[Cushman]

Nope. "Right now, over there" is a phrase which has no meaning on astronomical scales. There is no defined order for events which don't have a causal connection.

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

There exists a frame of reference in which the state that we see in that image takes place at the same time as we read about it today. There's another frame of reference where it happens billions of years ago, before the formation of the Earth; another in which it happens in our future, long after our Sun has evaporated into nothingness (Ed: There isn't actually, see below). All of these perspectives are literally, completely equally valid. They're all real.

There was a good discussion about this last time it came up here: http://news.ycombinator.com/item?id=1907271

[sliverstorm]

Your parent's point was simple and practical, not bound up in relativity. Parent was merely observing that even though we see it (please do not launch off into a metaphysical discussion about "what is seeing"), if we were to go visit it, it could no longer be there for use to "use".

[Cushman]

You seem to think I'm quibbling over pedantry. I want to make it clear I'm not talking about wacky theoretical physics here, let alone metaphysics— this is hard science. You can't avoid getting "bound up in it"; relativity is, definitely is, how our world works.

In our universe, "It could all be gone by now" not only isn't true, it doesn't even make logical sense. There is no one such thing as "now".

Now, "It could all be gone by the time we get there" is a different, totally practical and valid statement. But that's just because of the amount of time it would take us to cross that distance; it has nothing to do with the distance that light has already traveled.

[sliverstorm]

I understand relativity is hard science.

While I agree you are not talking about wacky unproven theory, I am not sure I agree that you are not being a pedant. Your parent's assertion is most definitely plausible from the right frame of reference, and mainly written to elicit amusement, but you insist on getting bogged down in the details discussing how it is NOT plausible in other frames of reference, which is quite frankly, besides the point.

[Cushman]

I guess I disagree it's beside the point. You say my parent's assertion is plausible from the right frame of reference— what frame of reference are you assuming my parent was referring to? Certainly not the one we're all in, where we can see there remains plenty of water.

I'm inferring you understood it as something like "a frame of reference which agrees with the theoretical frame of reference of a spaceship we launch today, at the time it arrives at the reservoir". That seems far from obvious to me— I find it much more likely that xhuang (like many other commenters on this article, I should add, not to mention the public at large) has been misled by pop-science into the thinking that something being twelve billion lightyears away means that right now, whatever we're seeing is actually twelve billion years in the past.

But that's exactly wrong! There is no actually, and what we see is as valid to call "right now" as anything else. And this is the universe we live in! Maybe I am a pedant, but this stuff is mind-bogglingly amazing when you can actually wrap your head around it, so I try not to miss out on a relativity teaching moment.

Anyhoot, we obviously both get it, so I'll wait on xhuang to say whether my comment was relevant to his point or not.

[sbov]

I would take the frame of reference of an observer at the location of the water because that is your goal - it's what is meant by "over there".

Could you resolve the "right now" issue if you took a picture of the event to said observer, who then compared it to a history of images and then said "oh yeah, 5 minutes later it was all gone, never to return"?

These seem like a closer representation of what people mean when they say "it's all gone now."

[Cushman]

See, you're making the same mistake— an observer at the location of the water when? You want to say "now", but there is no such thing. Taking a picture doesn't help the issue— you only wind up tangling yourself up in it.

This stuff was all figured out in the first place through thought experiments, so let's try one to demonstrate what I mean. Let's say there are two real events we can talk about here. Someone at the reservoir poses, the light takes twelve billion years to get here, and we snap a photo. Then say there's a second event, the draining of the reservoir, which happens (at the reservoir) one year from the time the person poses.

What we want to know is, has that event happened yet, even though we observe there to be water? So on your suggestion, as soon as we take the picture, I jump on the spaceship with a copy and fly over there at near to the speed of light. Twelve billion years after they posed, the person at the reservoir will look up and see me lift off from Earth, and then very soon after I will arrive carrying the picture to ask them about. (Of course I'll already have seen the event I want to ask them about, but set that aside for now.)

Okay, so here's the fun part. While I'm travelling there, at very near to the speed of light, time stops for me, or very nearly. I perceive the journey to take a matter of hours, say. I arrive at the reservoir carrying a still-wet photograph to ask them about. They say, "Oh, yeah, that happened about twelve billion years ago— a year before we drained the reservoir, wasn't it?"

"Ah hah!" I exclaim. "I had a hunch the water was already gone when I left."

