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> What blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago. Well... Not really. It t

> What blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago. Well... Not really. It took the entirety of the book A Brief History Of Time to even grasp a cusp of this, so I know I won't convince you in a short HN comment, but "now" is the collection of events that currently affect us. Due to causality, events outside our light cone have not occurred yet. They have no more influence than does an egg falling off the counter that has yet to hit the floor. The egg breaks only when it hits the floor - events only occur in our reference frame when they could affect outcomes in our reference frame. C is the propagation of causality, the fact that light travels at that speed in a vacuum doesn't affect that.

anonzzzies 19d ago
But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so we cannot say if it’s now, past or future in reference to us as observer.
- grumpopotamus 19d ago
  I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".
  - anonzzzies 19d ago
    Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
  - ViktorRay 19d ago
    Wait then what about events that happen on Pluto? Pluto is around five light hours away from us. If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us. So you’re saying that it is not correct to say her recorded activity happened five hours ago?
    - philipov 19d ago
      The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree. This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
      - NamlchakKhandro 19d ago
        A quantum entangled data stream of video between Pluto and earth would mean your now observing it in real time
        skirmish 19d ago
        Quantum entanglement does not enable FTL communication: [1]. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
    - herbstein 19d ago
      If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
      - ViktorRay 19d ago
        Ah yes that’s indeed true. But from the Earth’s reference frame wasn’t the Pluto activity I mentioned earlier simultaneous with what happened 5 hours ago on Earth? So couldn’t you say from Earth’s reference frame this black hole’s activity happened simultaneously with events 20 million years ago?
  - tomrod 19d ago
    It's a semantic saying that misses what people typically mean when we speak about simultaneous events. Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
    - philipov 19d ago
      It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
  - dotancohen 19d ago
    You are getting downvotes, but you are correct. People are voting by intuition, not by physics. Relativity is not intuitive.
    - andrewflnr 19d ago
      No, I took a physics class that covered relativity of simultaneity and that's how I know this is BS.
  - andrewflnr 19d ago
    > One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem. Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.
ckcheng 19d ago
> but "now" is the collection of events that currently affect us. Just to see if I understand what you’re saying… So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now? A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago? (Dinosaur = photon from a star or something. Decay = red shift. Or a real dinosaur— guess it’s the same now.)
- drfloyd51 19d ago
  “Now” is weird. It doesn’t exist in a cosmic sense. Check out book “the order of time”.
- dotancohen 19d ago
  > So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now? Yes. More specifically, the big bang would be happening now, at the location from where we perceive that photon. Hey, this stuff is not intuitive. However the early universe absorbed all the big bang photons. The furthest back we can observe photons from is the CMB. And yes, the phase change is happening now, there... Complicated by the Hubble expansion of course. > A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago? No, that is incorrect. But you already knew that. Analogies don't work for light and for time and for relativity - it is too different from our everyday experiences for analogies to hold.
  - ckcheng 18d ago
    [flagged]
steve_adams_86 19d ago
From a practical perspective this makes perfect sense to me. But we do know the photons travelled over a period of time, right? Where those photons came from, ‘now’ is something else. Though it doesn’t matter practically, those events did occur in the past, before they were a part of our now. Or am I missing something?
- dotancohen 19d ago
  No, actually, the photons did not travel over a period of time. Photons travel at C, and thus experience no time in their reference frame. As for _our_ reference frame, I've explained in other comments but a debate has developed between those few who do understand because they've studied physics, and those who think that their intuition is more correct than Einstein. I decline to elaborate further, HN comments are not long enough to cover it all. In short: you are in fact missing something but that is fine, everybody is. Einstein himself stated that in his day maybe three people on the planet understood, and he was uncertain if he was among those three.
  - andrewflnr 19d ago
    You've got some nerve to come in here with self-admittedly barely-understood BS from a pop science book and pretend it's other people's fault for being uneducated when they challenge you.
    - NamlchakKhandro 19d ago
      Lmfao
    - dotancohen 17d ago
      Quality of the information from the pop-sci book not withstanding, I've also taken physics courses on the subject. Albeit twenty years ago. But go ahead, you win the internet argument. When I thought this was a discussion, and I could potentially learn something or help somebody understand, I was happy to participate. But I have no interest in an argument consisting of insults and accusations and no exchange of information. You win, congratulations.
andrewflnr 19d ago
I think you misunderstood Hawking. "Now" in the relativistic sense is not the same as our past light cone. Reference frames for simultaneity are defined by relative motion. Galaxies without too big a redshift are close enough to moving at the same speed we are, and thus being in our same reference frame, that simultaneity (and times relative to the present) are reasonable to talk about.
- dotancohen 19d ago
  I may have misunderstood Hawking, but my Physics grades suggest that I do understand the principles. I tried to explain within the confines of an HN comment so that GGP could understand. Any answer that fits in this box will be incomplete. The most complete answer that is correct, would be a simple "No" but I felt that was too dismissive and not informative enough.
  - andrewflnr 19d ago
    Regardless of your past grades, your usage of terms in your comment like "now" and "reference frame" are plainly incorrect. A simple "no" would have been plainly incorrect, to say nothing of being dismissive. I'll repeat: reference frames in relativity are about motion, not any kind of distance, spatial or temporal.
    - dotancohen 18d ago
      I agree that my usage of the term "now" was incorrect. "Now" really isn't defined well when the concept of simultaneous events is not valid. I also used reference frame when referring to distant objects, for the simple reason that the conversation is (was at that point) confined to light traveling between those objects - the light being in a different reference frame from the two objects. I agree, now the morning after, that this was poorly phased and can not be generalized to two arbitrary objects no matter how distant. As I've stated, an answer in the confines of an HN comment on this topic will always be incomplete. We can answer very specific situations, but those answers can not be generalized to other situations.

