[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.

[Dylan16807]

But that sort of problem only comes up when you accelerate. If you maintain a frame of reference then you can handle things far more gracefully.

A shared reference frame over two points billions of light years apart can be established. You simply keep the distance between these points roughly constant, which holds the velocity of all points roughly constant, which keeps calculations of simultaneity roughly constant across the whole region.

I can then state that in the combined earth/reservoir plusorminus .01c reference frame that the water is already gone. And this is factually true. You still have to wait for information to propagate but everything works out. The timing has no disputes between any members subscribing to this reference frame.

Relativity doesn't cause major problems until you deal with matter moving at high relative speeds.

P.S. I'm deliberately ignoring the expansion of space in this argument.