What If Time Moved Backwards?

If time ran backwards, the second law of thermodynamics would reverse — entropy would decrease instead of increase. Broken things would spontaneously reassemble, heat would flow from cold to hot, and every effect would precede its cause. The most fundamental problem isn't physical: it's that memory only works in one direction, so in a reversed-time universe, you'd remember your future and have no knowledge of your past.

Why Time Has a Direction at All

Here's the strange part: most of the laws of physics work equally well in both directions. If you filmed a collision of billiard balls and played it backwards, there's nothing in the physics that makes the reversed version wrong — the math works either way. The same is true for gravity, electromagnetism, and even nuclear forces at small scales.

The exception is the second law of thermodynamics: entropy — disorder — always increases in a closed system. A dropped glass shatters; shattered glass doesn't spontaneously reassemble. This asymmetry is why we experience time as moving in one direction. It's not a fundamental law of physics so much as a statistical inevitability: there are vastly more ways for things to be disordered than ordered, so disorder always wins.

What Entropy Reversal Would Look Like

In a reversed-time universe, every natural process runs the other way:

• Heat flows from cold objects to hot ones. Ice cubes make hot coffee hotter instead of cooler.
• Living organisms grow younger rather than older.
• Explosions run in reverse — shrapnel converges back into a whole object.
• Stars absorb light rather than emit it.
• Smoke pulls back into chimneys; carbon dioxide reconstitutes into fuel and oxygen.

From the inside, none of this would feel strange to the beings experiencing it — because their brains would also run backwards, and what they experience as memory would be records of the future, not the past.

The Memory Problem

Memory is a physical process: neurons form connections, chemical changes encode information about past events. In reversed time, those connections would form before the events they encode — you'd "remember" things that haven't happened yet (from the external perspective) and have no memory of what already occurred.

Physicist Sean Carroll has argued that our experience of time's arrow is entirely a consequence of the Big Bang starting in an extremely low-entropy state — an exceptionally ordered initial condition that we've been moving away from ever since.

This creates an identity paradox: would a reversed-time version of you even be "you"? Your memories, your sense of self, your understanding of cause and effect — all of it would be inverted. You'd know exactly how you're going to die but have no idea how you got to this moment.

Causality Paradoxes

Reversed time obliterates normal causality. Effects would precede causes: the bullet hole would appear before the gun fires, the crash would happen before the cars meet. Decision-making as we understand it becomes incoherent — you can't "decide" to do something based on information about its outcome if that outcome is already in your memory as a past event.

Some physicists have speculated about regions of the universe where time runs opposite to ours — separated by the Big Bang — where the same logic applies in reverse. The beings there would experience their time as forward and ours as backward, and neither could communicate with the other without violating causality.

Play With Time on whatifs.fun

For a more playful take on time manipulation, Time Travel explores the classic paradoxes. What If Time Freeze asks what happens when time stops entirely rather than reverses. Time Machine puts you in control of navigating history, and Time Perception tests how accurately you actually experience time passing — which turns out to be surprisingly bad.

For the full philosophical treatment of time manipulation, what if you could stop time explores the freezing scenario in depth. And what if you could time travel covers the causal paradoxes that arise from moving through time in any non-standard direction.