Year: 2022

PREQUANTUM TIME. “Rather, one has to confront the extremely difficult problem of solving a collection of very non-linear, coupled (elliptic) partial-differential equations. So far, this has proved intractable. This, then, is the first form of the problem of time: the problem of finding a time ‘before quantisation’.” https://lnkd.in/dMm_YkR View in LinkedIn

THE EMERGENCE OF TIME. “The problem of time vs. the emergence of time. Perhaps the most obvious difference is that the problem of time is mostly about time rather than space; while as we said in Section 1, we intend ‘the emergence of time’ to also cover the emergence of spacetime, and so space. At a technical level, the tendency not to dwell on a ‘problem of space’ stems from the fact that, as noted above, in constrained quantization, the momen- tum constraint —which, from a spacetime perspective, shuffles spacetime points within a given 3-dimensional spacelike slice—is much easier to deal with, conceptually and technically, than the Hamiltonian constraint, which is related, at least classically, to diffeomorphisms that map points from one spacelike slice to another.” https://lnkd.in/dMm_YkR View in LinkedIn

TIME IN QUANTUM GRAVITY. “On the other hand, the problem of time is about ‘finding’ a time in quantum gravity theories: and so not, in the first instance, about finding some kind of classical limit of, or classical approximation to, such theories.” https://lnkd.in/dMm_YkR View in LinkedIn

HAWKING RADIATION. “The origin of the ‘Euclidean’ approach to quantum gravity lies in Hawking’s remarkable discovery in 1974 that a black hole will radiate particles with a thermal spectrum via a quantum-mechanical process.” https://lnkd.in/dMm_YkR View in LinkedIn

EMERGENCE. “The deceptive picture: Emergence is not a process in time. But this ‘emergence’ is not a process in time: Hartle and Hawking’s proposed ‘framework’, and others such as Vilenkin’s, stands in no temporal relation to classical spacetime, or any of its parts (regions or points), even very early ones.” https://lnkd.in/dMm_YkR View in LinkedIn

HAPPENSTANCE. “Laws and explanation are central controversial issues in philosophy of science, which of course we cannot enter here. But we only need two uncontroversial points. First, there is a strong temptation to think of the laws (in our special case: differential equations) as being truly explanatory, while the statements of particular fact (boundary conditions) are not explanatory, because they are matters of ‘mere happenstance’.” https://lnkd.in/dMm_YkR View in LinkedIn

THE LOGICAL STACK. “Second, this thought is indeed questionable! After all, one can explain a particular fact; and one can ask for an explanation of a law. Agreed, the explanation of a particular fact will typically invoke another such fact (usually an earlier one, as in what philosophers call ‘causal explanation’), and one can ask for an explanation of that fact—so that a regress beckons, and one is tempted to think that one must eventually accept a ‘mere happenstance’.” https://lnkd.in/dMm_YkR View in LinkedIn

THE TOWER OF ASSUMPTIONS. “But similarly, when one can asks for an explanation of a law, the explanation will typically invoke another such law (relating them deductively, or by the one being a limiting case of the other), and one can ask for an explanation of that law—so again a regress beckons, and apparently one must eventually accept a ‘mere happenstance’ of laws also.” https://lnkd.in/dMm_YkR View in LinkedIn

NO HOWLERS, YET. “Broadly speaking, our conclusion is twofold. First, we concede that there are large conceptual difficulties in both programmes’ attempts to have classical spacetime ‘emerge’ (as well as ferocious technical difficulties). But second, more positively, we see no knock-down conceptual errors, or philosophical howlers, in these attempts, as they have been developed so far. In that sense, there are good prospects for work for the future.” https://lnkd.in/dMm_YkR View in LinkedIn

SO ENDS this immensely interesting and difficult paper, where a reader like me is simply hanging on by their fingernails to follow this radiant stream of thought. Understanding how physicists see the world is somewhat of a rare sport: Einstein introduced relativity in 1905, and we are still choking on digesting what it all means, but be assured, while it will undoubtedly continue to change, it is a radical and important view of reality. View in LinkedIn