Five Scientific Ideas That Explain Most of Everyday Life

School gave most of us a long list of answers and not much sense of which ones carry the weight. A few do most of the work. If you understand what things are made of, where energy goes, why things run down, what electricity is and how living things copy themselves, you can account for a surprising share of an ordinary day.

It helps to be clear first about what kind of thing a scientific idea is. Science is not a pile of facts but a method for finding out which of our ideas about the world survive contact with it. Notice something, guess at an explanation, work out what that guess predicts, and then look, carefully, to see whether the prediction holds.

A claim that cannot fail any possible test is not telling you anything about the world.

Each of the five ideas below could have failed. None has yet, and that is why they are worth knowing.

1. Everything is made of atoms, and atoms prefer company

John Dalton was a Quaker schoolteacher in Manchester who kept a weather diary for 57 years. His interest in gases led him to a question the old four elements could not answer: why do substances combine in fixed proportions? On 21 October 1803 he read a paper to the Manchester Literary and Philosophical Society that ended with the first table of atomic weights. Matter, on this view, comes in small countable units, and those units join up by rule.

The joining is the part you meet every day. A chemical bond is what happens when two atoms find it cheaper, in energy, to stay together than to stay apart. Sodium has one spare electron; chlorine is one short. Sodium hands its electron over, both become charged, and the opposite charges lock together into salt. Two hydrogen atoms, each with one electron and each wanting two, solve the problem by sharing.

Small differences in bonding make large differences in the kitchen. The hydrogen bond, a weak attraction between molecules and not a true bond, is why water boils at 100 °C instead of around minus 80 °C, which is what its small size would otherwise predict. Without that weak attraction there would be no kettle to wait for, and no liquid water to put in it.

2. Energy is never used up, only moved

The first law of thermodynamics is bookkeeping with teeth: energy can change form but can never be created or destroyed. Pour it from motion into heat, from heat into steam, from steam into electricity and from electricity into light, and the total at the end equals the total at the start.

The law sounds obvious now. In 1840 it was not even clear that heat was a form of energy at all. Most chemists still treated heat as an invisible fluid that flowed from hot bodies to cold ones.

Once you accept the bookkeeping, everyday questions change shape. Rub your hands together and they warm up; the motion did not vanish, it became heat. Slam the brakes and the car stops for the same reason. The useful question is never "where did the energy come from?" on its own, but "where did it go?"

The idea also clears up a common muddle. Heat and temperature are not the same thing. Temperature is a measure of how vigorously the molecules of a substance are jiggling; heat is energy in transit from a hotter body to a colder one. A bathtub of warm water holds far more heat than a spark from a grinder, yet the spark is thousands of degrees hotter. Touch the bath and you are comfortable. Touch the spark and you are burned only briefly, because there is so little of it.

3. Things run down, and that is arithmetic

If energy is never lost, why does anything stop? Why does tea go cold and a room get untidy?

Drop a glass and it shatters; it never reassembles.

Milk stirred into coffee never unstirs. Yet every law of motion runs equally well backwards. Film a planet orbiting and reverse the film, and nothing looks wrong. Film the glass and reverse it, and everyone laughs. The quantity that tells the two films apart is entropy, and the reason it always rises is, at bottom, arithmetic. There are far more ways for things to be spread out and mixed than gathered and sorted, so that is the direction they drift.

This is the idea behind most domestic disappointment. Energy is conserved, but it spreads out, and spread-out energy is harder to use. It is why hot things cool to the temperature of the room and never the other way round on their own.

4. Electricity and magnetism are one subject

The steady electric current began with a dead frog. In Bologna in the 1780s Luigi Galvani noticed that a frog's leg twitched when touched by two different metals, and concluded that animals made their own electricity. Alessandro Volta disagreed: the frog was merely a detector, and the electricity came from the two metals meeting through a salty fluid. To prove it he stacked discs of zinc and silver separated by brine-soaked cardboard.

Then came the link to magnets. If a current makes a magnetic field, can a magnetic field make a current? Michael Faraday, a blacksmith's son and former bookbinder's apprentice, chased that question for a decade. The answer came on 29 August 1831. He wound two coils of wire on opposite sides of an iron ring, connected one to a battery and the other to a meter, and saw the needle kick at the instant he closed the switch, then kick back when he opened it. A steady current did nothing. Only a changing magnetic field pushed current through the second coil. Faraday wrote no equations, having left school at thirteen; the mathematics was supplied later by Maxwell.

The same subject covers light. Every band of the electromagnetic spectrum is the same thing, an oscillating electric and magnetic field travelling at the speed of light, and the bands differ only in wavelength. Visible light is a narrow slice. Nearly all the rest is invisible, which did not stop it becoming useful. Hertz, asked about the use of his radio waves, said they were "of no use whatsoever".

5. Life copies itself, with small differences

Three ideas from biology sit under most of what happens in a body, a garden or a hospital.

The first is where food comes from.

Almost every meal you eat and every breath you take traces back to one chemical reaction.

Photosynthesis is the process by which plants, algae and some bacteria use the energy of sunlight to turn carbon dioxide and water into sugar, releasing oxygen as a by-product. The sugar feeds the plant and, eventually, everything that eats plants.

The second is how instructions are passed on. The molecule that carries inheritance is a ladder twisted into a spiral, with rungs made of four chemical bases. A always pairs with T and G with C, so each strand is a template for the other. Pull the two apart, and each can rebuild its partner. That is how a cell copies its instructions before it divides.

The third is what happens when the copies vary. Natural selection is an argument so simple that Thomas Huxley, on first reading it, said how stupid he had been not to think of it himself. Living things vary. Some of that variation is inherited. More are born than can survive. So whichever variants help their owners survive and reproduce become more common, generation by generation. No designer is needed; the environment does the sorting.

The same thinking explains illness. Until the late nineteenth century, most doctors blamed disease on bad air rising from filth. Germ theory replaced that with something more precise and more useful: specific microbes cause specific diseases, they spread from person to person, and they can be killed. Washing your hands is that theory in practice.

What to take from this

• Matter is made of atoms, and bonds form when staying together costs less energy than staying apart.
• Energy changes form but the total stays the same. Ask where it went.
• Heat is energy in transit; temperature is how vigorously molecules are moving. They are different things.
• Entropy rises because there are more ways to be mixed than sorted. That is why things run down.
• A changing magnetic field drives a current, and light is part of the same subject as electricity and magnetism.
• Living things copy their instructions, the copies vary, and the environment sorts the variants.
• A scientific claim earns its place by being testable.

There are many more where these came from. Science: How we found out how the world works takes one hundred big ideas and tells each through the experiment, observation or instrument that revealed it, with a detailed plate in every chapter. It is a 313-page PDF and EPUB, and it costs £9 on Gumroad: matsiems.gumroad.com/l/science.