- Physics has no agreed definition of what time actually is.
- Einstein treated time as a coordinate, not something experienced.
- "Now" doesn't appear in our equations — only in our minds.
What if the moment you call 'now' doesn't actually exist in the universe's fundamental structure? Physicist Sabine Hossenfelder tackles one of physics' most perplexing puzzles: what time actually is and why we experience it the way we do.
The question splits three ways, and each answer gets stranger than the last.
Key figure
3
competing definitions of time debated by physicists
Time as Einstein's Coordinate System
In Einstein's theories, time works like a coordinate - a label to order events, just like using different map projections for the same territory.
You can change time coordinates without changing time itself. This coordinate time describes what happens in our universe but has zero connection to how we actually experience time passing.
The second definition gets more concrete: time is what clocks measure. But clocks are made of matter, and we identify some quantity from that matter - a pendulum's swing, a heartbeat, or as Hossenfelder notes with characteristic wit, "Kevin checking his phone."
In quantum physics, though, physicists haven't reached agreement on what time even means.
Where Time Comes From (Maybe)
Time might just be fundamental - one of the universe's basic ingredients. Or it could be emergent, arising from something else entirely. As Hossenfelder explains, "time is an illusion because it might come from something else."
Some physicists pursue causality-first approaches, arguing that time emerges because events need ordering. Others, like Julian Barbour, claim time doesn't fundamentally exist - what's real are just changing relationships between particles.
The most mind-bending possibility? The universe started as four-dimensional space, and at some boundary, one dimension transformed into time.
The Mystery of Now
Here's where physics hits a wall. Why do we feel time passing? Why does "now" feel special, separating past from future?
Einstein called this "the problem of now" - our math doesn't include a present moment. We remember the past but not the future, yet physics treats all moments equally.
Two solutions exist: either our sense of being in a particular moment corresponds to nothing fundamental in the universe, or there's a physical "now" created by quantum wave function collapse.
What is wave function collapse?
In quantum mechanics, particles exist in multiple possible states at once until they are measured. Wave function collapse is the moment those possibilities narrow to a single outcome. Some physicists think this transition could be what creates our experience of "now."
Hossenfelder leans toward causality being key, admitting:
I spend so much time thinking about time that I should be getting an interest rate.
Sabine Hossenfelder, physicist
Most physicists, she notes, don't take this question seriously enough - despite it being central to understanding how nature works.
Sources
- Primary Source: What is time? (Sabine Hossenfelder, YouTube)
- Additional Context:
- Measuring time in a timeless universe (arXiv, 2024)
Fact Check: Claim-by-Claim Verification Verified
The recap accurately summarizes the key points, quotes, and ideas from Sabine Hossenfelder's YouTube video without factual errors or misrepresentations.
Commentary
- Speculative ideas like 4D space turning into time or wave function collapse creating "now" are presented as possibilities by Hossenfelder, with appropriate hedging.
- Barbour's timeless universe from changing particle relations is correctly noted as hard to grasp, aligning with video.
Sources used for verification
Academic/Peer-reviewed:
Other reliable sources:
- What is time, really? (Sabine Hossenfelder video transcript) - YouTube
- Is time an illusion? (Sabine Hossenfelder blog) - backreaction.blogspot.com
Fact-checked by Perplexity Sonar Pro on 2025-12-24
