Monday, May 27, 2013

Quantum Mechanics...For Cyclists

Most cyclists have a pretty good understanding about mechanics – they are those grimy people (mostly men but also occasionally a lady) hidden away at the back of the bike shop trying to do their job (fixing your bike) whilst being continuously hassled by everyone who walks in. They particularly enjoy jobs that involve finding ‘random creaking noises’ and being interrupted.

Bicycle Mechanics – just trying to do their job.

Despite this, most cyclists do not have a very good understanding of Quantum Mechanics, probably because rather than working on their theoretical physics PhD they spend more time uploading to Strava…

…and studying the Weight Weenie forums…

Interestingly, it only takes “5 hours of shaving” to make your own 7g aluminium bidon cage.

So, why is there such a huge gap between the relatively simple idea of mechanics and the rather mind-bogglingly esoteric concepts of Quantum Mechanics? Quantum, by definition, just means a discrete amount of something (as opposed to an analogue or varying amount of something), so Quantum Mechanics is basically just a discrete amount of mechanics, right? Well, not really.

Some of the pioneers of Quantum Mechanics include Planck, Bohr, Heisenberg, de Broglie, Einstein, Schrödinger, Dirac, Fermi, Pauli, Bose & Feynman - just to name a few. You might recognise a few names here, but then again, you might not. And that’s fine because these 20th century physicists probably wouldn’t recognise the names of any cyclists from their era, mainly because the Internet hadn’t been invented yet making communication between people almost impossible.

Quantum Mechanics is a branch of theoretical physics (stay with me here) that was formulated in order to explain the behaviour and interaction of very, very small particles (usually sub-atomic particles such as electrons, photons, quarks and gluons) that cannot be explained with traditional, or Classical Physics. This differs from cycling, where all of the relevant things like aerodynamics, material properties (including strength and stiffness), rolling resistance and power output can be explained with plain, old, regular physics. However, underlying all of these principles are the quantum interactions between the tiny particles that make up the slightly less tiny particles that make up the stuff that make up cyclists and bicycles and explain how energy is transmitted between them.

The scale that these quantum interactions occur on is very small, so small in fact that they can be difficult to even imagine.

For example, one of the smallest professional cyclists is Frenchman Samuel Dumoulin who measures up at 159cm and 56kg. Yet on the quantum scale he would be considered extremely large. 

Each of Sammy’s bidons contains approximately 500ml of water. We (and most likely WADA) don’t know if there’s anything else in there besides water, but for the sake of simplicity let’s give him the benefit of the doubt. 

500ml of water contains approximately 3.3428x1025 hydrogen atoms. Now this is a big number. Obviously it has been written here in scientific notation to make things simpler but if you write out the entire number it would be:
          33,428,000,000,000,000,000,000,000 hydrogen atoms per bidon (I hope I got that right!)

Hydrogen, being the simplest atomic element, consists of a single electron orbiting a single proton (of course in the molecular form there are interactions between the different atoms but we won’t go in to that now). The diameter of each hydrogen atom is about 10,000,000,000 times more diminutive than Sammy Dumoulin and the hydrogen atom nucleus is approximately 100,000 times smaller than this (an incredible 1,000,000,000,000,000 times smaller than Sammy Dumoulin). And it is on this scale that quantum interactions occur.

The scale of quantum interactions is best related with a familiar reference point.

It is interesting to note that approximately 99.9999999999999% of each atom is nothing but empty space. Quantum Mechanics helps to explain how billions and billions of interactions between particles, which are composed almost completely of nothing, are required to form things with a tangible mass – even a very minor a 7g bidon cage.

Much of Quantum Mechanics involves finding the probability that a tiny particle will be located somewhere rather than describing its actual path (for example, the position of an electron orbiting a nucleus) and because it is so abstract, in its formative years, many of the fundamental principles were discovered using thought experiments because no actual measurable physical experiment could be performed. 

