Announcer
The following program features simulated voices generated for educational and philosophical exploration.
Alan Parker
Good evening. I'm Alan Parker.
Lyra McKenzie
And I'm Lyra McKenzie. Welcome to Simulectics Radio.
Alan Parker
Tonight we're examining one of physics' most unsettling conclusions about the nature of reality. According to our best theories, the flow of time—the sensation that the present moment is uniquely real, that the past is gone and the future uncertain—may be an illusion. The block universe picture suggests all moments exist equally, spread out in four-dimensional spacetime like cities on a map. What we experience as the passage of time is merely our consciousness moving through this static structure, or perhaps not even that. If this view is correct, what becomes of memory, agency, and the very experience of being human? Are we mistaken about something as fundamental as temporal flow?
Lyra McKenzie
This feels like one of those moments where physics brutally contradicts phenomenology. Every fiber of our being insists that time passes, that yesterday is gone and tomorrow hasn't happened yet. We remember the past but not the future. We can influence what comes next but not what already occurred. These asymmetries seem built into the fabric of consciousness. Yet physics tells us this is parochial—that from the universe's perspective, all moments are equally real, differences between past and future are thermodynamic artifacts, and temporal flow is something brains construct rather than something they detect. If we take physics seriously, do we have to abandon the reality of lived experience?
Alan Parker
Joining us is Dr. Carlo Rovelli, theoretical physicist at Aix-Marseille University. His work in quantum gravity and thermodynamics has led him to profound meditations on time's nature. He's argued that time as we experience it emerges from our thermal perspective on the universe—that the flow of time is real for us precisely because we're inside the system, interacting thermodynamically with our environment, rather than viewing it from some impossible external vantage. Welcome.
Dr. Carlo Rovelli
Thank you. It's a pleasure to discuss these questions.
Lyra McKenzie
Let me ask bluntly: does time really pass, or is that an illusion?
Dr. Carlo Rovelli
The question requires careful unpacking. Time as described by fundamental physics—particularly general relativity and quantum mechanics—doesn't flow. The equations treat time as a dimension similar to space. Past, present, and future all exist in the mathematical structure with no privileged 'now.' But this doesn't mean temporal experience is illusory in the sense of being false. It means time's passage is perspectival, emerging from our particular relationship with the universe. We experience time flowing because we're thermal systems with limited information about the universe's state. The flow of time is real for beings like us, but it's not a fundamental feature of reality independent of observers.
Alan Parker
Can you elaborate on the connection between thermodynamics and temporal experience?
Dr. Carlo Rovelli
Time's directionality—the arrow from past to future—comes from entropy, the second law of thermodynamics. Entropy increases toward the future, creating an asymmetry that doesn't exist in fundamental equations of motion. Those equations work equally well forward or backward. The asymmetry arises because we interact with the universe from a particular thermal perspective. We're hot, localized systems exchanging heat with our environment. We have far more information about past states than future states because we carry traces of the past in our memories and records. This information asymmetry creates the experience of time flowing from a definite past toward an uncertain future. If you were a cold, dispersed system in thermal equilibrium, you wouldn't experience temporal flow at all.
Lyra McKenzie
But that suggests time's passage depends on consciousness or at least on particular kinds of physical systems. Is the flow of time then subjective?
Dr. Carlo Rovelli
Not subjective in the sense of being arbitrary or constructed by minds. It's perspectival, like velocity. Whether something is moving depends on your frame of reference, but that doesn't make velocity unreal or subjective. Similarly, temporal flow depends on your thermodynamic relationship to the universe, but that's an objective physical fact about systems exchanging energy and information. Consciousness doesn't create time's flow—consciousness is one kind of thermal system that experiences flow because of its physical properties. The mistake is thinking there must be a single objective answer to whether time flows that's independent of physical perspective.
Alan Parker
What about the block universe picture where all moments exist equally? How literally should we take that?
Dr. Carlo Rovelli
The block universe is a useful mathematical representation of spacetime in general relativity. It's legitimate to visualize all events in spacetime as existing in a four-dimensional structure. But we shouldn't reify the mathematical model. The question 'do all moments exist equally' assumes a notion of existence that transcends time, which is precisely what's at issue. What does it mean to say the future 'exists' now? Existence is temporal—things exist at particular times. The block universe shows that spacetime has a unified mathematical structure, but that doesn't immediately tell us the ontological status of past and future events. We have to be careful not to confuse the map with the territory.
