The goal of physics is to describe the laws of nature precisely and perfectly. That would make the science of physics exactly as complex as nature itself.
Preferably, it would be in the form of a single equation that would anticipate everything that could ever happen, and even why.
No one knows if such an equation is possible. Maybe the laws are infinitely complex, or maybe they aren't really laws at all.
I'm interested in the reason for physics in the exact same way others might be fascinated by Tolkien or new age beliefs. David Foster Wallace once wrote a complex literary book about mathematics. He was criticized because natural language was too vague for the task. This article has a similar problem if not worse. Anyone can appear to write knowledgeably by combining things they partially understand. However, like any human activity, physics has many hidden assumptions.
Progress has been slow since the 1960s. Physics has not gotten better like computer science has, but a few general principles have proven oddly useful. No one understands them or knows why they are useful. They may hint at deeper laws:
Nature is consistent. Not once have the laws of physics been violated at any point.
Conservation of energy: if some hypothetical event would allow perpetually accelerating motion, it's automatically impossible. Perpetual motion machines of the first kind include Cavorite and frictionless Portals. The accelerating expansion of the universe is not considered a form of motion though.
The Flatland metaphor: three-dimensional beings such as humans can visualize the fourth dimension only by imagining how a two-dimensional being would interact with the third dimension.
The Bell Curve (not related to Art Bell. Although...): All random repeating phenomena can be plotted on a probability curve. By observing only a few instances, one can predict all instances of said phenomenon.
That is why UFOs and paranormal claims are thought to have natural explanations. A few alleged cases can't be explained, but that is only to be expected, given a normal distribution.
Action equals reaction: objects can't change their speed or direction by themselves, or at least the speed and direction of their center of mass. An object will keep moving in a straight line at the same speed forever. The only way they can appear to change direction or speed is by expelling matter. Even then, the center of mass of the combined system hasn't changed. The center of mass of the universe is the Big Bang.
Holographic principles: if you could perfectly understand the smallest possible area of space, you could understand the whole universe.
Invariance: the laws of physics and the properties of space, time, and energy look the same regardless of the speed of an observer. An observer can never tell whether they are moving or whether the matter around them is moving.
Invariance works on many levels. Sometimes the laws of physics even appear scale-invariant. The earth from space looks exactly like a much smaller object close by. One might think it would get hazier or dimmer because of the distances involved. Copernicus and Galileo were among the first to know better when they realized the moon was a world obeying the same laws as Earth.
Aesthetics: simpler is better, more universal is better, more symmetrical is better.
Mach Principle: all particles in the universe define each others' momentum. That's how the water in a bucket 'knows' the bucket is rotating, causing it to flow towards the rim, since the fluid's internal arrangement hasn't changed at all.
The same thing would happen if the universe somehow started rotating around the bucket.
The reversibility principle: according to the laws of physics, any event can be time-reversed. Every process can happen the other way around, though not in immediately obvious ways.
A centrifuge suddenly spinning in the other direction would not generate negative gravity. For that, it would have to 'spin' all the way around the earth.
However, it would not be infinitely impossible for a large fraction of the particles of Earth to suddenly fly off into space, cooling the rest to near-zero.
There are no known laws to prevent gasses in the earth's atmosphere from absorbing heat from around them, and to assemble themselves into a reverse meteor accelerating into space. It's just incredibly unlikely.
One way to make it seem like time has reversed direction would be to simply reverse the direction of every particle at once.
Entropy almost always increases: particles moving randomly will almost always find themselves in a more chaotic state. Even when they become somewhat more organized by chance, they tend to return to chaos as soon as possible.
For most possible states of matter (almost all, in fact), entropy isn't an issue. A detailed description or a movie of particles moving chaotically would be just as plausible played in reverse. Higher energy levels seem to erase the arrow of time. Even gravity has no obvious preferred direction. Particles bouncing off each other look exactly like particles pulled together played backwards.
This is not true for the tiny minority of matter existing in an ordered state, including the matter of our world when viewed from a sufficient distance.
Physics is smooth. There are no arbitrary interruptions, granularities, pauses, or barriers between events, other than the speed of light. It's impossible for a physical system to have so much going on that nature can't decide what to do next, causing a reality crash. At most, time may appear to stop when viewed from afar.
Kinetic energy: an object moving twice as fast has twice the total energy or momentum. However, during a collision it releases twice the energy in half the time in the same distance. This means it will do four times as much damage. An object thrown up at twice the speed will stay in the air twice as long, but climb four times as high: twice as long times twice as fast.
Restate any problem as a different but equivalent problem to understand it better.
Vastly exaggerate the core essence of a problem to make its true nature become obvious.
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