victims of Stalin’s terror.

Today, 75 years after Bronstein’s forecast, thousands of articles and dozens of books on quantum gravity later, the problem is still a great challenge. Although a thousand theoretical flowers have bloomed, they still lack a firm basis in physical principles. Never before in physics have so many people worked for so long with so little tangible success. Should not they pay their thalers to the memory of Matvei Bronstein?

Twenty years ago I met a man who had shared a prison cell with Bronstein and more than a hundred other inmates. This man miraculously survived labor camps on the Kolyma, in Magadan and Norilsk. But he remembered his first weeks in prison. Few of the inmates, he said, felt like talking about crazy accusations and torturous interrogations. They tried to escape through lectures and quizzes. Bronstein earned applause with his lecture on the relativity theory. And he knew more poetry by heart than anybody else in the cell.

About the Author: Gennady Gorelik is a historian of science at Boston University and author of a biography of Matvei Bronstein.

The views expressed are those of the author and are not necessarily those of Scientific American.

Take a look at the complete line-up of bloggers at our brand new blog network.






Comments 21 Comments

Add Comment
  1. 1. Broadside 9:23 am 07/14/2011

    Bronstein was in jeopardy for reasons he could do nothing about. Not only is Bronstein a Jewish name, but it was also the family name of Leon Trotsky, Stalin’s mortal enemy.

    Link to this
  2. 2. jtdwyer 9:27 am 07/14/2011

    Briefly, particle physicists presume that quantum mass must be a property of particles, mediated by particle interactions such as the Higgs mechanism.

    I notice that that an enormous amount of energy must be required to produce kinetic self propagation in wave state manifestations of matter. What happens to that energy in the localized particle state of matter? I suggest that propagation energy is reconfigured as self-opposingly directed (rotation/spin) potential energy known as mass. In this case the energy of mass may not be properly considered to be a property of particles but an external energy field.

    In this case, the potential energy of mass would simply dissipate along with the momentum imparted by experimental conditions in particle colliders. Since no mediating particle would be produced, just as no particle is produced by dissipating kinetic energy, there is nothing for particle detectors to track, except the momentum of the principal residual particles.

    This suggested external field characteristic of quantum mass may explain why the effects of gravitation in general relativity are imparted through the external dimensional coordinates of spacetime…

    Link to this
  3. 3. evosburgh 10:13 am 07/14/2011

    I not only see this as a tragedy for physics but also a a precautionary tale. We MUST always think for ourselves and question what we are told by others and their motives. Group think is ALWAYS going to end up doing more harm than good and while we cannot all be experts on all topics we can all question those who are telling us information and ascertain whether what they are telling us is based in fact or is their opinion.

    I believe that it was Einstein who said’Most of the fundamental ideas of science are essentially simple, and may, as a rule, be expressed in a language comprehensible to everyone.’ which is a rule that I try to live my professional life by.

    Link to this
  4. 4. Adam_Smith 11:48 am 07/14/2011

    Even layman dilettante I was quick to conjecture that the problem with Einstein’s image of an orbiting electron shedding gravitational energy lay in treating the electron, at any given energy level, as a particle. As a spread out wave or field instead it would be effectively static with respect to the location of its mass. The equivalence of mass and energy established by Special Relativity makes it all the easier to think of the electron as occupying an energy level rather than a position.

    Still, I had some difficulty following Bronstein’s explanation — the part about measuring devices of arbitrary mass. After that, however, I was not surprised by the idea that the nature of space-time as it is used in General Relativity is what needs to be changed to permit a successful theory of quantum gravity. I have long suspected that the resort to "imaginary" Hilbert spaces in quantum theory was a clue that there is more to real space-time than it appears.

    Link to this
  5. 5. rloldershaw 11:54 am 07/14/2011

    There is a simple and natural path to the unification of General Relativity and Quantum Mechanics, i.e., to quantum gravity, that involves expanding the fundamental symmetries of General Relativity to include discrete scale invariance (aka discrete conformal invariance).

    This give you a discrete self-similar model of nature’s hierarchical structure, i.e., a discrete fractal model.

    Two introductory papers on this new paradigm are:

    http://arxiv.org/abs/physics/0701132
    and
    http://arxiv.org/abs/astro-ph/0701006

    An extensive website at http://www3.amherst.edu/~rloldershaw is devoted to the paradigm and includes both technical and non-technical discussions of it.

