I’d like to spend a short time with you thinking about what physics actually is. When I taught physics in schools I was never really sure my pupils grasped the whole point of physics. I remember spending time at the beginning of a topic showing some pictures and video clips of things moving; from atoms in a lattice, to cells, dancing humans, orbiting planets, all the way up to the expanding Universe itself. I was emphasising that everything moves and that was why we were about to spend a term studying forces and motion. I have never had to introduce a biology topic in such a way.
The average person on the street has a much clearer understanding of what the other two sciences are all about. Ask about biology and they can easily tell you what this subject covers, it’s to do with living things. David Attenborough sitting amidst luxuriant flora while his hushed voice dramatises the snippet of some creature’s life cycle that you are about to witness, timelapsed for your entertainment, is obviously biology. Likewise chemistry is to do with mixing chemicals to make new chemicals. I remember my first lesson using a Bunsen burner, which was frankly terrifying! Having been so sternly warned by my parents never to play with matches I could barely bring myself to light the thing. I will always be able to conjure up in my mind the trays of glass stoppered bottles, the chink of test tubes and beakers, the smells and mysterious stains that lurk in a well used chemistry lab. The immediate appeal to our senses of biology and chemistry secures them in our minds.
Ask someone to describe physics though and you will often be given a list; electricity, forces, space those sort of things. Physics seems to be about stop watches and little light bulbs that don’t work, magnets, mirrors and masses. A hodge podge of rules about a lot of things that just don’t seem connected at all. This is all rather unsatisfactory. We’re talking about the passion of Newton and Einstein! Surely these people did more than just dabble in a pick-and-mix of the bits that are left over when chemicals and living things are taken out?
Please be assured they did! Physics is not a list of all the science bits that don’t neatly fit into biology and chemistry. There is a deep and fundamental connection between the magnets, mirrors and masses but it isn’t something that can be seen or touched or smelt. Maybe that is why we find it hard to sum up what it’s all about. Before you go about exploring any of the physics topics on this website, call to mind the big picture of physics I’m about to reveal to you, and you will set off on the right foot.
Physics is about Stuff
Let’s think about the Universe. The Universe is an amazing concept, it encompasses everything we know of. The Universe is huge, and old and full of things. It’s a bit like an enormous ancient mansion with an undetermined number of rooms in an unknown layout crammed with millions of objects, pieces of furniture, dust, cobwebs. You have inherited this sprawling domain. How can we get a handle on this enormous, complex entity? Think about it. What would you do to start sorting it out, to start describing the Universe? Can I suggest that we start by taking an inventory of the contents?
At school we are introduced to the idea that all the stuff in the Universe can be divided into solids, liquids and gases each of which is made up of very small, hard, round particles. I would like you to consider adding another group to this which takes into account the fact that particles are actually made up of smaller bits and that these bits can have important properties such as electric charge. We can’t leave them out so let’s add another group of stuff which is the particles themselves. So we have solids, liquids, gases and their constituent particles. This gives us a neat way of grouping all the lumpy matter in the Universe.
Sorting out the lumpy matter and putting it to one side would give us a big pile, a big puddle, a big cloud and trillions of whizzing particles. But the Universe is still not empty. There is more to it than lumpy matter. Back inside the Universe there are sparkles and flashes, some patches would feel warm, some would give you sunburn, infact in some places you would see through your hands to the bones, some places would burn right through your hands to the bones. There is a whole lot of radiation in there and it moves in waves rather than lumpy blobs. We can group all of this together too and unpick it into particular kinds of waves and radiation later on if we want to.
What is left is quite odd. What is left is quantum in nature. We are not even talking about particles anymore, there are fields. Take a big stick and swing it around in the Universe, the fields all begin to swirl up together like candy floss and you can pull them out. Physicists believe there are quantum fields in with the matter and radiation, you may have heard of one of these called Dark Energy which is believed to affect the expansion of the Universe.
Once we’ve sorted out the contents of the Universe into these broad groups we can start to address the nature of the mansion itself. How many floors? How many rooms? Maybe try and draw a floorplan of the Universe. Here we would hit upon a big problem.
