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Quantum in Review

 A short overview of quantum physics

Astronomy/Quantum Physics
Until recently, quantum physics was a totally arcane and exotic form of physics only known within the high-brow circles of academia and to those who studied both the macro and micro worlds.

Even today, champions of such say the theory is almost unknowable, or so exotic that it can’t be easy grasped or explained in just one sentence.

Background

In the old days of physics and astronomy, the old philosophers and observers such as Kepler, Ptolemy, Copernicus, and then Newton started to put their minds to how and why objects moved in the sky and what the forces might be which seemed to be controlling them.

In the Middle Ages of course, to the theologians and thanks to Catholic dogma, God -  and ‘man in His own image’ - lay at the centre of universe, and everything circled around such.

They believed that there was an innate order of things in which the planets and the Moon (and moons) – moved around the earth on invisible fixed orbits, against a background of the stars – or the Primum Mobile as they used to call it.

Newton realised there was a force involved and called it gravity and developed his laws of motion to describe not only how the planets moved in the sky (and seemingly remained suspended there), but also how things moved on earth among a myriad of other theories. He’s probably rightly described as being the father of modern science.
Einstein came up with two theories of relativity.

In the early 1900s– both the earlier Special and later General Theory – covered a wide range of subjects principally covering the constancy of the speed of light (in a vacuum); how everything is effectively relative to this; how to moving observers, (and that applies to all of us and every planet when you think about it) a given event can only appear to be relative; and the relationship between energy and mass; (they are interchangeable but crucially and ‘magically’ [my words] linked to the square of the speed of light).

His famous realisation of e=mc2 led physicists to understand the high energies or potential involved in a tiny amount of mass – hence nuclear reactors and the atom bomb development.

(Einstein famously wrote to President Roosevelt warning him of the potential of his famous equation – which came prophetically true).

But perhaps his greatest concept, in his second General Theory, was that of space-time. In this he took Newton’s studies of gravity and successfully amended them to incorporate them with his earlier Special Theory of Relativity.

Aside from the three dimensions of space we know, he added a ‘fourth’ dimension, called space-time in which gravity, light, time and the three physical properties of space were seemingly inter linked.

Until the beginning of the 20th century, time was believed to be independent of motion, progressing at a fixed rate in all reference frames; however, later experiments revealed that time slowed down at higher speeds of the reference frame relative to another reference frame (with such slowing called "time dilation" explained in the theory of "special relativity" ).

Many experiments have confirmed time dilation, such as atomic clocks on board a Space Shuttle running slower than synchronized Earth-bound inertial clocks.

The duration of time can therefore vary for various events and various reference frames.

The classic example of this is the so called twin paradox.

If you happen to have a moped that can travel at the speed of light, and you are sat with your friend in the city plaza having a coffee, imagine you run out of sugar.

Your pal, who happens to know the best sugar in the universe [say] lies on Proxima Centauri, says “Hang on, I’ll just nip and get some”.

Now Alpha Centauri is four light years away. So say your pal speeds on his hyper cycle, travelling at the speed of light. It takes him four year to get there and four years to get back.

Measuring the time he is gone on HIS watch, he has been gone eight years.

But on his return to the city plaza, not only has his waiting coffee gone cold, but you have turned to dust and perhaps a 100 years have passed in ‘earth time’, even though your pal seemed only to be gone for eight years.

These are just some of the strange effects of time dilation when objects approach the speed of light which Einstein predicted.

Einstein also predicted that heavy objects in space should actually ‘warp’ the fabric of space through their sheer mass and gravitational effect.  And not only that, but a distortion of time would be involved for observers of such.

As far-fetched as this may seem, he was proved to be correct in a famous experiment conducted later when astronomers and physicists observed the sun during an eclipse.

Einstein predicted that the best way to test the idea of the warping of space-time by heavy objects would be if we could observe the light coming from a star which lies beyond some huge beyond huge mass was warping the fabric of space-time.

A bit like the effect of dipping a spoon in water (which appears bent from certain angles), the light from the star beyond the mass (the sun say), he said, should be bent or distorted by the huge space-time gravity well in which the sun sits - and thus the star should actually appear to be in a different position.

(See my diagram below)
In the above, the light rays from the distant star are bent around the space-time gravity well created by the mass of the Sun, to such an extent that to the observer, the star beyond the sun, tracing back along the line of the light, appears to be in a different position.

Of course, you can’t see light from stars beyond the sun because the sun is too bright – it masks most stars.

But in a famous experiment by Arthur Eddington, the results were confirmed during a solar eclipse confirming the bending of light around massive objects.

