Season 17, Episode 24: Why Should Gravitons Even Exist?

Hey StarTalkians! Season 17, episode 24 was another Cosmic Queries edition, where Neil and Chuck sat down to answer viewer questions. This included an absolutely fantastic question about whether gravity is a true force and whether the graviton is even needed:

Cosmic Queries – Total Darkness - StarTalk Radio

(Question starts at 25:50)

Neil’s answer covered the basics but left the core idea hanging. So let’s add to it a little.

Virtual and Free Particles: The Photon and Electromagnetism

As argued in an informative Big Think article, we need to distinguish between free and “virtual” particles to understand why gravitons probably exist.

For electromagnetism, the “free” photon is the thing associated with light waves. Light waves are made of photons, similarly to how water waves are made up of water molecules.

But electromagnetic interactions – like two magnets attracting each other – are mediated by virtual photons. These are “virtual” because they can’t be observed; they’re more like abstractions that help us solve problems and predict experiments . They’re a manifestation of the underlying field.

Virtual and Free Gravitons

So if we can describe gravity in quantum terms – a big “if” – we’d use a virtual particle to carry the force too. This would be exchanged during gravitational interactions, and would arguably only exist in our calculations.

But gravitational waves also exist, and like a light wave, these are probably composed of free gravitons. You can even recreate the famous LIGO result in this framework. It works exactly like the photon example.

Answering the Question

Now we can finally address this fantastic question. It reflects a common misconception about gravity, that it is “not a force” in some way. Neil shared the key quote: “Spacetime tells matter how to move; matter tells spacetime how to curve.”

The thing the question (and Neil’s answer) misses is: how does matter tell spacetime how to curve? Once it is curved, the question is right – there’s no typical force at play. But the fact that matter curves spacetime at all is the effect of the force. Einstein’s equations don’t describe interactions between two masses, but the interaction between mass or energy and spacetime itself.

Matter’s interaction with the Higgs field (via the famous boson) gives it mass. Virtual gravitons, on the other hand, mediate the interaction between that mass and the fabric of spacetime.