Physics is about how stuff interacts
If we have indeed removed all the contents of the Universe, our vast mansion has been emptied then the building itself would be fundamentally affected. Einstein showed in his General Theory of Relativity, that matter and space are completely linked. Matter shapes the space around it. Imagine putting a table back into an empty room in the mansion and finding the room itself changed size and shape as a result.
But that’s not all, the shape of the space also affects how the matter moves through it. So anyone walking through the room with the table in it would find themselves walking faster and round a different route compared to a room without a table in it. The building we have when the matter is removed is not the same when all the matter is put back in.
This is an example of how the components of the Universe interact with each other. The matter-space interaction is very complicated as the influence works both ways. It’s also the largest scale interaction that we know of, affecting the whole Universe and caused by all the matter and waves in it. There are many other kinds of interactions occurring between the various chunks stuff of the Universe. What other interactions are there to consider and what signs are there that an interaction is occurring?
What is an Interaction?
Robinson Crusoe did not have much of a social life. He may have been the life and soul of a party, a gregarious flirt and amusing wit and raconteur but all these wonderful interpersonal skills were wasted as he sat sunburnt on the beach of his desert island counting the gulls. A social life requires others to socialise with. Likewise an interaction doesn’t happen to a chunk of stuff sitting alone by itself. There has to be at least two chunks.
An interaction doesn’t require the two chunks of stuff to be touching or even particularly close together. Some interactions occur between objects separated by large distances. The Sun is very far away from the Earth (93 million million miles in fact) but the heat from the Sun can still melt an ice cube in a cold drink on a summer’s day. What is needed to identify an interaction is not the proximity of the objects but a clear cause and effect relationship. Some behaviour of one chunk of stuff clearly influences the other piece. The heat radiation from the sun is influencing the ice cube, making the water change state.
Physics is about finding and describing these cause and effect interactions. We can go further by carefully measuring the characteristics of the stuff before and after interactions, then finding any patterns between these measurements. This is why physicists make use of equations, graphs and formula. Mathematics can express, very precisely, the relationships between the objects that are interacting.
Signs of Interactions
Water melting when exposed to a heat source is a clear interaction. We can see the change occurring in the ice cube right before our eyes. If we take away the heat source, the interaction and the accompanying changes stop. Physicists develop ways of seeing and thinking about situations which enable them to spot and isolate the interactions they are interested in.
One way to tell if an interaction is occurring is to look for visual signs of changes in size, state, position, speed, direction, colour, shape etc. If we want to be precise, measurements can be taken of the objects before and after an interaction to quantify exactly how much of a change has occurred. There are some changes that can’t be seen and require our other senses or some instruments to detect a change. Changes in temperature, musical pitch, pressure, elasticity, strength of a magnet, tension in a wire or expansion rate of the Universe can all be found from observations and measurements. If no change occurs then we can say that no interaction has taken place.
Types of Physical Interactions
By looking closely at the stuff in the Universe we can spot the changes that different interactions bring about. Having already sorted the stuff in the Universe into helpful categories we can start looking at the way these different types of stuff interact. We introduced the largest scale of interaction when considering the affect of matter on space itself. Let’s work our way down the scale and see if we can systematically log the other major interactions that are noticeable in the Universe.
Stepping down a scale from the space-matter interaction we can look at how very large lumps of matter interact with each other and by large I mean galaxies or groups of galaxies in space. These interact through gravity and the interactions occur over enormous distances. We can spot this by looking at the motion of galaxies and how these change when other galaxies move by.
Within the galaxies, the stars also interact with each other through gravity as do planets, comets, meteoroids, asteroids and all the other astrophysical objects floating around in a galaxy.
The large craters visible on the Moon and the Earth are evidence that collisions occur frequently between objects in the Universe. On the Earth we see collisions between many things, even a butterfly landing on a leaf is a sort of collision.
On a smaller scale we can begin to see interactions amongst the particles within solids, liquids and gases. Movements of particles against each other produce effects such as pressure, friction, air resistance. Movements within a substance produce material properties like elasticity, conduction of heat, transmission of sound. Bonds between particles affect melting points, tensile strength, hardness. Some particles attract or repel each other in proportion to a property called electric charge and this can lead to the movement of charged particles in a substance and the binding of charged particles together in groups. The entire of chemistry is the study of this sort of physical interaction.