    The curious thing is that although there is a high degree of interest in quantum gravity, 35 years of efforts to get people to seriously consider the basic concept of discrete cosmological self-similarity have been largely fruitless.

    Given the long-term failures of all other approaches to quantum gravity, and given the fact that the discrete fractal paradigm makes at least 10 definitive predictions (including the successful prediction of pulsar-planets before they were discovered), and given that the paradigm was derived from an empirical study of nature rather than Platonic musings, it is hard to understand the resistance to this unique, natural, and empirically testable path to reconciling General Relativity and Quantum Mechanics.

    Robert L. Oldershaw

    Link to this
  6. 6. Ancientpoet1 12:02 pm 07/14/2011

    Gravity:

    What if gravity is a figment of our imagination, even though it is measurable and has been measured by mapping and geodesy organizations for a very long time, like the old Defense Mapping Agency, now NGA, The National Geodesy and Information Agency, that I once worked for. I read an article recently that brought into question what keeps matter attracted, not by gravity per say, but by other forces of nature. If that is true, gravity may well be something we can only measure in combined forces, as it is not visible even under a microscope of the widest magnitude. I am not a scientist, but have studied physics and other sciences on my own for a very long time. Einstein may yet be right that the so called Universe is infinitely old as science is still in it’s infancy as I’m concerned. There are many possibilities for what forces exist beyond what has been discovered do far. Do we even know whether our Universe is expanding by the so called original big bang or is being pulled into the far reaches of space as we know it by as yet unknown forces?

    Ancientpoet

    Link to this
  7. 7. ShawnStone 10:50 pm 07/14/2011

    While most of this content is over my head in more then one regard, I find the tale of Bronstien compelling. Over all an exquisite article.

    Link to this
  8. 8. HubertB 5:31 pm 07/15/2011

    Since the gravitational constant is so much smaller than the coulombic constant, might the plank gravitational constant simply be so much smaller than the plank coulombic constant that we have simply not been able to measure it so far? When I took classes in a college lab, we were glad to get four place accuracy. I never worked with machines capable of measuring the difference between the coulombic constant and the gravitational constant.

    Link to this
  9. 9. debu 10:59 pm 07/15/2011

    Gravity is the manifestation of mono magnetic coupling effect of gravitoethertons on each proton and neutron of any molecule. As such the effect of gravity is proportional to the number of protons and neutrons of any molecule. But our universe is a sea of gravitoethertons –a pack of five god particles and density of gravitoethertons sea is varying from place to place by near by masses. As such gravity varies as we approach earth from sky and weight varies but acceleration is constant due to adjustment of force in a diluting sea of gravitoethertons. READ ETHER=DARK ENERGY=GRAVITOETHERTONS THEORY OF GRAVITY IN BALLOON INSIDE BALLOON UNIVERSES OF MATTER AND ANTIMATTER UNIVERSES ON OPPOSITE ENTROPY PATH PRODUCING GRAVITOETHERTONS AT COMMON BOUNDARY BY ANNIHILATION AND INJECTED INTO OUR UNIVERSE AS ANTIMATTER UNIVERSE IS NEGATIVE ENTROPY PATH AND OUR UNIVERSE GET MORE ENERGY THAN LOST BY ANIIHILATION MAKING OUR UNIVERSE TO ACCELERATING EXPANSION. THEREFORE QUARK THEORY IS TO BE CORRECTED AS THE FUNDAMENTAL FORCE PARTICLES ARE INJECTED INTO OUR UNIVERSE AND MATTER PARTICLES DANCE TO THE TUNE OF GRAVITOETHERTONS FIELD DENSITY VARYING FROM PLACE TO PLACE AND BY CHANCE WE GET AVOGADROS LAW TO ETC ETC FOR CHEMICAL COMBINATION TO MAKE LIFE POSSIBLE IN INFINITE SUCH MANIFESTATIONS IN MULTI VERSE INFINE SPACE AND TIME AND NUMBER OF BALLOON INSIDE BALLOON.–DURGADAS DATTA.–durgadas.ddatta@gmail.com.

