Quantum physics is a bizarre but widely accepted theory which states too much to be described in a single sentence. It all started with such ideas as wave-particle duality, Schrödinger's cat and the existence of photons. At first only a few radical scientists followed the theory, while most others felt it was just impossible, but as more and more experiments showed it might actually be true, more and more scientists started to pay attention to it.

History

It all started when a scientist called Max Planck suggested that energy comes in little packets called "quanta" - after the latin for "amount". The quantum was to energy what the atom was supposed to be to matter - unsplittable, indivisable, and the smallest amount of energy you can ever get. This upset the physics world quite badly. It also upset Planck quite badly, and he resisted this theory until he died - his theory had originally been created to explain an experiment he carried out, but he kept trying to introduce alternative explainations.

What's so bad about quanta?

Well let's focus on light quanta, or photons. If you produce a single photon and fire it at a semi-silvered mirror (ie a beam splitter) then which way does it go? You cannot pre-determine this, since you cannot have half a photon passing through like you can have half a beam of light. This event cannot be predicted; it is random.

This obviously was a blow to physicists, because under classical physics they'd been able to predict more or less what they wanted. But now, all this randomness was just going to confuse them. Thus was born quantum theory.

Implications of improbability

One of the most daunting attributes of quantum theory to a classical physicist is the fact that nothing is real until you try to measure it. For example, when you shot that photon at the beam splitter, the photon went one way or the other, but until you check which way, you don't know. This is pretty obvious, but the point is that in classical physics, everything is predictable, and given the all the original information about where the light is, and which direction it is moving in, you could predict how much light would go one way and how much the other (because in classical physics light is a wave, and can always be split in half). But, in quantum physics, some things are just random and unpredictable, no matter how much you know about the original situation. The only way scientists have of describing this is that the photon 'splits' into two semi-photons - not actual pieces of energy, but the possibility of them - such that each 'ghostly' photon has a 50/50 chance of being detected as a photon.

Two types of information

The information about photons - and, indeed anything - which is clear-cut - that is, the information which has already been measured and has a fixed value, is known as the "classical" information. For example, if you know that a photon's colour is blue, it is classical information. The information which has yet to be measured, and so is therefore a combination of all possibilities for that property, is "quantum" information - but this is fragile stuff. Measuring what the info is destroys the information - for example, before you discovered what the colour of the aforementioned photon was, it was a combination of all possibilities, from any part of the spectrum. Measuring it forced it to commit to a specific colour and so it lost all the others - and up until the point you measured it, it had no specific colour.

The EPR conundrum

Albert Einstein himself could not and would not get to grips with quantum theory because he believed in cause and effect - that, given all the information about an object, you could predict what it would do with complete accuracy.

So he and a few of his friends came up with this experiment to try to disprove quantum theory. Ironically, it actually helped to reinforce it.

Entanglement

Suppose you have 2 photons which you know are polarised at 90 degrees to each other. You don't know exactly what the polarisations are, but you do know that they are perpendicular. Thus they both have a sort of ghostly state of polarisation which covers every possible angle, similar to the ghostly photons discussed earlier. You send these photons off in opposite directions, and into the path of 2 polarising filters (which let through only verically-polarized photons) you placed there beforehand.

When one photon reaches its filter the other is just about to reach its filter. If the first one has gone through, the second one will be blocked, as it is horizontally polarised. But until the first one reaches the filter, neither you, nor either of the 2 photons, can know whether it will pass through. And so when the first one does pass through, the second must get a message from the first to set it's polarisation accordingly, even though the 2 photons are physically very far apart. Because of this communication, they are said to be an "entangled" pair of photons. However, what if the time between one photon reaching its filter and the other photon reaching its filter, is too small to allow something travelling at the speed of light to carry the message from the first photon to the second in time?

This means, Einstein and company concluded, you have broken the speed of light, and thus quantum theory is impossible. What Einstein proposed instead was that each photon, rather than conspiring with each other, had some undefined property that, if you could measure it, would tell you what the outcome of a particular polarisation measurement might be.

But, the only way to find this property would be to measure each photon's polarisation, which would be a bit pointless in an experiment to help you predict a photon's polarisation.

Today's majority belief is that Einstein did not prove that Quantum Theory was invalid due to its violation of the theory of relativity (that says nothing can go faster than light), but in fact quantum theory can in fact break the speed of light. However, it is disputable, whether or not anything intelligable can be sent faster than light, due to all the randomness brought along with quantum theory.

The Future

So far, quantum theory remains just that - a theory, because there is still too much controversy and hypotheses and not enough evidence to prove or disprove it. But, the world is advancing technologically and scientifically faster than it has ever seen before. Perhaps soon, quantum theory will be a lot simpler. On the other hand, while many find this theory interesting, noone has yet found a real-world application of it.