I was surprised to learn, last week, that my profession has gone to the dogs. I’d introduced myself to a nonscientist as a theoretical physicist.
“I think,” he said, “that theoretical physics has lost its way in symmetry and beauty and math. It’s too far from experiments to be science.”
The accusation triggered an identity crisis. I lost my faith in my work, bit my nails to the quick, and enrolled in workshops about machine learning and Chinese.
Or I might have, if all theoretical physicists pursued quantum gravity.
Quantum-gravity physicists attempt to reconcile two physical theories, quantum mechanics and general relativity. Quantum theory manifests on small length scales, such as atoms’ and electrons’. General relativity manifests in massive systems, such as the solar system. A few settings unite smallness with massiveness, such as black holes and the universe’s origin. Understanding these settings requires a unification of quantum theory and general relativity.
Try to unify the theories, and you’ll find yourself writing equations that contain infinities. Such infinities can’t describe physical reality, but they’ve withstood decades of onslaughts. For guidance, many quantum-gravity theorists appeal to mathematical symmetries. Symmetries, they reason, helped 20th-century particle theorists predict experimental outcomes with accuracies better than any achieved with any other scientific theory. Perhaps symmetries can extend particle physics to a theory of quantum gravity.
Some physicists have criticized certain approaches to quantum gravity, certain approaches to high-energy physics more generally, and the high-energy community’s philosophy and sociology. Much criticism has centered on string theory, according to which our space-time has up to 26 dimensions, most too small for you to notice. Critics include Lee Smolin, the author of The Trouble with Physics, Peter Woit, who blogs on Not Even Wrong, and Sabine Hossenfelder, who published Lost in Math this year. This article contains no criticism of their crusade. I see merit in arguments of theirs, as in arguments of string theorists.
Science requires criticism to progress. So thank goodness that Smolin, Woit, Hossenfelder, and others are criticizing string theory. Thank goodness that the criticized respond. Thank goodness that debate rages, like the occasional wildfire needed to maintain a forest’s health.
The debate might appear to impugn the integrity of theoretical physics. But quantum gravity constitutes one pot in the greenhouse of theoretical physics. Theoretical physicists study lasers, star formation, atomic clocks, biological cells, gravitational waves, artificial materials, and more. Theoretical physicists are explaining, guiding, and collaborating on experiments. So many successes have piled up recently, I had trouble picking examples for this article.
One example—fluctuation relations—I’ve blogged about before. These equalities generalize the second law of thermodynamics, which illuminates why time flows in just one direction. Fluctuation relations also provide a route to measuring an energetic quantity applied in pharmacology, biology, and chemistry. Experimentalists have shown, over the past 15 years, that fluctuation relations govern RNA, DNA, electronic systems, and trapped ions (artificial atoms).
Second, experimentalists are exercising, over quantum systems, control that physicists didn’t dream of decades ago. Harvard physicists can position over 50 atoms however they please, using tweezers formed from light. Google has built a noisy quantum computer of 72 superconducting qubits, circuits through which charge flows without resistance. Also trapped ions, defects in diamonds, photonics, and topological materials are breaking barriers. These experiments advance partially due to motivation from theorists and partially through collaborations with theorists. In turn, experimental data guide theorists’ explanations and our proposals of experiments.
In one example, theorists teamed with experimentalists to probe quantum correlations spread across space and time. In another example, theorists posited a mechanism by which superconducting qubits interact with a hot environment. Other illustrations from the past five years include discrete time crystals, many–body scars, magic-angle materials, and quantum chaos.
These collaborations even offer hope for steering quantum gravity with experiments. Certain quantum-gravity systems share properties with certain many-particle quantum systems. This similarity, we call “the AdS/CFT duality.” Experimentalists have many-particle quantum systems and are stretching those systems toward the AdS/CFT regime. Experimental results, with the duality, might illuminate where quantum-gravity theorists should and shouldn’t search. Perhaps no such experiments will take place for decades. Perhaps AdS/CFT can’t shed light on our universe. But theorists and experimentalists are partnering to try.