    Link to this
  10. 10. Wilhelmus de Wilde 10:19 am 07/16/2011

    I have a great respect for Robert G Oldershaw, but why do we have to integrate all scales to one ? , just in order to unify different theories from different scales ? Why not accept that quantum theory is dealing with another scale as relativity ? Why is gravitation that we knew of first as the attrction of our earth, not a result of the "existance" (read collapse of wave function) or the consciousnes experience of "reality", holographic theory, I think that we wish too much to achieve a result which is based only on the observed 5% of reality that surrounds us, folow OCCAM’s RAZOR , it is simple.

    keep on thinking free

    Wilhelmus

    Link to this
  11. 11. pkalmar 11:53 am 07/16/2011

    "The views expressed are those of the author and are not necessarily those of Scientific American."
    What are you afraid of? Stalin is dead!

    Link to this
  12. 12. kenkoskinen 12:28 pm 07/16/2011

    Well, it is amusing (taken together) to read all of the differing ideas and by that I do not mean to slight anyone’s opinion. The problems inherent in understanding gravity is that although we know what it does (via Newton & Einstein) we do not know why and how it really works. Sure Einstein added the space/time bending feature but we were not much closer to understanding the who & why questions. We were in the same position with electromagnetism but then we discovered its boson or force carrier i.e. the photon. Ditto to the nuclear forces. The three are now included in the Standard Model. The models tell us what the forces do and we know how and why they do it. The agreements between theory and detection are remarkable!

    It is natural to assume that gravity should also have a boson i.e. the graviton. However not only has it not been detected, mathematical models that include it break down into pesky infinities. More over, it is amongst the wildest infinity problems known to theoretical physics.

    This leaves us in the dark as per how & why gravity works. The problem of combining general relativity with quantum mechanics suffers from form discrepancies. Both theory have very different forms. Both work well independently but it is as if one speaks English and the other Chinese and no one knows how to translate.

    We have to assume our universe is in some way unified. This follows from common anthropic reasoning. We are contained in space/time and are the observers asking questions. Our universe works and that means the pieces/forces must be some how linked.

    In other words we are looking into the heart of some unknown physics. Much is said about the WMAP constituents (i.e. approximately: baryonic matter & neutrinos 5%, dark matter 22% and dark energy 73%).This is fine but space/time is the container and is therefore the largest constituent of our universe. My bet is the answer to quantum gravity lies in some unknown qualities of space/time.

    Link to this
  13. 13. Zephir 8:29 pm 07/16/2011

    From perspective of AWT the deterministic formulation of theory of quantum gravity is impossible, it’s like the observation of water surface from perspective of both surface, both transverse waves. It would require to introduce infinite number of dimensions into equations. Some approximative theory could work with finite number of dimensions.

    Link to this
  14. 14. carlorovelli 10:24 am 07/17/2011

    Matvei Bronstein has been honored by the institution of a Prize for your scientists working towards the solution of the problem of quantum gravity. The Prize is named after him: it is the "Loop Quantum Gravity Matvei Bronstein Prize". The new Prize has been announced this summer at the LOOPS11 conference in Madrid. So, his memory is well alive in the research community …

    Link to this
  15. 15. gmusser 10:58 pm 07/18/2011

    Hi Carlo,
    Who was the recipient?
    George

    Link to this
  16. 16. Wayne Williamson 7:54 pm 07/20/2011

    Didn’t read the article or comments…bad..bad..on my part…but gravity is just a difference in time change between two points…ie a warping of space…

    Link to this
  17. 17. Bjimmer 10:43 pm 07/20/2011

    ~
    > "What if gravity is a figment of our imagination, even though it is measurable and has been measured by mapping and geodesy organizations for a very long time….
    ….what keeps matter attracted, not by gravity per say, but by other forces of nature."

    You are correct.
    There is no such thing as a separate force that we refer to as gravity.
    The force we are seeing and call "gravity" is the electrostatic force imbalance caused by the fact that the charges are not point sources, with the positive charge of the nucleus being closer to being a point source than the negative charge of the electron shell(s), thus a tiny imbalance.
    The deviation is tiny, approximately 40 orders of magnitude smaller than the electrostatic charge force at any given distance, which matches the "gravitational" force perfectly for every species of matter that I have calculated.

    ~

    Link to this
  18. 18. taffazull 12:08 am 07/21/2011

    Matvei Bronstein’s life story is in accord with my long held view that birth and death are predetermined and built into the structure of the universe itself and not to be understood as isolated events.We either understand the universe as a whole or not understand anything at all.

