6 ms·

isn't it also means there was that much water xx billion light years ago, it could be all gone now!

by xhuang 15y ago

deleted 15y ago
[deleted]
Cushman 15y ago
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 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 http://news.ycombinator.com/item?id=1907271
- sliverstorm 15y ago
  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 15y ago
    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 15y ago
      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 15y ago
        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 15y ago
        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."
- omarchowdhury 15y ago
  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 15y ago
    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?
    - sliverstorm 15y ago
      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.
- thret 15y ago
  Have you considered exactly which frame of reference would be required for it to appear to happen in our future?
  - Cushman 15y ago
    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 15y ago
  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 15y ago
    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.

isn't it also means there was that much water xx billion light years ago, it could be all gone now!

by xhuang 15y ago

isn't it also means there was that much water xx billion light years ago, it could be all gone now!

deleted 15y ago

[deleted]

Cushman 15y ago

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 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 http://news.ycombinator.com/item?id=1907271

sliverstorm 15y ago
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 15y ago
  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 15y ago
    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 15y ago
      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 15y ago
        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."
omarchowdhury 15y ago
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 15y ago
  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?
  - sliverstorm 15y ago
    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.
thret 15y ago
Have you considered exactly which frame of reference would be required for it to appear to happen in our future?
- Cushman 15y ago
  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 15y ago
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 15y ago
  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.