Thought experiments are like philosophical logic riddles for very, very smart people and one of the most famous is known as Schrödinger’s Cat, which was used to try and refute some of the probabilistic foundation principles of Quantum Mechanics. Basically, it states that if quantum theory holds true then a cat in a sealed box would be both alive and dead simultaneously until such time as the box is opened and the state of the cat observed.

This might all seem pretty absurd, and it is, and I’m not exactly sure why a cat was necessary (why couldn’t he just have used a slug or a cockroach?) but it does highlight some of the complex philosophical questions that the field of Quantum Mechanics raises. It is, perhaps best summed up by God, in this Tweet:

We can take some comfort in the fact that the simpler laws of classical physics still remain accurate for predicting the behaviour of the vast majority of large objects on the order of the size of large molecules or bigger (which includes all cyclists, even Sammy Dumoulin) at velocities much smaller than the velocity of light (which is around 20 million times the speed of Fabian Cancellara over a 50km time trial).

As well as probability functions, Quantum Mechanics also uses the concept of wave-particle duality to help describe the behaviour of small particles. The famous Double-Slit Experiment was used to illustrate an incredible phenomenon of photons (the particles that make up light) and since then other small objects.

The Double-Slit Experiment showed that photons could act as either discrete particles (left) or as an interfering wave (right).

In the experiment, photons are randomly directed at a screen via two narrow slits with a 50% chance of passing through either one. They produce a wave-like interference pattern on the screen (even if individual photons are sent through the slits one at a time) but only if it is not known via which path each photon travelled. 

When a detector is incorporated that determines which path (slit) each photon emanated from then the interference pattern disappears and the photons act like discrete particles, independent of each other and with no wave-like properties.

In some ways this is analogous to a professional cyclist, who can be doped up to their eyeballs until becoming aware that they will be tested at an upcoming race, at which point they can immediately (and quite magically) appear clean

This experiment shows that the observer plays a role in determining the reality of a quantum event.

The impressively titled Delayed Choice Quantum Eraser experiment takes this even further and demonstrates the phenomenon of quantum entanglement. In this experiment, individual photons are sent through a conventional double-slit apparatus before being split in to an entangled pair of photons (A & B) that share certain characteristics. Photon A is sent directly to a detector screen and there is no observation made about which slit it came from.

The Delayed Choice Quantum Eraser experiment - elegant and powerful.

Photon B is sent down one of two different paths depending on which slit it came from before passing through a beam splitter that gives it a 50% chance of being detected in a way where its path is known and a 50% chance of this being unknown.

The key part of this experiment is that Photon A is sent directly to a screen (with no information about where it came from) while Photon B takes a much longer and slower route before it is detected (with a 50% chance of its path being known).

When scientists looked at the A Photons whose entangled B Photons had an unknown path it was found that an interference pattern was exhibited on the screen indicating wave-like characteristics. When they looked at A photons whose entangled B photons had a known path (it was known which slit they came from) then the interference pattern disappeared.

The incredible phenomenon exhibited here is that the A Photons reached the screen before it was decided if their entangled B partners had a known or unknown path. So how did the A photons know which way to behave?

This is a very confusing concept that kept scientists busy for many years (so it is likely to keep cyclists puzzled for centuries, if not eternity). Einstein called it “spooky action at a distance” as it appears that information about the state of each entangled particle is transmitted instantaneously between them when one is observed – even when they are separated by large distances.

Obviously, these concepts start to get very involved, and needless to say the mathematics underpinning it is not for the faint-hearted. Unfortunately, unless you are willing to dedicate your life to studying these principles, it is difficult to garner a broad understanding. I already feel like a breather, even after this brief taster of just a few of the most fundamental concepts of Quantum Mechanics.

Although quite overwhelming, the key thing to take from all of this is that the principles of Quantum Mechanics are so fundamental that they can, in theory, be used to explain everything in the universe…except for the presence of a rear shock on this bike:

1 comment:

  1. Would that bike count as a "mongrel" for the purposes of classification at the Dirty Gran Fondo? Would it need to be observed first?