The "nature of time" is arguably the single most frustrating open problem in science. It sits at the exact intersection where our most successful physical theories crash into each other, and where our subjective human experience directly contradicts our objective descriptions of reality.
Here is a detailed breakdown of why physicists and philosophers are still struggling to define what time actually is.
1. The Clash of the Titans: Relativity vs. Quantum Mechanics
The biggest reason physicists struggle with time is that their two best rulebooks for the universe basically hate each other, and they disagree primarily about time.
General Relativity (The Rulebook for the Big): In Einstein's universe, time is malleable. It is a dimension, woven together with space into a 4D fabric called "spacetime." It is not a steady background beat; it stretches and warps. If you are near a black hole, time moves slower. If you move fast, time slows down relative to someone standing still. In this view, the "past, present, and future" are all equally real, existing simultaneously in a "Block Universe."
Quantum Mechanics (The Rulebook for the Small): In the quantum world, time is rigid. It is treated as a universal, absolute background parameter—a steady drumbeat that marches on regardless of what particles are doing. It is not a dimension you can move back and forth in; it is just a label for when things happen.
The Crisis: When physicists try to combine these two into a "Theory of Everything" (Quantum Gravity), the math breaks. Specifically, in the Wheeler-DeWitt equation (often called the master equation of quantum gravity), the variable for time ($t$) completely cancels out. The equation suggests that the universe, at its most fundamental level, is static and timeless. This is known as the "Problem of Time"—the math says time shouldn't exist, but everything we experience says it does.
2. The Philosophy Wars: Presentism vs. Eternalism
While physicists fight over equations, philosophers (and philosophically minded physicists) fight over what parts of time are "real."
Presentism: The view that only the "now" exists. The past is gone, and the future hasn't happened. This aligns with our intuition but conflicts with relativity. If "now" is relative (simultaneity is relative to your speed), then there is no single "now" across the universe, making Presentism very hard to defend scientifically.
Eternalism (The Block Universe): The view that all time exists at once. The past, present, and future are like coordinates on a map. Just because you aren't in "Paris" doesn't mean Paris doesn't exist; similarly, just because you aren't in "1920" doesn't mean 1920 isn't real. In this view, the "flow" of time is an illusion of consciousness, similar to how a movie is just static frames but looks like motion.
The "Growing Block": A middle ground where the past and present are real, but the future is not yet real—the "block" of reality is constantly adding new slices.
3. The Paradox of the "Arrow of Time"
If the laws of physics are so smart, why can't they tell the difference between a movie played forward and backward?
Time-Symmetry: Almost all fundamental laws of physics (Newton’s laws, Electromagnetism, Relativity) work equally well forward or backward. If you reversed the velocity of every particle in the universe, physics says the movie should just play backward perfectly.
Entropy: The only major law that cares about direction is the Second Law of Thermodynamics, which states that entropy (disorder) tends to increase. This gives us the "Arrow of Time." We break eggs (low entropy $\rightarrow$ high entropy), but we never see eggs un-break.
The Mystery: Physicists still struggle to explain why the universe started in such an incredibly low-entropy (highly ordered) state in the first place. If it hadn't, we wouldn't have an arrow of time at all.
4. Radical Modern Solutions
Because the standard theories are stuck, prominent modern physicists have proposed radical alternatives:
"Time is an Illusion" (Carlo Rovelli): Rovelli, a founder of Loop Quantum Gravity, argues that time is not a fundamental ingredient of reality. He proposes the "Thermal Time Hypothesis," suggesting that what we perceive as time is just a statistical blur. Just as "heat" is just the average jiggling of atoms, "time" is our macroscopic, blurry perspective on a timeless quantum reality.
"Time is the ONLY Reality" (Lee Smolin): On the opposite side, Lee Smolin argues in Time Reborn that the "Block Universe" is a mistake. He believes laws of physics are not timeless truths but actually evolve over time. For him, time is the most fundamental thing there is, and even space might just be an illusion emerging from time.
5. The Brain's Deception
Finally, neuroscientists warn that our feeling of time might have little to do with physics.
The Specious Present: We don't perceive the "now" as an infinitesimally small point. Our brain constructs a "thick" present (roughly 2-3 seconds long) where we hold the immediate past and anticipated future together. This is why we can hear a "melody" rather than just isolated notes.
Predictive Coding: The brain is a prediction machine. The feeling of "flow" is likely the brain constantly updating its internal model of the world. When we are bored, we pay more attention to the updates (time drags); when we are having fun, we ignore the updates (time flies).
Summary
We struggle to understand time because we are trying to reconcile three things that don't fit:
Quantum Mechanics: Says time is a rigid background stage.
Relativity: Says time is a flexible dimension in a static block.
Human Experience: Says time flows, has a direction, and has a special "now."
Until a theory of Quantum Gravity can bridge these gaps, the nature of time will remain a mystery.