Category: Physics

Recently I have been debating in my head the following question: Does the four page limit for papers in Physical Review Letters squash physics?
Benefits of the four page limit: (1) brevity enforces a focused article, (2) experiemental results can often be described in four pages, (3) you can tell when a paper is submitted to PRL on the preprint server by counting the number of pages.
Problems with the four page limit: (1) brevity means much is left out or compressed to near unreadability, (2) experimental techniques are rarely described in enough detail, (3) the compression to unreadibility means that general readership across the different sections of PRL, one of the supposed goals of the journal, is difficult if not impossible, (4) the papers are often light on citations since it is easier to cut citations to get to the page limits than to cut the content, (5) theories of any complexity are impossible to present in four pages without obmitting or skimming major portions of the work.

The nice thing about this New York Times article about the Einstein and the World Year in Physics, is that they provide a picture of Einstein when he was rather young. Isn’t it strange how the picture of Einstein many of us have in our head–the picture many of associate withe genius–is of the older Professor Einstein, and not of the young man working in a patent office?
Many argue that today no one “working in a patent office” could produce the same quality papers as Einstein did in 1905. But I doubt this. Maybe it’s because I still believe that our next fundamental breakthroughs in physics will be both revolutionary and simple. I guess this sits me on the opposite side of much progress in theoretical physics, which has seen a progressive increase in the level of sophistication, from special relativity to quantum theory to general relativity to quantum field theory to the standard model to superstring theory. But I’m a sucker for simplicity, and especially for conceptual simplicity. There are many ideas which are conceptually simple, but whose consequences are very difficult to sort out. And if it’s a simple idea with deep consequences which leads to the next new set of ideas in physics, then why can’t it come from someone passionate who isn’t sitting behind the standard academic bandwagon?

Every living thing follows along a set path. And if you could see your path or channel, then you could see into the future, right? Like err… that’s a form of time travel. – Donnie Darko

Time is nature’s way to keep everything from happening all at once. – John Archibald Wheeler

The past exists only as recorded in the present.

What is time? This question, in various forms, has been pondered by physicists and philosophers for eons. No doubt, various advances in our understanding of time have been made (the relativity of time for different observers, the symmetry breaking of time invariance as demostrated directly by K^0-K^0-bar experiments, etc.) but there are still enough troubling aspects of time for a good theoretician to get lost in. For example, the role of time in quantum theory (and in particular in a theory of quantum gravity) is one question which has consumed the soul of more than a few physicists.
In physics, a question which often bothers theoreticians is the origin of an arrow of time. The problem roughly is that we have underlying laws which are time symmetric, yet the universe seems to pick out a particular direction for the evolution in time. One explanation for the arrow of time is that it comes from thermodynamics. If we start with a universe which has a very low entropy, then the forward march of time can be marked by the upward increase in entropy. If we had started in a universe with maximal entropy, presumibly there would be no advance of time. But if the increase in entropy corresponds to an increase in the forward direction of time, does this mean that we can measure time in the same units of entropy? Can we measure time, then, in bits?
At first sight, this seems wrong. Take, for example a reversible computer. This computer acts according to reversible rules and so the entropy of the computer does not increase, even though, time is increasing as the computer runs a program. But maybe there is a way out of this puzzle. One possibility is that it is impossible to construct a truely reversible computer. This might seem silly, since we think the laws of physics are reversible, and so we can think about some physical system as enacting a reversible computation. But it’s not clear to me that robust computation is possible with a totally reversible system (more specifically without some effective irreversibility, such as cold ancilla bits which are discarded.)
Another possibility is that it might be true that a reversible computer can be constructed, but that it is impossible to construct a clock without irreversible evolution. I.e. to see the evolution of a reversible computer with respect to time, we need a clock around. Here things get rather tricky. Can’t I can think of a simple reversible two state system which simply cycles between the two states as a clock? I don’t think so. The reason is that a clock isn’t really just a system which counts, but it’s really a way in which we callibrate the basic units of time. So I use a cesium atom as a clock by using it to calibrate what a second is. Thus I run an experiment which performs measurements on the cesium clock which gives me a basic calibration upon which all clocks can be run. But why can’t this callibration be made totally reversible? I’m not sure, but it’s a good homework problem. I suspect that the callibration experiement cannot be made reversible (whenever I try the simple methods to make it reversible, I run into “effective” irreversibilities.)
So it seems that we can measure time in bits, or at least thermodynamic time in bits. What about other arrows of time (such as the arrow of time arrising from K^0-K^0-bar experiments or a cosmological arrow of time?) It would be fun to try and design a K^0-K^0-bar experiment which acts as a clock. And what of the relationship between time being measured in bits and the holographic principle, where surface areas are measured in bits?
See I told you a theoretical physicist could lose his soul thinking about time.

If you really want your head to spin, I recommend Phys. Rev. Lett. 94, 011602 (2005), “Spontaneous Symmetry Breaking Origin for the Difference Between Time and Space” by C. Wetterich:

In this Letter we pursue the perhaps radical idea that the difference between time and space arises as a consequence of the ‘‘dynamics’’ of the theory rather than being put in by hand. More precisely, we will discuss a model where the ‘‘classical’’ or ‘‘microscopic’’ action does not make any difference between time and space. The time-space asymmetry is generated only as a property of the ground state and can be associated to spontaneous symmetry breaking.

I think this is the first time I have ever seen anyone use SO(128,C).

Philip Anderson (“More is different!”) in the New York times today on “What do you believe is true even though you cannot prove it?”

Is string theory a futile exercise as physics, as I believe it to be? It is an interesting mathematical specialty and has produced and will produce mathematics useful in other contexts, but it seems no more vital as mathematics than other areas of very abstract or specialized math, and doesn’t on that basis justify the incredible amount of effort expended on it.
My belief is based on the fact that string theory is the first science in hundres of years to be pursued in pre-Baconian fashion, without any adequate experimental guidance. It proposes that Nature is the way we would like it to be rather than the way we see it to be: and it is improbable that Nature thinks the same way we do.
The sad thing is that, as several young would-be theorists have explained to me, it is so highly developed that it is a full-time job just to keep up with it. That means that other avenues are not being explored by the bright, imaginative young people, and that alternative career paths are blocked.

On the plane I got quized by a neighboring passenger about tsunami dynamics (“oh, you’re a physicist?”) Here is what I recall from Physics 12:
The waves created by tsunamis are very long wavelength. While typical ocean waves are around a hundreds of meters long, tsunamis produce wavelengths of up to a hunderds of kilometers. Since the wavelength of the tsunami is on the order of the depth of the ocean, tsunami waves are shallow water waves (most ocean waves have wavelength of hundreds of meters and are so are different beasts called deep water waves.) The speed of this type of wave (if you want to be fancy you say “celerity” here) is around the square root of the accleration due to gravity times the water depth (typically a few kilometers). This is why tsunami waves move at speeds of a few hundred meters per second and is also why tsunamis which hit the land aren’t moving at this speed (because the ocean depth gets shallower as you approach land and so the tsunami slows down.)