What is time / what are we measuring?

The question

When we measure "time" with any clock, what physical thing are we actually measuring? Relativity shows time is not a uniform flow — it changes with velocity and gravity — but it does not say what time is. michelle-thaller names this as a likely site of "the next revolution in physics."

Why it matters

It sits under gravitational-time-dilation, quantum-entanglement, and cosmology. Einstein's view that past, present, and future may all coexist ("persistently annoying illusions") follows from relativity but has no settled mechanism; Thaller says modern physics has nothing better yet and doubts it "will get any less weird than that."

What we currently believe

• Time is real and variable: measurably different for your head vs. feet, for satellites vs. ground, near black holes vs. far. This is engineered around daily (GPS).
• Any way of measuring time (bouncing-light clock, vibrating atom, unwinding spring) changes identically under motion — so it's "time itself" changing, not just the instrument.

Evidence we have

• michelle-thaller in 2026-05-28-youtube-powerfuljre-joe-rogan-experience-2506-michelle-thaller: "I don't think we have an answer to what time is. What are we measuring? I think you're asking for the next revolution in physics that we don't have yet."
• Einstein's two-mirror clock thought experiment; measured time dilation; GPS correction (see relativistic-time-dilation-to-gps-correction).

Evidence we need

• A physical theory that says what time is, not just how it transforms — likely requires reconciling general relativity with quantum mechanics.

How to resolve

• Watch for progress in quantum gravity; whether the "block universe" (all-time-at-once) view gains testable consequences.

Related

• gravitational-time-dilation
• quantum-entanglement
• expanding-universe
• michelle-thaller