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### The Data Structure of the Universe

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Physicists manipulate the interactions among these objects so the system resembles an interaction among more fundamental particles. For example, in circuit quantum electrodynamics, researchers use tiny superconducting circuits to simulate atoms, and then study how these artificial atoms interact with photons.

Or in a lab in Munich, physicists use a superfluid of ultra-cold atoms to settle the debate over whether Higgs-like particles can exist in two dimensions of space the answer is yes. These simulations are not only useful to overcome mathematical hurdles in theories we already know. In a recent experiment, for example, Raymond Laflamme , a physicist at the Institute for Quantum Computing at the University of Waterloo in Ontario, Canada, and his group used a quantum simulation to study so-called spin networks, structures that, in some theories, constitute the fundamental fabric of space-time.

And Gia Dvali , a physicist at the University of Munich, has proposed a way to simulate the information processing of black holes with ultracold atom gases.

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A similar idea is being pursued in the field of analogue gravity, where physicists use fluids to mimic the behavior of particles in gravitational fields. In addition, physicists have studied hypothetical fundamental particles by observing stand-ins called quasiparticles.

## Theoretical Nuclear and Particle Physics

These quasiparticles behave like fundamental particles, but they emerge from the collective movement of many other particles. Understanding their properties allows us to learn more about their behavior, and thereby might also to help us find ways of observing the real thing. This line of research raises some big questions. First of all, if we can simulate what we now believe to be fundamental by using composite quasiparticles, then maybe what we currently think of as fundamental — space and time and the 25 particles that make up the Standard Model of particle physics — is made up of an underlying structure, too.

Quantum simulations also make us wonder what it means to explain the behavior of a system to begin with.

The classical field theory of electromagnetism breaks down at very short distances, or in the presence of very strong fields. It becomes necessary to assume that this field is not just a number at every point of space and time, but a "quantum operator", having definite but somewhat complicated mathematical properties. The quantum field reduces to the classical one under familiar circumstances, but differs sharply from it in some regimes of distance or energy.

In quantum theory, a field is not just something associated with waves, but is also related to particles by virtue of the well-known wave-particle duality. An elementary particle is a sort of coherent excitation of a quantum field. Thus, the electromagnetic field must be associated to a fundamental particle occurring in nature.

Indeed, such a particle exists and is called the "photon". An intuitive picture of an electromagnetic interaction, as described by quantum theory, is that the field quantum is exchanged between the objects that interact.

Thus, a pair of magnets, when they approach each other, exchange photons, and it is this exchange that leads to the force between them. One may say that the existence of the photon is predicted by the very existence of quantum electromagnetic interactions.

## Aspen Center for Physics

Remarkably, all the interactions that are needed to explain chemistry and, as far as we know, biology are electromagnetic in nature. Atoms interact electromagnetically to form molecules and compounds. In a sense, therefore, we could claim that electromagnetism which is correctly described by a quantum field theory is a "unified theory of chemistry".