zondag 9 november 2014

On future physical theories

Philosopher of physics Dennis Dieks (http://en.wikipedia.org/wiki/Dennis_Dieks) gave a talk in Dutch on expectations he has for the future of physical theories:

http://www.sg.uu.nl/opnames/van-kern-tot-kosmos/van-kern-tot-kosmos-de-wereld-volgens-natuurkunde-en-filosofie (start around 2:10:00)

My translated, approximate transcription is below. Any errors due to interpretation and translation are my responsibility. Topics are: 
  • The increasing abstraction and generality of physical theories.
  • The disappearance of classical explanatory categories like space and time from theories (like in Verlinde's novel gravitational theory: http://en.wikipedia.org/wiki/Entropic_gravity). 
  • The increase of the number of theories that explain observations equally well (like the different interpretations of quantum mechanics).

    “The unreasonable success of physics” - Dennis Dieks

    Let me start with a kind of hymn to physics: physics has offered many impressive results. Some decades ago physicist and Nobel prize winner Eugene Wigner wrote a famous paper "The unreasonable success of mathematics in the natural sciences." Of course it is miraculous that mathematics thought up somewhere in a garret is applicable in physics. Like the differential geometry utilized in Einstein's theory which stemmed from several decades before that theory. However, I am not much impressed by this argument. Lots of mathematics appears from garrets. Nevertheless, only a small part of it is used in physics. It would be interesting to conduct research to find out if the “unreasonable success”is really as miraculous as often assumed.

    For me the real miracle is that physics is so successful. That there are regularities such that things proceed as expected. That buildings stay exactly in place, and that I am able to trace back a pair of spectacles I lost. The predictions are very accurate, to many decimal places. Much of modern physics – like Erik Verlinde's new gravitational theory - deals with only some last decimal places. Hence the discussion about a theory of everything is in some sense a non-discussion, because we already have such a theory for all ordinary applications of physics in biology, psychology, chemistry, etc. The new developments - strings, super gravitation - will make practically no difference for this type of applications. Discussions about existing applications can all be had on the basis of existing physics. Modern physics is a matter of steadily adding digits after the decimal place. In this sense the growth of physics is completely cumulative. The accuracy of our predictions steadily grows. I agree with former speaker Fred Muller: with regard to structure new theories are extensions of older ones. This is a cumulative process. During this steady accumulation revolutions take place. For example, the character of space and time in Newton's theory is completely different from that in the one of Einstein. The same holds for mass. For Newton it is the property of a particle, of a physical system. In relativity it is completely different, because here mass depends on speed. The character of mass has changed. In a certain sense we live since relativity theory in a different world than before. That is what I call a revolutionary transition. At the same time there is a cumulative growth of structures. How can these two things be reconciled? Below some more about this.

    First some more examples of revolutionary changes. In classical physics objects are localized. Space and time are very important as a kind of stage on which matters take place. From a philosophical viewpoint space is a principle of individuation: points act to discern things. Objects at different positions are different. On the contrary, in quantum mechanics particles do not have exact positions and velocities. This is a revolutionary change. Words (like "particle") do not change. Notwithstanding, if one looks more closely, they refer to different objects. At the same time certain relations keep unchanged in different theories. One will understand that if the objects change, the relations have to become more abstract. They have to cover more, because each theory includes the preceding one and the character of things changes continuously. The structures – the end products of the cumulative development - become more general and in some sense also more meaningless. More vague, because they have to include more. More possible realities should fit in.

