Question: What is Quantum Mechanics

Michael Nolan answered on 20 Nov 2013:

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Afternoon,

Nice question! I better do a good job as I use quantum mechanics every day!

We describe our Universe at present with one of two theories: quantum mechanics, which treats the atomic world and classical mechanics which treats the large scale world such as people, tables, cars, stars, galaxies.

Quantum mechanics (QM) is our scientific theory to describe the atomic world, which begins at around one 10 millionth of a meter (0.00000001 metre) and goes down to the quarks that make up protons and neutron. Around 1900 a bunch of experiments that happend to probe the atomic world showed that classical mechanics of Newton was totally wrong and a new theory had to be developed. This is QM and we use it to describe things like this

1. How atoms are formed from collections of electrons, protons and neutrons and why atoms are stable at all.
2. How sub atomic particles like electrons and photons interact with each other (Q electrodynamics, QED)
3. How protons and neutrons are built out of quarks and how quarks behave.
4. What happens to atoms when they smash into each other (scattering)
5. Why objects are coloured differently
6. How chemistry works

Number 6 is my area and here the idea is that particles, like electrons, can only take certain discerete energies or momentum, whereas in classical you can take any energy or momentum you like.

The profound consequence of this is that electrons in atoms take certain energy states and no other and the movement of electrons between these energy states when you hit them with light gives different elements and obejcts their characteristic colour.

You need a particular type of mathematics to describe this quantum behavour (quantum is latin for How Much, so we describe a quantum of energy as the minimal energy in a system) which I wont go to here, but it is very powerful.

We can describe how electrons behave in metals (so why copper conducts electricity) and in say plastics (so why they do not conduct electricity) or in silicon (the basis of the computer and electronics in general).

It turns out that QM also says light shows particle and wave properties – so it can be seen as a particle in scattering off electrons (Compton effect) but a wave in that it shows interference patterns through a diffraction grating (Young’s slit).

Electrons can also do this – you can bounce, scatter, electrons off objects, but you can also make them show wave properties and this is the basis of a tool to examine the atomic structure of materials.

This is very wierd behaviour when you are used to the classical world, which most of us are, and a wave is a wave and a particle a particle. In fact, you can say the nature of an electron depends on the observation you make on it – if you look at particle properties, like scattering, then that is what you see. But if you look at wave properties, then you see a wave. We do not see this as we are not atomic obejcts; the lengths over which this happens are controlled by Plancks constant which is around 0.000000000000000000000000000000000000626 – so it is TINY. If it were, say, 1, then we would see these wierd effects in the everyday world.

Sive mentions some wierdness that also pops out. QM says that until you observe and object it actually exists in a combination of all possible states (the cat is alive and dead if you put it, inaccurately, into words) and when you make a measurement you select one of those states and that is what you observe. So the electron is in both a wave and a particle state and your experiment then selects one of those states. The Cat thing was a vain attempt to try to explore this wierdness but the important point to remember is that the wierdness is really only there because we try to put OUR classical, large world, ideas and language onto something that has no link to our classical world. The mathematics in QM is correct and describes the world as we observe it and has no wierdness. It is our attempt to describe what the maths says that produce this wierdness.

some other stuff QM does for you:
1. you can create two light photons in a given state and allow them to separate as far as you like. measuring the properties of one photon gives you the information you need about the second – even if the second one is so far away that it cannot be probed (due to the finite speed of light). People find this wierd.
2. You can use the properties of electrons to do computing – instead of an on or off switch, you can have a combination of on and off, alllowing twice as many operations to be performed. Quantum computing
3.Hot on the heels and 1 and 2 is cryptography where the quantum states of photons can be used to encrypt a message
4. The MRI machine, the PET scanner, the CAT scanner, the ECG/EEG all use the quantum behaviour of electrons/positrons and nuclei in the medical field.

A final consequence is that act of observing something actually disturbs it. As I mention above, and the bit the Sive said did her head in, an object exists in a combination of possible states until you observe it. Well observing something is in fact not passive – you use photons of light reflected off the object, for example, to see it and if it is small, like an electron, and the photons are fast, then you actually do disturb the system quite a lot. The point that needs to be remembered is that the wierdness really only comes into play at the level of electrons and at our length scale it disappears.

There is nothing wrong with being confused – many physicists including nobel prize winners, are confused, as am I and it has been said that if you think you understand QM, then you dont understand it at all.

hope this helps you see this crazy QM world!

M