Laws of Quantum

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.

 

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25 responses to “Laws of Quantum

  1. Thanks for such a concise and useful summary Mekhi. Is ‘entanglement’ another principle or part of superposition, and am I correct in thinking its ‘spooky action’ has supportive experimental data? (Maybe I missed an earlier post on this?)

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      • Ah yes, thanks for the reminder as it was a follow-up on SHM, about which I’d done an intro in early 2012 when referring to the theory of cycles (soon after starting my blog).

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  2. Since I am confused about this stuff perhaps I am permitted to ask stupid questions. If space is granular, how far apart are the grains and what is between them? And can that be variable? Is there something that might be squeezed space and is space denser in places?

    If a single quanta of light behaves differently whether or not it is observed, would one person who observes it see something different from somebody who wasn’t looking but examines the results later? And what if a horsefly was looking instead of a physicist?

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    • I apologize for the obvious idiocy of my questions but they are nevertheless real in intent. My understanding of how false our fabrication of reality is since it is manufactured from highly limited abstractions produced by our crude sense apparatus and further shorn by a nervous system whose fundamental function is survival is worrying. It seems to me that a physical universe that can be modified merely by glancing at results is on the level of a comic book whether or not it has a firm basis in physics. Our current civilization is nutty enough without physics falling off the edge of some kind of graspable sense. I have the feeling this life was invented by Lewis Carroll after over indulging in the bottle.

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    • Hi Jii,

      While i make no claim to being an expert i believe i understand enough to give you rudimentary answers that might meet with some satisfaction here.

      Regarding quantised space: a quanta of space would be the smallest possible sub-division of space – there could be nothing measurable less than the quanta itself so there would be ‘nothing’ between them: they fill space like tiny little equal-sided cubes touching in 3D. They would be incompressible since if they were compressed they would be smaller than a single quanta which is, by definition, impossible. Similarly if you were able to decompress them you could only do so a single quanta at a time leaving a gap exactly one quanta in size between any two which is then just another quanta sized space which is measured as another quanta. You can’t have half (or any fraction) quanta sized gaps or fractionally sized quanta. it’s either all or nothing.

      As for the observation, it is not the form of the observer that affects the observation/particle – it is the method used. Clearly, no human can directly measure the movement of photons, for example, with their naked eye alone, neither could a housefly. Some form of equipment is needed to capture and translate the information being gathered about the particle being observed and whatever electromagnetic or other types of fields that are used to directly measure the particle are what affect the particle in some form or another in a way they would not do were the particle allowed to move independent of said observation.

      Humans and houseflies can of course detect photons with their eyes although i’m not sure if they are sensitive enough to detect a single one at a time but the action of detecting them causes them to no longer be ‘independent’, freely moving through spacetime photons and they become transformed into another form as they are so detected by the atoms making up whatever ‘eye’ is being used at the time.

      One person may well see something different from another but that would be down to the method of observation being used, the energy /frequency of the photon being observed and the differences introduced via different timescales/points in time and not because of intrinsic differences in human/animal physicist observers.

      Finally, these effects are far more noticable way down at the sub-atomic level rather than at our vastly larger scales of everyday observation in the same way that we might more easily radically alter the movement/trajectory of a single water molecule floating in air than a 100 kg block of ice travelling at 200 km/h falling on us from the sky. 🙂

      Hope that make things a little simpler?

      love.

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      • Jiisand i’m sorry for not getting round to replying earlier things have been a bit hectic. But it seems lwbut has written an wonderful response – so thank you. I’ve added a caveat in my post about the observer as I realised it really was a bit mysterious before. So thank you both for that! p.s. Always remember no questions are stupid!

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  3. Thank you, easily one of the best explanations for the lay person. Two things always strike me about quantum science. Firstly no wonder there are so many different explanatory theories and secondly it almost makes parapsychology seem mainstream ! Fascinating, the effort you and Joseph make to bring us this kind of information is greatly appreciated. Nigel

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    • Thank you very much Nigel, making exciting ideas in science accessible to everyone is one of our main aims so this comment means a lot. And doesn’t it just! Enjoy the rest of your day, Mekhi

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  4. What a confusing realm to exist in – at least the first one I get and the last one kind of implies we could manipulate the larger world by interfering with the quantum by observing it.

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  5. Maybe this ‘other’ world is actually very spiritual in nature. Maybe we really are co-creators, but have been taught in our world that we are not. Maybe we need to open our minds to the possibilities. Thank you for the clarity.

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  6. Let me promote you to a philosopher of physics. You encapsulated the quantum theory into a handful of almost understandable paragraphs.
    If light can be considered as a wave or a proton does it follow all electromagnet radiation has a dual personality? My difficulty is that a wave is a travelling disturbance or if you like virtually nothing concrete but a photon is something concrete that moves. When waves disturb space they can cancel out by the trough and peak mechanism ; does star light from multiple sources so cancel?

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    • Hi kertsen, 🙂
      You’re right! Light (visible to humans) is a small part of a much larger spectrum of electromagnetic waves and the whole spectrum has the particle/wave duality.

      When you say a photon is something concrete: a) how do you know? 😉 and b) since photons have no actual mass how concrete could they be? And if they do have no mass how exactly is it that they can make the vanes on a Crookes radiometer move back away from them when they are absorbed by the vanes??

      Be careful when you say waves cancel out because although two waves may result in a zero net movement at a point where both a peak and a trough from two waves of the same amplitude coincide, both waves continue on unaffected as the peak and trough pass each other. The waves don’t actually disappear when they ‘cancel ‘ out! Which is fortunate for us as the light from many stars might not be visible to us if their light was cancelled by the opposing amplitude of a light from another star whose light crossed the path of the first.

      A visible form of this is the reflected waves from a sea wall – you can see the incoming waves pass through the reflected waves and vice versa even though at many points they ‘cancel each other out’. 🙂

      love.

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      • Yes it’s all a bit of a mystery however you chose to look at it , but then most things when you boil them down seem to dissolve into nothing.

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