From the very small to the very large, physics is the study of how the world around us works. Here are our top ten greatest discoveries in physics.
1 – The Law of Falling Bodies
Aristotle, the Greek philosopher, convinced the world that lighter objects fell faster than heavier ones. This made perfect sense and so the idea persisted for around two thousand years. You have probably seen this for yourself. Drop a pen and a piece of paper and the pen hits the ground first.
Then, in the seventeenth century, Galileo said that the great Aristotle was wrong. Allegedly, he dropped balls of different weights from the top of the leaning tower of Pisa although Galileo himself did not describe any such experiment. Whatever the situation, he realised objects should fall at the same speed. In the paper and pen drop mentioned above, it is air resistance that causes the paper to fall more slowly as it reaches its terminal velocity much sooner than the pen.
The significance of this was that ultimately, it led to the knowledge of acceleration caused by gravity. Centuries later, the human race has discovered how to overcome the force of gravity in order to head into space. And, whilst he was on the surface of the Moon, on the 2nd August 1971, David Scott, commander of Apollo 15, dropped a feather and a geological hammer simultaneously. As there was no air resistance, as predicted by Galileo, they hit the ground at the same time.
2 – Universal Gravitation
Isaac Newton was born in the same year that Galileo died and is credited with the ‘discovery’ of gravity. A falling apple set him thinking about why it fell to the ground whereas the Moon stayed in the sky. His great leap of reasoning was to realise as the Moon travels through space in a straight line, as it passes the Earth, it is attracted by the same force that attracted the apple, trapping it in orbit.
But Newton also realised that the Moon would also have gravity, as would everything – gravity was universal. The effect of the Moon’s gravity on the Earth can be seen twice a day as the tides. The Moon attracts the water of the oceans and creates a tidal bulge, one on each side of the Earth. As the Earth rotates, the bulge remeains beneath the Moon. As the Earth rotates, different coastlines encounter the tidal bulge and the water level rises and falls.
3 – The Laws of Motion
But Newton didn’t stop there. Amongst many other discoveries came his laws of motion which he described in his Philosophiae Naturalis Principia Mathematica. These describe simply how everything moves.
His first law states that everything will remain either at rest or in a state of unifom motion i.e. no change in speed, unless an external unbalanced force is applied. This is sometimes referred to as the Law of Inertia.
His second law asserts that acceleration is produced when a force acts on a mass. The greater the mass, the greater the amount of force needed. In other words, it is easier for one person to push a bike than a car. This law also explains why falling objects accelerate. The force of gravity acts constantly whilst they are falling. They do reach their terminal velocity at some point when the force of air resistance balances the force of gravity.
His third law states that for every action there is an equal and opposite reaction. When you push an object, it pushes back. The classic example of this is a rocket. The burning fuel rushes out of the motor. As it does so, it pushes on the rocket. If the force is greater than that of gravity, the rocket can leave the surface and reach orbit.
Between them, Newton’s two greatest discoveries, gravitation and the laws of motion, established what is called ‘classical physics’.
4 – The Second Law of Thermodynamics
Thermodynamics is the science of transforming heat into movement. For industry, maximising the output from fuels is crucial as it leads to greater profits so people began to look closely at this branch of physics.
In the mid 19th century, German physicist Rudolf Clausius stated “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time”. The implication of this is, that during the process, some heat will always be wasted and therefore the efficiency of any heat engine will always be limited.
In general terms, it means that heat will only flow from hotter bodies to colder bodies and never in the other direction unless external work is applied to the system.
5 – Generation of Current Electricity
Although there appear to be primitive forms of voltaic cells as far back as Babylonian times, current electricity was effectively discovered only in 1808 when Alessandro Volta made his ‘Voltaic Pile’. Twelve years later, Danish scientist Hans Christian Oersted discovered that a needle was deflected by electricity passing through a wire – that was the moment when the link between electricity and magnetism was discovered.
A decade later, Michael Faraday made possibly one of the most greatest discoveries in physics that is felt directly today – electromagnetic induction. In 1831, he found that by moving a magnet near a wire (or vice versa), an electric current was induced in the wire. He had discovered how to generate electricity. It took 60 years of development by many people until the Faraday’s idea became commercial – the first electrical generator to supply mains electricity in the UK was installed at Godalming in Surrey.
6 – Special Relativity
Albert Einstein, a clerk in a Swiss patent office, had been thinking about light, time and space since he was a teenager. He came to realise that conventional Newtonian ideas would not apply as you reached the speed of light.
Time was regarded as being the same everywhere, passing at a constant pace wherever you were in the universe. Einstein saw it as being more like a river, speeding up and slowing down depending on where you were and the situation. Since his ideas were so grand, it was pretty much impossible to carry out experiments in order to test his ideas. So he carried out ‘thought experiments’.
One of the best known is the twins paradox. Take two identical twins, send one off into space to travel at near-light speeds and keep the other on Earth. When the twin returns from the space journey, he or she will be younger than the Earthbound twin. The faster you move, the slower time passes for you.
