What relativity did to 'now'
Start with Einstein. Special relativity showed that whether two distant events happen 'at the same time' depends on how you are moving; there is no single, universal present that everyone shares. Different observers slice spacetime into 'now' differently, and no experiment can crown one slicing as the true one.
Already this dents the everyday feeling that the whole universe marches forward together through one shared instant. The intuition that there is a cosmic present ticking everywhere at once turns out to have no place to live in the physics.
The block universe versus the flowing present
This pushes many physicists and philosophers toward eternalism, often pictured as the 'block universe': past, present, and future all exist, laid out in a four-dimensional whole, and 'now' is no more metaphysically special than 'here'. The rival view, presentism, holds that only the present moment is real, with the past gone and the future not yet.
The tension is that relativity, taken at face value, fits eternalism more comfortably, since it treats all of spacetime on equal footing. On the block picture, the vivid sense that time flows, and that the future is open while the past is fixed, is a feature of how conscious creatures experience the world, not a feature of the world's basic structure.
Rovelli: time as emergent, not fundamental
The physicist Carlo Rovelli, in The Order of Time, pushes further. In his relational and quantum view, time is not part of the deep furniture of reality at all: the fundamental equations can be written without it. What we call the passage of time, he argues, emerges from two things, thermodynamics and perspective.
The direction of time, the sharp difference between past and future, does not appear in the microscopic laws (which run the same forwards and backwards); it appears only in the second law of thermodynamics, the tendency of entropy, or disorder, to increase. We remember the past and not the future because entropy was lower in the past. The felt 'flow' is tied to our blurred, large-scale view of a world whose every microscopic detail we cannot track. Reality and change stay real, Rovelli insists; it is 'time' as a single cosmic parameter that dissolves.
So is it an illusion?
The careful answer is that time is real, but several things we assume about it may not be fundamental: a universal present, an objective flow, perhaps even time's status as a basic ingredient of physics. Change and events are entirely real; what dissolves is the naive picture of one cosmic clock ticking the same 'now' everywhere.
On this reading, the arrow and rush of time are less like bedrock and more like temperature: perfectly real to us, yet emerging from deeper, timeless-looking laws. This remains an open, actively debated question, and one of the places where physics and philosophy genuinely meet rather than talk past each other.