Tag Archives: Thomas Kuhn

Black holes, bananas, and falsifiability.

Previously I gave a poor man’s description of the concept of `falsifiability‘, which is a cornerstone of what most people consider to be good science. This is usually expressed in a handy catchphrase like `if it isn’t falsifiable, then it isn’t science’. For the layperson, this is a pretty good rule of thumb. A professional scientist or philosopher would be more inclined to wonder about the converse: suppose it is falsifiable, does that guarantee that it is science? Karl Popper, the man behind the idea, has been quoted as saying that basically yes, not only must a scientific theory be falsifiable, a falsifiable theory is also scientific [1]. However, critics have pointed out that it is possible to have theories that are not scientific and yet can still be falsified. A classic example is Astrology, which has been “thoroughly tested and refuted” [2], (although sadly this has not stopped many people from believing in it). Given that it is falsifiable (and falsified), it seems one must therefore either concede that Astrology was a scientific hypothesis which has since been disproved, or else concede that we need something more than just falsifiability to distinguish science from pseudo-science.

Things are even more subtle than that, because a falsifiable statement may appear more or less scientific depending on the context in which it is framed. Suppose that I have a theory which says that there is cheese inside the moon. We could test this theory, perhaps by launching an expensive space mission to drill the moon for cheese, but nobody would ever fund such a mission because the theory is clearly ludicrous. Why is it ludicrous? Because within our existing theoretical framework and our knowledge of planet formation, there is no role played by astronomical cheese. However, imagine that we lived in a world in which it was discovered that cheese was naturally occurring substance in space and indeed had a crucial role to play in the formation of planets. In some instances, the formations of moons might lead to them retaining their cheese substrate, hidden by layers of meteorite dust. Within this alternative historical framework, the hypothesis that there is cheese inside the moon is actually a perfectly reasonable scientific hypothesis.

Wallace and Gromit
Yes, but does it taste like Wensleydale?

The lesson here is that the demarcation problem between science and pseudoscience (not to mention non-science and un-science which are different concepts [2]) is not a simple one. In particular, we must be careful about how we use ideas like falsification to judge the scientific content of a theory. So what is the point of all this pontificating? Well, recently a prominent scientist and blogger Sean Carroll argued that the scientific idea of falsification needs to be “retired”. In particular, he argued that String Theory and theories with multiple universes have been unfairly branded as `unfalsifiable’ and thus not been given the recognition by scientists that they deserve. Naturally, this alarmed people, since it really sounded like Sean was saying `scientific theories don’t need to be falsifiable’.

In fact, if you read Sean’s article carefully, he argues that it is not so much the idea of falsifiability that needs to be retired, but the incorrect usage of the concept by scientists without sufficient philosophical education. In particular, he suggests that String Theory and multiverse theories are falsifiable in a useful sense, but that this fact is easily missed by people who do not understand the subtleties of falsifiability:

“In complicated situations, fortune-cookie-sized mottos like `theories should be falsifiable’ are no substitute for careful thinking about how science works.”

Well, one can hardly argue against that! Except that Sean has committed a couple of minor crimes in the presentation of his argument. First, while Sean’s actual argument (which almost seems to have been deliberately disguised for the sake of sensationalism) is reasonable, his apparent argument would lead most people to draw the conclusion that Sean thinks unfalsifiable theories can be scientific. Peter Woit, commenting on the related matter of Max Tegmark’s recent book, points out that this kind of talk from scientists can be fuel for crackpots and pseudoscientists who use it to appear more legitimate to laymen:

“If physicists like Tegmark succeed in publicizing and getting accepted as legitimate mainstream science their favorite completely empty, untestable `theory’, this threatens science in a very real way.”

Secondly, Sean claims that String Theory is at least in principle falsifiable, but if one takes the appropriate subtle view of falsifiability as he suggests, one must admit that `in principle’ falsifiability is rather a weak requirement. After all, the cheese-in-the-moon hypothesis is falsifiable in principle, as is the assertion that the world will end tomorrow. At best, Sean’s argument goes to show that we need other criterion than falsifiability to judge whether String Theory is scientific, but given the large number of free parameters in the theory, one wonders whether it won’t fall prey to something like the `David Deutsch principle‘, which says that a theory should not be too easy to modify retrospectively to fit the observed evidence.

