Free 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 Page A

we have enough energy. The insight that makes particle physics possible is Einstein’s famous equation, E = mc 2 , which tells us that mass is actually a form of energy. In particular, the mass of an object is the minimum energy that object can have; when something is just sitting perfectly still, minding its own business, the amount of energy it possesses is equal to its mass times the speed of light squared. The speed of light is a big number, 186,000 miles per second, but its role here is just to convert units of measurement from mass to energy. Particle physicists like to use units where speed is measured in light-years per year; in that case c is equal to one, and mass and energy become truly interchangeable, E = m .
    What about when an object is moving? Sometimes discussions of relativity like to talk as if the mass increases when a particle approaches the speed of light, but that’s a little misleading. It’s better to think of the mass of an object as fixed once and for all, while the energy increases as it goes faster and faster. The mass is the energy that the thing would have if it was not moving, which by definition doesn’t change even if it happens to be moving. Indeed, energy grows without limit as you get closer and closer to the speed of light. That’s one way of understanding why the speed of light is an absolute limit to how fast things can go—it would take an infinite amount of energy for a massive body to move that fast. (Massless particles, in contrast, always move at exactly the speed of light.) When a particle accelerator pushes protons to higher and higher energies, they are coming closer and closer to the speed of light, never quite getting there.
    Through the magic of this simple equation, particle physicists can make heavy particles out of lighter ones. In a collision, the total energy is conserved but not the total mass. Mass is just one form of energy, and energy can be converted from one form to another, as long as its total amount remains constant. When two protons come together at a large velocity, they can convert into heavier particles if their total energy is large enough. We can even collide perfectly massless particles to create massive ones; two photons can smack together to make an electron-positron pair, or two massless gluons can come together to make a Higgs boson, if their combined energy is larger than the Higgs mass. The Higgs boson is more than a hundred times heavier than a proton, which is one of the reasons it’s so hard to create.
    Particle physicists enjoy using units of measurement that make no sense to the outside world, as it lends an aura of exclusivity to the endeavor. Also, it would be a pain to use one set of units for mass and another for other forms of energy, since they are constantly being converted back and forth to one or the other. Instead, whenever we’re faced with an amount of mass, we simply multiply it by the speed of light squared to instantly convert it into an energy. That way we can measure everything in terms of energy, which is much more convenient.

Scale of energies. Particle physicists measure temperature, mass, and energy on the same scale, using electron volts as a basic unit. Common expressions include milli-eV (1/1000 eV), keV (1000 eV), MeV (1 million eV), GeV (1 billion eV), and TeV (1 trillion eV). Some values are approximate.
    The energy unit favored by particle physicists is the electron volt, or “eV” for short. One eV is the amount of energy it would take to move an electron across one volt of electrical potential. In other words, it takes nine electron volts’ worth of energy to move an electron from the positive to the negative terminals of a nine-volt battery. It’s not that physicists spend a lot of time pushing electrons through batteries, but it’s a convenient unit that has become standard in the field.
    One electron volt is a tiny bit of energy. The energy of a single photon of visible light is about a couple of