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Mass mystery unmasked?

July 9, 2012

Picture by AAAS – The Standard Model of Particle Physics

Our current understanding of the basic building blocks of the universe is the “Standard Model” of particle physics. It is a mathematical theory which basically says that all the matter (“stuff” that makes up you, me, planets, stars and everything else that we can see) and their interactions (forces like electrostatic attraction and repulsion) are made up of a few fundamental particles. This theory was built up starting from the formulation of quantum mechanics in 1920s and completed as a full-fledged description in the 1970s.

According to the “Standard Model”, there are a set of 12 fundamental particles out of which atoms and all matter is built up. These include the electron, the quarks (which are inside protons and neutrons found in the nuclei of atoms) and some exotic particles like neutrinoes. These are truly fundamental particles in the sense that they can’t be broken down into simpler components. Further, the theory says that interactions between these particles (forces between matter) are carried by another set of 4 fudamental particles, which includes the photon (electric and magnetic forces, carrier of light), W- and Z-bosons (responsible for radioactivity) and gluons (responsible for holding quarks together inside protons and neutrons).

This set of 16 components (12 + 4) completely describes all visible matter in the universe and their interactions. Just a note here that the Standard Model does not describe the gravitational force (which attracts two massive objects towards each other and makes the planets orbit around the sun). Physicists have not been able to come up with a consistent theory of gravity which is compatible with quantum mechanics, the foundation on which the Standard Model is constructed. So the Standard Model describes everything except gravity, and we sort of have a separate theory of gravity. The two have not been made mathematically consistent with each other yet.

The Standard Model is the most successful scientific theory ever. It has been tested by various experiments over the past 50 years to unprecedented accuracy. The theory has predicted the existence of various particles and phenomena, and experiments have found them years later. All the 16 fundamental components have been experimentally verified. So everything has been great. Except for one thing, one pretty major thing. In the simple version of the Standard Model, these 16 fundamental particles are all massless (that have zero mass) and all travel at the speed of light. But this is not what we observe in nature. Some of them are massless (like the photon and gluons), but all the others have been measured to have widely ranging masses. The electron is very light but has a non-zero mass. The lightest quark has a mass almost the same as the electron, but the heaviest quark has a mass 100,000 times that of the electron. What gives fundamental particles mass of such a wide range? What endows particles with mass? This was the question that the original Standard Model failed to address.

In the 1960s, six different theoretical physicists (one of which was the Englishman Peter Higgs) working at different places tried to answer this question. They postulated that all of space is filled with this “field” which interacts with the fundamental particles. Think of it as sand or some viscous liquid like honey. Over time, for some reason this became popularly known as the “Higgs field”. The basic idea is that fundamental particles moving through space are like beads being pulled through honey. Light particles like the electron interact with this field negligibly, so can fly across space fast and have less mass. Massless particles like the photon donot see this field at all. Heavy particles feel a massive drag from this field and acquire a large mass. The Higgs field (which permeates all of space, from inside the atoms making up you and me to the farthest galaxies) seemed to be a good mathematical description and a good explanantion for why particles have mass. In accordance with quantum mechanics, this Higgs field was supposed to be composed of fundamental particles called Higgs bosons.

So in order to explain why particles have mass, the Standard Model had to include one more component to the earlier 16, the Higgs boson. And this was the only component not confirmed by experiments, until now. Two independent experiments at the LHC (Large Hadron Collider) have reported the discovery of a previously unknown particle whose behavior is consistent with the Higgs boson. The LHC is a 27-km long underground tunnel beneath the Franco-Swiss border where protons travelling at nearly the speed of light collide head-on with each other. With the discovery of the Higgs, the Standard Model stands complete as the decription of physical reality in the visible universe (with the exception of gravity of course). The final piece of the puzzle. A true testament to human endeavour that we can understand so much about the workings of nature given that we ourselves are a product of it.

Regarding the term “God-particle” in reference to the Higgs boson, most physicists really consider it as a misnomer. It was coined by a Nobel prize winning physicist, Leon Lederman, in his popular science book of the same name. He sort of equated the importance of the Higgs, which endows all particles with mass, to something akin to a God who endows the universe with its properties. The popular media is of course just waiting for a catchy phrase like this, and jumped on the bandwagon. The Higgs and its discovery doesn’t really say anything about the existence or non-existence of God (if you can define the term “God” in the first place).

Lastly, scientists – unlike the media – are very careful regarding their claims and discoveries. So the status right now is that they say that a “Higgs-like” particle has been discovered. The only certain thing they are claiming is that a new particle has been discovered. Experiments so far suggest that it is most likely the Standard Model Higgs, but to confirm that more data is required. There are also various different versions of the Higgs theory (in one case, there are 5 different Higgs bosons postulated) and it is not yet clear which exactly the discovered particle corresponds to. To summarize, a great achievement for humankind and a cause for celebration and reflection. However, the quest for deeper knowledge continues in earnest!


From → Physics

One Comment
  1. Aaron Slepkov permalink

    Great summary, Vivek. Thoroughly informative and considered.

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