# How to understand Carlo Rovelli's notion that time “does not exist”?

Media coverage of Carlo Rovelli's book The Order of Time has had headlines like "There is no such thing as past or future", or "Carlo Rovelli: 'Time does not exist'." Is there a way to explain what he means that is more concrete than "the dance of nature does not develop to the rhythm kept by the baton of a single orchestral conductor"? (Rovelli's words from the second linked reference.) To be fair, he precedes that by "elementary processes cannot be ordered along a common succession of instants", which is fairly concrete but leaves a lot of questions open. In particular, what is an elementary process?

Can anybody who is familiar with Rovelli's work clarify what he's getting at? I would also like to know just how speculative the ideas in these papers are seen to be within the physics community.

There are a lot of other questions about whether time is real, but I think this one is different because it regards a specific author's claims about time.

• It is difficult to tell what exactly he means based on the description in the second link (I did not read the first link, it's a bit long). The equations in QM and QFT both have time as parameters in e.g. the wave function $\Psi=\Psi(\vec{r},t)$, so he must be talking about some effort at quantizing gravity. Unfortunately, I can not discern which of the myriad efforts currently underway is being referred to. – enumaris Jul 5 '18 at 20:26
• A start, wikipedia: Relational quantum mechanics. – AccidentalFourierTransform Jul 5 '18 at 20:31
• And a related paper from 20 years ago: xxx.lanl.gov/abs/quant-ph/9609002. Personally, I think this is a chat room question. – user198207 Jul 5 '18 at 21:12
• I think the scientific concept that Nature is timeless originated with Minkowski's 1907 spacetime interpretation of Einstein's special theory of relativity. Special relativity implied that Nature is really four-dimensional, so it exists at all spaces and all times. – Michael B. Heaney Jul 6 '18 at 0:02

There's also a separate but related concept of thermal time by Rovelli and Connes (the one with the non-commutative Standard Model), which is basically a conjecture that the macroscopic physical arrow of time is of thermodynamic origin. More precisely, to each quantum gravity density matrix $\rho$ we can associate the "thermal Hamiltonian" $$H = - \ln \rho,$$ for which by definition $\rho$ is stationary. However, fluctuations around $\rho$ can be shown to live in the thermal time associated with the Hamiltonian flow generated by the thermal Hamiltonian.