While the core idea of falsifiability is here to stay, I agree with Scott Aaronson that remarkably little progress has been made since Popper on building upon this idea. For all their ability to criticise and deconstruct, the philosophers have not really been able to tell us what does make a theory scientific, if not merely falsifiability. Sean Carroll suggests considering whether a theory is `definite’, in that it makes clear statements about reality, and `empirical’ in that these statements can be plausibly linked to physical experiments. Perhaps the falsifiability of a claim should also be understood as relative to a prevailing paradigm (see Kuhn).

In certain extreme scenarios, one might also be able to make the case that the falsifiability of a statement is relative to the place of the scientists in the universe. For example, it is widely believed amongst physicists that no information can escape a black hole, except perhaps in a highly scrambled-up form, as radiated heat. But as one of my friends pointed out to me today, this seems to imply that certain statements about the interior of the black hole cannot ever be falsified by someone sitting outside the event horizon. Suppose we had a theory that there was a banana inside the black hole. To check the theory, we would likely need to send some kind of banana-probe (a monkey?) into the black hole and have it come out again — but that is impossible. The only way to falsify such a statement would be to enter the black hole ourselves, but then we would have no way of contacting our friends back home to tell them they were right or wrong about the banana. If every human being jumped into the black hole, the statement would indeed be falsifiable. But if exactly half of the population jumped in, is the statement falsifiable for them and not for anyone else? Could the falsifiability of a statement actually depend on one’s physical place in the universe? This would indeed be troubling, because it might mean there are statements about our universe that are in principle falsifiable by some hypothetical observer, but not by any of us humans. It becomes disturbingly similar to predictions about the afterlife – they can only be confirmed or falsified after death, and then you can’t return to tell anyone about it. Plus, if there is no afterlife, an atheist doesn’t even get to bask in the knowledge of being correct, because he is dead.

We might hope that statements about quasi-inaccessible regions of experience, like the insides of black holes or the contents of parallel universes, could still be falsified `indirectly’ in the same way that doing lab tests on ghosts might lend support to the idea of an afterlife (wouldn’t that be nice). But how indirect can our tests be before they become unscientific? These are the interesting questions to discuss! Perhaps physicists should try to add something more constructive to the debate instead of bickering over table-scraps left by philosophers.

[1] “A sentence (or a theory) is empirical-scientific if and only if it is falsifiable” Popper, Karl ([1989] 1994). “Falsifizierbarkeit, zwei Bedeutungen von”, pp. 82–86 in Helmut Seiffert and Gerard Radnitzky. (So there.)

[2] See the Stanford Encyclopedia of Awesomeness.


Is physics in a crisis?

We live in very interesting times, especially if you are a theoretical physicist like me. To understand what kind of time we are living in physics-wise, it will be helpful to review some ideas of Thomas Kuhn, a famous philosopher of science. Kuhn described science as proceeding through a series of paradigms. A `paradigm’ is a sort of established framework in which scientists work to solve problems using an agreed-upon set of tools. The paradigm provides both the puzzles to be solved and the tools to solve them. Over time, scientists discover that the tools of the paradigm cannot solve every puzzle. The problems that lie beyond the reach of a paradigm are called anomalies. When enough serious anomalies are discovered, scientists begin to lose confidence in the existing paradigm and a crisis occurs. Historically, each crisis has been resolved by a subsequent scientific revolution, in which the old paradigm was replaced by a new paradigm that is capable of resolving the anomalies [1].

Interestingly, although the new paradigm solves more problems than the old paradigm, it also represents a complete change in perspective, so that even those problems that were solved by the old paradigm have to be `re-solved’ by the new paradigm, from a completely new point of view. As a result, there might be the odd puzzle that was solved by the old paradigm but suddenly cannot be solved by the new paradigm! This phenomenon is known as `Kuhn-loss’. The new paradigm is successful so long as it solves more important puzzles than the ones it loses through Kuhn-loss. I mention this only to illustrate how significant a change in paradigm is from Kuhn’s point of view: it is not merely a period of accelerated science, but a complete reworking of how scientists see the world.

We are currently in a period of crisis. Some physicists might disagree with me, but I think one can make a strong case that the paradigm that has taken us this far is showing cracks. In this post, I won’t directly compare current events to Kuhn’s description of a crisis, nor will I spend effort trying to define what the present paradigm is. For the moment I will content myself by pointing to some (just a few!) of the major puzzles that are facing us, and explain why they may represent `anomalies’ that require a new paradigm in order to solve them [2].

