Question: What will happen if nobody believes with quantum physics ? I mean if all scientist use classical physics to identify a phenomena.

Jack Miller answered on 22 Jun 2013:

Hi Gemabaskara13,

Great question! In short, we’d have a picture of the world similar to that before the invention of quantum mechanics.

This means that, in many situations, we’d either be unable to explain what was going on, or making predictions that were provably wrong.

We wouldn’t understand nuclear decay at all, we wouldn’t understand the structure of matter, and we wouldn’t be able to make lots of devices such as digital cameras or printed circuit boards. More puzzling, though, would be a large number of contradictions between what we think should happen, and what various experiments told us.

There are several classic examples of this. The first is the fact that the Bohr model of the atom — with a positive nucleus at the centre and electrons orbiting — only makes sense if the the electrons have discrete energy levels or ‘orbitals’ that they can sit in, otherwise the slightest perturbation would cause them to fly into the proton’s nucleus, or escape from the atom. Additionally, if you look at the spectrum of light from pretty much anything, you find that it isn’t continuous — it’s discrete, and made up of lots of different levels (this is why street lamps are orange — they’ve got sodium in them, which has two particularly bright levels with energies that correspond to yellowy orange, and when you electrically excite them you cause transitions from those levels). This lead to the term ‘quantum theory’, as the energy levels are ‘quantised’.

Secondly, if this wasn’t the case, if electrons could have continuous energy in atoms, you’d also find out that hot lumps of metal (‘black bodies’) radiated infinite power in the high energy (ultraviolet) end of the spectrum. This lead to the ‘ultraviolet catastrophe’ of classical physics.

Thirdly, there’s a good experiment called the ‘Stern-Gerlach’ experiment that shows that electrons have spin, and that spin can either be up or down. There’s an excellent video explaining it, the classical and quantum predictions, and the experimental result, on wikipedia: http://en.wikipedia.org/wiki/Stern-Gerlach is the page with a detailed explanation, and http://en.wikipedia.org/wiki/File:Quantum_spin_and_the_Stern-Gerlach_experiment.ogv is the video.

Fourthly, and finally for my fingers (and your eyes) are probably getting sore: there’s the double split experiment. If you fire a beam of electrons at two closely spaced slits, and the slits are spaced closely enough, you observe the same pattern of bright and dark fringes on the other side of the slits as you would expect to see for light of the same wavelength as the electrons. This isn’t what you’d expect classically — electrons are particles, so you’d expect to see the distribution you’d get from firing a machine gun at the two slits, namely two black bands.

Interestingly enough, if you fire one electron at a time through the slits, and keep track of where they end up, you still see the distribution of fringes I described. If, however, you add on a device that measures _exactly which one_ of the two slits the electron went through (and they do exist), the pattern goes away — you end up with two black bands. This is probably the best proof that quantum mechanics works, and that measurement does ‘collapse the wavefunction’, or have an effect on your experiments.

Ain’t small things weird?

Hope this helps,

— Jack