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As we understand physics today, all the matter we see in the world is made up of not the four classical elements of the Greeks, but just three elementary particles: the ‘up’ quark, the ‘down’ quark, and the electron. That’s it. Everything else is just detail.
three ‘pillars’ of modern physics: relativity, quantum mechanics, and thermodynamics.
The world of physics only really came of age in the seventeenth century, thanks to a large extent to the invention of the two most important instruments in all of science: the telescope and the microscope.
When physicists say that a physical system has a symmetry, they mean that some property of that system stays the same when something else changes.
Noether’s theorem tells us that we don’t ‘invent’ the mathematics in order to have a way of describing the world, but rather, as Galileo observed, that nature speaks the language of mathematics, which is ‘there’, ready and waiting to be discovered.
Individual perspectives of space and time separately are relative, but combined spacetime is absolute.
According to his general theory, matter and energy create a gravitational field, and spacetime is nothing more than the ‘structural quality’ of this field. Without the ‘stuff’ contained within spacetime, there is no gravitational field and hence no space or time!
On the microscopic scale, materials are held together by the electromagnetic forces between atoms. On the cosmic scale, it is gravity that holds matter together.
That the Big Bang happened everywhere within already infinite space, rather than at some particular ‘place’, is an important concept to grasp.
The entropy of a system, if left alone, will always increase: that is, a system will always relax from a ‘special’ (ordered) state to a less special (mixed-up) one. Physical systems unwind, cool down, and wear out. This is referred to as the second law of thermodynamics, and at its heart it is no more than a statement of statistical inevitability: if left alone, everything always eventually returns to a state of equilibrium.
