3 ms·

> However, this is still an open area of research That's exactly what I've feared. You just spent writing a full page of text how it takes several decades to u

> However, this is still an open area of research That's exactly what I've feared. You just spent writing a full page of text how it takes several decades to understand the theory, then I ask a simplest of questions about it, one that any bright primary schooler will come up - and the understanding immediately implodes on itself because the answer is simply not there. All of these decades of studying led to a model that barely supports itself but cannot hold even lightest useful load. Imagine how a person will study mechanical springs for several decades, then you ask them what happens if you add some load to the spring currently hanging idle, only to hear that it's an "open area of research". But it would probably go up. Something surely would. I understand the idea of local conservation. Imagine an empty region of space where there's just light, and neutrinos, and some dust. Some time passed and some of that light (and neutrinos) lost energy due to red shift. Where did that energy went, stress-tensor or not? Where's the $100? Whose pocket does it line? What I'm hearing from you is "it can't go nowhere so it went somewhere" but that's not good enough. I suspected that already. It can be that it's the same $100, they just worth much less than when they were printed because of all the other money which were in circulation between then and now. But that still needs to be described to the point.

pdonis 2y ago
> I ask a simplest of questions about it, one that any bright primary schooler will come up - and the understanding immediately implodes on itself because the answer is simply not there. No, the answer is there, but it's not simple. Again, you want a quick sound bite: yes or no to your question. And reality is not like that. Reality is not simple sound bites. Reality is complicated and it takes work to figure out what is really going on. That's why it takes years or decades to get a real understanding of any useful model of reality on the scale of the universe. That's why it takes years or decades to do research to expand the boundaries of our knowledge. You want me to just bypass all that and give you a simple answer. Sorry, no. There isn't one. That's not because scientists are stupid or because our models don't work. It's because you don't want to accept the actual cost of understanding them. > Imagine how a person will study mechanical springs for several decades But nobody does that. Physics students learn a simple model of how mechanical springs work in a lecture or two. Mechanical engineers learn more detailed models that work for things like designing car suspensions in maybe a semester. People might certainly work with mechanical springs for decades, but they won't spend that whole time just getting an understanding of our best current model of them. They will spend most of that time using the model that that already understand. But the whole universe is not a mechanical spring. And learning our best current model of the whole universe is a lot harder than learning our best current model of a mechanical spring. So you are simply being unrealistic if you expect someone to explain the whole universe to you with the same amount of effort it would take to explain a mechanical spring. > I understand the idea of local conservation. No, you don't. You may think you do, but if you did, you would not even have to ask: > Imagine an empty region of space where there's just light, and neutrinos, and some dust. Some time passed and some of that light (and neutrinos) lost energy due to red shift. Where did that energy went, stress-tensor or not? Local conservation says no stress-energy went anywhere. The redshift due to the universe's expansion does not lose any stress-energy. Stress-energy is conserved. A better question would be, if stress-energy is locally conserved, why is there a redshift as the universe expands? And the answer to that is that the local conservation law does not say the energy density stays the same. It says that stress-energy is not created or destroyed. But "stress-energy" is not the same as energy density. The density of stress-energy, for matter (or radiation, since you asked about light) is the same with the scale factor slightly smaller and the energy density slightly higher, as with the scale factor slightly larger and the energy density slightly smaller. That's what the equations tell us. (For dark energy, as I've said, they tell us that the energy density does stay the same as the universe expands. But that's only true for dark energy.) All of these statements are consequences of the Friedmann equations, or, if you want to look at it that way, of the relationship that is enforced by those equations between stress-energy and spacetime geometry. That's about the best I can do at describing what the equations say in ordinary language. There's a reason why physicists do physics with math: because math is a much better tool for the job. I've already told you what math to look up if you want more information.
- thriftwy 2y ago
  > The density of stress-energy, for matter (or radiation, since you asked about light) is the same with the scale factor slightly smaller and the energy density slightly higher, as with the scale factor slightly larger and the energy density slightly smaller. So you are saying, there is a semi-reversible process where energy and volume are correlated? That's what I've started with. Should there actually be some specific amount of energy in place in order this density-to-scale transmutation to happen? What kinds of energy participate in it in our real world scenario? Does it depend on the specific local density (though I've already asked that)?
  - pdonis 2y ago
    > So you are saying, there is a semi-reversible process where energy and volume are correlated? My original objection was to you thinking of it as a "process", or at least a process involving energy and volume. It's a relationship between stress-energy and spacetime geometry. > Should there actually be some specific amount of energy in place in order this density-to-scale transmutation to happen? There is no such "transmutation". That's not what's going on. Your questions aren't answerable because they are based on false premises. At this point I've done my best to explain what is and isn't going on. All I can do is recommend reading more detailed treatments, which you can find in any cosmology textbook, or in briefer form in most GR textbooks. Sean Carroll's lecture notes on GR are available for free online [1], and Chapter 8 discusses the basics of our best current model of the universe in cosmology. [1] https://arxiv.org/abs/gr-qc/9712019 https://arxiv.org/abs/gr-qc/9712019
  - DEADMINCE 2y ago
    At this point why not interact with an LLM to get a better base understanding? Disagreeing with someone more knowledgable whose answers you can't properly grok isn't a great way to learn, surely?
    - pdonis 2y ago
      I would not advise trying to learn physics (or any subject, for that matter) from an LLM. They will confabulate confident sounding nonsense just as easily as they will give you reasonable information--and if you don't already know the subject, you won't be able to tell the difference.

