the awe-inducing realization (once grasping the whole concept of light years) that looking up at the night sky is looking at something that doesn't exist at this very moment, but is a panorama of particular points in time, depending upon the specific distance of each object. I suppose knowing the spectral classification of each star, the speed and direction of each nebula, one could construct "realities" at different points in time.

but it just all boggles the mind so much....to think we can literally see back countless years (unless one is of the ilk that thinks that man and dinosaur pulled up a rock and had a weenie roast together!)...

So don't feel like the Lone Ranger here. But let me attempt.

We have reasonable certainty that distant objects are still in existence because throughout our observations we see similar objects at different ages in their respective lifetimes. So we can infer that the mind numbingly distant objects would obey the same physical laws and evolution as we observe elsewhere.

Distances are problematic, too. The Big Bang occurred about 13.7 billion years ago. But since that happened the universe has gone through 13.7 billion years of expansion. So when one measures the distance to a galaxy 10 billion years distance, its actual distance now is much greater.

How big is the universe? It depends how one measures it. I don't recommend delving into this question unless you want a mind twisting answer. Here's one answer from Astronomy Cast

It's all fun, though.

That's what Brian Green was on about, when he threw me into my current confusion. I think I grasp what he's saying, and I understand that we can form statistical projections about how things might be out there. Green is dealing with the fact that the universe we see out there isn't what it's really like now, but only with regard to the expansion and motion away from us.

I need one of those Keanu Reeves meme thingies. "What if a wave of destruction is sweeping through space toward us, but we can't see it yet?" Something like that.

First, the Copernican assumption that there's nothing special about our location, or at least not extraordinary. That the universe out there resembles the universe here.

Second, that nature doesn't have a separate version of reality in different places in the universe. The scientific principles active here equally apply out there. We can test this by our distant observations. The fact that we can make sense of what we see out there with the models that work here attests to the probability that it's true.

But Brian Green talks a lot about multiverses. I would say that that is mere speculation, as is the String Models that predict it. But Green is invested in Strings too, so that's not surprising. No comment on that other than to state that I am not a very big fan of Strings. I do like Brian Green, though. Funny about that.

It is kind of accepted, and thus not dwelled upon when writing articles. It would be the same thing as trying to define every term in a paragraph as it's written. Pretty soon everything bogs down in the description of the details and the meaning of the total document is lost.

Every astronomer knows what you are describing, that yes, potentially many of the objects we observe now may no longer exist, and we wouldn't know for thousands or even millions of years that they didn't. And of course it's not just existence, but anything we know about them determined from its light/EMF signals.

These are difficult concepts to grasp, but reality is what it is. We can see now what we can see, but yeah, what we see is really much older or back in time, if you will. We know that 'local' objects are fairly concurrent, time-wise, with us. But even in the case of the nearest stars, we wouldn't know if they 'blinked out' for several years. Hell, sunlight takes 8 minutes to get here.

That helps answer the question of why it isn't discussed anywhere. Thank you. It seems like something that should be brought up now and then, because most people I've encountered seem to accept the idea that what we see out there is how things are now, even those who have a good deal more formal education than I do. But I can see why it isn't mentioned regularly. I'm surprised there wasn't a Carl Sagan essay explaining the basic idea to the general public or something, though.

I don't recall, and I'm not sure I saw all episodes. I think it's mentioned on occasion, but you're probably right, it maybe should be mentioned more often for the general public.

It does annoy the 'Young Earth' crowd, though, so maybe that's why it isn't brought up so often. For all of the ' LW political correctness' accusations from the RWers we hear, I seem to hear more than a fair share from the right, too.

We can make pretty good predictions, then, and be sure we've gotten it pretty much right. There shouldn't be any tremendously huge surprises, and most things should be more or less where the math would tell us. Umm. Right?

I end up with questions. Maybe I need to drink less coffee. Heh. How many stars we see are still there? One would assume many of them would have burned out by the time we see the light they sent our way. How sure can we be about the effects of objects colliding or interacting? Two galaxies crunching into one another, say, would be... complicated. We couldn't begin to project a lot of the specific outcomes. Or interactions between objects we can see with matter we can't see. Wouldn't that complicate things? I end up thinking our predictions would be less accurate as we deal with more distant objects. Overall, we should expect that the general state of one area should be more or less like the others, but some of the specifics would be hard to predict.