Dark matter / energy: One of the best-known puzzles of our time is the mystery of dark matter and dark energy in cosmology. Briefly, the matter that we can see in the universe (galaxies, nebulae and so on) is moving around as though it is being pushed and pulled by gravitational forces that have no visible source. In fact, there seems to be 95% more `stuff’ in the universe that we can’t actually see directly – we can only deduce its presence by its gravitational interactions with visible matter. The fact that we don’t know what this stuff is has been called the most embarrassing problem in physics for good reason: if somebody asks me what kind of matter and energy there is in the universe, I have to admit that, for the most part, I have no freaking idea.

Quantum gravity: Going by Kuhn’s picture of science, the key tool of the present paradigm is the Standard Model (SM) of particle physics. This model is impressively accurate down to really tiny scales and has been spectacularly confirmed time and time again in the world’s big particle accelerators, right up to the recent discovery of the Higgs Boson at the Large Hadron Collider (LHC). However, a major limitation of the Standard Model is that it does not tell us how gravity fits into the picture. While we have brought electromagnetism and the nuclear forces up to date with quantum mechanics, our theory of gravity is still straggling behind by over a hundred years. All the other forces have been given a quantum makeover, but gravity remains the shy stepsister, cloaked in a classical veil. Despite some pioneering attempts to get behind that veil, most notably String Theory and Loop Quantum Gravity, there is still no agreement among the community about which approach is correct or whether we have to try something else entirely [3].

Quantum foundations: It is often said that nobody understands quantum mechanics. This would be very worrying if it were true, since much of today’s technology is based on it! So what is the situation really? Well, obviously we understand the theory well enough to use it in practical applications. The trouble is more on the philosophical side: physicists can’t agree on why quantum mechanics works so well. In fact, we still can’t agree on why the universe should be quantum mechanical in the first place! John Wheeler’s famous question `why the quantum?’ still keeps many of us awake at night. There is an ongoing body of research on quantum foundations, whose goal is to improve our understanding of quantum mechanics to the point where most of us can agree on a single interpretation. This interpretation (it is hoped) would reveal quantum mechanics in such a way that nearly every physicist will reflexively slap their forehead and declare `of course! It had to be that way’! The interpretation should be so compelling that classical physics will look absurd by comparison and quantum mechanics will be the most natural way to describe the world.

As an example, since Einstein, the gravitational force is now widely interpreted as the curvature of space and time. However, technically it is possible to explain gravity in terms of fields operating in flat spacetime, in a way that agrees with current experimental data – yet if you ask any physicist what gravity is, nearly all of them will say `the curvature of space-time due to matter’. By contrast, if you ask them what the wave function of quantum mechanics is, you will get all kinds of different answers, and probably an invitation to a conference on foundations where such matters are still being hotly debated. Whereas curved space-time seems like an elegant, simple and compelling way of visualizing gravity, we have no similarly compelling paradigm for visualizing quantum mechanics.


One of the tasks a physicist faces during a crisis is to identify which anomalies deserve our attention and which ones are less important. This decision is guided by one’s intuitions and one’s chosen philosophy, hence a physicist must embrace some philosophy in order to make progress. For my part, I am most interested in the latter two anomalies: quantum gravity and quantum foundations. I think that the two are deeply connected. Since the regime of quantum gravity is still far from being accessible to experiments, the success of a theory of quantum gravity will be decided by the intuitive appeal of the physical principles on which it is based, as well as its elegance and explanatory power. We cannot hope to meet these demands all the way down at the level of quantum gravity (the Planck scale) if we still can’t do it up here on our home turf for quantum mechanics. Indeed, it is embarrassing that we cannot claim to have such a compelling picture of quantum mechanics, given that we have so much experimental data to guide us!

In upcoming blog posts I intend to elaborate on quantum gravity and quantum foundations and their possible connection to one another. I will also present my own ideas about how we should try to resolve the connected anomalies, using a philosophy based on a modern revival of operationalism and ideas from the exciting new field of quantum information. Stay tuned!

[1] This is a very rough version of Kuhn’s picture of scientific progress. The reader is encouraged to read the entry on Thomas Kuhn in the Stanford Encyclopedia of Philosophy: http://plato.stanford.edu/entries/thomas-kuhn/ . The less lazy reader is referred to Kuhn’s seminal work The Structure of Scientific Revolutions, University of Chicago Press, 2nd ed. (1970).

[2] There are of course far more anomalies in physics than the three listed here, although many of them can be linked to the same broad categories. For a more thorough list, see John Baez’s `Open Questions in Physics’: http://math.ucr.edu/home/baez/physics/General/open_questions.html .

[3] Some people have gone as far as to argue that String Theory is a failure. As an ignoramus, my own stance on this is more cautious, but that is a topic for another blog post.