4 ms·

I don’t think I’ll ever understand the astrophysics use of the verbiage “causal” and “observable” in lieu of just using “light/ERM” and “spacetime”. The only a

I don’t think I’ll ever understand the astrophysics use of the verbiage “causal” and “observable” in lieu of just using “light/ERM” and “spacetime”. The only argument I could see for the need of these terms in addition to just light/ERM would be the ability to detect the existence of something beyond the furthest edges of the universe from where light reaches us through phenomena such as gravitational pull from beyond the edges on the bodies at the edges.

isaacg 6y ago
In the early universe, light could not travel freely, because the ambient temperature was hot enough that the universe was filled with plasma. The end of this period is called "recombination", and the light from that point in time is called the Cosmic Microwave Background. That light is the earliest light we can observe, but it is not the earliest event to have had casual effects on us, nor is the earliest event we can observe. In particular, that light was emitted about 370,000 years after the Big Bang, while events within the first second after the Big Bang have had casual effects on us. So there is an important difference between "light" and "casual" in physics.
- throwaway_kufu 6y ago
  Nice, my initial impression of your comment still fits squarely within the door I left open for using causal...except when I was thinking phenomena outside the edges physical space where light reaches us you expand it to outside the edges of time where light still reaches us. Unfortunately I’m now just sort of stuck thinking about causality, determinism and free will...not exactly knowing where you are on Earth I guess it’s a possibility you have never been and never will in my light cone, yet my comment was the causality of your reply in some sense. And in maybe an HN 1st you’ve made me reconsider my position, and Recognize I need to more deeply consider causality as having good use outside phenomena from outside light at the edges of space and time and spacetime.
  - panda-giddiness 6y ago
    Of course they've been in your lightcone -- otherwise it would've been impossible for you two to have a back-and-forth exchange!
sandgiant 6y ago
I'm not sure I understand. Something can be causally connected through something other than light. Say gravitational waves. So using "light" in this case would be wrong. Causality refers to the underlying constraints from the theory (nothing travels faster than the speed of light), this is more general than talking about any particular type of particle/field. The observable universe is the set of points from which information can reach you. That is not the same as spacetime?

isaacg 6y ago

In the early universe, light could not travel freely, because the ambient temperature was hot enough that the universe was filled with plasma. The end of this period is called "recombination", and the light from that point in time is called the Cosmic Microwave Background. That light is the earliest light we can observe, but it is not the earliest event to have had casual effects on us, nor is the earliest event we can observe. In particular, that light was emitted about 370,000 years after the Big Bang, while events within the first second after the Big Bang have had casual effects on us. So there is an important difference between "light" and "casual" in physics.

throwaway_kufu 6y ago
Nice, my initial impression of your comment still fits squarely within the door I left open for using causal...except when I was thinking phenomena outside the edges physical space where light reaches us you expand it to outside the edges of time where light still reaches us. Unfortunately I’m now just sort of stuck thinking about causality, determinism and free will...not exactly knowing where you are on Earth I guess it’s a possibility you have never been and never will in my light cone, yet my comment was the causality of your reply in some sense. And in maybe an HN 1st you’ve made me reconsider my position, and Recognize I need to more deeply consider causality as having good use outside phenomena from outside light at the edges of space and time and spacetime.
- panda-giddiness 6y ago
  Of course they've been in your lightcone -- otherwise it would've been impossible for you two to have a back-and-forth exchange!

sandgiant 6y ago

I'm not sure I understand. Something can be causally connected through something other than light. Say gravitational waves. So using "light" in this case would be wrong. Causality refers to the underlying constraints from the theory (nothing travels faster than the speed of light), this is more general than talking about any particular type of particle/field. The observable universe is the set of points from which information can reach you. That is not the same as spacetime?