To a physicist, the literal answer to the question 'what makes the world go around?' is - angular momentum!
Angular momentum is the momentum carried by an object due to its rotation about some axis (think of a ceiling fan or airplane propeller). Angular momentum actually plays a deep role in the physics of the universe and is tied to the stability of our own existence in crucial ways. In this post let us look at it broadly in the context of classical and quantum physics:
Classical physics
Classical physics concerns, roughly speaking, the physics of large objects.
i) Days of Our Lives We would have much longer days and nights - each a half-year long! - if the Earth did not rotate around its own axis and just revolved around the sun. I am sure you would not like to have such a long work day.
Why does the Earth rotate about its own axis? Because the dust and gas particles from which it was formed 4.5 billion years ago had some initial momentum about that axis. They could not suddenly come to a stop when earth formed, because, according to the laws of physics angular momentum - like energy - has to be conserved.
ii) How Not to get Fired We would fall into the Sun, attracted by its gravitational pull, if the Earth did not revolve around it. It is the centrifugal force on the Earth due to its revolution that balances the Sun's pull and stops it from barbecuing us.
This is true for all the planets in the solar system. All of them rotate around the sun in the same direction, indicating the angular momentum of the particles - from which the solar system was formed - about the Sun. Thus, angular momentum is responsible for the stability of the solar system. I hope you are impressed.
Afterword:
i) The Sun itself is not immune from angular momentum. It rotates about its own axis, once every 27 days. And it revolves around the center of our galaxy - once every 220 million years. In other words, in the 4.5 million years of its existence, the Sun has completed about 20 orbits around the galactic center.
ii) Awe-inspiring evidence of the role of angular momentum in the formation and evolution of large scale structures in the universe can be found in images of spiral galaxies.
Quantum physics
Quantum physics concerns, roughly speaking, the physics of small objects.
i) Use with Discretion In quantum physics also, angular momentum is conserved, just as in classical physics. But surprisingly, unlike in classical physics, where angular momentum can take continuous values (for example, increasing smoothly in speed), in quantum physics, angular momentum can only take certain discrete values (for example, increasing in steps of speed).
Such discrete values are taken on, for example, by electrons revolving around a nucleus in an atom. The rules obeyed by angular momentum - conservation and discreteness - determine the structure of atoms, the organization of matter in the periodic table, the interaction of light with matter, and much of chemistry, all vital to our existence.
ii) Take Me for a Spin Quantum physics presents a second surprise, with regard to rotation. This is the existence of a kind of angular momentum - called spin angular momentum - that cannot be thought of in terms of mechanical rotation (which is often referred to as orbital angular momentum).
Some scientists have tried to make sense of spin by recalling that according to quantum physics, every object can be thought of as a particle as well as a wave. These researchers relate orbital angular momentum with the particle aspect of the object and spin with the wave aspect.
In any case, every particle in the universe has a spin associated with it, which is denoted by an integer (for bosons) or half-integer (for fermions). The famous Pauli principle forbids two fermions (such as electrons or neutrons) from occupying the same quantum state. Since matter is mostly made of fermions, it is the Pauli principle that prevents matter - and therefore us - from collapsing on itself, since the fermions (assuming that they otherwise have the same attributes) are not allowed to occupy the same location in space.
Afterword: Black holes cause matter to collapse gravitationally. Is the Pauli principle overcome in that case? The short answer is we do not really know, as currently there is no theory which successfully combines quantum physics, which informs the Pauli principle, and gravity, which creates black holes.
However, at the singularity, or center, of a nonrotating black hole, for example, all laws of physics are suspended. So the Pauli principle likely no longer holds there and cannot stop the implosion of matter.
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