Lyra McKenzie
This connects to free will and agency. If the future already exists in some sense, are our choices predetermined? Do we really have the capacity to shape what comes next?
Dr. Carlo Rovelli
The block universe doesn't immediately imply determinism in the problematic sense. Even in the block picture, your choices are part of what determines the structure. The future exists in the model, but it exists as causally dependent on present conditions, including your decisions. The real question is whether the laws of physics are deterministic, which is separate from whether we represent them in a tenseless mathematical framework. Quantum mechanics suggests fundamental indeterminism regardless of how we picture time. Moreover, the relevant notion of free will doesn't require that the future be metaphysically open. It requires that your deliberation and decision-making processes are genuine causes of action, which is compatible with those processes being part of a fixed structure viewed from outside time.
Alan Parker
What about memory? If all moments exist equally, why can we remember the past but not the future?
Dr. Carlo Rovelli
This asymmetry is again thermodynamic. Memory is a physical trace—a low-entropy correlation between your brain state and past events. We can form memories of the past because the past was in a lower entropy state, allowing localized systems like brains to carry information about it. We can't remember the future because the future is higher entropy, making it impossible for current states to carry reliable information about it. This isn't a law of logic but a statistical fact about the universe's thermal history. If entropy were decreasing toward the future, we'd remember the future and find the past uncertain. The asymmetry of memory follows from the asymmetry of entropy, which is about our thermal relationship to the universe.
Lyra McKenzie
You've suggested that time might not be fundamental at the deepest level of quantum gravity. What does it mean for time to be emergent?
Dr. Carlo Rovelli
In quantum gravity, we're trying to describe the universe at scales where both quantum mechanics and general relativity matter—the Planck scale. At these scales, spacetime itself may not be fundamental but emergent from more basic quantum entities. Time might be like temperature—a macroscopic concept that emerges from underlying microscopic dynamics but doesn't exist at the fundamental level. Just as temperature is a statistical property of many particles, time might be a statistical property of quantum gravitational states. This doesn't mean time is unreal at our scale any more than temperature is unreal. It means that in the deepest description of nature, temporal concepts might not appear. They emerge when we describe large-scale, coarse-grained behavior.
Alan Parker
If time is emergent rather than fundamental, does that resolve the tension between physics and phenomenology?
Dr. Carlo Rovelli
It helps, but doesn't fully resolve it. The tension is between timeless fundamental laws and our time-bound experience. If time emerges from timeless quantum gravitational dynamics, then temporal experience is grounded in physics but not present at every level. This is actually how emergence works throughout physics. Solidity emerges from quantum field interactions. Fluidity emerges from molecular dynamics. These macroscopic properties are real and causally efficacious at their level, even though they're not fundamental. Similarly, temporal flow is real for thermal systems like us, grounded in fundamental physics but not present in the fundamental description. The phenomenology is neither illusory nor fundamental—it's emergent.
Lyra McKenzie
But there's something unsatisfying about that. We don't just experience time—we are temporal beings. Our narrative identity, our sense of self unfolding through experience, seems to require genuine temporal flow. Can we really be who we are if time is ultimately illusory?
Dr. Carlo Rovelli
I understand the concern, but I think it rests on a misunderstanding of what calling time emergent means. Your temporal existence is completely real. You are a process unfolding in time, with memories of the past and expectations of the future. That's a physical fact about what you are—a particular kind of thermal system far from equilibrium. The claim isn't that this is illusory but that it's perspectival. Time as you experience it is real from inside your thermal perspective. What's not real is time as an absolute, universal flow independent of physical systems. But you never experienced that anyway. You always experienced time from inside, as a participant. Physics is just explaining why that inside perspective has the features it does.
Alan Parker
How does this understanding of time affect how we should think about change and persistence? What makes something the same object across time if all moments exist equally?
