Saturday, 10 May 2008

Reading continued - String theory

As I delved deeper into Smolin's Trouble with physics, with its critique of the specific status of string theory (at the cost of other possible approaches to quantum gravity and ToE) I found a very recent paper on arXiv. This is "So what will you do if string theory is wrong?" by Moataz H. Emam. Written by an active proponent of the theory, it contains some very defensive statements. Is it a preparation for defeat?

Let's look at some quotes:

String theory occupies a special niche in the history of science. It is the only theory of physics with no experimental backing that has managed to not only survive, but also become “the only game in town” (to quote Sheldon Glashow). In addition, the theory has gained much popularity with the general public, spurred on by accessible online accounts and popular TV programs. Judging by amateur web sites and personal discussions, there seems to be a rising belief that it is a correct theory of nature. [...]

In fact, string theory has so far failed to conform to the definition of a scientific theory. In his classic work Karl Popper gives several criteria that a scientific theory must satisfy. These may be summarized as “the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability”. [...] So far string theory has failed to meet Popper’s criterion. It might be argued that this situation is temporary. Eventually technology will catch up with string theory and allow us to test its assumptions directly or someone will find a way to test the theory using current technology. This hope is what keeps string theory on the list of scientific theories, saving it from the fate of astrology and creationism. The failure to satisfy Popper’s definition is however a serious drawback that string theory critics will, justly, continue to point out.
So why do people continue to work on string theory? There are several reasons. We often hear that the theory is aesthetically attractive and that it would be a shame if nature had not picked such an elegant structure to use as the basis of the universe. [...]

People like myself who are interested in some small segment of the string theory landscape that might not relate to the universe naturally are asked: “Why do you work on this theory? Shouldn’t you, as a physicist, be interested in what describes nature? Why waste your time on something
that you know a priori to be wrong?” Another closely related question is “What if someone proves that subatomic particles cannot possibly be made of strings? In that case not only is the particular theory you are working on wrong, the whole edifice has collapsed! What will you do then? Will you drop your research and switch to something else? Or will you stubbornly continue to work on the (now incorrect) string hypothesis? What will happen to all of your careers? And why take the
risk in the first place?” These questions are reasonable and may be rephrased as “Are there any accomplishments of string theory that would survive such a total collapse?

This is indeed, an important question. In fact, no critics of string theory deny its beauty and mathematical accomplishments. But the question rests on limited resources. With the decrease of funding for basic science, including physics we must ask ourselves a question is this the best avenue to follow. And, if because there are no physical references to guide us there is no option but to explore the whole landscape (as Emam suggests). But how can we hoe to explore bu a tiny part of the 10^500 Landscape? The analogy with a Persian rug that Emam uses is misleading: the ratio of the single thread to the whole rug, the image of `already woven patches' is wholly misleading: the real ratio of known/unknown is ... unimaginably low. And unless the research is given to some future quantum computers, I see no chance of exploring all the alternatives. On the other hand if we give the task to computers, would it still be human science?

The lack of experimental results to guide us through the vast string landscape leaves string theorists with no choice but to systematically explore all of it! These explorations, even within theories that we already know are not related to nature, have resulted in the discovery of deep and elegant mathematics. Mathematicians today work in parallel with string theorists to explore the frontiers that the latter have opened.

Studying the large number of theories in the landscape and how they are related to each other has provided deep insights into how a physical theory generally works. The string theory landscape may be likened to a vast range of samples collected and studied in detail for the purpose of understanding why theories of physics behave the way they do and perhaps guide us into answering
deep questions about such things as symmetry and its origins. So even if someone shows that the universe cannot be based on string theory, I suspect that people will continue to work on it. It might no longer be considered physics, nor will mathematicians consider it to be pure mathematics. I can imagine that string theory in that case may become its own new discipline; that is, a mathematical science that is devoted to the study of the structure of physical theory and the development of computational tools to be used in the real world. The theory would be studied by physicists and mathematicians who might no longer consider themselves either. They will continue to derive beautiful mathematical formulas and feed them to the mathematicians next door. They also might, every once in a while, point out interesting and important properties concerning the nature of a physical theory which might guide the physicists exploring the actual theory of everything over in the next building. Whether or not string theory describes nature, there is no doubt that we have stumbled upon an exceptionally huge and elegant structure which might be very difficult to abandon.

The difficulty of abandoning one's own brainchild is obvious. But is there enough scientific justification? Today, one may assume that there is some hope that string theory would not remain forever disconnected from real world and experimental physics. But how long can we (should we) wait? Another 30 years? Who can say?

1 comment:

Andrew Daw said...

I say we'll have to wait for as long as it takes for enough physicists to accept the validity of a radical alternative theory of everything.

Such a theory needs to examine the evidence of quantum mechanics together with enough observable natural evidence of where it acts so as to sufficiently justify and describe enough details of a cause that acts non-locally in addition to the known forces.

This being a theory that would explain everything that cannot be explained as the effects caused by the forces alone.

How long this acceptance will take I've no ides really. But it's been quite a long slog up to now.