. The Higgs boson is a boson, with integer spin 0.
Well that is just one type of boson; are there others? In , we mentioned that the electromagnetic interaction is mediated by photons (particles of light) and the weak nuclear interaction by the W and the Z bosons—they are all bosons of spin 1. It is rather interesting that all these interaction-mediating particles are bosons and not fermions; if fermions are like people holding on selfishly to their corner of space, bosons are like the hugs and kisses, gifts, and interactions that can be stacked up, with no limits on their numbers, keeping the fermions together. Bosons are responsible for every physical interaction we know of, acting as the glue holding the universe together. That makes perfect everyday sense—you cannot expect an introverted individual averse to socializing to keep a party together.
, electron orbits can only have certain particular radii (or distance from the center of the nucleus). Now if we start filling in these orbits with electrons, then each orbit can have only two electrons, one of spin up and one of spin down, because of the exclusion principle. So once we have two electrons in, then the next electron is forced to be in the next higher or larger orbit, and so it goes on as we fill in more and more electrons. This is shown schematically in .
This is sort of like how bus seats fill up—people tend to occupy the seats at the front first, closest to the driver and the entrance. If the bus were to have only one seat per row on either side of the aisle, and also happened to operate in some conservative country where men and women sit on opposite sides of the aisle, then the similarity can be striking, as we see in : Each row can be occupied by one man on the left and one woman on the right, just like a single spin up electron and a single spin down electron in each orbital!
So much about fermions, what about bosons? The gregariousness of bosons actually leads to some very interesting behavior with important real-life applications. Because photons are bosons, we can put as many of them in the same region of space as we want with all in the same state, and when we succeed in doing that very efficiently, the result is something quite amazing that we all know about: laser!
, we mentioned the wave-particle duality, a lingering mystery in quantum mechanics, which indicates that all particles in the universe, fermions or bosons, behave both as particles and as waves, depending on the situation or experiment involved. So it is with light—which is comprised of particles called photons and also can be understood as electromagnetic waves. We can visualize the situation as is shown in . In ordinary light, the photons, although crowded together, are not quite in sync with each other, as shown in . The waves associated with the photons are in diverse orientation, and there is a lot of destructive interference among the waves (see ), and this results in relatively low intensity and very diffuse light. In laser light, on the other hand, all the photons are in identical states, with their associated waves oriented the same way and in generally constructive interference, as shown in . This results in very efficient and high-intensity, focused light. The difference is easy to see: Ordinary light can light up a whole room, while laser light creates a single very bright spot, because all the light is oriented the same way and therefore focused toward a very specific direction in space. In lasers, the bosonic nature of light truly becomes manifest, since it results from zillions of photons essentially being in the same state. If photons were fermions, lasers would have been impossible.
Now, one might say dismissively, “Come on now, this is not so surprising, after all photons have no mass, they have no material substance, so of course we can pile them into the same state.” True, but then I need to tell you about the Bose-Einstein condensates (BEC). Something very similar to what happens in lasers can be made to happen with atoms as well, which are the building blocks of all matter, including human beings. But that can happen only at very low temperatures, close to absolute zero; creating BEC in the laboratory was one of the major triumphs of experimental physics in the last decade of the twentieth century. The idea of BEC has a long history: It was first predicted by Einstein in the mid-1920s, but it took seventy years until it was finally created in the lab in 1995, because of the challenges of creating such low temperatures. As the BEC experimentalists like to brag, the coldest spots in the universe are right here on earth, in their labs!
, resulting in reduced intensity and rather inefficient light. (b) In laser light, all the waves are oriented the same way, and they are almost all in phase with each other (meaning crests and troughs of the waves are aligned), and this results in very high-intensity light.
—like well-distinguishable particles with tiny wavelength (see for its definition). As temperatures get lowered, the wavelengths of the atoms get longer and longer until waves sort of merge with each other, as shown in , and all the atoms behave like one single entity! That’s a BEC or Bose-Einstein condensate.
While there are obvious similarities between BECs and lasers, it is worthwhile to point out some key differences. First, lasers obviously work at room temperature—we use them all the time in optical drives, DVD players, and laser pointers—but BEC requires ultracold temperatures near absolute zero. That’s partly because the wave-like properties of atoms become manifest only at those very low temperatures. Second, a BEC usually stays in one place—where it is created—unless it is physically moved by some complicated machinery, but laser light, like all light, can never truly stand still and is always moving at its very high velocity.
Lasers and BEC simply reinforce certain essential qualities necessary for human success but so hard to harness: discipline and teamwork. One can think of lasers as an incredibly disciplined bunch of photons moving together as a team in a precisely coordinated way—sort of like a highly disciplined army. The military and successful businesses are known for being disciplined and for coordinated teamwork, but both discipline and teamwork do not usually come naturally to us and require particular effort. Well, so does creating laser light even though it involves bosons. Perhaps we can blame our difficulties with teamwork and discipline on being constitutionally made of fermions that prefer to be by themselves! But as far as our individual personalities go, there is indeed a broad range, and so you might ask yourself, are you bosonic or fermionic in nature?
The allowed states of the electrons (as discussed in ) orbiting the atomic nucleus depend on other factors besides the size of their orbits, such as angular momentum that defines how fast and in what orientation each electron is orbiting. But the basic idea is the same, each allowed state called an “orbital” about the nucleus can be occupied by only two electrons—one of each spin. To keep it simple, here we will just think of each distinct state being defined only by the size of the radius of the orbit as shown in .
Absolute zero is the lowest temperature possible. It is approximately–273.15º Celsius or–459.67º Fahrenheit. As mentioned in , temperature relates to the disorderly or random motion of atoms and molecules. At absolute zero, all such motion stops (from the classical physics perspective), and the atoms come to a standstill; therefore, no further lowering of temperature is possible. However, absolute standstill would imply precise knowledge of the atom velocities (exactly zero velocity), which conflicts with the uncertainty principle discussed in ; therefore, in the quantum view, there is always some residual movement even at absolute zero, appropriately called zero-point motion.
By the way, such quantum mechanical waves associated with material particles like atoms are called matter waves and their wavelengths called de Broglie wavelength after an aristocratic French physicist who first suggested the idea of matter waves in 1924 in his doctoral thesis, for which he got the Nobel Prize.