What is time?
Based on Sabine Hossenfelder's video on YouTube. If you like this content, support the original creators by watching, liking and subscribing to their content.
In relativity, time is a coordinate used to label and order events, and changing time coordinates doesn’t change the underlying physics.
Briefing
“Time” in physics splits into at least two distinct ideas—time as a coordinate that orders events, and time as what clocks measure—yet none of those meanings directly explains why humans experience a flowing present. The core takeaway is that the sensation of “now” and the feeling that time passes may not be fundamental to nature; instead, it could emerge from deeper structures such as causality, while the physics of time itself may be largely about bookkeeping rather than a universal flow.
In Einstein’s framework, time behaves like a coordinate alongside space. It’s a labeling system for events: you can choose different coordinate conventions for time (just as you can switch between coordinate systems in space) without changing the underlying physics. This “coordinate time” is therefore not tied to subjective experience. Separately, physics also uses time in a constructive sense: time is what a clock measures. But clocks are physical systems made of matter, and the measured “passage” of time must be connected to how that matter evolves. In Einstein’s theory, the relevant quantity is tied to the length of a particle’s path through spacetime, often called “proper time.” Quantum physics complicates the picture further: defining what a clock means at the quantum level is an open problem, with no widely agreed method for extracting time from quantum systems.
That leaves the question of where time comes from. One possibility is that time is fundamental—an ingredient of the universe that doesn’t need a source. Another is that time is emergent, arising from something deeper. A prominent route links time to causality: if events must be ordered for cause-and-effect to make sense, then “time” could be a consequence of that ordering. The transcript points to several causality-first approaches, including Rafael S. Sorkin’s causal sets, Steven Wolfram’s hypographs, and Felix Finster’s causal fermion systems. A different stance appears in Julian Barbour’s view that time is not fundamental; what’s real are relations among particles, and “time” is merely the ordering that makes those relations intelligible.
Even if time emerges from causality or from relational structure, the hardest issue remains the human experience of a special present. Mathematics in physics lacks a privileged “now,” yet memory preserves the past and not the future—an asymmetry that bothered Einstein and is framed here as the “problem of now.” Two broad resolutions are offered: either the “now” feeling corresponds to nothing fundamental (so every moment is equally real when it occurs, without a special status in the underlying laws), or there is a physical notion of now. The transcript cites George Ellis’s suggestion that the present is created by wave-function collapse. The speaker leans toward causality-first thinking, arguing that “now” likely isn’t fundamental, and that physicists often underplay how big the question really is.
Cornell Notes
Physics uses “time” in two ways: as a coordinate in Einstein’s relativity (a label that orders events and can be re-coordinatized without changing physics) and as what clocks measure (linked to proper time along a particle’s spacetime path). Quantum physics makes the clock idea harder because there’s no general consensus on how to define time from quantum systems. Time may be fundamental or emergent; one well-trodden idea is that time arises from causality, with examples including causal sets, hypographs, and causal fermion systems. The most difficult part is the “problem of now”: physics lacks a privileged present, yet humans experience a special “now” and remember the past but not the future. Possible fixes include treating “now” as non-fundamental or introducing a physical mechanism such as wave-function collapse to define the present.
How does Einstein’s relativity treat time compared with space?
What does “clock time” mean in physics, and why does it raise a deeper question?
What does it mean to say time might be emergent?
How does Barbour’s relational view differ from causality-first approaches?
What is the “problem of now,” and why does it matter for theories of time?
What physical mechanism is cited as a candidate for defining the present moment?
Review Questions
- What distinguishes coordinate time from proper time, and why does that distinction matter for connecting physics to human experience?
- Which approaches treat time as emergent, and how do causality-first and relational views differ in what they treat as fundamental?
- What are the two broad strategies for addressing the “problem of now,” and what would each imply about whether “now” is fundamental?
Key Points
- 1
In relativity, time is a coordinate used to label and order events, and changing time coordinates doesn’t change the underlying physics.
- 2
Proper time links “clock time” to the spacetime path length of a particle, tying measured time to matter’s evolution.
- 3
Quantum physics lacks a universally accepted method for defining clocks and extracting time from quantum systems.
- 4
Time may be fundamental or emergent; emergent views often derive time from deeper organizing principles like causality or relational structure.
- 5
Causality-first programs include causal sets, hypographs, and causal fermion systems, each attempting to build time from event ordering.
- 6
The “problem of now” arises because physics lacks a privileged present while human experience treats “now” as special and remembers the past but not the future.
- 7
One proposed way to make “now” physical is to associate it with wave-function collapse, though the transcript favors a causality-first interpretation where “now” is not fundamental.