Rethinking multi-grain timestamps [LWN.net]

Rethinking multi-grain timestamps

Posted Oct 9, 2023 22:13 UTC (Mon) by mjg59 (subscriber, #23239)
In reply to: Rethinking multi-grain timestamps by Wol
Parent article: Rethinking multi-grain timestamps

> To add, if the time between two events is less than the time for a photon to travel between the locations, then the question "which came first" does not make sense.

If the light from a distant supernova reaches me shortly after I've taken a sip of tea, I can pretty confidently assert that the supernova happened first even though the time between the two events was less than the time for a photon to travel between the locations.

Rethinking multi-grain timestamps

Posted Oct 10, 2023 14:27 UTC (Tue) by Baughn (subscriber, #124425) [Link]

First from your frame of reference, sure, but there'll be a frame of reference in which the events are reversed.

In the supernova case those are all far away from you in phase space, but for high-frequency networking there's a lot more chance of ambiguity.

Rethinking multi-grain timestamps

Posted Oct 10, 2023 20:30 UTC (Tue) by ianmcc (guest, #88379) [Link] (2 responses)

If two events are separated in space by a distance that is more than cΔt, where Δt is the difference in time between the events and c is the speed of light, then it is known as a "space-like interval". The events are closer together in time than they are in space such that it is not possible for light to travel from one event to the other, and there is no causal connection between the events (i.e. it is not possible to say that event 1 caused event 2, or vice versa).

It is a theorem in special relativity that if two events are space-like separated, then there exists a (possibly moving) reference frame where the two events are simultaneous. Moreover there are also reference frames where event 1 occurs before event 2, and reference frames where event 2 occurs before event 1.

Although different observers will genuinely disagree about the order of events, since there is no causal connection between them there is ultimately no ambiguity in observable effects. I.e. both observers would be able to calculate and agree that event 1 could not have caused event 2, and vice versa. So although there will be a reference frame where you sip your tea before the supernova explodes, you can rest assured that you didn't cause it.

Rethinking multi-grain timestamps

Posted Oct 11, 2023 9:59 UTC (Wed) by Wol (subscriber, #4433) [Link] (1 responses)

> The events are closer together in time than they are in space such that it is not possible for light to travel from one event to the other, and there is no causal connection between the events (i.e. it is not possible to say that event 1 caused event 2, or vice versa).

Just to throw a spanner into the works, quantum mechanics would beg to differ :-) That was Einstein's "Spooky action at a distance", which appears to be a real thing.

Just like (if I've got it right) quantum mechanics says black holes can't exist.

The latest I knew, we have some evidence that says relativity is correct, we have some evidence that says quantum mechanics is correct, and we have loads of evidence that they can't both be right. Where do we go from here :-) Has somebody found the GUT? Or the TOE?

Cheers,
Wol

Rethinking multi-grain timestamps

Posted Oct 11, 2023 11:07 UTC (Wed) by mathstuf (subscriber, #69389) [Link]

There's still no causality broken in QM with entanglement. You can observe some measurement of an entangled entity and know what result would occur if measured somewhere else at the same moment (and outside the light cone), but causality is not broken because to *use* the information, you must actually communicate with the other side (as you cannot influence the result without breaking entanglement; you're just learning things at the same time as elsewhere).
Note that the "interpretations" (e.g., Copenhagen, many worlds, etc.) are about *how* entangled particles do this.

QM doesn't have anything to say about black holes as it does not have a model for gravity at all. The problems are that black holes represent a situation where gravity is strong enough to matter (heh) on the QM scales.

And yes, there are gaps in the theories for what happens here. We don't know what it is.

PBS Space Time is a good source of information on these topics: https://www.youtube.com/c/pbsspacetime/videos