    Link to this
  19. 19. Wilhelmus de Wilde 12:24 pm 07/21/2011

    Dear Dr Rovelli

    I went to the IOP but cannot find the txts of Loops11, is there a possibillity to get them, thank you.

    Link to this
  20. 20. RussOtter 4:10 pm 07/21/2011

    Sense Gravity is relative to its environmental mass, it is ubiquitous and unified between Classical and Quantum Physics or so it would seem to me…

    Note: The four forces may be unified, but not all is unified between Classical and Quantum Physics. This is a twain that will never truly meet. Similar to the Finite vs. Infinite.

    Very plausibly Quantum Gravity, is real, it is just so near massless, being microscopically small and in virtual proximately to the other electrons, and protons/neutrons, that it behaves as Quarks do when they are together. When Quarks are together they are assumed to be virtually massless. When separated Quarks gain mass, hence gravity is more directly strengthened between the quarks.

    In other words, Gravity is not a weak force per se’, as nearly everyone seems to portray it. It’s relative strength is derived from its environmental mass. As an example it’s strength and weakness is directly derived by its level of mass, in combination with possible other forces such as dark matter and dark energy. The greater the mass the greater the strength of Gravity. The less the mass the weaker the strength of Gravity. A simple Classical Physics reality.

    For example: If Gravity is related to the area of a Black Hole, it would be 100 fold or more exponentially the strength, than any other of the natural forces. Certainly far more than allowing any electromagnetic force to move it, as is certainly true on our Earth. For which I continue to see examples of this fact to explain that Gravity is the weakest of the four known forces. NOTE: “Well Yes: relative to its environment.”
    Gravity’s strength is purely relative, to its environment. A point I never hear discussed, but appears implicitly obvious.

    Thus, at a micro level gravity is fundamentally weak, but not non-existent. The fact that electrons do not collapse onto the nuclei, could also be due to dark energy in play, and the other 3 known forces, counter-acting the extraordinary weakness of gravity in a quantum mechanical micro-environment. This is mathematically a proof.

    Hence Gravity is unified within both Classical and Quantum Physics by its very nature of being relative to its environment.

    Russ Otter

    PS… If this does not make sense, please explain why? If it does seem correct, please review some other ideas I have regarding “Natural Force” Energy, versus the requirement to use fossil fuels and its surrounding rhetoric involved with a host of issues regarding both theoretical physics and philosophy… http://www.otterthink.blogspot.com Thanks, Russ

    Link to this
  21. 21. Jan Jitso 8:05 am 07/26/2011

    In 2003 Vasily Yanchilin wrote The Quantum Theory of Gravitation in which he took as hypothesis that the mass of the universe determines its constellation and the Heisenberg uncertainty. That allowed to develop a new theory, based on the principle of least action and worked out mathematically very well, in which gravity is perceived as a purely quantum-mechanic process. In short: imagine a mass at x=0 and a tiny particle at x=10. A next moment due to the Heisenberg uncertainty this particle will be at x=9,9999…. and from there it may go back to x=10, but less frequent since at x=9,9999… the incertainty is smaller due to the external mass. The result is movement of the particle towards the big mass. This is perceived as gravitational attraction.
    I like to add the following: Keep old words like electron for the whole of tiny particles and call those transitions say iets. There may be some ressemblance with strings in certain theories. In the big mass of course also lots of iets are busy but the net result will be zero (except for the extremely small attraction of nearby tiny particles like above). When pushing the mass the balance gets disturbed, the net result becomes non-zero. This is noticed as inertial mass. In the old general theory of relativity inertial mass and gravitational mass are equivalent. Such is not so in the new theory as (the potential of) the total mass of the universe determines speed of light and Planck. Which change with the expansion ot the kosmos. (A consequence is that Ia supernovae info has to be adjusted, cosmological constant, inflation, negative energy and accellerated expansion become superfluous). In the new theory gravitational mass constitutes the universe and speed of light changes, while in an inertial system it remains the same.
    Recently Vasily Yanchilin prepared a manuscript on pro and contra of black holes. Therefore SciAm may have an interview with him and ask also his opinion on the presumed Higgs particle which seems not necessary in the new theory.

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American MIND iPad

Give a Gift & Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now >>

X

Email this Article

X