Irregular Webcomic! #3552

Gravitons are hypothesised elementary particles related to gravity. According to the hypothesis, gravitons mediate the gravitational force. What this means is that gravitons play the same role for gravity that photons (particles of light) play for the electromagnetic force.

We know about photons, and they are easy to detect. Heck, our eyes detect them. A photon can be thought of essentially, as a tiny packet of electric and magnetic field, fluctuating in time and space as it propagates at the speed of light. (For more about photons and their relationship to electromagnetism, see the annotations to comics #1420, #3219, and #3255.)

When two objects interact electromagnetically, you can understand it in terms of the objects exchanging photons. The electric and magnetic fields the objects produce result in probing fingers of photons heading outwards. When these photons meet the other object, they can be absorbed, thus transferring momentum from one object to the other. In this way, two electrically or magnetically charged objects can either attract or repel one another. but One object gains momentum in one direction, and the other gains the same amount of momentum in the opposite direction, so overall momentum is conserved. The photons that mediate the electromagnetic force in this way are often termed virtual photons, because they exist only for fleetingly short periods of time.

Besides the familiar electromagnetic force, the strong nuclear force and weak nuclear force are also mediated by exchange of particles. In these two cases, the exchanged particles are gluons (for the strong force) and W and Z bosons (for the weak force). Both of these particle types (gluons and W/Z bosons) are different from photons, but in different ways, and these differences manifest in the behaviour of the forces.

An important difference between electromagnetism and the weak force is that the W and Z bosons have mass whereas photons don't. Electromagnetism has an infinite range, though it gets weaker with distance according to the inverse square law. This is because the virtual photons generated by charged objects are massless, and so can propagate to limitless distances without violating conservation of mass.^[1] The weak nuclear force, however, is mediated by particles with mass, and these virtual massive particles can only travel a limited distance from their source before vanishing. The result is that the weak force has a limited, and quite short range.

On the other hand, the strong nuclear force is mediated by gluons, which are massless like photons. However, gluons carry what is known as colour charge. As gluons travel further away from a particle, this colour charge potentially becomes more visible to outside particles. Here a phenomenon known as colour confinement kicks in, making it harder to pull the gluon away, as the colour charge requires more and more energy to pull it away. Eventually the energy gets so high that it creates quark-antiquark pairs of appropriate colour charges to neutralise the bare colour of the gluon. The result is like stretching a rubber band until it snaps... but then replacing it with two rubber bands tied together. Each time you stretch beyond a certain point, a new rubber band (i.e. a new quark-antiquark pair) gets inserted into the gap. The result of all this is that the strong force is essentially constant with distance, not falling off in strength at all. Which is why it keeps the subatomic particles it acts on really close together. Pull them apart to any distance at all, and they just snap back together, unlike pulling masses apart and overcoming the weakening force of gravity with distance.

Despite these vast differences in how they work, the weak and strong forces both only effectively apply in the realm of subatomic sizes (for completely opposite reasons!), doing their thing to keep atoms together (or in the case of the weak force, enable radioactive decay to split atoms apart).

Similarly, we think that gravity might be mediated by particles in the same way. We know the range of gravity is infinite and it falls off as an inverse square law, like electromagnetism, because it's clearly in charge of what happens in the greater universe when we examine it with telescopes. So this means any particle which mediates gravity has to be massless. There are some other considerations that tell us other properties of gravitons, like their quantum spin property and so on. Basically, we know everything about gravitons... except how to detect them. Well, we have some idea, but the equipment we'd need to run the experiments would be larger than our solar system.

And you thought the Large Hadron Collider was big.

[1] It's a little more complicated than this, since photons carry energy (which we know from Einstein is equivalent to mass). But it's virtual energy, so it's all kind of okay. Suffice it to say that it's more complicated, but I don't want to go further into the details today.