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> Our supermassive black hole is in a quiet chapter of its life, but astronomers suspect that it has previously experienced this active phase. That's what alwa

by lolc 7y ago

> Our supermassive black hole is in a quiet chapter of its life, but astronomers suspect that it has previously experienced this active phase. That's what always freaks me out about astronomy. We don't know what Sagittarius A is up to at the moment. We will know in 26400 years.

coldtea 7y ago
>That's what always freaks me out about astronomy. We don't know what Sagittarius A is up to at the moment. We will know in 26400 years. Doesn't it make more sense to freak out because we won't know in 26400 years? Or, for that matter, not even get to know what will happen in 100 years?
- lolc 7y ago
  Sure, humanity might die out. And nobody will see it. That freaks me out too :-)
dTal 7y ago
If it makes you feel any better, the concept of "at the moment" doesn't really have a meaning at relativistic distances - simultaneity isn't defined. In any sense that practically matters, what Sagittarius A is up to "right now" in our frame of reference is what we see it doing in our telescopes. Although there is a sense in which it happened 26400 years ago, any event further into the future than that is, for us, just as nonexistent as next week. Ironically, HN seems to be confused as to whether you made this comment 15 minutes or 1 hour ago, depending on whether I'm in "Add Comment" or the main thread. Different frames of reference, perhaps? (edit: all comments are incorrectly marked as being 20 minutes old in the main thread) (edit2: and my comment is only visible to me, apparently. HN is broken.) (edit3: huge lag between edits and public visibility of said edits from a non-logged-in browser)
- deleted 7y ago
  [deleted]
- bcaa7f3a8bbc 7y ago
  > huge lag between edits and public visibility of said edits from a non-logged-in browser As the saying goes, "there are 2 great unsolved problems in computer science:, naming objects, cache invalidation, and off by one errors." In case you haven't noticed, to reduce server pressure, a logged-in user sees comments in real time, but a guest only sees the cached version of the page that only updates slowly. When Hacker News sees an explosive amount of traffic, logging out is a way to help. The last time it happened was when Donald Trump was elected.
sandworm101 7y ago
Either way, it wont suddenly turn "on" overnight. These things ramp up and down instantly on cosmological scales, which means many thousands/millions of years in our world.
- privong 7y ago
  > Either way, it wont suddenly turn "on" overnight. These things ramp up and down instantly on cosmological scales, which means many thousands/millions of years in our world. Some supermassive black holes have been caught briefly turning on, over timescales on the order of weeks-months. Tidal disruption events, where a star wanders too close to a SMBH, can result in luminous accretion with significant accretion rates (e.g., Figure 3 of [0]). As the material of the star falls into the SMBH or is blown away, the luminosity drops off, with a predicted decay of t^(-5/3). [0] https://physicstoday.scitation.org/doi/full/10.1063/PT.3.2382 https://physicstoday.scitation.org/doi/full/10.1063/PT.3.238...
mirimir 7y ago
For sure. But we're pretty far off axis, so I doubt that we'll get heavily irradiated.
thirdsurf 7y ago
There may be a form of information transfer that’s much faster than light, on the longitudinal axis of gravity.
- solstice 7y ago
  Could you elaborate? This would be news to me
  - chongli 7y ago
    Light gets slowed down whenever it passes through any medium. For example, it can take thousands of years for the light produced by fusion in the Sun to reach the surface of the photosphere. Neutrinos, on the other hand, can pass through the Sun largely unimpeded and reach us far more quickly. This is in spite of the fact that neutrinos do not travel as fast as light in a vacuum. The same possibility holds true for gravitational waves, since they are unimpeded by matter as well. They even have the advantage of propagating at the speed of light, unlike neutrinos.
    - the8472 7y ago
      The premise of your argument is flawed. > For example, it can take thousands of years for the light produced by fusion in the Sun to reach the surface of the photosphere This is not due to free travel of light through a transparent medium with a high index of refraction or due to gravity. Through a large part of the sun energy is transported by convection and even in the radiative zone photons get frequently absorbed and remitted, the mean free path is below 1cm.
    - sudhirj 7y ago
      This seems to be a common misconception because of the way the property is named. What we’re referring to is the maximum speed of information through the fabric of space time. Light (photons) travel at that speed when unimpeded, as do neutrinos, gravity waves, magnetic effects and anything on the electro magnetic spectrum. ‘Slowing’ down light by putting it in a reflective cylinder, water or fiber optic cable doesn’t change the nature of the universe or help achieve faster than light information exchange - it just means some photons are experiencing a traffic jam.

