# In the Causal Set theory, what actually is a causal set?

The causal set theory is an approach to quantum gravity. But don't understand what it claims spacetime to be made of. What is a causal set? Is it a physical object or just a spacetime event? I read the wiki page and watched a few videos on it, but I still just don't understand it.

• To improve your chance of an answer, you should include research you have done yourself, or cite books you have read.
– user146020
Mar 23, 2017 at 22:48
• @JohnForkosh So, according to the Causal Sets spacetime emerges by a set of events?
– 2117
Mar 24, 2017 at 0:32
• Yeah, as I understand it, which isn't much. It's the causet structure (poset ordering) that's axiomatically fundamental, and everything else emerges (is constructed) from that. But, to repeat, I know next-to-nothing about it (and that's approaching zero from the negative side:) I'd started reading arxiv.org/abs/gr-qc/0106024 mostly just for its first two chapters as an intro to causets. Or you might try arxiv.org/abs/gr-qc/9909075 for an even briefer intro. Personally I found Dou's better. The introductory parts of theses often seem to have meaty-yet-accessible discussions.
– user89220
Mar 24, 2017 at 5:00

A causal set builds a universe from just two things: particles and actions. This means that spacetime is made up of particles and each particle has a family history of causes from which it emerged.

Light traveling through space is a set of causally linked particles even though we think of space as empty. Of course, quantum space is actually chock full of vacuum oscillators and so space is already a causal set. All matter comes from other matter and so matter is also a causal set.

The real issue is what exactly does a causal set do for explaining reality. Quantum charge is already a causal set and so making quantum gravity a causal set seems to set the stage for unification of charge and gravity. However, that has not yet been the case.