Umm. I need to listen to that podcast. I'm just repeating variations on my original puzzlement. Also, I think this post was meant as a reply up-thread, but I goofed and responded to the main thread. Oopsie.

In time.

You are correct in your understanding that "The longer ago the light was cast from a star, the less we should know about its current state." In fact, if the moon were to suddenly, magically vanish from space, then you could look up and see it, and 1.2 seconds later it would disappear. Or, if there is a mountain 10 miles away visible from your house and it suddenly, magically vanished, then you could see it and about 52 microseconds later it would disappear.

Sometimes the press will report something like: "Yesterday astronomers discovered a supernova of star xyzzy-23 in the Foo galaxy 200,000 light years distant.". What this really means is "Yesterday the first observation was made of a supernova of star xyzzy-23 which occurred 200,000 years ago in the Foo galaxy.".

Of course, scientists assume that the laws of physics as we currently understand them are the same throughout the universe. So, just as physics tells us that the moon or mountains won't 'magically' vanish, we are able to extrapolate our knowledge of stars and galaxies and the universe based on our current understanding of physics, astronomy, and cosmology. We have classifications of stars and understand the typical life-cycles that various star types go through. Thus , just as we can predict that the sun will consume its hydrogen and become a red giant in 4 to 5 billion years, so can we predict, based on our estimates of age, mass, and other parameters for a distant star, predict its current state and possible likely outcomes of its stellar evolution.

Note that all estimates of the mass, age, distance, and other properties of distant astronomical features have inherent uncertainties in them.

There is a reasonably good summary of various techniques for measuring astronomical distances here Distance Measurement in Astronomy:



Each entry in the list of distance indicators has a link to an explanation of the method used, for example Hubble constant and red shifts will take you here.



Hope this information helps.



BTW, Prince Myshkin, I make no claim to being wiser or better educated than anyone. I know that we are all naive idiots.

You re-stated my whole babble-fest of an original post in one phrase, which is pretty cool.

Interestingly (umm... maybe), I ended up reading up on Hubble's Law last week, after I was told that an acquaintance of a family member might contact me in hope that I could serve as a "math whiz" to help him with his project to disprove Hubble's Law. (Luckily I am no math whiz, so I can dodge out of this easily. You learn to throw around quaternions and cross products for 3D programming and suddenly everyone thinks you can solve complex physics problems. Umm.)

Anyway. It turns out that there is some effort by creationists to attack the big bang theory by invalidating Hubble's Law. I'm pretty sure this fellow must be one of them. If he's a serious scientist, even amateur, I would hope he could do the math himself. An odd and troubling business....

I did really identify with the protagonist in The Idiot, for various reasons.

Time is relative to the location of the sentient being and without sentience there is no time.

Time then being, presumably, some aspect of existence which doesn't necessarily resemble our perception of it? "Time" as we think of it is a construct of consciousness? I am led to understand that we don't ultimately know what time is, but there are various ideas about the matter. Actually, I think there's a chapter in, umm, one of Brian Greene's books. Umm. I'm starting to seem like a Greene groupie, here.

But while we don't know what it is, time as a construct is treated a certain way within the context of science, sort of taking for granted that time as we perceive it is valid and we can accept the evidence of our observations. Or... so I understand it. The nature of time is an interesting discussion, in itself, really.

Along with space, it is the foundation of our perceptive apparatus. Other than that it has no independent existence. We -and anything else capable of perceiving - bring time and space to the universe. The universe does not bring them to us.

I do so quite often. I don't dispute the possible validity of your thought. I just sort of babble with the air going in and out, is all.

I do wonder whether your thought has some scientific grounding, or is more in the philosophical realm. Which is to say, is the question of the actual nature of time really relevant when considering a matter like the one in this thread? Does science bother with such considerations, whether they are ultimately valid or not? I don't know how to answer that (not the brightest person in the lamp, me), but it seems like one ends up with a philosophical conversation about what is real or how can we know anything, if we follow the idea of time as a purely perceptual phenomenon very far. Is the question relevant to the science under discussion? I ask less to challenge the idea than to try to understand something about which I may not know as much as I've thought.

That's how the metaphysics came into being. Because we need to bother ourselves with them.

I was ready to panic. I'm always halfway prepared to believe that everything I've ever thought I knew was just dead wrong. It's happened to me before.