Physics explains a tremendous amount about how we see the world, but it has limits. One mystery physics can shed very little light on: why time seems to go forward. And it turns out this mystery has immense implications to how we live our lives.
Football season has started, and games are filmed from many different angles. Sometimes when a scene is re-played, we'll see the film go in reverse.
Like the re-wind, the equations of physics are reversible. They work just as well whether you define time as going forwards or backwards.
When the quarterback throws, energy stored in his muscles is transferred to the ball. A tiny bit is lost as heat from air resistance as the ball travels, but most of the energy is still within the ball when the receiver catches it.
If you've ever caught a ball thrown by someone who really knows what they're doing, you feel the energy. A stinging slap against the skin of your hands, immediately followed by a shock traveling up the bones of your arms. Less felt but still present, the energy transfers down through your feet.
It's equally valid to describe stored energy coming out of the ground, travelling through the receiver's skeleton into the ball, which then flies backwards (absorbing heat out of the air) into the passer's hand. The energy then fuses the carbohydrate back into the passer's muscles.
(If this seems impossible, remember everything has to go in reverse. The game timer ticks up instead of down, a hot dog comes out of someone's mouth and goes back to concession to un-cook, the grass on the field un-grows as light streams out of it back to the Sun, etc etc).
Reversability holds true from the highest to lowest levels. A photon strikes an electron, the energy excites the it, and a tiny moment later, another photon gets re-emitted.
Our Milky Way galaxy is on a collision course to hit the Andromeda galaxy in a few million years, but it's just as valid to say that the galaxies already collided, and now are rebounding away from each other. We could go on to say that the energy from that collision will go on to fling the Milky Way into particles of dust, eventually collapsing into a single point – the "gnaB giB" in reverse.
Some physicists have dedicated their careers to finding any sign of the 'arrow of time' – some tiny clue why time appears to go one direction and not the other. Bless their hearts, they're not having very much luck.
In our everyday lives, it seems there must be some difference, because we feel time pass. But it also seems like my house stays in the same place and the Sun moves over it.
Today, we only know the opposite is true because of hundreds of years of difficult work. Work which caused tremendous doubt, grief, anger, as people had to gradually let go of what seemed obvious and accept what was inescapable.
If time is truly and completely reversible, it means today is tomorrow's yesterday. There would be no difference between past, present and future.
Only moments of experience. Just mysteries.