The most convincing argument pro QCD is the “outstanding success” of lattice QCD. Lattice QCD explains masses of light hadrons with precision of nearly 2% – precision that was never seen by any theory (S. Dürr, Z. Fodor, J. Frison et al. Ab Initio Determination of Light Hadron Masses. Science 322 (2008)).
What is the probability that an invalid theory would explain so many hadron properties so precisely? Practically zero. This is probably the main (and legitimate) argument scientists use to not even consider competing models.
What is lattice QCD?
QCD equations are extremely complicated and practically almost unsolvable, even with the most advanced supercomputers. Lattice QCD was invented by Ken Wilson during the 1980s as a computational tool to approximate QCD equations.
Even with lattice QCD, the computation is highly time-consuming. When using lattice, the practice is to define several physical parameters for the lattice computing engine, and then define a discretization. The discretization density controls the computation time vs. accuracy.
Lattice QCD in practice
One thing that one should know is that, in spite of several decades of work, lattice QCD still doesn’t have any significant success of predicting something.
Two of the most amazing and illustrative articles are the article which discusses how lattice QCD can explain the strong nuclear force (N. Ishii, S. Aoki and T. Hatsuda, The nuclear force from lattice QCD, Phys. Rev. Lett. 99 (2007)) and a further discussion by Wilczek (Frank Wilczek, Hard-core revelations, NATURE, Vol. 445 156 (2007)).
Ishii et al demonstrated how Lattice QCD can be used to explain the strong nuclear force. To do so, they set as input the masses of several particles, set the lattice QCD density, and let the machine do the computation. The Lattice output was found to be compatible with experimentally known parameters of the strong nuclear force. This is indeed very convincing, and many readers of the article probably stopped reading at that point.
Those who continued to read had a little surprise waiting. It turns out that the particles masses used as input to the lattice engine were very different than their experimentally known values. For example, the pion mass in Ishii et al paper is 530 MeV while in reality it is less than 140 MeV.
This article is considered as a breakthrough – finally QCD is able to explain the strong nuclear forces!
Well – not even close, according to my judgment. In fact, the work of Ishii et al actually proved that QCD is incompatible with the strong nuclear force. In fact, I believe, they proved that QCD is incorrect.
Why? Simply because a computational engine like lattice QCD can be used not only to confirm something that we already measured. It can also be used to disqualify a theory.
Ishii et al could, theoretically, publish an outstanding paper which demonstrates that by using lattice QCD they were able to calculate the forces between nucleons, and the results contradict reality. They probably tried first the real and well established physical properties of the pions, and probably started with low discretization in order to know what the results of such computation might be. The results didn’t offer the well-known properties of the strong nuclear force, so it seems. Therefore, I guess, they played with these parameters until they got something they can publish.
In order to get the strong nuclear forces, Ishii et al proved that we must have pions with mass which is about 4 times bigger than the real value. My understanding is that they proved that QCD is inconsistent with the strong nuclear force. They proved QCD is inconsistent with reality.
Physicists told me that Ishii et al did a legitimate and common usage of lattice QCD – they tried first unreal values in order to speed up processing. However, 4 years has passed since this publication and no other calculation proved that the real physical values lead to the desired results. Furthermore, we will see in the next blog that this is not the only problem with this paper.
The bible code
There are people who believe that everything is written in the bible, including events which didn’t occur yet. For example, after the Israeli prime minister Rabin was assassinated in 1995, people found that if you take a sequence of letters from one of the chapters of the bible you may find a hint about an assassination that will occur (here).
Some Harry Potter fans demonstrated that the Hebrew translation of Harry Potter can also explain any event, like the bible can do. (Look here if you are really interested in that…)
Lattice QCD has a close similarity to the bible code technique. You can explain everything but you cannot predict anything. In particular, you should never dare to use it in order to explain something that you do not really want.
Is that science?