pdonis 2y ago

> I ask a simplest of questions about it, one that any bright primary schooler will come up - and the understanding immediately implodes on itself because the answer is simply not there. No, the answer is there, but it's not simple. Again, you want a quick sound bite: yes or no to your question. And reality is not like that. Reality is not simple sound bites. Reality is complicated and it takes work to figure out what is really going on. That's why it takes years or decades to get a real understanding of any useful model of reality on the scale of the universe. That's why it takes years or decades to do research to expand the boundaries of our knowledge. You want me to just bypass all that and give you a simple answer. Sorry, no. There isn't one. That's not because scientists are stupid or because our models don't work. It's because you don't want to accept the actual cost of understanding them. > Imagine how a person will study mechanical springs for several decades But nobody does that. Physics students learn a simple model of how mechanical springs work in a lecture or two. Mechanical engineers learn more detailed models that work for things like designing car suspensions in maybe a semester. People might certainly work with mechanical springs for decades, but they won't spend that whole time just getting an understanding of our best current model of them. They will spend most of that time using the model that that already understand. But the whole universe is not a mechanical spring. And learning our best current model of the whole universe is a lot harder than learning our best current model of a mechanical spring. So you are simply being unrealistic if you expect someone to explain the whole universe to you with the same amount of effort it would take to explain a mechanical spring. > I understand the idea of local conservation. No, you don't. You may think you do, but if you did, you would not even have to ask: > Imagine an empty region of space where there's just light, and neutrinos, and some dust. Some time passed and some of that light (and neutrinos) lost energy due to red shift. Where did that energy went, stress-tensor or not? Local conservation says no stress-energy went anywhere. The redshift due to the universe's expansion does not lose any stress-energy. Stress-energy is conserved. A better question would be, if stress-energy is locally conserved, why is there a redshift as the universe expands? And the answer to that is that the local conservation law does not say the energy density stays the same. It says that stress-energy is not created or destroyed. But "stress-energy" is not the same as energy density. The density of stress-energy, for matter (or radiation, since you asked about light) is the same with the scale factor slightly smaller and the energy density slightly higher, as with the scale factor slightly larger and the energy density slightly smaller. That's what the equations tell us. (For dark energy, as I've said, they tell us that the energy density does stay the same as the universe expands. But that's only true for dark energy.) All of these statements are consequences of the Friedmann equations, or, if you want to look at it that way, of the relationship that is enforced by those equations between stress-energy and spacetime geometry. That's about the best I can do at describing what the equations say in ordinary language. There's a reason why physicists do physics with math: because math is a much better tool for the job. I've already told you what math to look up if you want more information.

thriftwy 2y ago
> The density of stress-energy, for matter (or radiation, since you asked about light) is the same with the scale factor slightly smaller and the energy density slightly higher, as with the scale factor slightly larger and the energy density slightly smaller. So you are saying, there is a semi-reversible process where energy and volume are correlated? That's what I've started with. Should there actually be some specific amount of energy in place in order this density-to-scale transmutation to happen? What kinds of energy participate in it in our real world scenario? Does it depend on the specific local density (though I've already asked that)?
- pdonis 2y ago
  > So you are saying, there is a semi-reversible process where energy and volume are correlated? My original objection was to you thinking of it as a "process", or at least a process involving energy and volume. It's a relationship between stress-energy and spacetime geometry. > Should there actually be some specific amount of energy in place in order this density-to-scale transmutation to happen? There is no such "transmutation". That's not what's going on. Your questions aren't answerable because they are based on false premises. At this point I've done my best to explain what is and isn't going on. All I can do is recommend reading more detailed treatments, which you can find in any cosmology textbook, or in briefer form in most GR textbooks. Sean Carroll's lecture notes on GR are available for free online [1], and Chapter 8 discusses the basics of our best current model of the universe in cosmology. [1] https://arxiv.org/abs/gr-qc/9712019 https://arxiv.org/abs/gr-qc/9712019
- DEADMINCE 2y ago
  At this point why not interact with an LLM to get a better base understanding? Disagreeing with someone more knowledgable whose answers you can't properly grok isn't a great way to learn, surely?
  - pdonis 2y ago
    I would not advise trying to learn physics (or any subject, for that matter) from an LLM. They will confabulate confident sounding nonsense just as easily as they will give you reasonable information--and if you don't already know the subject, you won't be able to tell the difference.