3 ms·

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 rela

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 13d 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 13d 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 12d 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.
    - andrewflnr 12d ago
      You keep using the length of HN comments as an excuse, when you're the one injecting complexity that doesn't apply. Even your "corrections" here are still wrong. I'll repeat once more: reference frames are about motion, not distance. A nearby (relatively) fast-moving object has a different frame, but a far away object that's (relatively) motionless has the same frame. Simultaneity is defined per reference frame. The reference frame of the light traveling between objects is entirely a distraction for our purposes. This isn't actually that hard to explain in a comment. See also https://news.ycombinator.com/item?id=49691488 https://news.ycombinator.com/item?id=49691488
      - dotancohen 12d ago
        The point I've been trying to make is that one can not travel from one cosmically-distant place in the Earth's reference frame to another cosmically-distant place in the Earth's reference frame without breaking the reference frame. And specific to the GGGGP's question - traveling at the speed of light breaks that reference frame. Please, pedantically correct my phrase "break the reference frame". I'd like to know how to phrase that properly. "Change reference frame" maybe?
        andrewflnr 12d ago
        There's no way to phrase it correctly because the concept is incoherent. Using light to send information from one part of a reference from to another is a non-issue. In fact that's the best way to do it, and it's a standard part of the formulation of "reference frames" that I learned in class. We don't care about the light's reference frame along the way, only its externally observable trajectory.
        dotancohen 11d ago
        But that has nothing to do with the GGGGGP's question about things happening in the past. I anticipate your reply that the light is just sending information about what happened. But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object. There is no "now" and no relative, definite "past" for distant objects, even if they are in the same reference frame. It is not intuitive, I understand that. I didn't make the rules. I do appreciate you insisting that I improve my terminology. And I do appreciate being made to defend myself, I had to go look some of this stuff up. It is fascinating - especially the pre-Einstein work that left everybody puzzled. LATE EDIT: > According to the special theory of relativity introduced by Albert Einstein, it is impossible to say in an absolute sense that two distinct events occur at the same time if those events are separated in space. If one reference frame assigns precisely the same time to two events that are at different points in space, a reference frame that is moving relative to the first will generally assign different times to the two events. That's from here: https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity I'll not say that Wikipedia is a particularly good source, I'm happy to be corrected if you have something that contradicts that.

dotancohen 13d 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 13d 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 12d 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.
  - andrewflnr 12d ago
    You keep using the length of HN comments as an excuse, when you're the one injecting complexity that doesn't apply. Even your "corrections" here are still wrong. I'll repeat once more: reference frames are about motion, not distance. A nearby (relatively) fast-moving object has a different frame, but a far away object that's (relatively) motionless has the same frame. Simultaneity is defined per reference frame. The reference frame of the light traveling between objects is entirely a distraction for our purposes. This isn't actually that hard to explain in a comment. See also https://news.ycombinator.com/item?id=49691488 https://news.ycombinator.com/item?id=49691488
    - dotancohen 12d ago
      The point I've been trying to make is that one can not travel from one cosmically-distant place in the Earth's reference frame to another cosmically-distant place in the Earth's reference frame without breaking the reference frame. And specific to the GGGGP's question - traveling at the speed of light breaks that reference frame. Please, pedantically correct my phrase "break the reference frame". I'd like to know how to phrase that properly. "Change reference frame" maybe?
      - andrewflnr 12d ago
        There's no way to phrase it correctly because the concept is incoherent. Using light to send information from one part of a reference from to another is a non-issue. In fact that's the best way to do it, and it's a standard part of the formulation of "reference frames" that I learned in class. We don't care about the light's reference frame along the way, only its externally observable trajectory.
        dotancohen 11d ago
        But that has nothing to do with the GGGGGP's question about things happening in the past. I anticipate your reply that the light is just sending information about what happened. But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object. There is no "now" and no relative, definite "past" for distant objects, even if they are in the same reference frame. It is not intuitive, I understand that. I didn't make the rules. I do appreciate you insisting that I improve my terminology. And I do appreciate being made to defend myself, I had to go look some of this stuff up. It is fascinating - especially the pre-Einstein work that left everybody puzzled. LATE EDIT: > According to the special theory of relativity introduced by Albert Einstein, it is impossible to say in an absolute sense that two distinct events occur at the same time if those events are separated in space. If one reference frame assigns precisely the same time to two events that are at different points in space, a reference frame that is moving relative to the first will generally assign different times to the two events. That's from here: https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity I'll not say that Wikipedia is a particularly good source, I'm happy to be corrected if you have something that contradicts that.