These illustrations demonstrate that theoretical physics, on the whole, remains healthy, grounded, and thriving. This thriving is failing to register with part of the public. Evidence thwacked me in the face last week, as explained at the start of this article. The Wall Street Journal published another example last month: John Horgan wrote that “physics, which should serve as the bedrock of science, is in some respects the most troubled field of” science. The evidence presented consists of one neighborhood in the theoretical fraction of the metropolis of physics: string and multiverse models.
Horgan’s article reflects decades of experience in science journalism, a field I respect. I sympathize, moreover, with those who interface so much with quantum gravity, the subfield appears to eclipse the rest of theoretical physics. Horgan was reviewing books by Stephen Hawking and Martin Rees, who discuss string and multiverse models. Smolin, Woit, Hossenfelder, and others garner much press, which they deserve: They provoke debate and articulate their messages eloquently. Such press can blot out, say, profiles of the theoretical astrophysicists licking their lips over gravitational-wave data.
If any theory bears flaws, those flaws need correcting. But most theoretical physicists don’t pursue quantum gravity, let alone string theory. Any flaws of string theory do not mar all theoretical physics. These points need a megaphone, because misconceptions about theoretical physics endanger society. First, companies need workers who have technical skills and critical reasoning. Both come from training in theoretical physics. Besmirching theoretical physics can divert students from programs that can benefit the economy and nurture thoughtful citizens.1
Second, some nonscientists are attempting to discredit the scientific community for political gain. Misconceptions about theoretical physics can appear to support these nonscientists’ claims. The ensuing confusion can lead astray voters and parents who face choices about vaccination, global health, national security, and budget allocations.
Last week, I heard that my profession has wandered too far from experiments. Hours earlier, I’d skyped with an experimentalist with whom I’m collaborating. A disconnect separates the reality of theoretical physicists from impressions harbored by part of the public. Let’s clear up the misconceptions. Theoretical physics, as a whole, remains healthy, grounded, and thriving.
1Nurturing thoughtful citizens takes also humanities, social-sciences, language, and arts programs.
Quantum Frontiers 
Completely agree with you that theoretical physics is flourishing as a scientific discipline. Apart from some of the amazing results you mentioned there are several others – which you, of course, know about such as: topological insulators, the complete classification of topologically ordered systems in terms of K-theory; in astronomy the coming launches of the thirty meter telescope and the synoptic sky telescope; classification of many body phases via machine learning; entanglement themalization and localization …. the list goes on.
To those who wish to cast “quantum gravity” and its associated difficulties as representing all of physics are of course doing a disservice by giving the general public a distorted view of the scientific process.
Even in quantum gravity itself the situation is hardly dire. The Ryu-Takayanagi conjecture and holography have opened up an entirely new approach to constructing space-time from entanglement. Holographic superconductors and black hole chemistry and showing us completely new aspects of space-time and geometry.
Just one small correction. When Smolin and others criticize string theory, they are not criticizing “quantum gravity”. String theory is just ONE approach to quantum gravity. There is an entirely separate branch of loop quantum gravity and associated approaches. Sadly LQG was dismissed and sidelined by string theorists for the longest time, whose hubris is now coming back to bite them.
These are just growing pains of a new paradigm shift in high energy theory. When the dust settles both LQG and string theory will be viewed as precursors of a full theory of quantum gravity. Till then, of course, proponents of both camps must keep up their respective campaigns, for doing and defending what we understand best – isn’t that just human nature?
Thanks for your reflections, the extra examples, and the feedback. I agree that that phrase was misleading: It didn’t clearly mean “string theory” or “the quantum-gravity community’s sociology” there. I’ve edited the post accordingly. Thanks for the feedback!
Thank you! All too often in the public press, “theoretical physics” is meant to imply string theory (or else to hover again and again over Einstein’s” objections to quantum mechanics). We need more focus on the many other things going on.
