Warped Passages - Lisa Randall [73]
Bosons act in exactly the opposite fashion to fermions. They can and will be found in the same place. Bosons are like crocodiles—they prefer to pile up on top of one another. If you shine light where there is already light, it behaves very differently from your hand karate-chopping a table. Light, which is composed of bosonic photons, passes right through light. Two light beams can shine in exactly the same place. In fact, lasers are based on this fact: bosons occupying the same state allow lasers to produce their strong, coherent beams. Superfluids and superconductors are also made of bosons.
An extreme example of bosonic properties is the Bose-Einstein condensate, in which many identical particles act together as a single particle—something that fermions, which have to be in different places, could never do. Bose-Einstein condensates are possible only because the bosons of which they are composed, unlike fermions, can have identical properties. In 2001, Eric Cornell, Wolfgang Ketterle, and Carl Wieman received the Nobel Prize for Physics for their discovery of the Bose-Einstein condensate.
Later on I won’t need these detailed properties of the way that fermions and bosons behave. The only facts I will use from this section are that fundamental particles have intrinsic spin and can act as if they were spinning in one direction or another, and that all particles can be characterized by whether they are bosons or fermions.
What to Remember
Quantum mechanics tells us that both matter and light consist of discrete units known as quanta. For example, light, which seems continuous, is actually composed of discrete quanta called photons.
Quanta are the basis of particle physics. The Standard Model of particle physics, which explains known matter and forces, tells us that all matter and forces can ultimately be interpreted in terms of particles and their interactions.
Quantum mechanics also tells us that every particle has an associated wave, known as the particle’s wavefunction. The square of this wave is the probability that the particle will be found in a particular location. For convenience, I will sometimes talk about a probability wave, the square of the more commonly used wavefunction. The values of this probability wave will give probabilities directly. Such a wave will appear later on when we discuss the graviton, the particle that communicates the force of gravity. The probability wave will also be important when discussing Kaluza-Klein (KK) modes, which are particles that have momentum along the extra dimensions—that is, directed perpendicular to the usual dimensions.
Another major distinction between classical physics and quantum mechanics is that quantum mechanics tells us that you cannot precisely determine a particle’s path—you can never know the precise path a particle took as it traveled from its starting point to its destination. This tells us that we have to consider all the paths that a particle can take when it communicates a force. Because quantum paths can involve any interacting particles, quantum mechanical effects can influence masses and interaction strengths.
Quantum mechanics divides particles into bosons and fermions. The existence of two distinct categories of particles is critical to the structure of the Standard Model and also to a proposed extension of the Standard Model known as supersymmetry.
The uncertainty principle of quantum mechanics, coupled with the relations of special relativity, tell us that, using physical constants, we can relate a particle’s mass, energy, and momentum to the minimum size of the region in which a particle of that energy can experience forces or interactions.
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