Last year news about the "God's Particle" was everywhere. The Large Hadron Collider (LHC) at CERN published the news that the latest data pointed to the existence of the predicted Higgs boson (God's) particle. There isn't any confirmation yet, but it seems it's getting close (read here and here).
So what's Higgs boson actually? You can find lot's of stories about how it got the different names and all the media buzz around it, but it's not easy to find information understandable by "common mortals". As a physics enthusiast, that's what I'll try to do in this post.
If you've studied physics in High School or even in College, you start out with the Atom model created by Ernest Rutherford in 1909. The atom has a nucleus in the center and the electrons going around it much like planets go around the Sun. However it had a few short-comings and in 1913 Nils Bohr found a solution, by adding that electrons can go around the nucleus only on certain orbits. It's elegant and pretty easy to understand :) However it's not the end of the story and here the complications start. It turns out that even the Bohr model had shortcomings, the biggest one that it was able to describe only the Hydrogen atom. In the mean time Max Planck and Albert Einstein had worked on Quantum Theory and had postulated the concept of "quanta", meaning light is emitted not continuously but in discrete chunks. Taking the concept of Louis De Broglie that all particles have wave-like behavior and combining it with the ideas of Quantum Theory, Erwin Schrodinger created an equation that described elementary particles using a wave function. This is known as the Schrodinger's equation. Instead of thinking of electrons as particles that orbit the nucleus, the concept of wave-particle duality was introduced and the Schrodinger's equation gives the probability that an electron is located somewhere (potentially it can be located anywhere, but it's more likely to be found in some places). At it's core this is the model still in use today (more generally called Quantum Mechanics), but many new concepts have emerged, to complicate things further (continue reading :))
Quantum Mechanics does a great job in explaining the micro World (sub-atomic particles), but it's useless in the macro World (planets, stars, galaxies and ultimately the Universe). On the other hand The General Theory of Relativity explains perfectly the macro World, but it's useless in the micro World. Albert Einstein spent the last 30 years of his life trying to unify both Worlds under a single theory (called the Theory of Everything). Ultimately Einstein was unsuccessful in his attempt and this is still the biggest unsolved problem in Physics today. However, efforts made by lots of physicists made it possible to create a theory that would unify Electromagnetism with the Weak (responsible for radioactive decay) and Strong (responsible for holding protons and neutrons together at the atomic level and responsible for holding quarks together inside hadrons) nuclear forces. This was completed somewhere in the mid-70s and it's called the Standard Model. Until today this is the most complete theory that explains perfectly all the experiments at the atomic and sub-atomic levels. It has predicted successfully the existence of several particles that were later confirmed by experiments. The mathematical model behind the Standard Model is pretty complicated, but in total it includes 61 particles and about 20 constants. From the 61 particles, 60 have been observed and measured successfully and the last one is "our" Higgs boson. It has eluded physicists for about 40 years, but now finally it seems the quest is over.
So what's the role of the Higgs boson and why it's so difficult to observe it? Higgs boson plays a unique role in the Standard Model because it explains why other particles have mass (how it does so, it's complicated). The difficulty of observing it is related with two facts: First the Standard Model cannot predict the mass, so the physicists do not know exactly where to look for it; second it is known that it is a very big particle, so a huge amount of energy is needed to create one. Now to give a better idea of the mass and energies that are used in atomic and sub-atomic scale the concept of electron-volt is needed (eV). By definition one eV is the amount of energy gained by the charge of a single electron moved across an electric potential difference of one volt. Converted to joule it's about 1.6 x 10-19 J. Since the energies, momentum, masses and distances in the atomic scale are very very small and using the Standard Units (J for energy, kg* m/s for momentum, kg for mass and m for distance) would result in very small numbers, physicists prefer to use eV for all the above. It's very practical for them, but very confusing for "mortals". Since it has confused me a lot, I'll try and explain simply how they use the same unit for measuring different things.
- Measuring energies in eV it's straightforward because it's native. To get a better idea of the scale of eV, here are some examples:
- The energy of a photon in visible light is about 2-3 eV.
- The energy released in the nuclear fission of one atom of U-235 is about 200 MeV.
- The kinetic energy of a flying mosquito is about 1 TeV.
- The energy needed to power a standard (100 W) light bulb for one second is about 624 EeV (Exa electronvolt) or 624,000,000 TeV.
- The unit of energy is kg x m2/s2. The unit of momentum is kg x m/s so to get momentum from energy you have to divide by m/s which is the unit of speed. Since particle physicists use the speed of light as a reference, by dividing the energy by the speed of light you can measure the momentum in units of eV/c. Furthermore by using natural units, where c = 1 you can measure momentum in eV. To convert from kg x m/s to eV the formula is 1 eV = 1.6 x 10-19 J / (3 * 108 m/s) = 5.4 x 10-27 kg x m/s.
- The mass-energy equivalence makes it possible to use eV as a unit of mass. Again using the speed of light as a reference you can express mass in eV/c2 and by using natural units, you can use eV for mass. To convert from eV to kg the formula is 1 eV = 1.6 x 10-19 J / (3 * 108 m/s)2 = 1.79 x 10-36 kg or 1 TeV = 1.79 x 10-24 kg. As a reference here are some examples:
- The rest mass of electron is 9.1 x 10-31 kg = 0.51 MeV
- The rest mass of proton is 1.67 x 10-27 kg = 938 MeV
- Estimated rest mass of Higgs bosson is 125 GeV = 224 x 10-27 kg. *
- When using the natural unit where c = 1 and the reduced Planck constant h' = 1, it's possible to use eV as a unit of distance. To convert to standard units we would have h' = 1.0545 x 10-34 J x s = 6.58 x 10-16 eV x s.
Now back to the Higgs boson mass, which as stated above is estimated at about 125 GeV. Compared with other elementary particles it is huge. In order to experimentally create such a huge particle you need a lot of energy and to achieve a lot of energy you need to accelerate particles at very high speed (E = m x c2). At LHC protons are accelerated to very near the speed of light in order to have energy high enough to create the Higgs. The maximum energy the LHC can achieve is 7 TeV for particle so the collision will produce 14 TeV and the result of it sometimes will produce a Higgs boson. To achieve an energy of 7 TeV the proton must have a speed of 0.999999991 c, or about 3 metres per second slower than the speed of light.
On a last note, although the Standard Model is very accurate and precise, it is not elegant. It includes 61 particles (all of them having different values for mass, spin and/or electrical charge) and about 20 arbitrary constants. On top of that it still doesn't explain gravitation and "new" concepts such as dark matter and dark energy. So from a philosophical point of view there must exists another more general theory that would explain all the laws of the Universe and for which the Standard Model would be a special case. The "problem" is that we have to wait for the next Einstein to make the next big leap in physics and introduce new concepts that would revolutionize the way physicists think. I have the feeling that physicists now are too focused on the mathematical models and not on the physical phenomenons. The genius of Einstein was the ability to think outside the box, to question all the existing knowledge and to look for elegant solutions (his thought experiments are truly genial).
Happy reading,
Albert
* Thanx to Gopolang for pointing out my mistake. Rest mass of Higgs bosson is around 125 GeV, not TeV like I had written in the original article.