Entropy counts possible arrangements of a system and is tied to missing information. Its tendency to rise is the usual basis for time's direction, though the details remain actively studied.
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The square-cube law says volume grows faster than surface area as things get bigger. Kleiber's law says metabolic rate scales roughly with mass to the 3/4 power, though that exponent is debated.
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Bell's theorem rules out local theories matching quantum predictions; the measurement problem shows quantum theory alone does not say why single outcomes occur. Neither is settled in interpretation.
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Maxwell's demon is a 1867 thought experiment in which a tiny sorting being seems to defy the second law of thermodynamics. Information theory and the cost of erasing memory resolved it.
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Landauer's principle says erasing one bit of information must dissipate at least k_B T ln 2 of energy as heat. At room temperature that is about 2.9×10^-21 joules.
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Ordinary computers use bits that are 0 or 1; quantum computers use qubits that can be in superpositions and be entangled, which helps with some tasks but is hard to engineer.
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Special relativity says light's speed is a fixed limit: massive objects would need infinite energy to reach it, and faster signals would break cause and effect. Why nature works this way is unexplained.
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It says that position and momentum cannot both be sharply defined at once; their uncertainties have a minimum product of ħ/2. This is not just a limit of instruments.
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Moore's Law is the observation that chip transistor counts double about every two years. Transistor shrinking has slowed sharply, but whether that means the end depends on how you define the law.
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A Turing machine is an abstract mathematical model of computation: a head reading and writing symbols on an unbounded tape according to a finite rule table. It defines what is computable.
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Machines that run forever without an energy source would break the laws of thermodynamics, and real systems also lose energy to friction and drag.
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Error-correcting codes store extra check bits alongside data. When data is read, the check is recalculated, so a flipped bit can be located and fixed automatically.
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Hawking's calculation implies that black holes destroy information as they evaporate, which conflicts with quantum mechanics. Most physicists now think information survives, but how is debated.
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The halting problem is proven unsolvable by any algorithm. P versus NP asks a different question, whether efficient finding is as easy as efficient checking, and it remains open.
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