Light cone == gravity cone?


I’ve heard of a light cone, but never heard the term gravity cone. Seems to me a light cone is kind of misnamed or arbitrary. It’s more like an information cone. There’s nothing special about light. You just can’t feel things that happen outside the light cone. So if a big object moves outside your light/gravity cone, you’ll never feel the gravity wave. Should it just be thought of as a gravity cone? Or a causality cone? Or an information cone?


Am I misunderstanding this concept?

In: 10

No. You are not misunderstanding the concept at all. It’s just called a light cone because the speed of light in a vacuum is a convenient way to measure the maximum speed of causality.

I think you’ve got a decent grasp of the concept; it’s just that naming things is hard. You’re right that it’s not a *light* cone specifically, but then again, it isn’t really a cone either. The names are based on specific thought models (“imagine 2D space as XY coordinates and time as Z; now imagine the cone that extends upwards along the Z axis that corresponds to how far a photon could reach over time from any given point on the XY plane…”). There’s always going to be some slippage between the model and the reality it describes.

Gravity cone? Sure. Causality bubble? Why not! The name itself is ultimately a little less important than the concept it’s gesturing at.

As you noticed, there are many things moving at c: gravity, light in vacuum, more generally an electro-magnetic field, information, causality, …; and absolutely everything is (locally) bounded by c, nothing can exceed it.

Ultimately, pick whatever name you prefer for yourself. But in communication with other people, it is better to use established names, in this case “the speed of light (in vacuum)”. However:

> causality cone

This one probably is closest to the truth, as it is the most general.

One should now probably talk about entanglement, local realism, and a bunch of other things that sound like they either violate causality and/or are faster than c, but this probably leads to so many other things… anyway, there are already some ELI5 on those.

It is more to do with the fact that the first actual measurements of “stuff” going at this speed happened to be based on visible light, and even as the concept got expanded to include other phenomena, the name stuck

In a way, its similar to how to “horsepower” is applied to things that have never even involved horses, but happen to use an engine that was originally measured in terms of how many horses it could do the work of

It’s a causality cone. You’re right, there’s nothing “special” about light except that it’s massless, which allows it to move at the speed of causality.