There is a simple solution to the QM-GR conundrum. Allow gravity to be a field in its own right with its own field energy density analogous to electric or magnetic fields. Let this energy density be a source term for the right member of the Einstein field equations and you will find that the metric coefficients are functions of the gravitational potential rather than having to take on the role of the potentials in addition to geometry. The potentials then provide a natural framework for a quantum field theory of gravity unscathed by the vanishing of the covariant derivatives of the metric. Huseyin Yilmaz worked this out many years ago. His theory passes all of the same tests that have been taken as confirmation of GR and correctly encompasses cosmological redshifts. https://arxiv.org/pdf/1507.07809.pdf
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No doubt there are still many exciting things happening in physics, some of which are even comprehensible to us laymen: I haven’t been as blown away by anything like LIGO’s kilonova since the Higgs Boson.
Still, as I approach my 75th birthday, it is a bit depressing to think that I might die not knowing what 95% of the universe is made of.
You should know that 72 of the 95 percent is made up of a cosmological constant that Einstein later considered to be his biggest blunder. You can go peacefully knowing that he was correct in his second thought.
Well, Stan, I don’t know how you know with such confidence. What I know is that there is strong evidence for accelerating expansion. Nor is the cosmological constant the only proposed explanation. Of course there is no experimental or observational support for any of them. So your certainty is no help to me.
Science is part experiment and part theory, induction and deduction, theory determines what’s and observable. .. so what we need is a better focus in philosophy, because the ignorance is hilarious. A lot of modern researchers write “novel” papers every day proposing things that (they ignore) were established by Hume or Kant centuries ago.
Physics is in great shape, don’t care about the buzzwords and press.
Juan, examples from Hume and/or Kant please?
Hume’s understanding of the problem with deterministic laws, when the amount of data to form a conception of the world is always finite. That QM is probabilistic is a consequence of this, any fundamental theory must be about probabilities.
Kant understood that space isn’t fundamental. And Aristotle and Leibniz wrote about how space is a relationship between objects.
Von Weizsacker, being an educated fellow, knew all this from the start so he was 60 years ahead of the mainstream physics community. People working on his approach these days (Zeilinger, Bruckner) are on the cutting edge, theoretically and in the realization of actual experiments.
As an interested layperson I found this piece very helpful. It succinctly and clearly presented a general overview of the field with its successes and its current conflicts with an assured impartiality. It depersonalized the controversies that are symtomatic of strength not weakness in theoretical physics and enlarged my understanding of the field. Meanwhile I will enjoy following the dramatic struggle between loop quantum gravity proponents and their string theory adversaries.
Theoretical physics has not reached its end, mathematical physics might have. Some would want to treat physics purely in a mathematical way, that needs not be the case.
In subatomic world, strong nuclear force dominates. In macro world, electromagnetic force reigns. Within a galaxy, gravitational force controls.
Theory of everything remains far fetched mainly because it is near impossible to lump together very weak gravitational force and extremely strong nuclear force (physicists have done that for em force and weak nuclear force). Of course string theory could be mathematically sound in the ninth or tenth dimension. But what is 9D, 10D or even 11D?
In this well written article, paragraph 6 reads, regarding quantum gravity, “Try to unify the theories, and you’ll find yourself writing equations that contain infinities”.
Physical infinities including a black hole singularity can cause a physical theory such as general relativity to fail or become undefined. But rather than being a problem, it might point to one possible means of unification, based on a thought experiment.
In this thought experiment, the detection screen of a typical double-slit setup is extended from laboratory size to span the width of the observable universe.
Classical physics says it takes two photons, one from each opposing edge of the observable universe, to cover the diameter of the universe, now also equaling the width of the detection screen. Quantum physics, on the other hand, whether in the laboratory or in the thought experiment, encompasses the same width using a single wavefunction.