The reservoir people think I'm nuts. "We drained the reservoir almost twelve billion years ago. You only left Earth a couple of hours ago. Of course the water was gone."*

This is true. From my perspective, I have just taken a picture, flown for a couple of hours, and the reservoir has been gone for billions of years. But, no matter. Bearing my prize, a notarized letter affirming that the picture depicts events one year prior the draining of the reservoir, I hop back in my spaceship and make for Earth. Again, a journey of twelve billion lightyears seems to take a manner of hours. On arrival, I disembark, waving the letter triumphantly.

"See? We were right! The water was gone a mere year after we took the picture."

"We know," says Earth. "We saw it happen one year after you left."

"...I haven't been gone for a year," I say with a sinking feeling in my gut.

"You've been gone for twenty-four billion years. We watched you leave, and one year later the water was gone. A couple of hours ago we saw you arrive at the reservoir, get the notarized letter, and lift off again; now you're here."

"No, no," I protest. "You only think that just happened a couple of hours ago. It takes light twelve billion years to travel that far, it's ancient history by now. This letter is twelve billion years old."

Suddenly, it occurs to me that I am trying to argue that events that I personally witnessed a couple of hours ago are, in fact, ancient history. I am holding a still-drying photograph which Earth insists was taken twenty-four billion years ago, and they have twenty-four billion years of history to prove it; I am also holding a notarized letter which both I and the Earth have just seen signed mere hours ago, and saying that it is twelve billion years old. And yet I know that these objects are only hours apart in age.

I realize that if I flew back to the reservoir to get double-confirmation, hours later they would say that they had just seen me arguing with Earth, and they took my side, although according to their records the letter was actually notarized twenty-four billion years ago and not twelve as I had been claiming. I realize that no matter how many trips I make, I will never be able to make it so that I, the Earth, and the reservoir people agree about the order of events.

"You know," I mention to the Earth, "I could have saved a lot of time if I had just read the Wikipedia article on the Relativity of Simultaneity, couldn't I."

"Yeah, probably," the Earth says. "Sorry about all your friends and stuff."

* cf. "a frame of reference which agrees with the theoretical frame of reference of a spaceship we launch today, at the time it arrives at the reservoir"

[extension]

You would observe that history yourself, as you traveled toward it.

[Dylan16807]

It's fine to get excited about strongly relativistic arguments but I don't think they have a place in talking about events at distant celestial objects.

The amount of distortion you can have between distances and times is bounded by the amount you let your reference frames differ. Between stars there is not all that much velocity, so time is a relatively stable concept. Sure it'll wander a few percent but nothing nearly as drastic as having no sense of 'now'.

Don't forget that when you bring relativistic speeds into the mix you can't agree on distances either. Are you prepared to argue that it's actually one billion light years away?

[Cushman]

I just posted a fun little thought experiment to illustrate why this is actually a thing: http://news.ycombinator.com/item?id=2799908

When you read someone write "as I write this", they mean "simultaneous with the event of this writing in the reference frame that I occupy currently". I mean I doubt anyone conceptualizes it like that, but that's what they mean physically. It just wouldn't make sense any other way.

So what does it mean when someone writes "as I write this, the water is all gone"? That statement is factually false, since in their frame of reference the event in which the water disappears has not yet happened.

Now as I noted in my thought experiment, that statement is true in "a frame of reference which agrees with the theoretical frame of reference of a spaceship we launch today, at the time it arrives at the reservoir". But that's just not what people mean when they say "now".

Commenters on this article, and there are several, aren't saying that because they're mindful of relativity and are using a peculiar implicit reference frame in order to make the point that the water might all be gone by the time our ship gets there. If they were, they would never say "now", because that's nonsensical, and we have a perfectly straightforward way to say that: "It might all be gone by the time our ship gets there." (In fact, isn't "the frame of reference of a spaceship we launch today, at the time it arrives at the reservoir" the same as "the time our ship gets there"?)

They're saying it because they think that there's such a thing as simultaneity. They think conceptually we could travel there instantly. It's just not true.

And it's not not true in the sense that "little g is 10m/s^2" is not true, it's not true in the sense that "The sky is red" is not true. Somebody who says the sky is red misunderstands something fundamental about their universe, and I think that's worth teaching them about.

Now, sure, you could argue that, say, it's stormy out, so it's dangerous to fly, just like it would would be if the sky were red, so practically the statement is ok and I'm just being a pedant. But that would be ridiculous.

