Does the strong force distinguish between past and future ?-I

A popular introduction to the strong CP problem

My friends and family often ask me, "What are you working on now ?" and  despite my most sincere efforts at answering this question, I am always left with a dissatisfying feeling. I feel that what is very exciting to me sometimes comes across as distant and  abstract as they are not familiar with my area of work. This is the first of a series of blogposts that aims to rectify this situation.  I will try to explain my current area of research in a way that hopefully conveys its importance as well as my excitement for it.

Part I: An irreversible world from 'reversible' laws

My current area of research,  the strong CP problem,  is related to the question whether the strong force-- one of the four fundamental forces of nature that holds the nucleus of an atom together-- respects Time Reversal  symmetry or not. So let me first tell you what is Time Reversal symmetry in this post. I will come to the more specific issue of whether the strong force respects this symmetry only in  later posts. 

If a process respects Time Reversal symmetry (or simply T-symmetry),  it means that if we take a video of it and run it backwards, the reverse course of events would also obey all the laws of nature. By this definition the  following reversed video seems to  shows a process that definitely  breaks T-symmetry:

                                           

After all, none of us has ever seen an egg 'uncracking' as the video shows. Quite remarkably, the above process actually does not break T-symmetry at a fundamental level!  This is also true for a host of other irreversible processes from daily life, for instance, a ball rolling on a floor that gradually slows down and stops (we never see the reverse, i.e. the ball on a floor spontaneously beginning to move). If we could track each and every microscopic particle in the above processes we will see that their histories are perfectly time reversible. This is because the fundamental laws that apply for these microscopic constituents do not distinguish between past and future and are perfectly time symmetric. 

So how do the reversible laws of nature give rise to the the irreversibility seen so commonly in everyday life ? This has actually been one of the deepest and most intensely debated paradoxes in the history of physics. So much so that the Austrian physicist Ludwig Boltzmann-- whose resolution of this paradox is now standard-- was driven to severe depression partly because his ideas were not universally accepted.  Tragically, his life ended when he  committed suicide in 1906 while on vacation at Trieste with his family. 

    Ludwig Boltzmann (1844-1906)

In fact debates on this topic still continue amongst physicists and philosophers. Perhaps, I will return to the subtleties that are still being debated in another blogpost. For now, let me explain the standard resolution of this paradox for the simple example of the ball on the floor that stops rolling. A ball rolling on the floor stops because of friction. Due to friction the ball loses its energy as heat and sound. This energy is thus transferred to the floor and the surroundings. Now the reverse: the ball extracting energy from the floor and spontaneously moving, never happens. This process is however  is reversible at the microscopic level. This is because, as Richard Feynman famously observed, almost all of the natural phenomena we see in  everyday life can be understood as a microscopic billiard game where countless particles like atoms and molecules collide and interact with each other. In particular both heat and sound are, at a fundamental level, atoms in motion. Now the motion of particles has been very well understood ever since Newton. Newton's laws of motion are completely reversible. So if we track the motion of  every microscopic particle present in the ball, the floor and the surroundings, we will find that all of them follow these laws of motion. Consequently, if their motion is reversed in time everything will still be perfectly consistent with Newton's laws. 

Why then do we never see balls that spontaneously start moving ? The answer to this is that while such an occurrence would be perfectly compatible with the laws of nature, it is extremely unlikely or improbable. This is because the time reverse of this process will involve the particles in the floor all moving in a coordinated way in the same direction.  Such a coordinated motion is required for these particle to transfer their energy and momentum to the ball such that it starts moving. This is overwhelmingly improbable, as the different particles in the floor and the surroundings, will in general move in an unrelated way in different directions.  To give an analogy, it is extremely unlikely that after shuffling a pack of cards, the hearts, diamonds, clubs and spades get perfectly organised into groups. Such a reorganisation of the cards is, however, not impossible or prohibited by a  deep principle; in fact if we repeat the whole exercise many many times it is bound to happen eventually.

      

                       Boltzmann's tombstone bearing the entropy formula

Boltzmann's way of explaining the irreversibility in the above examples would be to say that disorder, or more technically entropy, always increases. Starting from an organised pack of cards, it is inevitable that shuffling will increase the disorder. The reverse: getting an ordered pack of cards by shuffling repeatedly, is practically impossible. Similarly the entropy of ball along with its surroundings is much greater once it dissipates all its energy via friction. In other words all  irreversible  phenomena are irreversible because disorder/entropy must always increase which is not the case in the time reversed process where it decreases. This is the so-called second law of thermodynamics.  Boltzman's tombstone in Vienna  bears the inscription of the  his entropy formula.

                             The energy released in a nuclear explosion is

                                  ultimately due to the strong interactions.

My present work, actually, does not concern the emergence of an irreversible world from reversible laws but investigating whether the laws are truly T-symmetric. Now, there is no doubt that the laws governing the phenomena of everyday life are indeed reversible. But time reversal symmetry  is not a sacrosanct rule of nature that cannot be broken. There are exotic processes in particle physics that break this symmetry. It is just that the fundamental processes underlying what we see in everyday life are perfectly T-symmetric. This is because the two fundamental forces of nature that underlie everyday phenomena,  electromagnetism and gravity, obey time reversal symmetry. There are actually two other fundamental forces, the so called weak and strong forces. While the weak force is responsible for radioactivity the strong force holds the nucleus of an atom together. It is the energy associated with the strong force that is released when a Nuclear Bomb explodes.  The exotic processes mentioned above that break time reversal involve the weak force. There is still no evidence that the strong force breaks time reversal symmetry and this is the focus of my present work.

The fact that the strong force preserves T-symmetry although the weak force is irreversible at a fundamental level is actually paradoxical and called the strong CP problem, the subject of my present research.  In the coming blogposts I will explain

(1)How do we experimentally know that the strong force preserves reversibility ?

(2) Why is the reversibility of the strong force paradoxical ?

(3) How can the strong CP problem be solved ?

Eventually I would also like to come back to the question of entropy and irreversibility of everyday life although that is not my current research topic.  Another fascinating subject I would love to discuss in the future is how a breaking of T-symmetry, beyond what we observe for the weak force, is necessary to explain the abundance of matter  relative to antimatter in our universe.  So stay tuned!