question about lasers

[Cazero]

Anyone who's been in a thunderstorm knows from experience that sound can be "late". It's only logical for light to be the same. Light is just fast enough that we can't notice in our everyday life.

[Rydiro]

Anyone who's been in a thunderstorm knows from experience that sound can be "late". It's only logical for light to be the same. Light is just fast enough that we can't notice in our everyday life.

Light is never late, nor is it early.
It arrives precisely when it means to.

[Devils_Advocate]

Oh absolutely, yes. I have no problem with the science, as far as that goes. It's the way people keep trying to explain it that I think is fraught with ambiguity, misunderstanding and downright error. In particular this "we are seeing the stars x years ago" meme, seems to me to rely on some very odd definitions of words like "seeing" and "ago", which are at variance with the way those words are normally used.

(See above example:

- that's simply not consistent with how anyone anywhere uses the word "see". You don't "see" light - well, maybe when you're trying to decide whether a building is occupied by looking at the windows, but that's not the same thing - you "see" the things that reflect/transmit/emit light.)

But the point is that "seeing something" in that sense isn't a direct interaction between viewer and viewed. The direct interaction is between the viewer and light.

It's the best explanation I've seen yet. For the most part I agree with the principle and I do understand the speed of light, honest. My issue is with the loose use of terms such as "ago" and "earlier".

Assuming the crew of the space station were interested in Earth, and assuming they speak English with usage and idiom that is something like what the people around me have always used, it seems to me that - ten years after the nova, when they're training the telescope in our direction, they'd probably be saying things to each other along the lines of "it'll hit them any minute now".

Someone watching a video recording may well refer to observed events as happening "currently", but if I ask "Wait, do you think this is a live broadcast?", I expect a response along the lines of "Oh, no, I was speaking non-literally in a sense that I thought was obvious from context" (unless they mistakenly believe that it is a live broadcast). And I'd expect the space station crew to similarly reply that, oh, no, of course they don't actually think that this is happening as they watch it, as a naive interpretation of their statements would suggest.

Similarly, if someone refer to how things "really are" in a work of fiction that they can reasonably be assumed to realize is fictional, it's normally understand from context that they don't think that the events actually happened, but are instead speaking about what happens in the story rather than what characters in the story believe. Words take on non-standard meanings in non-standard contexts!

"Now" can be defined as "on the light cone of an event".

Well, in the sense that any word can be defined in any way... obviously, yes.

It's a little unintuitive, but no more so than everything else we've been talking about, and it has the virtue of being consistent both with the normal everyday usage of the word and with relativity.

In normal everyday usage, "now" and "the present" refer to the times at which those phrases are used, with each usage corresponding to the time of that usage. It's just like how "here" refers to the location in which "here" is said.

You may not take the above for granted, but available evidence suggests that that makes you the weird one, and that the problem is your peculiar misunderstanding of common usage rather than widespread departure form common usage, which seems to me rather a contradiction in terms. At the very least, misunderstanding on your part seems like a parsimonious explanation for various people seeming to independently depart from normal usage of "now". What's the alternative explanation, the popularization of non-standard usage in science by academics? But I'd expect to have seen more complaints of this supposed misuse of "now" were that the case. I'm pretty sure that I've never encountered one before.

Like, I get that we're not a dreadfully representative sample of anything, but if the available theories are that you don't understand what "now" means or that no one else in this thread understands what "now" means, the odds very much are not in your favor.

But it doesn't make any sense, no matter how many times they say it. Basically, it makes no allowance for the relativity of "now".

Relativity of simultaneity, like much (all?) of special relativity, is based on events not happening at the same times as which they're observed.

When you say that "the light we see from a celestial object was emitted five years ago", you're presupposing the existence of some frame of reference that allows - something, clearly not us - to observe both the light leaving the distant star and the light arriving at our position, and measure the time between these events.

At this point I'm fairly baffled. Which of those things do you think isn't possible in our own frame of reference, and why? I'm guessing that you're assuming some sort of unusual definition of either "observe" or "measure".