Science and Interstellar

Salon has a brief excerpt from Kip Thorne’s book The Science of Interstellar, in which the author/physicist attempts to explain some of the scientific concepts used in the film. I still haven’t seen Interstellar, but I’ve heard mixed things from other physicists and astrophysicists (mostly that the visuals are cool but the plot is lacking and fairly nonsensical).

The excerpt isn’t bad but I think it’s probably a bit too high-level for most readers since all the material explaining many of the concepts is hopefully in the book but left out of the Salon piece.

It seems that the science behind the crazy things like wormholes and black holes in the film are based on the idea that our universe is a four-dimensional space embedded into a larger space. Think of this as a higher dimensional analog of creatures living on the surface of a ball. To those creatures, the universe (a sphere) is three dimensions, but we know that it is four (one of those is time in both cases). This idea shows up quite a bit in modern theoretical physics and in some sense is the basis of string theory and M-theory, a generalization of string theory. In both cases more than four dimensions are required to get sensible physics out of the theory, so these extra dimensions can be both small (compactified) – leading only to quantum effects – or large, where the particles and fields of the Standard Model are somehow confined only to our four dimensional space. In some theories, you can let gravity operate in all dimensions, which can explain why gravity is so much weaker than the other forces: it operates in more dimensions, diluting its apparent strength. This last points appears to be an important part of the physics of the movie.

A few things on terminology:

  1. The bulk: The greater dimensional space that the universe is embedded into. Gravity can travel throughout the bulk, so objects in other universes can have effects on ours through gravity.
  2. Branes: Higher dimensional objects. A particle is 0 dimensions, a string is 1, and a brane is an arbitrary number (up to the number of dimensions in the bulk). Our universe can be seen as a 4-dimensional (3 space + 1 time) brane embedded in the bulk.
  3. AdS: Anti-de Sitter space. This is the name for the spacetime with a uniform negative curvature. In the film, we are not creatures living on a sphere (which is the analog for de Sitter space), but rather creatures living on an infinitely large saddle or potato chip. In reality, as far as we can tell, space is flat, so any curvature would have to be incredibly small.
  4. Tidal forces: Non-uniform forces on different parts of an object. For example, the force of gravity on Earth from the moon is slightly stronger on the side of Earth nearest the moon, so fluids such as water are being pulled to the near side of Earth (instead of everything being pulled together by the same amount), creating tides.
  5. Action: The time integral of a Lagrangian or integral over space and time of a Lagrangian density. In field theories, the Lagrangian density is a function of fields and field derivatives that contains all the physics of the system. The system evolves in such a way that extremizes (maximizes or minimizes) the action, so the evolution of the system can be understood by applying calculus of variations to the action. Part of what theoretical physicists do is to come up with Lagrangians (either for fundamental theories or for effective theories applying only under certain conditions) that describe either novel or poorly understood physical systems in order to understand their properties. These are often designed to fix problems with our current incomplete theories. For example, the Higgs boson comes out of a Higgs field added to the Standard Model Lagrangian in order to fix some problems with unitarity and gauge invariance that are introduced when massive vector bosons (the W and Z) are added to the theory.
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