The quest for models and grand theories
Looking back, the whole course of the history of physics from Plato to Ptolemy to Copernicus to Newton to Einstein has been story of the moving on from one theory or model to a different or better one, and each advance shows that the version of reality we enjoyed under its predecessor has been superseded or modified [1].
Not all observers have been happy with this way of looking at things. In the 1920s and 1930s, the founders of quantum mechanics split into realist and antirealist camps. Albert Einstein and other realists held that the whole point of physics is to come up with some mental picture, a model in other words, no matter how imperfect, of what objective reality is. Antirealists such as Niels Bohr said that scientists should instead confine themselves to making and testing empirical predictions. Leonard Susskind is of much the same view[2].
Other contemporary observers have also asked whether there is really any need for an endless search for so-called Grand United Theories and Theories of Everything, such as that Kepler thought he had found, and which Einstein spent the last two decades of his life searching for?[3] Wouldn’t it be best instead, inquires Marcelo Gleiser to simply admit that our view of the world is a permanent work in progress, and that there is no Final Truth to be discovered for the simple reason that we will never have the totality of information to determine if we have arrived at this so-called “Final Truth”?[4] Are these endeavours yet another example of the West’s monotheistic religious tradition clothed in scientific garb which reason exorcised hundreds of years ago, and which Laplace despatched to the boundary in his commentary on the Universe? Should we not be asking instead why we need to believe so badly in this Final Truth? Is it really there to be discovered, or is it not time to admit that there is no cosmic blueprint, no master plan, no architect and no overarching explanation for the Universe?[5]
But if there is no such grand final theory out there, perhaps physicists may have to settle for several. Perhaps reality comes in a number of different varieties. The goldfish in its goldfish bowl sees a version of reality differently than ours, but is it any less real, ask Hawking and Mlodinow?:
“Their view is not the same as ours from outside their curved bowl , but they could still formulate scientific laws governing the motion of the objects they observe on the outside. For instance, because light bends as it travels from air to water, a freely moving object that we would observe to move in a straight line would be observed by the goldfish to move along a curved path. The goldfish could formulate scientific laws from their distorted frame of reference that would always hold true and that would enable them to make predictions about the future motion of objects outside the bowl. Their laws would be more complicated than the laws in our frame, but we would have to admit the goldfish view as a valid picture of reality. For all we know, we too may spend our entire lives staring out at the world though a distorting lens[6].
“Similarly with the contrast between Ptolemy’s Earth centred model of the cosmos and Copernicus’ sun-centred model. Although it is not uncommon for people to say that Copernicus proved Ptolemy wrong, that is not true. As in the case of our view versus that of the goldfish, one can use either picture as a model of the universe because we can explain our observations of the heavens by assuming either Earth or the sun to be at rest. Despite its role in philosophical debates over the nature of our universe, the real advantage of the Copernican system is that the equations of motion are much simpler in the frame of reference in which the sun is at rest”.
Other examples may be provided. The laws of special relativity teach us that different observers in space and time will observe the same events differently from the perspective of their respective reference planes, and both are perfectly valid ways of observing and describing the same event or events. The principle of black hole complementarity, and the “perspectival” depiction of reality in the holographic principle, both the subject of previous elaboration, are others.[7].
And finally for present purposes there is string theory. For something vaunted as the much desired theory of everything, by the 1990s physicists discovered that they had an embarrassment of riches in the form of five different string theories. However, string theorists are now convinced that these five different string theories are in fact but different versions of the same thing, denominated M-theory. As Hawking and Mlodinow point out, it seems that the traditional expectation of a single theory of nature may be untenable and that to describe the universe, we must employ different theories in different contexts. The different theories in the M-theory landscape may look very different, but they can all be regarded as versions of the same underlying theory, and they all predict the same phenomena where they lap, but none works well in all situations.
Hawking and Mlodinow conclude by noting that M-theory, like the goldfish example, shows that the same physical situation can be modeled in different ways, each employing different fundamental elements and concepts. “It might be that to describe the universe we have to employ different theories in different situations. Each theory may have its own version of reality, but according to model-dependent realism, that diversity is acceptable, and none of the versions can be more real than any other. It is not the physicist’s traditional expectation for a theory of nature, nor does it correspond to our everyday idea of reality. But it might be the way of the universe”[8].
Latterly, physicists have adopted a new approach to the problem: breaking it up into myriad parts. What started out as Einstein's grand vision of a united theory has "morphed into slow grinding labour carried out by different teams of physicists, each trying to solve a small piece of a vast cosmic puzzle"[9]: Rosenberg's work on axions as a putative component of dark matter, for example, which, if their existence is proved will force a revision of the Standard Model of particle physics, do away with the need to modify some of Einstein’s gravity predictions, and provide another link to the much sought after theory of everything.
Other researchers are designing experiments searching for the ubiquitous dark energy; and yet others to see whether our universe is in fact a three dimensional projection of a two dimensional reality.[10] Einstein failed in his search for a grand united theory partly because two of the fundamental forces - the weak and the strong nuclear force - had not then been discovered, but new researchers are continuing his quest for "a deeper truth, a higher enlightenment" aided by modern technology and a revised experimental approach to an old problem[11]
[1] Stephen Hawking and Leonard Mlodinow, The Grand Design, Bantam Press (2011), 54.
[2] Leonard Susskind interviewed by Peter Byrne, “Bad Boy of Physics”, Scientific American, Special Collector’s Edition, op cit, August 2013, 108.
[3] Marcelo Gleiser, op cit, 127.
[4] Gleiser, 127. Gleiser’s views on this topic appear to restate those of Russell Stannard (1989) and John Barrow (1991), as discussed in Paul Davies, The Mind of God, 166-167.
[5] Gleiser, 6-7, 98 (in words deceptively similar to Dawkins’, I might add). Vlatko Vedral is another who has commented upon the fact that the desire to reduce everything to a single simple cause is common to both a religious and scientific way of thinking. “While methods of investigation can vary, in the same way that in religion we reduce everything to a common deity, in science we strive towards a unifying theory of everything”: Decoding Reality – The Universe as Quantum Information, op cit, 6.
[6] Hawking and Mlodinow, “The (Elusive) Theory of Everything”, Scientific American, Special Collector’s Edition – Extreme Physics – Probing the Mysteries of the Cosmos, August 2013, 90 at 91, 92.
[7] These examples are both derived from Leonard Susskind’s interview with Peter Byrne, “Bad Boy of Physics” in Scientific American, op cit, 109-110.
[8] Hawking and Mlodinow, “The (Elusive) Theory of Everything”, Scientific American, op cit, at 93.
[9] Corey S Powell, "Cleaning up after Einstein", Scientific American, Special Issue -100 years of General Relativity, September 2015, 50 at 52.
[10] For these and other examples, including string theory, see Ibid. Rosenberg's work on axions is considered in more detail in the section on dark matter, above.
[11] Ibid.