That wavefunction refers to one unmeasured particle such as a photon. But if that unmeasured photon is assumed to be physical as in classical reality, it will need to exceed the speed of light to be able to appear, in the instant of measurement, at either end of the detection screen. In effect, one physical photon is asked to do the duties of two.
What the thought experiment shows is that if quantum reality is assumed to be physical, a conflict with classical physics arises. No one has a good handle on what quantum reality is. The above demonstration suggests at least one thing quantum reality is not. It cannot be the classical physical reality.
This finding is made possible by simply scaling a standard laboratory quantum physics experiment to classical physics size. Whereupon, the laws of both quantum and classical physics jointly apply in a single experiment, to a single particle, in a single instant, of measurement.
This brings us back to infinities. If quantum reality is “nonphysical” in the above sense, it could be free from the failures to which classical reality is vulnerable. As such, it might even be more fundamental, as pointed out by a comment in https://www.preposterousuniverse.com/blog/2018/01/17/beyond-falsifiability/comment-page-2/#comment-7295910552604315312
The title of Einstein’s “EPR” paper in 1935 reads, “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” A nonphysical quantum reality explains why, when applied to a physical reality, quantum mechanics appears incomplete. With that, a literally “physical” block in efforts of unification may be addressed.
Here from a Sean Carroll blogpost to inquire about something you said along the lines of: [instead Wave function evolves through time. Meaning, quantum reality evolves over time according to the Schrodinger equation, until the moment of interaction. Whereupon, whichever entity (playing the role of one) being measured switches from quantum reality to become classical reality.]
What empirical ways could this intangible switch be explained to reconcile the quantum & classical? What are the implications from the wave function evolving and what would that negate?
LGQ,
Thank you for your interest in my thought experiment.
Your “What empirical ways could this intangible switch be explained to reconcile the quantum & classical?” will be addressed in two parts.
The “intangible switch” refers to any measurement = entanglement = any interaction in the natural environment. As such, all three items, each by itself, need little explanation. As to why the three are grouped in this manner, I will be happy to elaborate if necessary.
As for “to reconcile the quantum & classical”, quantum reality is recognized to be the more fundamental reality (https://www.preposterousuniverse.com/blog/2018/01/17/beyond-falsifiability/comment-page-2/#comment-7295910552604315312), capable of giving rise to classical reality (https://www.preposterousuniverse.com/blog/2018/02/08/why-is-there-something-rather-than-nothing-2/comment-page-5/#comment-7295910552604315547).
And entanglement (aka measurement or natural interaction in the environment) is the mechanism mediating the emergence of classical reality. No claim is made that this reconciles quantum and classical. It is presented merely as one way for the two to work together.
Now to “What are the implications from the wave function evolving and what would that negate?”
That the wavefunction evolves is standard. The purpose is to account for all possible outcomes at measurement. What’s important to point out is that the wavefunction in this case is “nonphysical”, as required by the result of the thought experiment.
As such, there is no need to invoke a collapse (as in Copenhagen) or a splitting of worlds (as in Many-Worlds). Because there is nothing physical in the wavefunction to be disappeared by collapsing. Nor any physical worlds to split off, at measurement. Per quantum unitarity, one measurement result is obtained. Or, one nonphysical quantum reality becomes physical.
In that sense, it negates (using your word) needing to have a Copenhagen vs. a Many-Worlds interpretation. It could potentially “unite” all interpretations by making all share a nonphysical wavefunction. That, by the way, also sidesteps whether the wavefundtion is ontic or epistemic.
Lastly, since the idea “real = physical” is so ingrained (https://quantumfrontiers.com/2019/01/27/humans-can-intuit-quantum-physics/comment-page-1/#comment-123863), lest a “nonphysical” quantum reality sounds unreal, will briefly describe the nature of a nonphysical quantum field.
If a physical particle is the result of local excitation of a nonphysical quantum field, before that particle is observed, the underlying quantum field is indifferent. In other words, what a given nonphysical quantum field manifests (at measurement), depends on the “tool” involved.