Smaller still and we look into atoms themselves and see attraction caused by a different sort of charge, the colour charge, which attracts nuclear particles together. Subatomic interactions also cause particles to change from one type to another, to absorb and emit radiation waves. Waves themselves can interact producing larger or smaller waves or patterns of waves overlapping each other. Thus we can see the enormous scope of physics, all of these different types of matter and these different ways of interacting are the realm of the physicist. The elements of this mind-boggling array are not as disparate and unrelated as first appears, these interactions do all have some things in common; force and energy.
Physics is about Forces and Energy
Physicists use the word force in a slightly different way to its normal use. A news reporter may stand in a flack jacket in front of a column of tanks kicking up dust as they trundle past and talk about an invasion force, parents lament that they can’t force their kids to eat their greens. In each case the word force implies something powerful put into a situation that compels a change. The interactions discussed in the previous section bring about a change and they do so through the action of a physical force.
Whenever two objects interact a force exists between them. Each object experiences the force, imagine a cup resting on a table. The cup is pushing down onto the table, a light paper cup would have a smaller push then a large ceramic mug. The table is also pushing against the cup otherwise the cup would fall straight through the table and onto the floor. The two pushes are nicely balanced in this case as the cup doesn’t sink through the table and the table doesn’t shove the cup into the air. Careful study of the types of interaction listed previously have led physicists to conclude that there are actually only four types of force acting on objects in the Universe.
Far and away the most common force is the electromagnetic force. This is the force that stops the cup falling through the table. The tiny electrons in the atoms on the outside of the cup repel the tiny electrons in the atoms on the surface layer of the table, the two surfaces push away from each other. Whenever two solid objects touch the electromagnetic force comes into play. This force is also responsible for pressure, upthrust, friction, air resistance, elasticity, conduction, sound, all chemical bonding, all collisions, material properties as well as the two parts of its name, electricity and magnetism.
The gravitational force attracts matter together but the effect is only really obvious for large chunks of matter. Gravity doesn’t keep atoms together. Inside the nucleus of atoms a different very strong force must exist to hold all the positive charges close together as the electromagnetic force is trying to push them all apart. This is called the strong nuclear force, named presumably by a particularly unimaginative physicist. The final force actually allows things to come apart, it enables radioactive decay to occur and particles to change type. It exists between subatomic particles and is called the weak nuclear force. One of the things that all interactions have in common is the action of one of these four forces.
Every time a force acts on something, energy is also changed. The arrangement of energy ‘before’ the change is different to the ‘after’ situation and so each interaction also results in a reordering of energy. The precise amount of energy that gets reordered is linked to the force that causes the change. Finding ways to express this relationship between force and energy for different interactions is fundamental to physics. This then allows predictions to be made about the effect of a force. A theoretical physicists is happy to work out the relationship, an experimental physicists tests the relationship and an engineer goes and makes something useful using her knowledge of this relationship.
At the start of this introduction chapter we found that biology and chemistry can be easily summed up in a sentence; biology is the study of living organisms and their interdependence, chemistry is the study of the reactions between chemicals to make new chemicals. We are now in a position to explain what physics is actually about.
Physics is the study of the interactions between the stuff in the Universe; these interactions are caused by forces and lead to a change in how energy is stored.
Physics is the really fundamental science as it starts from the absolute basics, what is there in the Universe and how does it behave. Chemistry concerns only a small subset of these interactions. The part of chemistry that tries to explain why chemical reactions occur is actually chemists doing physics, the cataloguing of elements and chemicals and their properties and the recipes that make new chemicals is not physics. Biology results when these chemicals and chemical reactions build into complex, self-replicating, living units. Explaining how living things live requires the biologists to look at chemical reactions in cells and the explanation of these reactions is again physics. The cataloguing of what living things there are, what they look like, how they live and interact is not physics. Ernst Rutherford summarised this succinctly when he said “all science is either physics or stamp collecting”.