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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

by 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."
    - extension 15y ago
      You would observe that history yourself, as you traveled toward it.
    - Cushman 15y ago
      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"

by Cushman 15y ago

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."
  - extension 15y ago
    You would observe that history yourself, as you traveled toward it.
  - Cushman 15y ago
    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"