Original episode:https://youtu.be/8xp3Bs6nZ-Y · Timestamps are clickable — they seek the player in place
Physicist Jim Al-Khalili discusses the physics of time, outlining what he calls the four distinct "problems of time": whether time flows, how to reconcile quantum field theory with general relativity, the special nature of "now," and the origin of time's direction [00:18]. He distinguishes between "physical time" as described by physics and "manifest time," our subjective perception. The talk traces the evolution of the concept from Newton's absolute time to Einstein's relative spacetime, explaining time dilation through special and general relativity. It then explores the "block universe" model, the challenge of unifying physics (the Wheeler-DeWitt equation), the thermodynamic and quantum origins of the arrow of time, and finally, the physical possibilities and paradoxes of time travel.
[00:00] - [01:55] Introduction to the four problems of time and the distinction between physical time and manifest (psychological) time.[01:55] - [05:34] The subjective experience of time's flow, its perceived speeding up with age, and the classical physics view of absolute time (Newton).[05:34] - [08:48] How Einstein's theories of relativity overthrew the idea of absolute time, showing that time is relative.[08:48] - [14:36] Special relativity: how the constant speed of light leads to time dilation, with the example of atmospheric muons.[14:36] - [19:02] General relativity: how gravity slows down time, a real effect essential for the functioning of GPS technology.[19:02] - [21:51] The concept of time as the fourth dimension, leading to the "block universe" model and the idea of a "world line."[21:51] - [26:30] The problem of unifying quantum mechanics and general relativity, the time-less Wheeler-DeWitt equation, and the idea of time as an emergent property.[26:30] - [34:01] Philosophical views on time's reality: presentism versus eternalism (the block universe), and the implications for determinism and free will.[34:01] - [40:35] The problem of "now": relativity shows there is no universal present moment (relativity of simultaneity), and psychology shows our "now" is an extended period.[40:35] - [49:38] The fourth problem of time: the origin of time's direction, or the "arrow of time," from the second law of thermodynamics and entropy.[49:38] - [54:57] The speaker's view: the arrow of time is fundamental, baked into reality by irreversible quantum decoherence and entanglement, not just an emergent property.[54:57] - [1:05:33] The beginning and end of time: discussing the Big Bang as the start of time and speculating on the universe's ultimate fate (heat death, big crunch, big rip).[1:05:33] - [1:11:19] Time travel: traveling to the future is physically possible via time dilation, while traveling to the past is theoretically possible but leads to paradoxes (e.g., the grandfather paradox).[1:11:19] - [1:18:42] Potential resolutions to time travel paradoxes, including the many-worlds interpretation and the Novikov self-consistency principle, and a concluding thought on the limits of current knowledge.[00:18]Physicist Jim Al-Khalili starts by acknowledging that time is a uniquely tricky subject to study because, unlike anything else, we can't step outside of it to look at it objectively [01:20]. To get a handle on it, he makes a crucial distinction between "physical time"—the 't' that appears in physics equations—and "manifest time," which is our subjective, psychological experience of time's passage [01:34]. He then frames the entire discussion around four fundamental "problems of time" that physicists are still wrestling with [00:18].
First, he tackles our perception of time. We all feel time flowing, and many of us feel it speeding up as we get older [02:28]. A year is an eternity when you're five, but a flash when you're fifty. This feeling, he suggests, might be linked to how many new experiences we're laying down in our memories. Yet, our perception is also contradictory: a boring half-hour in a waiting room drags on, while an exciting half-hour at a party flies by, even though the party involves far more new experiences [03:48].
This subjective feeling of flow is very different from the classical physics view, established by Isaac Newton, which saw time as an absolute, universal clock, ticking at the same rate for everyone, everywhere, regardless of what they were doing [04:53]. This is our common-sense view, but it was completely overturned by Albert Einstein [05:34].
Einstein showed that time is not absolute; it's relative. His first theory, special relativity, was born from a deeply strange fact about light: everyone measures the speed of light to be the same, no matter how fast they are moving [10:27]. This seems impossible. If you're in a rocket chasing a beam of light, you'd expect the light to seem slower to you, but it doesn't. For this to be true, Einstein realized that something else must be changing: space and time themselves. The consequence is "time dilation": the faster you move, the slower your clock ticks relative to a stationary observer [11:28]. This isn't just a thought experiment. It's been proven. For example, short-lived particles called muons are created high in the atmosphere. They shouldn't live long enough to reach the ground, but because they travel near the speed of light, their internal clocks slow down so much that they make the journey [12:18].
A decade later, Einstein's general theory of relativity added gravity into the mix. He showed that gravity also slows down time [17:15]. The stronger the gravitational field, the slower time passes. This effect is also real and essential for modern technology. GPS satellites orbit high above Earth where gravity is weaker, so their clocks run slightly faster than ours on the ground. Engineers must constantly correct for this difference; without that correction, your phone's GPS would be inaccurate by miles [17:41]. So, in a very real sense, your head is aging slightly faster than your feet [18:41].