[omarchowdhury]

I fail to see how "right now, over there" fails to have any meaning when at this very moment that point in space does exist, the assumption is that we are moving between there and here without any lag (instantly) when comparing the sets of areas.

In our minds, where the comparison is taking place, we need not account for how long it takes light to travel. We can simultaneously hold the thought that we are here on Earth, and way over there, there once was a giant cloud of water vapor.

[Cushman]

First you should read the Wiki article I linked, since it goes over it much more thoroughly than I can, and then maybe I can help if you still have questions. Here's the introductory paragraph to get you started:

Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time. But where the events are separated in space, such as one car crash in London and another in New York, the question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in others (in a different state of motion relative to the events) the crash in London may occur first, and in still others the New York crash may occur first.

I emphasize again that this is the actual way the universe works, not theoretical physics. What you say about our brains holding the two ideas simultaneously in our head is true; our brains are wrong, and the statement "There once was a giant cloud of water vapor" is untrue, or rather, undefined.

Also: I fail to see how "right now, over there" fails to have any meaning when at this very moment that point in space does exist, the assumption is that we are moving between there and here without any lag (instantly) when comparing the sets of areas.

To move "instantly" from one place to another presupposes there is such a thing as "over there, right now", which there isn't. Your definition thus has no meaning. Isn't this stuff wild?

[omarchowdhury]

So what I take that relativity of simultaneity is showing, is that there is no "universal time" everywhere.

But this is all dependent on the positioning of the observers. The observers painted on the universal graph are all ones waiting for light to reach them. This is what is causing the confusion as to when is "now", as all the observers or points of awareness are separated in space. And I take it that, their separation in space is also what separates them in time... and thus each have different notions of when now is.

But can we not conjure an omniscient observer as our frame of reference? One who exists at all points, at all times.

Would he not be able to observe both the Earth on July 24, 2011 and what exists in the space of the water vapor cloud as Earth time is still July 24, 2011?

[Dylan16807]

Sure you can.

Pick a vector.

Any vector.

You can create an observer moving at this velocity and it'll give you a different idea of simultaneous from an omniobserver at a different vector.

Oh, and before you ask, no there is no 'unmoving' vector. They're all relative to each other with no reference frame.

[Cushman]

But can we not conjure an omniscient observer as our frame of reference?

Yes. We can not. In fact, that is precisely what the theory of relativity states: There is no privileged reference frame.

There is literally, actually no such thing as "what exists in the space of the water vapor cloud as Earth time is still July 24, 2011". As I look at my clock at noon on July 24, I see water. Someone at the reservoir will not see my clock strike noon for twelve billion years, my time, and they'll look around and maybe see no water. We disagree, but we're both right in our own frame of reference. Both perspectives are true.

Which makes sense if you think about it; your omniscient observer would have to agree with one or the other (or some other) disagreeing perspective. What criteria could you possibly use to choose between them?

[sliverstorm]

Tip: "I'd love to explain it, but (I won't)" comes across as extremely elitist, and to some people, quite repugnant. If that's not how you mean to come across, simply omit that first bit before the comma, and you will come across much more amicably.

[Cushman]

That's a great tip, thanks. I really meant that I'd love to explain it, but you're right that it doesn't come across that way :)

[thret]

Have you considered exactly which frame of reference would be required for it to appear to happen in our future?

[Cushman]

Whoops, I forgot my own rules. Relativity doesn't violate causality; Obviously no one will observe us to see the reservoir it before it happens.

Okay, so there's a frame of reference in which what we'll observe to happen to the reservoir tomorrow has already happened. It was much punchier the other way, though :(

[harshavr]

I am no expert on this, but if you assume the cosmological principle which states that our universe is roughly isotropic, there is a natural set of observers called comoving observers who are at rest with respect to the expansion of the universe. One can define a universal time using the clocks of these observers. Presumably, this is what gives meaning to statements like the universe is 14 billion years old.

[Cushman]

Well, again, relativity doesn't violate causality. No one sees an effect before a cause. That translates into the cosmological principle in that all observers will see the big bang happen before anything else, all observers will see stars form before planets, et cetera, so it all comes out in the wash.

Nothing about that privileges a comoving observer with relation to unrelated events, though; they'll have their perspective just like everyone else, and that will be that. There's nothing "universal" about comoving time.