The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World by Sean Carroll Read Free Book Online
Authors: Sean Carroll
fields.
This is where the “quantum” part of quantum field theory comes in. There’s a lot to say about quantum mechanics, perhaps the most mysterious idea ever to be contemplated by human beings, but all we need is one simple (but hard to accept) fact: How the world appears when we look at it is very different from how it really is.
The physicist John Wheeler once proposed a challenge: How can you best explain quantum mechanics in five words or fewer? In the modern world, it’s easy to get suggestions for any short-answer question: Simply ask Twitter, the microblogging service that limits posts to 140 characters. When I posed the question about quantum mechanics, the best answer was given by Aatish Bhatia (@aatishb): “Don’t look: waves. Look: particles.” That’s quantum mechanics in a nutshell.
Every particle we talk about in the Standard Model is, deep down, a vibrating wave in a particular field. The photons that carry electromagnetism are vibrations in the electromagnetic field that stretches through space. Gravitons are vibrations in the gravitational field, gluons are vibrations in the gluon field, and so on. Even the fermions—the matter particles—are vibrations in an underlying field. There is an electron field, an up quark field, and a field for every other kind of particle. Just like sound waves propagate through the air, vibrations propagate through quantum fields, and we observe them as particles.
Just a bit ago we mentioned that particles with a small mass take up more space than ones with a larger mass. That’s because the particles aren’t really little balls with a uniform density; they’re quantum waves. Every wave has a wavelength, which gives us a rough idea of its size. The wavelength also fixes its energy: It requires more energy to have a short wavelength, since the wave needs to change more quickly from one point to another. And mass, as Einstein taught us long ago, is just a form of energy. So lower masses mean less energy mean longer wavelengths mean larger sizes; higher masses mean more energy mean shorter wavelengths mean smaller sizes. It all makes sense once you unpack it.
Stuck away from zero
Fields have a value at every point in space, and when space is completely empty those values are typically zero. By “empty” we mean “as empty as can be,” or, more specifically, “with as little energy as it is possible to have.” According to that definition, fields like the gravitational field or the electromagnetic field sit quietly at zero when space is truly empty. When they’re at some other value, they carry energy, and therefore space isn’t empty. All fields have tiny vibrations because of the intrinsic fuzziness of quantum mechanics, but those are vibrations around some average value, which is typically zero.
The Higgs is different. It’s a field, just like the others, and it can be zero or some other value. But it doesn’t want to be zero; it wants to sit at some constant number everywhere in the universe. The Higgs field has less energy when it’s nonzero than when it’s zero.
As a result, empty space is full of the Higgs field. Not a complicated set of vibrations that would represent a collection of individual Higgs bosons; just a constant field, sitting quietly in the background. It’s that ever-present field at every point in the universe that makes the weak interactions what they are and gives masses to elementary fermions. The Higgs boson—the particle discovered at the LHC—is a vibration in that field around its average value.
Because the Higgs particle is a boson, it gives rise to a force of nature. Two massive particles can pass by each other and interact by exchanging Higgs bosons, just like two charged particles can interact by exchanging photons. But this Higgs force is not what gives particles mass, and it’s generally not what all the fuss is about. What gives particles mass is this Higgs field sitting quietly in the background, providing
This is where the “quantum” part of quantum field theory comes in. There’s a lot to say about quantum mechanics, perhaps the most mysterious idea ever to be contemplated by human beings, but all we need is one simple (but hard to accept) fact: How the world appears when we look at it is very different from how it really is.
The physicist John Wheeler once proposed a challenge: How can you best explain quantum mechanics in five words or fewer? In the modern world, it’s easy to get suggestions for any short-answer question: Simply ask Twitter, the microblogging service that limits posts to 140 characters. When I posed the question about quantum mechanics, the best answer was given by Aatish Bhatia (@aatishb): “Don’t look: waves. Look: particles.” That’s quantum mechanics in a nutshell.
Every particle we talk about in the Standard Model is, deep down, a vibrating wave in a particular field. The photons that carry electromagnetism are vibrations in the electromagnetic field that stretches through space. Gravitons are vibrations in the gravitational field, gluons are vibrations in the gluon field, and so on. Even the fermions—the matter particles—are vibrations in an underlying field. There is an electron field, an up quark field, and a field for every other kind of particle. Just like sound waves propagate through the air, vibrations propagate through quantum fields, and we observe them as particles.
Just a bit ago we mentioned that particles with a small mass take up more space than ones with a larger mass. That’s because the particles aren’t really little balls with a uniform density; they’re quantum waves. Every wave has a wavelength, which gives us a rough idea of its size. The wavelength also fixes its energy: It requires more energy to have a short wavelength, since the wave needs to change more quickly from one point to another. And mass, as Einstein taught us long ago, is just a form of energy. So lower masses mean less energy mean longer wavelengths mean larger sizes; higher masses mean more energy mean shorter wavelengths mean smaller sizes. It all makes sense once you unpack it.
Stuck away from zero
Fields have a value at every point in space, and when space is completely empty those values are typically zero. By “empty” we mean “as empty as can be,” or, more specifically, “with as little energy as it is possible to have.” According to that definition, fields like the gravitational field or the electromagnetic field sit quietly at zero when space is truly empty. When they’re at some other value, they carry energy, and therefore space isn’t empty. All fields have tiny vibrations because of the intrinsic fuzziness of quantum mechanics, but those are vibrations around some average value, which is typically zero.
The Higgs is different. It’s a field, just like the others, and it can be zero or some other value. But it doesn’t want to be zero; it wants to sit at some constant number everywhere in the universe. The Higgs field has less energy when it’s nonzero than when it’s zero.
As a result, empty space is full of the Higgs field. Not a complicated set of vibrations that would represent a collection of individual Higgs bosons; just a constant field, sitting quietly in the background. It’s that ever-present field at every point in the universe that makes the weak interactions what they are and gives masses to elementary fermions. The Higgs boson—the particle discovered at the LHC—is a vibration in that field around its average value.
Because the Higgs particle is a boson, it gives rise to a force of nature. Two massive particles can pass by each other and interact by exchanging Higgs bosons, just like two charged particles can interact by exchanging photons. But this Higgs force is not what gives particles mass, and it’s generally not what all the fuss is about. What gives particles mass is this Higgs field sitting quietly in the background, providing
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