    Elements disappearing more and more from these increasingly vague structures are space and time themselves. I do a prediction about a tipping point: in the newest theories space and time will lose the function they always had. Quantum mechanics itself already suggests this. Therefore we do not need a new theory as that of Verlinde to see this, in which space and time are emergent properties. A suggestive example from quantum mechanics is teleportation. According to Einstein, Podolsky and Rosen (and Bohm later on) it is possible to have a non-localised state. Think of a particle here and one far away. So they have very different positions, but can simultaneously share certain properties. The total spin can be defined (it is 0), however it cannot be reduced to those of the individual particles. For example to the sum of the individual spins. There is something holistic about the system, so to speak. This immediately suggests it can be utilised to send signals. When one determines the spin of particle A, than one knows for sure the spin of B. It turns out that this cannot be utilised straightforwardly. One cannot transmit signals at a speed exceeding that of light. So it appears as if the classical space and time ideals can simply be maintained. Nevertheless there is something strange going on. This is what we call teleportation and comes to light if one inspects the situation more closely. [ what follows is an explanation of the mechanism of quantum teleportation of which a translation does not make much sense. See http://en.wikipedia.org/wiki/Quantum_teleportation in stead ] It is possible with the help of the transmission of 2 classical bits (for example through a telephone line) to teleport the state of particle A to particle B. This is strange from a classical viewpoint, because the state contains much more information than 2 bits, which can be characterised by a real number between 0 and 1. Space and time apparently function as a stage on which everything happens and on which things propagate. However, when one takes a closer look at it one notices that teleportation cannot be done through the ordinary propagation of signals. It is as if the information goes through a back door from A to B – not through space and time. A signal is needed but it is not the signal itself that transmits the information. Hence from quantum mechanical computations processes follow that can not be interpreted as processes that propagate in space and time. Therefore I think that quantum mechanics already hints towards the idea that space and time are a kind of emergent phenomena. I think that quantum mechanical computations (in Hilbert space, for the informed) are more fundamental than what we can understand in terms of space and time.

    We can expand on this. There are indications that we can regard gravitation as a kind of statistical phenomenon emerging from a much deeper, microscopic level. However, the aforementioned phenomenon that one needs to be increasingly general now takes revenge. This is because one has to incorporate all previous views in the new view. The new view contains hardly information of the kind one likes to have. That's apparent from Verlinde's model. It is based on information, not on particles or other physical systems. However, what is information really? Is talking about information not a kind of second choice because one does not know exactly what it is all about? 0-s and 1-s are only ways to represent information. Information is always information about something. An advantage of Verlinde's theory is that this is not important any more. One is no longer interested in what type of fundamental process underlies gravitation. It is sufficient to assume there is such a process to which statistical mechanics can be applied. This is exactly a theory that is helpful as soon as one does not know what is going on exactly. So on the one hand Verlinde's theory is a giant leap forward, but at the same time it is a kind of failing to admit that one cannot go beyond a certain border. That it is not very important what is going on beyond that border.

    We are on the verge of developments in which space and time lose their ordinary function and that we focus on deeper lying matters that are to a certain extent unimportant. So the former question what the information is about remains unanswered. This is an example of a general trend in physics. Classical explanatory concepts like causality, space and time and propagation of things disappear from sight. Abstraction increases and one cannot explain matters any more in ordinary ways.

    There is another thing that amplifies matters: the philosophical problem of theoretical underdetermination. In physics and in science in general one tries to formulate theories. Observation is an important input and theory should be in accordance with it. The more theories get separated from our daily lives – dig deeper, so to speak - the more they contain what cannot be observed. However, in theories is much more we cannot observe. As physics evolves this unobservable part grows. Compare this to a sphere. What can be observed is at the surface, the remainder lingers below it. If the radius increases the observable surface increases, however, the unobservable volume increases even faster. This brings the problem of underdetermination to light. It is the same as in mathematics in which one has to estimate a number of parameters on ground of only a limited amount of data. The equivalent physical problem is is that one has too few observations to fix all the parameters of a theory. A logical consequence is that many theories are possible based on exactly the same data. This is already happening in quantum mechanics. All those different interpretations that mostly correspond with respect to structure but sometimes differ a bit are all able to explain all known observations. They are empirically equivalent. Quantum mechanics is only an example of this phenomenon. Future physical theories will be more and more abstract, predict more accurately (higher accuracy), while at the same time being less uniquely determined. The theory of Verlinde is an example of this. One can image that one can have many different implementations of the underlying 0-s and 1-s which will make no difference for the structure above.

    A tipping point will thus be the disappearance, the fading of space and time in physical theories which will make no difference for our daily lives. After all we have to deal with left and right, etc. A second point is that theories will be less uniquely determined. There will be more discussion possible about what is the true character of reality. It is a kind of enormous advance which is accompanied by a slow slipping away of the means to gain knowledge about details.

     Space-time stage.

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