anonzzzies 19d ago

But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so we cannot say if it’s now, past or future in reference to us as observer.

grumpopotamus 19d ago
I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".
- anonzzzies 19d ago
  Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
- ViktorRay 19d ago
  Wait then what about events that happen on Pluto? Pluto is around five light hours away from us. If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us. So you’re saying that it is not correct to say her recorded activity happened five hours ago?
  - philipov 19d ago
    The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree. This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity
    - NamlchakKhandro 19d ago
      A quantum entangled data stream of video between Pluto and earth would mean your now observing it in real time
      - skirmish 19d ago
        Quantum entanglement does not enable FTL communication: [1]. [1] https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
  - herbstein 19d ago
    If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
    - ViktorRay 19d ago
      Ah yes that’s indeed true. But from the Earth’s reference frame wasn’t the Pluto activity I mentioned earlier simultaneous with what happened 5 hours ago on Earth? So couldn’t you say from Earth’s reference frame this black hole’s activity happened simultaneously with events 20 million years ago?
- tomrod 19d ago
  It's a semantic saying that misses what people typically mean when we speak about simultaneous events. Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
  - philipov 19d ago
    It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
- dotancohen 19d ago
  You are getting downvotes, but you are correct. People are voting by intuition, not by physics. Relativity is not intuitive.
  - andrewflnr 19d ago
    No, I took a physics class that covered relativity of simultaneity and that's how I know this is BS.
- andrewflnr 19d ago
  > One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem. Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.

ckcheng 19d ago

> but "now" is the collection of events that currently affect us. Just to see if I understand what you’re saying… So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now? A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago? (Dinosaur = photon from a star or something. Decay = red shift. Or a real dinosaur— guess it’s the same now.)

drfloyd51 19d ago
“Now” is weird. It doesn’t exist in a cosmic sense. Check out book “the order of time”.
dotancohen 19d ago
> So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now? Yes. More specifically, the big bang would be happening now, at the location from where we perceive that photon. Hey, this stuff is not intuitive. However the early universe absorbed all the big bang photons. The furthest back we can observe photons from is the CMB. And yes, the phase change is happening now, there... Complicated by the Hubble expansion of course. > A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago? No, that is incorrect. But you already knew that. Analogies don't work for light and for time and for relativity - it is too different from our everyday experiences for analogies to hold.
- ckcheng 18d ago
  [flagged]

steve_adams_86 19d ago

From a practical perspective this makes perfect sense to me. But we do know the photons travelled over a period of time, right? Where those photons came from, ‘now’ is something else. Though it doesn’t matter practically, those events did occur in the past, before they were a part of our now. Or am I missing something?

dotancohen 19d ago
No, actually, the photons did not travel over a period of time. Photons travel at C, and thus experience no time in their reference frame. As for _our_ reference frame, I've explained in other comments but a debate has developed between those few who do understand because they've studied physics, and those who think that their intuition is more correct than Einstein. I decline to elaborate further, HN comments are not long enough to cover it all. In short: you are in fact missing something but that is fine, everybody is. Einstein himself stated that in his day maybe three people on the planet understood, and he was uncertain if he was among those three.
- andrewflnr 19d ago
  You've got some nerve to come in here with self-admittedly barely-understood BS from a pop science book and pretend it's other people's fault for being uneducated when they challenge you.
  - NamlchakKhandro 19d ago
    Lmfao
  - dotancohen 17d ago
    Quality of the information from the pop-sci book not withstanding, I've also taken physics courses on the subject. Albeit twenty years ago. But go ahead, you win the internet argument. When I thought this was a discussion, and I could potentially learn something or help somebody understand, I was happy to participate. But I have no interest in an argument consisting of insults and accusations and no exchange of information. You win, congratulations.

andrewflnr 19d ago

I think you misunderstood Hawking. "Now" in the relativistic sense is not the same as our past light cone. Reference frames for simultaneity are defined by relative motion. Galaxies without too big a redshift are close enough to moving at the same speed we are, and thus being in our same reference frame, that simultaneity (and times relative to the present) are reasonable to talk about.

dotancohen 19d ago
I may have misunderstood Hawking, but my Physics grades suggest that I do understand the principles. I tried to explain within the confines of an HN comment so that GGP could understand. Any answer that fits in this box will be incomplete. The most complete answer that is correct, would be a simple "No" but I felt that was too dismissive and not informative enough.
- andrewflnr 19d ago
  Regardless of your past grades, your usage of terms in your comment like "now" and "reference frame" are plainly incorrect. A simple "no" would have been plainly incorrect, to say nothing of being dismissive. I'll repeat: reference frames in relativity are about motion, not any kind of distance, spatial or temporal.
  - dotancohen 18d ago
    I agree that my usage of the term "now" was incorrect. "Now" really isn't defined well when the concept of simultaneous events is not valid. I also used reference frame when referring to distant objects, for the simple reason that the conversation is (was at that point) confined to light traveling between those objects - the light being in a different reference frame from the two objects. I agree, now the morning after, that this was poorly phased and can not be generalized to two arbitrary objects no matter how distant. As I've stated, an answer in the confines of an HN comment on this topic will always be incomplete. We can answer very specific situations, but those answers can not be generalized to other situations.