This is the first in a series of posts about particle physics, its theories and its experiments, and with only minimal math. You miss some of the awesomeness that way, but will be spared working your way through twenty-page integration problems.Goal of Chapter Zero: To define what in general particle physics is studying and review the main concepts from other areas of physics needed to understand particle physics.
To answer this question, let's start from something more familiar and work our way down. You may remember from a chemistry class at some point that the world is made up of stuff called elements. Elements may be combined into mixtures, but mixtures act differently from elements and can be split up into their component elements once again. If you split some amount of an element, each piece you're left with will act exactly the same; if you split a piece of gold in half, each half will still behave like gold regardless of how you test it. The smallest bit of an element that still retains its identity of an element is called an atom.
Atoms are not fundamental, which means they are made up of smaller parts. Atoms are made up of a nucleus with protons and neutrons. The nucleus is surrounded by electrons whizzing around. These little pieces of matter that cannot be split up any more are called particles.
A particle is a little bit of matter and energy that cannot be broken down any more. These are the smallest building blocks of the universe.
Force or Interaction: For physicists in the normal, macroscopic world, a force is a push or a pull. They change how fast an object is moving or what direction the object is headed in or something like that. In particle physics, we speak of forces or interactions interchangeably; the term interaction is used because sometimes forces make particles turn into other particles and other strange behaviors, which doesn't really fit with how people normally think of forces.
Quantum Mechanics: In quantum mechanics, particles are treated as wave-like objects described by probabilities. These little bits of matter can either bounce around like particles or bend around like waves, and we can't say for sure what a given particle will do in a certain situation. We can describe all of the possible outcomes of an interaction and with what probability each outcome will occur (or we try to, and when we're wrong we rewrite our theory). This is why experiments that play around with quantum mechanical objects must take many, many measurements, enough to accurately estimate what the probabilities are.
Another weird feature of quantum mechanics is that certain properties are quantized, or only exist in discrete amounts. This happens in normal physics as well; for example, an object can either spin clockwise or counter-clockwise, and there are no other possible ways of spinning. In quantum mechanics, there are several quantities like this, and each can be associated with some integer. These quantum numbers can be used to describe many of a particle's properties and behavior.
Relativity: The effects of relativity become important for things moving close to the speed of light, and since particles are so light it doesn't take a lot of effort to get them moving that fast. There are two main relativistic effects for fast-traveling objects: time moves slower for them, and distances are shorter for them. Also, for relativistic particles energy and mass are interchangeable. In particle physics, we often use the same units for mass and momentum and energy, since at the speeds they are traveling the difference between the three is slight.
Conversation of mass/energy, charge, and spin: Several areas of physics are built on conservation laws, where regardless of what craziness happens between particles, some property must be the same before and after. One example of this is electric charge. However much electric charge you had before the interaction happened, you will have the same amount afterward. Spin, which is one of the quantum numbers for fundamental particles, is also conserved during interactions. Conservation laws let you mathematically connect the before to the after and figure out what happened in the middle. The one we use most often is conservation of mass/energy. We have to conserve both mass and energy together because for relativistic particles, you get trades back and forth between the two (energy = mass x (speed of light)^2). This is incredibly useful because if you add up all the before contributions and all the after contributions and they don't match, you know something you didn't measure was involved in your interaction, and that's probably interesting.
Mass: This particle property determines how hard it is to accelerate a particle. Particles with higher masses take more energy to get moving. Since for a relativistic particle, the mass you measure depends on how fast the thing is moving, we normally report the particle's rest mass, or the mass you measure when the thing is holding still.
Charge: This determines what forces a particle interacts with. If a particle has a non-zero charge for that force, it will interact with that force in a way dictated by the charge. The term was coined for electric charge interacting with the electromagnetic force, and that is normally what we're referring to when we talk of "charged particles." However, the other forces can be described with their own charges, too, and these are amongst a particle's quantum numbers.
Spin: One of the intrinsic quantum numbers of a particle. It's named 'spin' because it comes from the angular momentum equations of a particle, which are mathematically the same as if the particle were spinning on its axis (these are quantum mechanical, wave-like objects, so there is no good answer to the question of how they spin). Every particle has a spin, and spin can affect how particles interact. Particles that aren't fundamental, but that are made up of other particles, also have a spin, which is the vector-sum of the component particle spins.
Helicity: This describe how a particle's spin vector (which is the mathematical way of keeping tracking of whether the particle is spinning clockwise or counterclockwise) relates to its momentum vector. Right-handed particles have the two vectors pointing in the same direction, while left-handed particles have them pointing in opposite directions. Most particles can exist as either right- or left-handed, but some cannot, which again effects what types of interactions a particle can have.
