Here at RTU we (mainly Joe) has written a lot about the interesting behaviour of the Quantum world. To play around in this world you need to follow a set of basic yet counter-intuitive rules about the way things work on the smallest of scales. Today I thought I would quickly present, or recap for some of our readers, the principles that govern the workings in this realm of the tiny.

**Law 1: Uncertainty**

In the macroscopic world, if you know the initial conditions of an object and you subject it to a force you can calculate its position at a later time. Let me give you can example, we have a plane which is currently over Canada and it is flying with a fixed velocity. Knowing this we will therefore be able to determine its position after a certain duration of time. However the case is not so straight forward when it comes to subatomic particles. In this world knowledge about one thing means a trade off with another. If we know the position of a particle accurately then it means we do not know its momentum (which to recap, is its mass times its velocity) well at all. Basically if we can pinpoint where an object is, we know very little about its mass or its velocity – seems counter-intuitive doesn’t it?! Welcome to the world of quantum.

The uncertainty that dominates this world is aptly named as the ‘quantum uncertainty principle’ and it is not limited to position and momentum. Uncertainty acts on a multitude of other quantities, for example energy and time. The more refined your measurement of the energy of a quantum particle the fuzzier your grip on the time duration in question. For example, in the classical world if a ball is trying to get over a wall it needs enough energy to do so, it needs enough energy to reach the height required to pass over the wall. *But *in the quantum world a particle which doesn’t classically have the energy to pass a barrier can ‘tunnel’ through, appearing on the other side given enough uncertainty in the time taken to do so. It is this phenomena that enables particle pairs to be produced out of a vacuum as we saw in Black Holes: Glowing and Shrinking post. So there’s law number 1, we basically can’t know much with certainty.

**Law 2: Quanta**

In the classical world things seem continuous. Take energy for example, before quantum theory, electromagnetic energy was though to be emitted continuously from a source. When looking at a light source it seems to be flowing out smoothly in all directions. However through the joint efforts of Planck and Einstein it was discovered that energy was instead emitted in individual packets called ‘quanta’. (See the photoelectric effect experiment!) Quanta is a generic term for discrete particles. In the case of electromagnetic radiation instead of energy coming out in a continuous flow it is emitted in quanta called photons. Quantisation is a fundamental aspect of the quantum world and applies to many more things than just energy.

Quantisation is believed to extend right down to the fundamental level and a quanta exists for every type of field. If you read my previous post ‘What is a field?’ you’ll remember that what we call space in everyday life is actually the gravitational field. Therefore according to Quantum Theory even space* itself *is quantised. In a nutshell, space is granular – if you analysed a region of space it would not be infinitely divisible, eventually one would get right down to the tiny granular points that make up the fabric of spacetime.

Any its not just quanta of fields that come in these little bite-sized chunks but other quantities that crop up in Quantum Theory such spin and charge – they all come with a minimum unit size. You can’t *keep *reducing the value of these properties, you eventually hit the minimum boundary.

**Law 3: Duality**

Now after Einstein realised that light came in discrete little chunks known as photons our understanding was thrown under the bus. Previous experiments had shown us that light very much acts like a wave – it can interfere with itself creating areas of constructive interference and destructive interference. (To understand this think of a water wave analogy, if the peak of two waves coincide perfectly it creates a bigger wave, but if the trough of a wave hits another’s peak they somewhat cancel out). This phenomena was seen with light, even though it was understood to be quantised into little chunks. Is that necessarily contradictory you may say, can’t all the particles act together to cancel each other out in certain areas and visa versa? Well the real spooky fact is that this wave-like behaviour even happened when we fired just out *one *photon/quanta at a time! To explain this it would almost have to be as though the single photon was interfering with itself!

Louis de Broglie came along and proposed ‘wave-particle duality’ quite simply that quantum objects can act as both particles and waves for they exist in a ‘wave-like superposition’ of all possible states. This leads us onto our next ‘pseudo’ law – superposition.

**Law 4: Superposition**

In the classical world things exist without doubt in one state – how confusing interactions with people would be if not! However, in the quantum world objects exist in a superposition of multiple states which is represented by a ‘wavefunction’. The wave function encodes all the different states the object could be in along with the associated probabilities. For example a photon could be in position A or position B and the wavefunction includes both these possibilities including the information of whether one is more likely than the other.

Whether particles* actually* exist in this plethora of states or whether this is just how we, from our macroscopic perspective, understand it is a question for philosophers of physics. However when we perform a *measurement *of a quantum object we can then pinpoint which of these multiple states it is. Read on for what is probably the weirdest law yet…

**Law 5: Measurement**

In the classical world if somebody told us our act of observation would have an effect on the outcome of an experiment we would think it very self-righteous! If a ball is fired from point A whether it hits point B or C does not depend on whether anyone is watching, it depends on its initial velocity alone – unless one believes they are telekinetic. However in the quantum world the observer* is *a crucial element in determining an outcome.

Take the example of a photon being fired a screen. (The full Double-Slit experiment is explained in full in my very first post *here* on RTU). If nobody monitors the path of the photons a wavelike pattern builds up on the screen. However if an observer actively measures the path the photon takes a different pattern of lines builds up on the screen. Our act of observation actively causes a change in the outcome of the experiment! The general belief in Quantum Theory is that our act of measurement causes the particle to the collapse from its superposition of multiple states to one finite position. The observer in quantum mechanics assumes a leading role.

Let me add a caveat here to take away some of the supernatural nature that currently seems to surround the observer. (A word of thanks to lwbut for explaining this so nicely in the comments below). Because quantum particles are so* tiny* any form of interference from us on the macroscopic scale can alter their behaviour. When we observe these particles, because we cannot do it solely with our human eye we must use some form of equipment to capture information about the particle. Whatever method we use, be it a beam of light to whatever will affect the particle in some way, which it would not have to endure if the particle allowed to move independent of said observation.

So there we have the five principles of the quantum world uncertainty, quantisation, duality, superposition and measurement – all counter-intuitive to our everyday experiences yet strong and sturdy laws when playing around in the realm of the tiny.