What do we really know about gravity?

Gravity is undeniable. We can enjoy it, thrilled by the sensation of leaving our stomachs behind as we accelerate down the steep slope of a rollercoaster; fear it, as we stand at the edge of a skyscraper so high that the cars look like dots, let alone the people walking below; or, as happens most of the time, we can take it for granted, and barely notice its presence.

But it’s still there. Pulling us into Earth’s blazing core, it keeps our feet on the ground and keeps our air, and our insides, where they should be. Since that infamous apple first fell on Newton’s head, scientists – particularly physicists – have been aware of gravity, and have studied it intensely. And yet, this most fundamental of forces still evades us.

True, we don’t know the actual origins of any of the fundamental forces. We know how they work (mostly) – we can predict them, calculate their effects – but we don’t know where they come from. Gravity, however, is proving even more mysterious. It just doesn’t seem to follow the few rules that the other forces obey.

There are four fundamental forces. The electromagnetic force covers all things electric (e.g., an electric field created by a Van der Graff generator) and magnetic (e.g., well, a magnet) – using its mathematical definition, we can calculate the effects of these fields. The weak force controls radioactive decay, affecting all particles that are fermions – particles with half-integer spin (for example, an electron has spin of 1/2). The strong force is what holds atoms together – it holds together the protons and neutrons in the atom, and also holds together the quarks that make up the protons and neutrons.

The final force that we have left, then? Gravity, of course. However, there are remarkable differences between gravity and the other three forces, and we’re not sure if that’s because we’re missing something, or because they are just different.

For example, each force has what’s known as a ‘mediating particle’. The electromagnetic force is ‘mediated’ (i.e., carried) by particles known as photons. These are often considered to be particles of light, as light is an electromagnetic wave, but they can ‘carry’ any kind of electromagnetic wave. Which kind of wave they are depends on their wavelength, which in turn depends on their energy.

In a similar way, the weak and strong forces have mediating particles: the weak force has “W+”, “W-” and “Z” bosons, particular bosons which are significantly heavier than the mediating particles of the other forces. The strong force has particles called gluons, unsurprisingly given that the force exists to stick stuff together!

 

Photon, gluon, W and Z bosons (courtesy of particlezoo.net)

The mediating particles we know exist: the photon (electromagnetic force), gluon (strong force) and W and Z bosons (weak force). Image: particlezoo.net

So it makes sense that gravity should have a mediating particle, and one has been postulated: the graviton. Unfortunately, no one can find any proof of its existence, as it doesn’t fit with either maths or experiment. So, unlike with the other three forces, we can’t yet use particle physics models to explain gravity, which makes things tricky!

Graviton

Do gravitons exist? Image: particlezoo.net.

Another difference between gravity and the rest of the forces is that gravity is a lot weaker than the rest. And by a lot weaker, I mean really, really weak: so weak that it seems impossible that it could have originated at the same time and place as the others. In theory, all the forces should have been combined into one super-force at the Big Bang, and then as the Universe spread out and cooled down, they separated. We know this to be true for the electromagnetic and weak forces (they’ve been shown to combine into one “electroweak” force), and it looks like the strong force will fit. But where does gravity come in?

That’s the big problem physicists are still facing. Gravity is so different, and so unknown, that we really don’t know all that much about it, despite the fact that it’s the force we experience most. We can calculate it using Newton’s laws; we experience it every second; and yet it’s still one of the least understood areas of physics.

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