• Question: What are Lasers and their properties? Is the Nuclear force a field force? What does Color Charge mean in particle physics?

    Asked by sanban to Chris, Dave, David, Fiona, Jack on 20 Jun 2013.
    • Photo: Chris Mansell

      Chris Mansell answered on 20 Jun 2013:


      I will have a try at answering the part of your question about lasers.

      Lasers are devices that emit light waves that:

      -have only one wavelength (rather than being a mixture of light waves that all have different wavelengths).

      -travel in only a very narrow range of directions (which is why people like to use laser-pointers to point at things).

      – are coherent. This means that the light waves all line up with each other: the peaks of each wave are all next to each other and the troughs of each wave are all next to each.

      Light waves emitted from light bulbs don’t have these properties. They have a range of wavelengths, travel in a wide range of directions and (mainly as a result of this) aren’t coherent.

    • Photo: David Freeborn

      David Freeborn answered on 20 Jun 2013:


      Hi Sanban,

      Those are some great questions! I’ll have a go at answering.

      Lasers are a type of light. The word stands for “Light Amplification by Stimulated Emission of Radiation”. The difference between lasers and other types of light is that it’s “coherent”. This means that all of the light is the same wavelength and the same phase, so the “peaks” and “troughs” in the wave fall at the same point.

      You can see what I mean here: http://c683966.r66.cf2.rackcdn.com/wp-content/uploads/multichromatic-sunlight-monochromatic-and-coherent-light-950×968.jpg?cda6c1

      The fact that the light in a laser is coherent means it is much easier to keep the beam very focussed- so we can transfer a lot of energy onto a small area. This is how laser cutting works. You can watch a video of laser-cutting here: https://www.youtube.com/watch?v=UeGVbtrrHjE.
      The coherence of the light also means it’s easier to use lasers to read and transfer information, which is why we use lasers in DVD readers and barcode scanners.

      We’re able to produce lasers because the electrons around atoms sit in fixed, separate energy levels. If we fire light at some materials in the right way, we can make the electrons “jump” from one energy level to a higher one. When the electrons fall back down again, they emit light in a very narrow band of energies.

      We now use lasers for a lot of day-to-day activities, from surgery to transferring information. They would make very, very inefficient weapons though- it would take a lot of power to get even a very weak laser.

      ***

      There are two fundamental nuclear forces in nature- the Weak and the Strong nuclear force. There is also what is called the “effective nuclear force” which is not a fundamental force.

      The Strong force is the force that uses gluons to bind quarks together into protons and neutrons, and other particles such as pions.

      The Weak force is responsible for some types of radioactive decay, such as the “beta” decay of some atomic nuclei.

      The “effective nuclear force” is a special case of the strong force. This force uses pions and gluons to bind protons and neutrons together to form atomic nuclei. But it’s really just a special case of the Strong force.

      All three of these are “field forces”. Each of the particles in these forces can be thought of as a quantum excitation, or vibration, of the field, either the strong field of the weak field.

      But we also know that the field theories we use to describe them cannot be fully correct- it is only a very, very good approximation. We know there must be an even better theory underlying the field theories that we use.

      ***

      The electromagnetic force has a property called “charge”. Charge can be positive or negative. Positive charges tend to attract negative charges, and vice versa. Because opposite charges tend to attract, we can think of the electromagnetic force as trying to create “electrically neutral” states, of no overall charge.

      The strong nuclear force has a property a bit like charge, that we call colour charge, though it doesn’t have anything to do with the real colours we see with our eyes. Unlike charge, which can be positive or negative, there are three possible colours: which we call “red”, “green” and “blue”. There are also three “anti-colours” that antiparticles can have: “anti-red”, “anti-green” and “anti-blue”.

      Just like the electromagnetic force tries to create “neutral” states, the strong force tries to create “colourless” or “white” states.

      There are two ways to do that: it can either create a red-green-blue or (antired-antigreen-antiblue) state, or it can create a colour-anti-colour state, like red-antired or green-antigreen.

      These different states tend to correspond to two different types of particles we see in nature:

      Baryons, which are composed of three quarks of different colours
      Mesons, which are composed of a colour-anti-colour pair

      We’re also searching for so-called “exotic” states- composed of 4 or 5 quarks in a colourless state. Right now, there’s some evidence that we might have found a 4-quark state, composed of colour-anticolour-colour-anticolour, but we’re not yet sure!

      I hope that all makes sense! Ask again if you don’t understand anything!

    • Photo: Jack Miller

      Jack Miller answered on 21 Jun 2013:


      Great answers from Chris & David. I’ll just add one thing about colour charge: whilst directly analogous to electrical charge, it’s also very different in that quarks don’t have a _fixed_ colour charge — it isn’t the case that all up quarks are red, say. Rather, when a given process occurs there are many different ways colour can be assigned, and the reaction will only occur if the quarks can take colours such that everything ends up in a colourless (i.e. neutral) state at the end. In fact, this is part of the evidence for colour charge in the first place!

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