Saturday, March 31, 2012

Applying quantum physical weirdness to the 'real world'

Ever since we have ventured into the subatomic world, the playground of quantum mechanics, we have found that particles behave in extremely weird ways. It is as if the classical view of the world does not apply to the world of quantum mechanics, and experimental results have repeatedly challenged our knowledge about reality. For example, we have discovered that the smallest things we can detect can behave as individual particles and as waves at the same time. Recently, scientists even proved that things can come into existence from what appears to be the nothingness we have come to know as vacuum. While the rules of quantum mechanics are almost impossible to imagine, because they are so radically different from what we experience on a day to day basis, we must accept them because they have been tried and tested. And even though we sometimes like to think that this weirdness only happens on very small scales, a few scientific groups have shown that it also applies to larger structures.

Double slit experiment
Perhaps the most famous experiment showing quantum mechanic weirdness is the double slit experiment. The experiment is set up by a beam of light that is fired at a 'wall' with two small openings. Behind this wall, there is a light-sensitive screen. Because light behaves like a wave, it creates a so-called interference pattern if the waves pass through the slits and hit the screen. Waves can reinforce each other or cancel each other out when they 'hit' each other: it depends on whether their 'phases' are in sync, as is shown on the picture below. At some point in time and space, they will reinforce each other, while at a different point they will cancel each other out. This creates a typical interference pattern, with alternating patches of dark and light, on the screen.
Sync: both waves reach peaks and troughs at the same time.
Not in sync: the top wave reaches its peak while the bottom peak reaches its trough, and vice versa.
Quantum weirdness occurs when scientists shoot a 'beam' of light that is so faint that individual particles of light, called photons, pass through the slits and hit the screen. Because there are no other photons around to interact with, one would expect the interference pattern to disappear: there should simply be one photon passing through one of the slits and hit the screen. Surprisingly, observations point out that there is an interference pattern. It is as if the individual photon goes through both slits at the same time, interacts with itself and creates an interference pattern.
S1: a light source showing waves shot at a screen at S2. The light passes through the slits, whereafter the waves get a chance to interfere with each other. The result is an interference pattern with alternating patches of light and dark on the screen at F, because waves either reinforce each other or cancel each other out.
It becomes even weirder when scientists attempt to catch the photon while it is moving through the slits. If they place a particle detector at one of the slits (at b or c in the picture above), the interference pattern disappears and it simply shows a pattern that is to be expected from individual particles moving through slits. It is as if our own measurement interferes with the outcome of the experiment. All in all, this has taught us two things, the first being wave-particle duality: things can behave both as particles and waves at the same time, but this disappears when you try to measure it.

That brings us to the second point, which is the uncertainty principle, as formulated by the famous Werner Heisenberg in 1927. It tells us that there is a limit to the certainty by which we can measure things. To put it simple, the more precisely one property is measured, the less precisely a second property can be determined. This becomes especially relevant at the small scale of quantum mechanics. It also tells us that the properties of a given particle are uncertain, until they are measured. To put it simple, if an electron can acquire the values 0 or 1, it will be both until one of the values is actually measured. This state is called superposition, and forms the basis for quantum computing. It is radically different from our classical view on the world: we do not see things in some sort of limbo state with undetermined properties, until we measure them.

Many worlds
The famous physicist Richard Feynman devised a mathematical tool to predict the quantum behaviour of these individual particles. It is often referred to as 'sum over histories', and it calculates the most probable path a particle took to get from point A to point B. It works by calculating all possible paths a particle can travel, even if that includes going to the other end of the universe and coming back. While that sounds ludicrous, his ideas made calculating the probable location of a quantum particle possible, with the help of a few mathematical tricks. And we can only get as far as calculating the most probable location: quantum physics does not work with certainties, merely with probabilities. Experiments have proven that calculating the sum over all the possible histories of a particle actually leads to solid predictions, which means we must accept the theory, even if it is unimaginable: at least it helps us to predict reality.
The classical view: in a straight line from point A to point B.
A few of the paths a particle might take in the quantum physics view. In reality, of course, the number of lines is infinite, and we need special tools to calculate its most probable location.
It is thought that particles do not simply have one history, when moving from the light source to the screen, but instead have all possible histories: that means they did not simply take one of the possible paths, but instead took all of them before reaching the screen. While again sounding ludicrous, this view helps us to understand quantum mechanical behaviour as observed in experiments. It has lead to the so-called many-worlds interpretation, arguing that all those possible histories we can think of or calculate, are real. As a consequence, that means for every quantum effect a parallel 'world' branches off.  It leaves us with an infinite amount of universes, wherein every possible event has happened, and will continue to happen. This, of course, cannot be proven, and it seems far-fetched, but quantum mechanics has taught us not to exclude anything. When trying to explain quantum rules, it is best to not try and understand the world in a classical way, but rather see the theories as tools that can be used to predict reality.

Large scale
Now, this double slit experiment has been tried and tested for many years and in many different shapes, and we simply have to accept that there is something called a wave-particle duality and the associated weirdness it implies on reality. We also have to accept that we cannot measure things with absolute certainty at such small levels. We could comfort ourselves by saying that such weirdness does not occur when things get bigger, but it seems scientists have been wrong to assume that. Experiments from a group of scientists from different labs point out the same thing happens at larger scales. They set up a variant of the double slit experiment and found wave-particle duality with rather large molecules.
The largest molecule that was used in the study.
Scientists have already shown that even relatively big things such as molecules show interference patterns. It is however hard to create something that can easily be observed, because of the small scale at which it all takes place. The aforementioned group of scientists, working at labs in Austria, Germany, Switzerland and Israel, shot large fluorescent molecules at a screen, and found the same, typical interference pattern. Using fluorescence microscopy, the researchers were able to determine the position of individual molecules, as they were launched onto the screen. It gives us unprecedented detail about how a wave-like interference pattern is created with individual molecules, and it is a prime example of wave-particle duality.
An interference pattern being built. a: 2 mins. b: 20 mins. c: 40 mins. d: 90 mins. e:  after the experiment. The typical dark-light bands are clearly visible.
According to the scientists, their version of the double slit experiment allows for more precise measurements. By using big fluorescent molecules, the individual particles are much more traceable than photons or electrons, which are unimaginably small. It also gives physical proof of quantum physics in the 'real world', as fluorescence molecules are things we, with a little help from a microscope, can see. The experiment gave a nice view on how a wave-particle interference pattern is built up, which we can use to further study this phenomenon. Additionally, we may try to find the boundaries between the classical and the quantum world by experimenting with larger fluorescent molecules.

Wave-particle duality is only one of the many weird rules that quantum physics have taught us. Another prime example is quantum entanglement, where two particles appear to get a bond for life, even if you move one of them to the other side of the world. If you measure the properties of one of the particles, the other one immediately assumes the opposite form, wherever it is. Therefore it seems that there is some sort of instant teleportation of 'data' taking place, as there is no physical connection.

Quantum physics has been shaking the foundations of reality ever since its inception in the twenties of the last century. By continuing our experiments, for example in the search of the famous Higgs boson or recreating the Big Bang, it is likely that quantum physics will continue to marvel us and challenge the foundations of reality. 

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