To illustrate, a stationary observer, one that is accelerating, a black hole, or two closely placed parallel plates, would yield, in the order of the tools listed, a particle, Unruh radiation, Hawking radiation and quantum fluctuations seen in the Casimir effect, all from the same nonphysical quantum field.
The above was touched on in Sean Carroll’s Gifford Lectures blog posted on November 23, 2016 (http://www.preposterousuniverse.com/blog/2016/11/23/gifford-lectures-on-natural-theology/#comment-7295910552604313840).
Incidentally, all four “tools” mentioned above are means to effect that “intangible switch” mentioned at the beginning of this reply.
Had your questions been misinterpreted, and to the extent this reply partially overlaps my comment on November 18, 2019 (http://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/#comment-7295910552604316840), my apologies.
Best,
KC
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Theoretical physics does have a major problem which it has so far addressed with nothing but ad hominem and ignorance: http://www.baur-research.com/Physics.
Nice article. I have read Lee Smolin’s “The trouble with Physics”. Long live theoretical physics.
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Theoretical physics would be much healthier if its participants recognized–(1) There is one and only one universe–one thing turned. If there are separable parts, iterations, eras, dimensions, bangs, or whatever, then please show that. But in any case, all that exists is still one universe. (2) There is no possibility that the (one) universe began about 14 billion years ago. The universe is eternal–definately.
With respect, you have offered statements, not arguments. E.g. how do you know that the universe is eternal?
The universe, all that exists, is eternal because at no time did something not exist, and it is utterly impossible that time itself ever “began”.
At no time did something not exist because something exists now–and something NEVER comes from nothing.
Time never began because that could only happen it NOTHING was changing, and then things were changing–but if nothing had been changing, that situation would never have changed.
I agree with you at least in the assumption of time. Physics presupposes time, but not space, so General Relativity´spacetime is a correct approximation. Space doesnt equals time, space is emergent.
Theoretical physics is nothing. It applied mathematics. No physics only mathematics
For example every physician has to be blamed for accepting infinite density at the center of a lack hole.
Give me 10 concrete accomplishments theoretical physics has produced solely on it’s own, in the last 10 years…..
These misconceptions are spread by Hossenfelder who claims that “physics” or “theoretical physics” is stagnant, because for her it seems that physics and theoretical physics is “high energy physics” aka string theory, supersymmetry or quantum gravity. Wheras gravitational physics, cosmology, condensed matter physics, quantum computing, etc have produced spectacular discoveries and progress. Much more than those communities she’s adamant with.
Her so-called “stagnation” covers such a short period of time that her definition of stagnation is ludicrous. But she claims that is not about time but about the amount of people dedicated to those theories, and their lack of self-criticism and that “they” don’t learn anything, and bla bla. She should look closer to the history of physics and science. Sometimes It takes years, decades, centuries, to make a breakthrough. Only because she thinks that physics is stagnant it doesn’t mean it really is. Many people are working hard in different experiments, theories, phenomenology etc, and those works are not based in beauty, naturalness, etc but on hard evidence. I’d say that quantum theory is rather ugly, and yet it’s so precise. So, it’s not true that everybody is lost in math and beauty. Maybe some of the string theorist and supersymmetry proponents are, but the physics community is much more larger than that.
There is no intention to engage in detailed discussions on the merits or lack thereof regarding Hossenfelder’s views. But thought that it is appropriate to point out what Penrose and others have
noted, on the diversion between application and basic understanding of quantum mechanics.
For convenience, will quote from a comment made in John Preskill’s blog on June 2, 2018:
“Difficult as quantum mechanics (QM) is to understand, we make it even harder, by being successful in using QM in our daily classical (“GR”) world, not the least in quantum computing. The more successful we are, the more QM mimics GR, and the farther away from basic QM understanding such as “QM properties unmeasured need not exist”.”
From: https://quantumfrontiers.com/2018/05/20/a-quantum-podcast/#comment-99061
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