Roberto Emparan: “Black holes are physics at its limits”

What is the relation between black holes and the theory of relativity?

Black holes are the simplest and most basic objects in which we can see the most relevant aspects of Einsteinʼs theory, and even though this theory is already a hundred years old, it is rich and complex enough for us so we have not taken everything it offers yet -not only experimentally but also theoretically.

Black holes are a prediction of the theory of relativity which took a long time to be accepted and understood: actually, Einstein never accepted it. What is more, he even wrote some articles denying the possibility of black holesʼ existence, although they didnʼt have that name then. Einstein died without knowing about the existence of these astronomic objects and not knowing that they showed the most dramatic consequences of his theory.

In what simple way can we understand how black holes behave?

The project I presented is based on an idea we started to elaborate a few years ago. The limit we took to understand black holes better -albeit it seems strange- consists on considering that the number of space dimensions, instead of being the usual one (three spatial and one temporal dimension) is endless. In previous works we realized that, with this idea, black holes get simplified.

Although it looks strange, it is not very different from what we do in physics, for example when we study the launch of a projectile ignoring air resistance.

Physics are the art of making approximations, like the joke about the spherical cow, in which in the first approximation a cow is a sphere to which some details are then added. This allows us simplifying the problem and making corrections.

Therefore, what we found is a way to change the theory of relativity in a way that black holes turn into that kind of spherical cow, and although we simplify its dynamics, we can later calculate properties and make corrections to get closer to their real behaviour.

Another result that was relevant regarding the black holes we saw some years ago is the fact that Einsteinʼs theories allow us understanding black holes as soap bubbles, since they satisfy the same elastic membrane equations. Specifically, the horizon of the black hole -an area with a complicated dynamic- is what we could see behaving as a soap bubble.  Therefore, it is a useful simplification and… the prettiest one!
Does this approximation have consequences on the idea we have about the Universe?

Maybe! It is a mathematic method that allowed us guessing how a black hole works and what its dynamics are. This limit allows us getting the essence of the black hole.

This is a new idea, not inspired on previous advances. The advanced grant our project received will allow us elaborating this idea from a theoretical perspective to know all about of its implications. The project will allow us contracting post-doctoral researchers to elaborate on the theory.

What role do black holes have in the Universe?

We donʼt know it for sure, but probably black holes have a more important role. The main thing is that, regarding the theory of gravity, they allow us going further than what Einstein said. For instance, when we try to have quantum mechanics in mind. Einsteinʼs theory was classy, and Stephen Hawking is the one who introduces quantum mechanics into gravitation.

Beforehand, it seems quite contradictory to introduce quantum mechanics to a gravitation theory.

The important effects in quantum theory are very small when in objects of a mass similar to the Sunʼs, or of a bigger mass, like black holes; but we think that in the origin of the Universe there could have been some microscopic black holes. In the primitive Universe, temperature and density were much higher and we can introduce the theory of quantum space-time. Somehow, black holes are physics at its limits.

To what point does the general relativity apply to systems that go further than the common gravitation systems?

This is one of the surprises of Einsteinʼs theory: it has been proved that the theory of relativity is not only useful to describe the cosmos but also to describe systems without gravity in a subtle way. Just like we use the theory of the soap bubbles to describe black holes making certain assumptions.

This is the case of the experiments that are being carried out on particle accelerators such as LHC, with particle systems that can be described in an easier way -and though it seems strange- as if this plasma was a black hole in a 5-dimension space.

Does this go with the idea that physics are the same at all levels?

Actually, good ideas work in lots of places, and Einsteinʼs theory of relativity is a very good theory, and it can be applied in lots of systems: with this theory we can understand phenomena related to superconductivity.

Personally, the theory of relativity is the cleverest theory we have, smarter than all of us. Sometimes theories are smarter than their creators, and Einsteinʼs case is clear: with the expansion of the Universe, with the black holes, his theory was telling him things he didnʼt fully accept.