• Question: what is quantum physics??

    Asked by kt1710 to Ceri, James_M, Arttu, Monica, Philip on 16 Jun 2011. This question was also asked by qwantumfisixlver101, ciaradevine28, anmla, ishy05, smileysara, jokasom.
    • Photo: Ceri Brenner

      Ceri Brenner answered on 13 Jun 2011:

      The word quantum means that light can be considered as packets of light, called photons. It’s one of the great things that Einstein came up with. It’s a way of saying that light energy can be quantized, as well as being a continuous wave. The idea that light can be described as a wave as well as a particle is quite a difficult concept to get your head around, but quantum physics is area of physics that helps us to explain why this can be so. It relies heavily on complicated mathematics and solving hard equations and explaining things in terms of wavefunctions and so on.
      But the beauty of quantum physics is that it reveals a world that couldn’t be explained in any other way unless photons (light particles) existed. I’m not sure if you’ve done it in class yet, but one of the wonderful examples of quantum physics is the photoelectric effect, whereby electrons are emitted from a material if a strong, electromagnetic wave (light basically) is shone onto it. ask your teacher about it, they might even have a demonstration of this in the lab somewhere.

    • Photo: James M Monk

      James M Monk answered on 14 Jun 2011:

      Imagine that we have a single particle system, perhaps an electron in a box, that we wish to describe. How are you going to do it? If we try and write down the electron’s position then, even if we have a perfect measuring device, we will need an infinite amount of information to write down the position to infinite precision. Given that we can’t do that, we end up writing down the electron’s position as a function of its probability to be at a particular place.

      That in itself doesn’t sound so strange, but there are other quantities that can be known about the electron other than its position. In particular it has momentum, so whatever function we use to encode its position will also have to encode its momentum. It turns out that if the function is very sharply peaked in position, then the spread of possible momenta will be very wide. There is a trade off between the precision to which we encode the particle’s position and the precision to which we encode its momentum. Note that this is a feature of the mathematics used to describe the system, not a result of measurement uncertainty.

      It is called quantum physics because, with the particle in a box (or any other bound system) there are some place where the probability to find the particle is zero (the very edge of the box, for example). This restricts even further the functions we can use to describe the particle, and it turns out that the momentum can only take certain discrete values. When quantum physics was first discovered, it was this behaviour of momentum not being continuous that was new.

    • Photo: Philip Dolan

      Philip Dolan answered on 14 Jun 2011:

      Just in case you needed an extra answer I’ll give it a shot too (although Ceri and James’ answers are already very complete)!

      In the late 1800’s a Scottish chap called Maxwell discovered all the laws that govern the ‘electro-magnetic force’. These were four great equations that described everything to do with charge (+’s and -‘s) and also explained magnetisation to boot! Combined with Newton’s laws of Gravity, this is always referred to as ‘Classical Physics’.

      But then a Kiwi, by the name of Rutherford discovered that atoms have internal structure (a dense, central ‘+’ surrounded by small, mobile ‘-‘s). According to Maxwell, the ‘-‘ electrons should spiral into the ‘+’ (since like charges attract) give out a bunch of light and then stop. But they don’t.

      This was a problem, one that Albert Einstein (as Ceri said), and Danish bloke called Niels Bohr partly solved by saying that the electrons can only exist in certain levels, and in the very lowest level there is no way that they can give away the right ‘packet’ of energy (a photon) to fall into the nucleus.

      But the theory wasn’t as complete and nice and mathematical as Maxwell’s was. That is until Heisenberg, Schrödinger and Dirac showed up, and, each using different and funky maths completely described it all (actually Schrödinger and Heisenberg showed it in two different ways and Dirac showed they were actually equivalent).

      It is worth saying (as it confused me when I was younger) that ‘photons’ can actually have any energy what-so-ever, but it is only when they have the right amount of energy that they can jump electrons up in energy levels.

    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 16 Jun 2011:

      As the others have written, the quantum theory describes nature at a more fundamental level than classical physics. So far we have not been able see any violations of quantum mechanics in any experiments or observations, so it seems to work really well. The non-classical effects are usually strongest when you look at small systems or low temperatures, but they can also be very important in some large and hot objects such as white dwarf stars.

      The main difference between quantum and classical physics is that the quantum theory is not deterministic. In classical physics, if you know the state of the system exactly, you can in principle calculate its whole future time evolution, but in quantum physics you can only calculate probabilities of different outcomes. Furthermore, these probabilities do not behave in the way we are used to, so rather than a particle somehow picking one of the possible trajectories randomly, it seems to follow all of them at the same time.

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