I do not understand the concept of quantum gravity. As mass comes in particles and mass has gravity will gravity not naturally be quatised? If gravity has its own quanta and if this does not coincide with mass quanta won’t we get funny effects? A particle too small to meet the 1 gravity quantum will not have gravity while two might. Am I just being thick?

Nyall Davies from Suffolk (Age 55+)

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Filed under: age 55+, Answered Big Questions, Gravity Big Questions, Jo Benjamin's Big Answers, Quantum Physics Big Questions |

IPPPlings, on March 15, 2008 at 6:33 pm said:Hi there Nyall! The question of quantum gravity continues to stump some of our most clever scientists, so you’re certainly in good company!

By Einstein’s famous equation, mass is just a form of energy. The thing that gravity couples to is energy, not just mass. For example, photons (i.e. light), which are massless but have energy, are affected by gravity — this is why black holes are black! 🙂

In fact, the mass of a particle depends on the [quantum] interactions it has with other particles! (The question of `where does mass come from’ is related to something called `Electroweak Symmetry Breaking,’ which scientists hope to study in the near future with experiments at the Large Hadron Collider.)

Because if this, the relevant thing that gravity `sees’ is the energy of a particle, not just its mass.

One of the biggest open questions in physics is how to quantise gravity. In Einstein’s picture of gravity, which is experimentally verified on classical and cosmological scales (e.g. on the scales between the size of people and the known universe), gravity is related to the geometry (shape) of the universe.

However, at a quantum level, we think about forces as being mediated by [quantum] particles. (The technical statement would be that we think of these as “excitations of quantum fields,” but we can ignore this subtlety.) So, just as electromagnetism is mediated by particles called photons (the same particles that make up light!), gravity is mediated by particles called gravitons.

Gravity, however, is extremely weak! The fact that we can have magnets that pick up paperclips is an example of this. The little magnet can overcome the gravitational force of the *entire Earth* acting on the paperclip. Because of this weakness of gravity, it is impossible to measure gravitons in the lab in the forseeable future.

This, however, isn’t where quantum gravity becomes problematic.

From a conceptual point of view, it’s unclear how to reconcile the Einstein picture of gravity as geometry with the quantum picture as gravity as a quantum particle.

Secondly, even looking only at the quantum picture, gravity ends up being “nonrenormalisable.” This is a fancy word that means that in order to make definite predictions, one would have to make an infinite number of measurements. Part of the problem does, in fact, include the self-interactions of gravity — as you alluded to in your question! (i.e. gravitons themselves emit gravitons…)

So with regards to the model you’ve described of gravity naturally being quantised, there are a few points to make. Since it’s not the mass of a particle that couples to gravity, but rather the particle’s energy, it’s not quite correct to say that gravity is naturally quantised. Further, the phrase `quantisation’ refers to the discrete excitations of a `quantum field’ forming `particles’ — i.e. what we mean by quantum gravity is that gravity can be thought of as being mediated by graviton particles. The `size’ of a particle doesn’t determine how it interacts with gravitons, just its energy.

I should note that quantum gravity is seen by many to be the `holy grail’ of theoretical physics. Some people refer to this as a `theory of everything’ because gravity is the only force that has escaped a proper quantum description. Doing so would unite the two major discoveries in physics in the past century (quantum theory and general relativity).

Currently, the most promising approach to quantum gravity is string theory. But this field is, itself, still very much in its infancy and there’s a lot left for scientists and mathematicians to understand.

So, once again, you’re in quite good company!