What's a particle?
To answer this question, let's start from something more familiar and work our way down. You may remember from a chemistry class at some point that the world is made up of stuff called elements. Elements may be combined into mixtures, but mixtures act differently from elements and can be split up into their component elements once again. If you split some amount of an element, each piece you're left with will act exactly the same; if you split a piece of gold in half, each half will still behave like gold regardless of how you test it. The smallest bit of an element that still retains its identity of an element is called an atom.
Atoms are not fundamental, which means they are made up of smaller parts. Atoms are made up of a nucleus with protons and neutrons. The nucleus is surrounded by electrons whizzing around. These little pieces of matter that cannot be split up any more are called particles.
A particle is a little bit of matter and energy that cannot be broken down any more. These are the smallest building blocks of the universe.
Some necessary theories and concepts
Force or Interaction: For physicists in the normal, macroscopic world, a force is a push or a pull. They change how fast an object is moving or what direction the object is headed in or something like that. In particle physics, we speak of forces or interactions interchangeably; the term interaction is used because sometimes forces make particles turn into other particles and other strange behaviors, which doesn't really fit with how people normally think of forces.
Quantum Mechanics: In quantum mechanics, particles are treated as wave-like objects described by probabilities. These little bits of matter can either bounce around like particles or bend around like waves, and we can't say for sure what a given particle will do in a certain situation. We can describe all of the possible outcomes of an interaction and with what probability each outcome will occur (or we try to, and when we're wrong we rewrite our theory). This is why experiments that play around with quantum mechanical objects must take many, many measurements, enough to accurately estimate what the probabilities are.
Another weird feature of quantum mechanics is that certain properties are quantized, or only exist in discrete amounts. This happens in normal physics as well; for example, an object can either spin clockwise or counter-clockwise, and there are no other possible ways of spinning. In quantum mechanics, there are several quantities like this, and each can be associated with some integer. These quantum numbers can be used to describe many of a particle's properties and behavior.
Relativity: The effects of relativity become important for things moving close to the speed of light, and since particles are so light it doesn't take a lot of effort to get them moving that fast. There are two main relativistic effects for fast-traveling objects: time moves slower for them, and distances are shorter for them. Also, for relativistic particles energy and mass are interchangeable. In particle physics, we often use the same units for mass and momentum and energy, since at the speeds they are traveling the difference between the three is slight.
Conversation of mass/energy, charge, and spin: Several areas of physics are built on conservation laws, where regardless of what craziness happens between particles, some property must be the same before and after. One example of this is electric charge. However much electric charge you had before the interaction happened, you will have the same amount afterward. Spin, which is one of the quantum numbers for fundamental particles, is also conserved during interactions. Conservation laws let you mathematically connect the before to the after and figure out what happened in the middle. The one we use most often is conservation of mass/energy. We have to conserve both mass and energy together because for relativistic particles, you get trades back and forth between the two (energy = mass x (speed of light)^2). This is incredibly useful because if you add up all the before contributions and all the after contributions and they don't match, you know something you didn't measure was involved in your interaction, and that's probably interesting.
Particle properties
Mass: This particle property determines how hard it is to accelerate a particle. Particles with higher masses take more energy to get moving. Since for a relativistic particle, the mass you measure depends on how fast the thing is moving, we normally report the particle's rest mass, or the mass you measure when the thing is holding still.
Charge: This determines what forces a particle interacts with. If a particle has a non-zero charge for that force, it will interact with that force in a way dictated by the charge. The term was coined for electric charge interacting with the electromagnetic force, and that is normally what we're referring to when we talk of "charged particles." However, the other forces can be described with their own charges, too, and these are amongst a particle's quantum numbers.
Spin: One of the intrinsic quantum numbers of a particle. It's named 'spin' because it comes from the angular momentum equations of a particle, which are mathematically the same as if the particle were spinning on its axis (these are quantum mechanical, wave-like objects, so there is no good answer to the question of how they spin). Every particle has a spin, and spin can affect how particles interact. Particles that aren't fundamental, but that are made up of other particles, also have a spin, which is the vector-sum of the component particle spins.
Helicity: This describe how a particle's spin vector (which is the mathematical way of keeping tracking of whether the particle is spinning clockwise or counterclockwise) relates to its momentum vector. Right-handed particles have the two vectors pointing in the same direction, while left-handed particles have them pointing in opposite directions. Most particles can exist as either right- or left-handed, but some cannot, which again effects what types of interactions a particle can have.
Congratulations! You reached the end of the introductory material!
Coming soon - 25 days of Particles!
Coming soon - 25 days of Particles!
Picture taken from greece.mrdonn.org.
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