What’s A Graviton? Or: How I Learned to Stop Worrying and Love Quantum Gravity

Part One of a Series on N=8 supergravity

This blog is called four gravitons, so I ought to explain what a graviton actually is. Starting from that, I can begin to explain N=8 supergravity, gravity’s highly supersymmetric cousin.

Let’s start like we often do, with a quick answer that will take some unpacking:

Gravitons are the force-carrying bosons of gravity.

I mentioned force-carrying bosons briefly here. Basically, a force can either be thought of as a field, or as particles called bosons that carry the effect of that field. Thinking about the force in terms of particles helps, because it allows you to visualize Feynman diagrams. While most forces come from Yang-Mills fields with spin 1, gravity has spin 2.

Now you may well ask, how exactly does this relate to the idea that gravity, unlike other forces, is a result of bending space and time?

First, let’s talk about what it means for space itself to be bent. If space is bent, distances are different than they otherwise would be.

Suppose we’ve got some coordinates: x and y. How do we find a distance? We use the Pythagorean Theorem:

d^2=x^2+y^2

Where d is the full distance. If space is bent, the formula changes:

d^2=g_{x}x^2+g_{y}y^2

Here g_{x} and g_{y} come from gravity. Normally, they would depend on x and y, modifying the formula and thus “bending” space.

Let’s suppose instead of measuring a distance, we want to measure the momentum of some other particle, which we call \phi because physicists are overly enamored of Greek letters. If p_{x,\phi} is its momentum (physicists also really love subscripts), then its total momentum can be calculated using the Pythagorean Theorem as well:

p_\phi^2= p_{x,\phi}^2+ p_{y,\phi}^2

Or with gravity:

p_\phi^2= g_{x}p_{x,\phi}^2+ g_{y} p_{y,\phi}^2

At the moment, this looks just like the distance formula with a bunch of extra stuff in it. Interpreted another way, though, it becomes instructions for the interactions of the graviton. If g_{x} and g_{y} represent the graviton, then this formula says that one graviton can interact with two \phi particles, like so:

graviton

Saying that gravitons can interact with \phi particles ends up meaning the same thing as saying that gravity changes the way we measure the \phi particle’s total momentum. This is one of the more important things to understand about quantum gravity: the idea that when people talk about exotic things like “gravitons”, they’re really talking about the same theory that Einstein proposed in 1916. There’s nothing scary about describing gravity in terms of particles just like the other forces. The scary bit comes later, as a result of the particular way that quantum calculations with gravity end up. I’ll say more about that later in this series.

2 thoughts on “What’s A Graviton? Or: How I Learned to Stop Worrying and Love Quantum Gravity

  1. Mark Ritchie

    Hi, is there anyway gravity can or could have changed on earth between today and say 12,000 years ago?
    With effects of ice age or the shift of ice age or anything further back that could have changed the gravity field or graviton components ?

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    1. 4gravitonsandagradstudent Post author

      So, it depends what you’re asking about.

      How much gravity you feel on the surface of the Earth depends on the shape of the Earth around you. So an ice age or the like can lead to very small changes, because the weight of the ice changes the distribution of rock below. People actually use this sort of effect today to look for things like underground lakes.

      This doesn’t change the force of gravity itself, of course. The same amount of mass still exerts the same gravity. It’s just a change in how much mass is near you, and thus how much gravity it’s exerting.

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