by lolc 7y ago

coldtea 7y ago

>That's what always freaks me out about astronomy. We don't know what Sagittarius A is up to at the moment. We will know in 26400 years. Doesn't it make more sense to freak out because we won't know in 26400 years? Or, for that matter, not even get to know what will happen in 100 years?

lolc 7y ago
Sure, humanity might die out. And nobody will see it. That freaks me out too :-)

dTal 7y ago

If it makes you feel any better, the concept of "at the moment" doesn't really have a meaning at relativistic distances - simultaneity isn't defined. In any sense that practically matters, what Sagittarius A is up to "right now" in our frame of reference is what we see it doing in our telescopes. Although there is a sense in which it happened 26400 years ago, any event further into the future than that is, for us, just as nonexistent as next week. Ironically, HN seems to be confused as to whether you made this comment 15 minutes or 1 hour ago, depending on whether I'm in "Add Comment" or the main thread. Different frames of reference, perhaps? (edit: all comments are incorrectly marked as being 20 minutes old in the main thread) (edit2: and my comment is only visible to me, apparently. HN is broken.) (edit3: huge lag between edits and public visibility of said edits from a non-logged-in browser)

deleted 7y ago
[deleted]
bcaa7f3a8bbc 7y ago
> huge lag between edits and public visibility of said edits from a non-logged-in browser As the saying goes, "there are 2 great unsolved problems in computer science:, naming objects, cache invalidation, and off by one errors." In case you haven't noticed, to reduce server pressure, a logged-in user sees comments in real time, but a guest only sees the cached version of the page that only updates slowly. When Hacker News sees an explosive amount of traffic, logging out is a way to help. The last time it happened was when Donald Trump was elected.

sandworm101 7y ago

Either way, it wont suddenly turn "on" overnight. These things ramp up and down instantly on cosmological scales, which means many thousands/millions of years in our world.

privong 7y ago
> Either way, it wont suddenly turn "on" overnight. These things ramp up and down instantly on cosmological scales, which means many thousands/millions of years in our world. Some supermassive black holes have been caught briefly turning on, over timescales on the order of weeks-months. Tidal disruption events, where a star wanders too close to a SMBH, can result in luminous accretion with significant accretion rates (e.g., Figure 3 of [0]). As the material of the star falls into the SMBH or is blown away, the luminosity drops off, with a predicted decay of t^(-5/3). [0] https://physicstoday.scitation.org/doi/full/10.1063/PT.3.2382 https://physicstoday.scitation.org/doi/full/10.1063/PT.3.238...

mirimir 7y ago

For sure. But we're pretty far off axis, so I doubt that we'll get heavily irradiated.

thirdsurf 7y ago

There may be a form of information transfer that’s much faster than light, on the longitudinal axis of gravity.

solstice 7y ago
Could you elaborate? This would be news to me
- chongli 7y ago
  Light gets slowed down whenever it passes through any medium. For example, it can take thousands of years for the light produced by fusion in the Sun to reach the surface of the photosphere. Neutrinos, on the other hand, can pass through the Sun largely unimpeded and reach us far more quickly. This is in spite of the fact that neutrinos do not travel as fast as light in a vacuum. The same possibility holds true for gravitational waves, since they are unimpeded by matter as well. They even have the advantage of propagating at the speed of light, unlike neutrinos.
  - the8472 7y ago
    The premise of your argument is flawed. > For example, it can take thousands of years for the light produced by fusion in the Sun to reach the surface of the photosphere This is not due to free travel of light through a transparent medium with a high index of refraction or due to gravity. Through a large part of the sun energy is transported by convection and even in the radiative zone photons get frequently absorbed and remitted, the mean free path is below 1cm.
  - sudhirj 7y ago
    This seems to be a common misconception because of the way the property is named. What we’re referring to is the maximum speed of information through the fabric of space time. Light (photons) travel at that speed when unimpeded, as do neutrinos, gravity waves, magnetic effects and anything on the electro magnetic spectrum. ‘Slowing’ down light by putting it in a reflective cylinder, water or fiber optic cable doesn’t change the nature of the universe or help achieve faster than light information exchange - it just means some photons are experiencing a traffic jam.