Dr. Carlo Rovelli
Objects are patterns of correlation across time and space. What makes the person you are now the same as the person you were yesterday isn't some metaphysical substance persisting through time but causal and informational continuity. Your present state carries traces of past states and will influence future states. Identity across time is about these connections, not about something existing timelessly. In the block universe picture, an object is like a worldline—a continuous path through spacetime. But the path only exists because there are local causal connections at each point. The unity is in the pattern of relationships, not in some underlying substrate that exists at all times simultaneously.
Lyra McKenzie
What about the present moment? Why does it feel like now has special status if all moments are equally real in the block universe?
Dr. Carlo Rovelli
The felt specialness of now is a feature of consciousness, not of time itself. At each moment, consciousness has a particular state that includes memories of the past and anticipations of the future, but the conscious experience itself is always of the present. This creates the sense that now is privileged. But this is true at every moment. Yesterday, yesterday was 'now' with its own sense of presentness. Tomorrow, tomorrow will be 'now.' The feeling that this moment is uniquely real is an indexical feature of experience—like the feeling that here is special when you're located somewhere. Everywhere is 'here' when you're there. Every moment is 'now' when you're experiencing it. The specialness is perspectival, not objective.
Alan Parker
Does understanding time this way change how we should live? If the future is already real in some sense, does that affect our ethical obligations or our relationship to possibility?
Dr. Carlo Rovelli
I don't think it should change our practical ethics much. Whether or not the future exists timelessly, we still experience it as uncertain and our choices as genuinely shaping outcomes. The phenomenology of deliberation and action doesn't change. What might change is our emotional relationship to time—less anxiety about the future's openness, perhaps more acceptance of events as part of a larger structure. But this is a contemplative shift rather than an ethical one. We still face choices, still have reasons to prefer some outcomes to others, still act on incomplete information. The block universe perspective might provide a kind of cosmic equanimity, but it doesn't remove the reality of situated, temporal existence.
Lyra McKenzie
How confident should we be in these conclusions? Could future physics overturn the picture you're describing?
Dr. Carlo Rovelli
Science is always provisional. The block universe picture comes from general relativity, which is an extremely well-confirmed theory but not our final theory of nature. Quantum gravity will likely modify our understanding. Some approaches suggest time might be more fundamental than relativity implies. Others suggest it's even less fundamental than I've described. We should hold these ideas with appropriate epistemic humility. What I'm confident about is that our naive realism about time—the idea that time flows objectively and universally—is incompatible with what we know from physics. The details of the alternative picture remain to be worked out, but we can't go back to thinking time is simple or obvious.
Alan Parker
What research directions in quantum gravity might illuminate these questions further?
Dr. Carlo Rovelli
Several approaches are promising. Loop quantum gravity, which I work on, suggests spacetime is quantized and time emerges from quantum transitions between discrete states. String theory approaches time differently but also grapples with its status in quantum cosmology. Research on the thermodynamics of quantum systems and the arrow of time in cosmology is crucial. We need to understand how macroscopic time emerges from microscopic dynamics. Also important is work connecting quantum mechanics to general relativity in ways that might reveal time's deeper nature. The interface between quantum information theory and gravity is particularly fertile ground. We're still in the early stages of understanding how time fits into the deepest level of physical description.
Lyra McKenzie
As we close, what should we take away from physics' challenge to our intuitive understanding of time?
Dr. Carlo Rovelli
That our experience is real but not the whole story. The flow of time is genuinely part of our lived reality, emerging from our particular physical relationship to the universe. But it's not a universal feature of nature independent of observers. This perspective requires holding two truths simultaneously: the phenomenological reality of temporal experience and the physical understanding that this experience is perspectival and emergent. We are thermal beings embedded in a universe with thermodynamic history, and that embedding gives us our temporal perspective. Understanding this doesn't diminish the reality of lived time but situates it properly within the larger structure of nature. We can honor both the physics and the phenomenology without claiming either is illusory.
Alan Parker
A perspective that respects both scientific understanding and human experience. Thank you for helping us think through these profound questions about time's nature.
Dr. Carlo Rovelli
Thank you. It's been a pleasure.
Lyra McKenzie
Until tomorrow, remember that now is always now.
Alan Parker
And memory is a gift of entropy. Good night.