The result in about 1919 ‘proved’ this aspect of relativity and made headline news across the world.

Newton had originally come up with the theory, but Einstein’s predictions were so accurate – confirmed later by Eddington – that relativity   - rather than replacing Newton’s theory’s – simply built on top of them.

Enter the quantum world.

So everything seemed quite predictable on the classical large scale – Newton and Einstein’s theories  - building on top of other theories - , seemed to give science the upper hand.

For the first time in the history of mankind in the early 1900s, science seemed to have in place a series of theories which not only approached an approximation of how the observable universe worked (or certainly the motion of things within it), but which enabled a set of theories and laws which actually seemed remarkably accurate.

Scientists in a sense for the first time, could perhaps legitimately describe themselves as having divined the mind of God [whether or not he existed].

Through sheer reasoning, mathematics and observation – a so-called pragmatic approach – man could seemingly fathom the vastness of space.

But then in the 1920 something happened which would literally throw the cat among the pigeons.

The discovery of atomic structure and the protons, neutrons and electrons which underlie the principle elements of nature caused scientists to probe deeper into the heart of things.

Atomic structure revealed that the basic elements were made of protons, neutrons and electrons.

The protons and neutrons (collectively known as hadrons) have within them quarks, which to date seem to be the fundamental constituents of matter.

 Quantum mechanics is based upon the concept that subatomic particles can have both wave-like and particle-like properties.

This phenomenon is known as wave–particle duality.

 The explanation stems from a theory proposed by French physicist Louis de Broglie in 1924, that subatomic particles such as electrons are associated with waves.

The concept of waves and particles, and the analogies which use them, are mechanisms of classical physics.

Unfortunately, quantum mechanics, which seeks to explain nature at a level underlying that of the atoms which comprise matter, cannot be understood in such terms.

Suppose that we want to measure the position and speed of an object -- for example a car going through a radar speed trap.

Naively, we assume that (at a particular moment in time) the car has a definite position and speed, and how accurately we can measure these values depends on the quality of our measuring equipment.

If we improve the precision of our measuring equipment, we will get a result that is closer to the true value.

In particular, we would assume that how precisely we measure the speed of the car does not affect its position, and vice versa.

In 1927 German physicist Werner Heisenberg proved that in the sub-atomic world such assumptions are not correct.

Quantum mechanics shows that certain pairs of physical properties, such as position and speed, cannot both be known to arbitrary precision i.e. at the same time.

He showed that the more precisely one of them is known, the less precisely the other can be known.

This statement is known as the Uncertainty Principle (or Heisenberg's uncertainty principle).

It is not a statement about the accuracy of our measuring equipment, but about the nature of the system itself – i.e. our naive assumption that an object has a definite position and speed, is incorrect.

Scientists now realise that on a scale of cars and people and planets, i.e. the classical level, these uncertainties are still present but are too small to be noticed; yet these uncertainties are large enough that when dealing with individual atoms and electrons they become critical.

Simply put, on the large scale, the classical laws of Einstein and Newton seem to hold.

But at the heart of things, on the very subatomic level, not only do these laws seem to break down or be wildly inaccurate, but chance and chaos seems to reign.

 In other words, there seems to be a fundamental uncertainty at the heart of things, and it was this concept which Heisenberg encapsulated in his Uncertainty Principle.

Einstein disliked the ideas of quantum theory claiming that ‘God does not play dice’; and yet he later acknowledged – and sought to bring together  - his theories of relativity with quantum mechanics.

Just as Newton was never satisfied with his theory of gravity, Einstein was never satisfied with General Relativity (his second theory).

Einstein was disturbed by two problems: he believed that there should be just one theory to account for both gravity and electro-magnetism, and he believed that this "unified field" theory should get rid of quantum mechanics.

Although Einstein himself helped create quantum mechanics, he hated the very notion until his death.

One interpretation of quantum mechanics is that everything is uncertain, and everything is fundamentally governed by the laws of probability.

Much has happened since Einstein’s day – principally the realisations of the four fundamental forces in the universe (or the four fundamental forces of ‘Nature’).

These are: gravity; the strong nuclear force (the binding energy or glue which holds protons and neutrons together); the weak nuclear force (which is crucial to radioactive decay), and electromagnetism.

Many people [and certainly older high school children] are probably familiar with the link between magnetism and electricity.

An understanding of the relationship between electricity and magnetism began in 1819 with work by Hans Christian Oersted, a professor at the University of Copenhagen, who discovered more or less by accident that an electric current could influence a compass needle.