This connection between space, time, and gravity leads to a powerful and strange model of the universe called the "block universe" [20:06]. In this view, time is a fourth dimension, woven together with the three dimensions of space into a single fabric of "spacetime." The block universe model suggests that all of time—past, present, and future—exists simultaneously, laid out like a loaf of bread where each slice is a complete moment in time. Our sense of a moving present is just our consciousness traveling along this pre-existing timeline [28:56]. This idea, called "eternalism," is what most physicists would subscribe to, according to Al-Khalili [29:11].
If the future already exists, it raises profound questions about free will. But the model gets even stranger when physicists try to unite general relativity with quantum mechanics. An early attempt at this unification resulted in the Wheeler-DeWitt equation, which describes the quantum state of the entire universe but, bizarrely, contains no variable for time [22:53]. This has led to the radical idea that time might not be fundamental at all. Instead, it could be an "emergent property" [23:53]—much like the "wetness" of water emerges from the collective behavior of H2O molecules, none of which is individually wet [24:09]. Time, as we know it, might arise from a deeper, timeless reality.
This brings us to the problem of "now." In the block universe, the present moment has no special physical significance [33:50]. Relativity also tells us there is no single, universal "now." Two events I see as simultaneous could be seen as happening at different times by someone moving past me at high speed [35:14].
The fourth major problem is the "arrow of time": why does time move from past to future? Most fundamental laws of physics are time-symmetric; they work just as well forwards as they do backwards [43:28]. The standard explanation for time's direction comes from the second law of thermodynamics, which states that in an isolated system, entropy—a measure of disorder—always increases [45:39]. A scrambled egg won't unscramble itself. This provides a clear directionality.
However, Al-Khalili argues this isn't the full picture. He believes the arrow of time is more fundamental, baked into reality at the quantum level. He points to a process called quantum decoherence, where a quantum system becomes entangled with its environment [52:11]. He argues this process is truly irreversible and is happening constantly, everywhere. This, he proposes, is what gives time its fundamental direction [53:16].
So, did time have a beginning? General relativity suggests it did, at the Big Bang [55:16]. Asking what came "before" is like asking what's south of the South Pole. But more speculative theories, like the multiverse or a cyclic universe, leave open the possibility that time is eternal [56:49]. As for its end, the universe's accelerating expansion could lead to a cold, empty "heat death," a re-collapse in a "big crunch," or a "big rip" where space itself is torn apart [1:00:23].
Finally, he addresses time travel. Traveling to the future is physically possible [1:07:29]. Thanks to time dilation, you could take a trip near the speed of light or near a black hole, and when you return, far more time will have passed for everyone else. Traveling to the past, however, is a logical minefield. While general relativity doesn't forbid it, it creates paradoxes like the "grandfather paradox": if you go back and prevent your own birth, you could never have existed to go back in the first place [1:10:36].
There are a couple of theoretical ways around this. The "many-worlds" interpretation suggests that when you travel back, you enter a parallel universe, so your actions don't alter your own past [1:12:46]. Another idea is the "Novikov self-consistency principle," which states that you can go to the past, but the laws of physics will prevent you from creating a paradox; your actions will only ever fulfill the history that was already written [1:13:40]. Al-Khalili concludes with a dose of humility: we should never assume we have all the answers. What seems like science fiction today could be the science of tomorrow [1:18:30].
[08:48] - [14:36] The explanation of time dilation in special relativity. The speaker clearly walks through the counterintuitive premise—that the speed of light is constant for all observers—and uses the real-world example of atmospheric muons to show this isn't just a theory, but a measurable reality.[19:02] - [21:51] The introduction of the "block universe" model. This is a core philosophical and physical concept in the episode. Hearing the speaker describe it with the metaphor of a book's pages helps visualize this profound and disorienting idea about the nature of time.[40:35] - [45:39] The discussion of the "arrow of time" and its connection to entropy. This is the standard scientific explanation for why time has a direction, and it's laid out with exceptional clarity, explaining why an egg can scramble but not unscramble.[51:19] - [54:05] The speaker's personal hypothesis for the origin of the arrow of time, rooting it in the quantum processes of decoherence and entanglement. This is a key moment where the talk shifts from established consensus to a more speculative but fundamental argument, revealing the cutting edge of this debate.[1:10:23] - [1:16:18] The breakdown of time travel paradoxes (like the grandfather paradox) and the potential solutions (many-worlds, self-consistency). It's a fascinating and accessible exploration of the logical knots that arise when we take the physics of time to its most extreme conclusions.A faithful reconstruction and plain-language retelling of the episode, generated by PodLens.