Energy is conserved, but it gets spread out
Conservation says energy doesn't disappear. Thermodynamics adds a second statement: energy spreads out as time moves forward. A hot cup of coffee gets cooler. A bedroom gets messier without active cleaning. A rocket's chemical fuel turns into heat, sound, and slightly warmer atmosphere — the same total energy, but now distributed across many more particles, each with less of it. The universe runs in one direction: more spread, never less.
The technical name for this spread-out-ness is entropy. The second law of thermodynamics: in any closed system, entropy never decreases. Energy still totals the same; it's just dispersed across more components.
Why this is the arrow of time
Almost all the laws of physics work the same forwards or backwards in time. F=ma doesn't care about direction. Conservation of momentum doesn't care. The only law that cares is the second law of thermodynamics. Time has an arrow because entropy increases. If you watched a movie of a shattering glass played backwards, you'd know — because shards reassembling into a glass would be an entropy decrease, and that's not how the universe runs.
The asymmetry of past and future is, at its base, the asymmetry of how energy spreads. Memory exists because entropy was lower yesterday. Anticipation exists because entropy will be higher tomorrow. Life is a temporary local pocket of low entropy, kept going by an external energy source (the Sun, mostly) — and even life ends and gets reabsorbed.
Why life feels like an exception (and isn't)
Living things look like they're decreasing entropy — they're highly organized, they grow, they replicate. But they're a local decrease at the cost of a much larger global increase. Plants take ordered sunlight and use it to organize molecules into themselves; the Sun pays the entropy bill (and pays it big). Animals eat plants and other animals. The chain ends in the heat your body radiates and the carbon dioxide you exhale, both of which are higher-entropy than what came in.
Earth as a whole is an entropy-increasing system. Life is the local fluctuation that uses the Sun's energy gradient to keep itself organized. The Sun will eventually exhaust, the gradient will close, life on Earth will end with it. The second law sets that horizon.
Why this is one of the most beautiful frames
The arrow of time, your sense of past and future, the irreversibility of broken glass, the inevitability of mortality — all of these are one law, said differently. The universe isn't running down because something's wrong with it. It's running in the only direction running is possible. The forward direction is just the direction in which energy spreads. Everything else — memory, biology, civilization — is what happens along the way.