Several other experiments followed, with André-Marie Ampère, who in 1820 discovered that the magnetic field circulating in a closed-path was related to the current flowing through the perimeter of the path.

Michael Faraday, in 1831 found that magnet passed through a loop of wire, induced a voltage; James Clerk Maxwell synthesized and expanded these insights into Maxwell's equations, unifying electricity, magnetism, and optics into the field of electromagnetism. In 1905, Einstein used these laws in motivating his theory of special relativity, requiring that the laws held true in all inertial reference frames.

The weak nuclear force, the second of the four fundamental forces, is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars – the massive nuclear conversion process which lies at the heart of glowing stars, which essential convert hydrogen into helium.

In 1968, the electromagnetic force and the weak nuclear force or interaction were ‘unified’, when they were shown to be two aspects of a single force, now termed the electro-weak force.

Many theorists today are trying to unify fully the four fundamental forces in the universe – and the Large Hadron Collider (which literally smashes protons together at high energies) is proving crucial in unveiling both new particles and new realms in which the fundamental forces of everything start to come together.

Physicists refer a ‘Standard Model of Physics’ which incorporates all the major force particles and sub atomic particles which seem to fit together well. The missing jigsaw piece currently is the Higgs Boson which they hope the LHC will unveil and which is crucial to understanding ‘mass’.

However, the full realisation of many is that the final unification of the four forces – known as the Theory of Everything – can only truly have happened at the moment of the Big Bang when the energies involved were absolutely colossal. In fact language probably has no means to encapsulate what actually happened there.

The Theory of Everything

Just as years ago scientists realised a link between electricity and magnetism to form electro-magnetism, today they are trying to unify the four fundamental forces in the universe.

On the ultimate level, a ‘Theory of Everything’ would unify all the fundamental interactions of nature: gravitation, strong nuclear force, weak nuclear force, and electromagnetism

Several (lesser!) so called Grand Unified Theories (GUTs) have been proposed to unify three of these forces - electromagnetism and the weak and strong forces.  Remember two of these themselves have already been unified (electromagnetism and the weak nuclear force) – the so called electro weak theory.

Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force.

Above the unification energy, on the order of 100 GeV, they would merge into a single electroweak force.

Thus if the universe is hot enough (approximately 1015 K, a temperature exceeded until shortly after the Big Bang) then the electromagnetic force and weak force will merge into a combined electroweak force.

Grand unification would imply the existence of an electronuclear force; it is expected to set in at energies of the order of 1016 GeV, far greater than could be reached by any possible Earth-based particle accelerator.

Yet GUTs are clearly not the final answer; both the current standard model and all proposed GUTs are quantum field theories.

i.e. the final step in the graph requires resolving the separation between quantum mechanics and gravitation, often equated with general relativity.

Numerous researchers concentrate their efforts on this specific step; nevertheless, no accepted theory of ‘quantum gravity’ – and thus no accepted theory of everything – has emerged yet.

In addition to explaining the forces listed in the graph, a TOE must also explain the status of at least two candidate forces suggested by modern cosmology: an inflationary force and dark energy.

Furthermore, cosmological experiments also suggest the existence of dark matter, supposedly composed of fundamental particles outside the scheme of the standard model. However, the existence of these forces and particles has not been proven yet.

 

MH 2011

Note on Schrodinger’s Cat.

One of the oddest bi-products of quantum discussions – and the realisation that on the small scale particles can appear to be in a quantum or seemingly random state or position (called superposition) is the arcane thought experiment known as Schrodinger’s Cat.

The thought experiment serves to illustrate the bizarreness of quantum mechanics and the mathematics necessary to describe quantum states.

Schrödinger's Cat: A cat, along with a flask containing a poison and a radioactive source, is placed in a sealed black box.

At one side inside the box is a tiny atomic source which over the course of an hour may or may not decay.

If an internal Geiger counter detects radiation, the flask is automatically shattered by a hammer mechanism releasing the poison that kills the cat.

If it doesn’t decay in the hour, the flask remains intact and the cat remains alive.

Schrodinger argued that over the course of one hour, in essence, an external observer one could not for certain predict whether the cat was ‘alive or dead’ (without opening the box). He argued to all intents and purposes it was in a ‘quantum state’ i.e. alive/dead.

Only by opening the box do we actually ascertain for certain whether the cat is alive OR dead – but the downside of all this is that we have broken the ‘quantumness’ of the system.

However esoteric this might seem, many volumes have been written on this hypothesis. It has also been used to question where the reality boundary lies between the ‘quantum world’ and the ‘classical’ macro boundary of the likes of Einstein and Newton. Nobel prizes lie in wait for those who can discover such.

MH

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