Quantum entanglement and the illusion of time, in 79 minutes | Jim Al-Khalili: Full Interview

Our perception of time's passage is notoriously unreliable and paradoxical. Years seem to accelerate as we age: a single year when you're five feels eternal, stretching between birthdays, while that same year at fifty vanishes in a flash. One theory attributes this to the laying down of new experiences — childhood is dense with novel sensations that make time feel extended, while adult routines compress our sense of duration.

Yet shorter intervals behave oppositely. Half an hour in a dentist's waiting room with nothing to occupy your mind drags interminably, while half an hour at an enjoyable party — where you're actively laying down rich experiences and meeting people — flies by. This contradiction reveals that psychological time operates by different rules than the clock time physicists study. The subjective flow we experience so vividly has no counterpart in the equations that describe the external world.

Einstein's special relativity predicts that time slows down for objects moving at high velocity — an effect called time dilation. A clock on a spacecraft traveling near light speed ticks more slowly when viewed by a stationary observer, though the traveler aboard sees nothing unusual. This isn't science fiction: cosmic ray muons created in the upper atmosphere live long enough to reach Earth's surface only because their internal clocks run in slow motion relative to us. Without time dilation, they would decay before completing the journey.

General relativity adds another layer: gravity itself slows time. The stronger the gravitational field, the slower time runs. Your head ages faster than your feet, though by imperceptible fractions of a second. This isn't academic — GPS satellites orbit where gravity is slightly weaker than on Earth's surface, so their clocks tick faster. Engineers must account for this relativistic effect; without correction, GPS location accuracy would degrade rapidly. Every smartphone user relies daily on the reality of time dilation.

Special relativity's most counterintuitive consequence is the relativity of simultaneity. Two events that appear simultaneous to one observer occur at different times for another observer moving at high velocity. If I see two distant light flashes happen «at the same time,» someone speeding past me will disagree — they might see one flash before the other, or even in reversed order if the events are spacelike separated (far enough apart that no light signal could connect them).

This shatters the concept of a universal present moment. If observers can't agree on what «now» means across distances, then now has no absolute physical meaning — it's merely a local experience. Our strong psychological sense of a privileged present dividing past from future turns out to be a feature of consciousness, not of spacetime itself. In the block universe, «now» is just wherever your consciousness happens to be located along your world line, with no more fundamental significance than «here» has in space.

Even psychological time doesn't offer a sharp present moment. When an event occurs, light must travel to your eyes, signals must propagate to your brain, and neural processing must occur before you become conscious of it — a delay called perceptual latency that can exceed a third of a second. When exactly did the event happen for you: when photons left it, or when awareness dawned?

Moreover, we don't experience time as discrete instants. Listening to music, we don't hear isolated notes replacing each other; we perceive melody as a continuum. This requires episodic memory stitching together recent moments into an extended present. We simultaneously hold the immediate past in memory and anticipate the near future, creating what philosophers call the «specious present» — a duration with temporal thickness, not a knife-edge dividing past from future. Our experienced now is a construction, not a physical given.

Time travel to the future is straightforward physics. Travel near light speed or enter a strong gravitational field, and your clock slows relative to everyone else's — when you return, less time has passed for you than for them. You've «fast-forwarded» to their future. The film Interstellar depicted this accurately: astronauts near a black hole experienced one hour while seven years passed on Earth. This isn't hypothetical; it's proven by particle accelerators and GPS satellites.

Backward time travel is another matter. General relativity permits «closed timelike curves» — paths through spacetime that loop back to your own past — but these create paradoxes. Kill your grandfather before he meets your grandmother, and you erase your own existence, meaning you never traveled back to kill him, so he survives… and the loop continues. Proposed solutions include parallel universes (you slip into another timeline) or Novikov's self-consistency principle (you can only make the past turn out as it did, eliminating free will). Stephen Hawking joked that if backward time travel were possible, where are the tourists from the future? The answer may be that you can only travel back to when the time machine was first built — and we haven't built one yet.

One of the most striking results in theoretical physics is the Wheeler-DeWitt equation, an attempt to describe the quantum state of the entire universe by combining quantum mechanics with general relativity. The equation's most remarkable feature: it contains no time parameter. It describes a static, timeless universe — everything simply «is,» with no notion of change or evolution. Of course, this describes the universe as a whole from an external God's-eye view we can never occupy; we only perceive the universe from within.

The Wheeler-DeWitt equation has led physicists to propose that time is an emergent property, like temperature or wetness — something that appears when you zoom out from a more fundamental, timeless reality embedded in the quantum realm. Whether this is «weak» emergence (deducible from microscopic laws, like temperature from molecular motion) or «strong» emergence (genuinely novel, like consciousness from neurons) remains hotly debated. How a timeless quantum universe gives rise to our experienced time is still an open question, one of the deepest in physics.