This can actually be seen now in practice with extremely accurate clocks. Orbiting gps satellites travel at about 18,000mph so their clocks will be running fractionally slower than on Earth. This would ultimately compromise the accuracy but the technology includes in-built compensation for the relativistic effects.
7 – Probably the Most Famous Equation Ever Written
This is E=mc2, the equation that links energy and matter and can tell you exactly how much energy will be released in a nuclear explosion. We will spare you the maths but Einstein derived the concept from kinetic energy equations applied to a system that emitted two light pulses in opposite directions. When he published his findings, it wasn’t actually in the famous format, that became popularised after the second World War following the two atomic bomb explosions above Nagasaki and Hiroshima. In 1990, Fritz Rohrlich derived the equation in a different way, based on the Doppler effect.
The implication of this equation is far-reaching. It means that anything that emits energy will lose mass and anything that absorbs energy will gain mass. In everyday terms, these losses and gains are incredibly small, despite the fact that c2 is a huge number.
In actual fact, Einstein wasn’t the first to come up with the idea that mass and energy are equivalent and that mass changes will occur when energy is absorbed or emitted. The first person recorded to have said this is Newton and there have been many others working along the same lines since.
8 – Quantum Theory
Around the turn of the nineteenth and twentieth century, new particles were being found that violated classical physics. A different theory was required. For example, the Curies discovred radium which glowed in the dark. It appeared that energy and particles were seemingly coming out of nothing, which clearly violated the principles of conservation of matter and energy.
At the time, energy was seen as being a continuous entity, that is, you could chop it into ever smaller chunks ad infinitum. Max Planck shook the world of physics by saying that this wasn’t so. He postulated that energy came in small ‘packets‘ which he called ‘quanta‘ (singular – quantum) as this ‘quantum effect‘ that occured at the fundamental particle level seemed to explain all the different phenomena that they were seeing. This simple sounding idea also meant that matter had wave like properties, the so-called wave-particle duality of the quantum theory.
The structure of the atom was not fully unravelled at the time and it was thought of as being like a miniature bowling ball i.e. solid. Over the course of the next 25 years, various big name mathematicians and physicists including Einstein, Dirac and Fermi worked on the quantum theory and more was discovered about the atom. In 1925, Austrian Erwin Shroedinger came up with a wave equation that described the electron, vindicating the theory.
The next significant development was the Uncertainty Principle, named after Heisenberg and expressed mathematically by Max Born. You cannot know the precise location of a particle, as well as its momentum, but you can work out the probability of where it can befound. On the scale of an electron, this means that it could be in two places at the same time and this is in fact the basis of many electronics devices.
Some of the ideas are very strange but it is currently the best model for the behaviour of sub-atomic particles.
9 – What is Light?
The study of light effectively got underway with Isaac Newton who wanted to try to prove Christian Huygens wave theory of light. He passed ‘white light’ i.e. daylight, through a prism and made the discovery that it could be split into the seven colours of the rainbow. He also showed that these colours could be re-combined to form white light again. This led him to suggest that light was ‘corpuscular’ in nature i.e. made from particles.
Newton’s prestige ensured that this theory was accepted widely but, in 1801, Thomas Young achieved what Newton had originally set out to do. He carried out experiments using closely spaced slits in an opaque material. When he shone light through these, he obtained a diffraction pattern, much like the patterns obtained when water waves cross one-another. Where crests meet, the wave heights are combined, where a crest and trough meet, the waves cancel out. This could not readily be explained using a particle theory of light.
Moving forward now to the start of the twentieth century, Einstein suggested that perhaps both theories were correct. His idea of ‘photons‘ of light was not particularly widely accepted until Robert Millikan’s work a decade or so later.
10 – The Neutron
The last of our ten greatest discoveries in physics led directly to the establishment of the nuclear age when, in 1939, a team lead by Enrico Fermi used neutrons as missiles to split the atom.
By the end of the nineteenth century, scientists had an idea that atoms could be represented as small spheres throughout which the protons were spread, rather like the plums in a plum pudding or currants in a currant bun. In 1909, working with two of his students, Hans Geiger and Ernest Marsden, Ernest Rutherford fired alpha particles at a thin sheet of gold foil. Their results were totally unexpected – some of the alpha particles bounced back. The only way they could explain this was by assuming that atoms were mainly empty space with a small nucleus of very dense material at the very centre.
One of Rutherford’s former students, James Chadwick, together with Rurtherford, had predicted the existence of a neutral fundamental particle, the neutron but had not proved its existence. Then Chadwick heard about work carried out by the Joliot-Curies who believed that they had knocked protons from paraffin wax. Chadwick and Rutherford didn’t agree, the method they had used wasn’t sufficiently energetic they said. Chadwick then carried out several weeks worth of experiments and finally tentatively announced the discovery of the neutron. Since it explained several anomalies concerning atomic structure, the discovery of the neutron was accepted.