[ben174]

I can't make any sense of this. It has happened over there - so it's happened. How does it make a difference where I am standing. I'm not talking about my reality. I'm talking about reality. Things which have taken place and nothing can change that.

[adharmad]

But how do you know that it has taken place without the light reaching you?

Without the light (or radiation), all you can say is that "something may have taken place everywhere".

[aceazzameen]

I like to think that the light that reached me is just a record of things that already happened.

[adharmad]

Yes but you cannot tell exactly what happened without the light reaching you. Sure you can guess. For example, if you know the state of a star a few billion years ago, with no other information after that, you can calculate at what point it would become a supernova. But without the light reaching you, there is no way to tell whether it actually became a supernova or something else happened (eg - it got swallowed by a black hole etc.)

[kortilla]

“I can’t see it” != “it hasn’t happened”

[ianai]

It might actually be useful to think of that as an equivalency because “c” is the fastest rate by which one part of the universe interacts with another part. I think of the universe branching off by the further in spacetime these interactions have to travel. The whackiest QM interactions are still bounded by “c.” The distances between events are called space-like precisely because light/etc would have to travel faster than light to go from A to B.

Then there’s ER=EPR to try to conceptualize.

[adharmad]

“I can’t see it” == “I don't know whether it happened or not”

[tyre]

The person(s) you're talking too are not talking about epistemology. If something happened yesterday and I am not aware of it until I read the paper today, the thing still happened yesterday. If I never read about it, the thing still happened yesterday.

Whether/when I "know" about it is separate, and not what they're talking about.

[mistermann]

"I don't know" very often (uncertain topics, for certain people) cannot be implemented, that's "just" how it "is".

[aceazzameen]

I agree you can't tell what happened after a observing a recorded event. If you were to read someone's diary from a decade ago, you wouldn't know what they were doing now. You need new observations.

The OP was referring to when a event happened though.

[heavenlyblue]

It's like saying that when you see a dead body with a sawed-off head nobody knows whether that body was born this way or somebody is a psycho who decapitated it.

For the sake if the argument let's allow this body to be an light-hour away from us when it happened.

I think your logic is very weak here.

[adharmad]

What I said in no way prevents us from guessing or inferring what could have happened using knowledge known to us. But they are all possibilities, some more likely than others - there is no way of knowing 100% which one of them actually happened without evidence or information in the form of light or radiation.

[heavenlyblue]

It has nothing to do with the speed of light though

[tcgv]

> But how do you know that it has taken place without the light reaching you?

Things happen without us knowing about it, and without light from such events ever reaching us. We don't need to know it has happened for it to have happened.

[ben174]

I don't. But that doesn't matter. If it has happened it has happened. Whether I know about it has no bearing on reality (not my reality).

[the8472]

In its reference frame light moves instantaneously.

[raattgift]

tl;dr Neighbouring points on a null worldline are not instantaneous because c is not infinite; distant points on the same worldline are even less instantaneous. There are many useful ways of capturing the "distant" in "distant points".

An element of light traces out a worldline. On any worldline we can apply whatever labelling-of-points we want since relativity is a coordinate-independent theory. We can label the points of a worldline with greek letters, hieroglyphs, roman numerals, natural numbers, real numbers, whatever we like and in any order we like.

One can build an infinity of calculationally-useless or misleading sets of coordinates on these worldlines for things heavier/slower than light. But one can also build an infinity of calculationally-useful and non-misleading coordinates for them, and many of those make use of the invariant spacetime interval. The same applies to coordinates for massless things / things that move at the speed of light, even though the invariant spacetime interval for light in free-fall is always 0, even if it is in free-fall for billions of years (like light from distant quasars, or the cosmic microwave background).

A calculationally-useful ordering applies a monotonically increasing order from the start to the end of a worldline in a time-orientable manifold (our universe is time orientable: smaller and denser in the past, bigger and sparser in the future). For timelike worldlines (i.e., anything that is always slower than light), almost always the most useful ordering is proper time.

But we cannot calculate proper time on a null (lightlike) worldline, so we will want some other monotonically increasing ordering function on the worldline, and ideally one with which we can solve the geodesic equation. Such a family of orderings is not only known, but has been textbook material since 1970 (Spivak's introduction to differential geometry). It's the affine parametrization.

For lightlike observers there is thus a useful and well-defined notion of time: the affine (parameter) time. This is different from but analogous to the proper time available to timelike observers. We can do standard vector physics on a photon using affine time, e.g. we can calculate its phase at various points along its trip from point A to point B. (Indeed, talking about a photon's quantum wavefunction, the affine parameter is proportional to its phase). We can also take the derivative of position with respect to affine time as a momentum that accurately captures the gravitational redshift or blueshift between two points on the null worldline.

[worewood]

There is no reality because there is no central authority to dictate what has happened and when/where. Your reality IS the reality.

[mistermann]

"The"?

"Is"?

[andrewflnr]

That's because it actually doesn't make sense. They're confused, not you. Source: college physics.

[edgyquant]

Reality is relative

[mistermann]

Can you expand on this perchance?