Ten of the greatest: 30-second theories

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Ten of the greatest: 30-second theories
JOHN GRIBBIN, Visiting Fellow in Astronomy at Sussex University
26th June 2010

From the principle of least action, to Richard Dawkins' memetics theory through to the placebo effect, John Gribbin discusses the greatest 30-second theories

1. THE PRINCIPLE OF LEAST ACTION

This is a fancy way of saying that nature is lazy. Things happen in the way that requires least effort, which is why, among other things, light travels in straight lines. In fact,particles, and light, travel by the path which takes the least time. This explains why an angled light ray bends towards the perpendicular when it passes from air into glass, through which it moves more slowly (this reduces the amount of glass it has to pass through).The principle is especially important in quantum theory. At first sight quantum theory seems to imply that a particle such as an electron can go by any path from A to B, even if that means going to Mars and back.This would make it impossible to operate things like computers, which depend on electrons being well behaved. But the great physicist Richard Feynman proved that all 'indirect' routes interfere with each other, leaving only one viable path - the quickest.

2. MEMETICS

Whenever we copy habits, skills, stories or any kind of information from person to person, we're dealing in memes. The term was coined by Richard Dawkins to describe 'cultural replicators' that copy and transmit biological information. Humans copy memes, including ideas and skills, through imitation and teaching; but they get changed, accidentally or on purpose, so that culture evolves. This echoes the way species evolve as genes mutate. Like genes, some memes are successful, while others aren't. It's obvious why some memes spread - they're useful, or aesthetically pleasing, like melodies. But some spread even though they confer no clear benefit - things like computer viruses.

3. QUANTUM FIELD THEORY

We're all familiar with the idea of a magnetic-field, the region around a magnet where its influence is felt. The way fields behave is described by field theory. Scottish physicist James Clerk Maxwell found the equations that describe magnetic and electric fields, and Einstein found the ones that describe a gravitational field. But in the Twenties physicists realised the 'fields' actually consist of particles moving through space: the 'quanta' of the field. When two electrically charged particles exert a force on each other, they do so by exchanging photons, the quanta of the electromagnetic field. The resulting quantum field theory (quantum electrodynamics, or QED) explains everything about the behaviour of the atoms that make up our world.

4. PANSPERMIA

Is all life on Earth descended from the bacteria left behind by aliens after a picnic? It sounds far-fetched, but it's an idea that was put forward seriously by the maverick astronomer Thomas Gold. He was building on a proposal made by the Swede Svante Arrhenius a century ago. Arrhenius suggested that life was brought to Earth by spores floating through space, an idea he called panspermia, meaning 'seeds everywhere'. A variation on this idea is the notion that the spores were sent deliberately by an alien civilisation - 'directed panspermia'. And then there's Gold's suggestion that the aliens happened to be passing by the Earth billions of years ago and stopped off for lunch. Crazy? Maybe. But one person who took the idea of panspermia seriously was Francis Crick, who co-discovered the double-helix structure of DNA.

5. PLACEBO EFFECT

If you're given a pill and told it'll cure your headache, and your condition improves even though the pill contains nothing but chalk, you have experienced the placebo effect. If you believe in it strongly enough, almost anything can have a placebo effect.

The placebo effect explains why treatments such as homeopathy work for many people - simply because the patients believe they will work. Scientific tests under controlled conditions have shown that placebo effects can be enhanced by giving people bigger pills, by giving them pink pills rather than white ones, and by the perceived seniority of the doctor who's prescribing the pill.

Until the 20th century, most medicines were completely useless, but consciously or unconsciously, the placebo effect was exploited to the fullest, especially when dealing with rich patients.

6. SNOWBALL EARTH

For millions of years, over 635 million years ago, the Earth was shrouded in ice. At that time, most land was clustered around the equator, but the glaciers left their mark on the rocks even there. Nobody knows what caused the Earth to freeze, but once it did, the shiny white surface of the ice reflected heat and maintained the 'snowball' conditions, until CO2 released by volcanoes created a greenhouse effect strong enough to melt the ice. If the ice had covered the entire planet, life may have died out. But there's evidence slushy pools survived during this period, and primitive but hardy life forms survived in the pools. When the Earth thawed, life exploded out across the planet in a burst of evolution. We may owe our existence to this Snowball Earth event.

7. RARE EARTH

Life in the universe may be common, but intelligent life may be rare. Proponents of the Rare Earth idea point to a chain of circumstances that allowed our civilisation to emerge, even though that required nearly four billion years of evolution. The sun is a relatively stable star, which has allowed life to evolve steadily over all that time. The giant planet Jupiter protects us from comets. Our large moon's gravitational pull stops the Earth from wobbling and tipping over, causing extreme changes in climate. And the Earth has an unusually strong magnetic field, shielding us from harmful radiation. The combination of these and other unusual features of our planet constitutes a chain of coincidences so unlikely that some astronomers think we may be the only intelligent form of life in the universe.

8. THE EKPYROTIC UNIVERSE

Here's a wild idea. Some cosmologists think our universe is one of a pair of three-dimensional universes, separated by a tiny distance (less than the diameter of an atom) in an extra dimension. Every point in space is next door to a point in the other universe, but the two are slowly moving apart. Billions of years from now, though, a spring-like force will pull them back together in a collision generating huge amounts of heat and light - a big bang. The two universes will then bounce apart and the whole process will repeat. This is the leading alternative to the Big Bang theory, and it asserts that the universe in a sense gives birth to itself.

9. ENTANGLEMENT

When any two quantum objects, such as electrons or photons (light particles) come into contact, they become 'entangled in a quantum sense. This means that forever after what happens to one of them affects the other, instantly, no matter how far away it is. If one particle is given a prod, the other twitches. This leads some people to hope that entanglement could be used for faster-than-light communication. The snag is, although you know from watching one particle that the other has been prodded, you don't know how it was prodded. But if that information is sent to us by conventional means, we can interpret the twitching - which means that quantum prodding could be used to send uncrackable coded messages.

10. FINITE

If the universe were finite, it could be shaped like a ring doughnut. And if that were the case, if you looked one way round the ring you'd see the same galaxies you could see by looking the other way round the ring, but from the other side. A more complicated topology would be a cube in which opposite faces are connected to one another. If you could travel up through the 'roof', you'd come back into the cube through the 'floor'. Some simple computer games work like this. Observations show that our universe doesn't have such a simple topology as this. But it could be shaped like a multi-dimensional dodecahedron.


John Gribbin is a contributor to '30 Second Theories: The 50 Most Thought-Provoking Theories in Science, Each Explained in Half a Minute' (Icon Books)