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Comment on perspective

observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago

Astrophysicist here. Yes, space is crazy, but interesting things to keep in mind:

The size of a star is determined by something called the photosphere. With those extremely massive stars, you can be hundreds of millions of kilometres “inside” and not yet know it.
Similar story with supermassive black holes, from the perspective of an astronaut falling in, they wouldn’t really be able to tell when they cross the horizon because the tidal forces there are very small (they will inevitably fall towards the centre and get spaghettified at some point)

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shneancy@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
:0 how can you be inside a star and not know it? I thought they all had a surface like our Sun

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- observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  See my response below to Captain Aggravated about how dilute those large stars are.
  
  It’s an interesting question whether anybody would actually feel spaghettification 😁 I actually don’t know. You can use physics to calculate the proper time derivative of the tidal forces, but you need biology to define the start (and end…) of the process. My intuition says that it probably happens too fast, so once the tidal forces are strong enough to be perceptible, they grow strong enough to rip you apart before you realize (again, just a hunch).
  
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- pool_spray_098@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
  Right? How can you not know it?
  
  Hmm, am I in the star yet? I mean my body is now made of million degree hot plasma, but I’m still not sure…
  
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KillingTimeItself@lemmy.dbzer0.com ⁨5⁩ ⁨weeks⁩ ago
as a non astrophysicist, or just a non astronomer in general. it weirds me out every time i remember that there is literally a part of the universe that apparently exists, of which we will never be able to see, because the light from that part of the universe, quite literally hasn’t reached us yet.

The observable universe is inconceivably massive. But it just keeps going.

And to think it’s not an improbable concept for humanity to recreate the physics behind a big bang in a controlled setting, somewhere down the line from here, is certainly an interesting thought.

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- psud@aussie.zone ⁨4⁩ ⁨weeks⁩ ago
  
  the light hasn’t reached is yet
  
  The light will never reach us, space is expanding faster than light between here and there
  
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  - KillingTimeItself@lemmy.dbzer0.com ⁨4⁩ ⁨weeks⁩ ago
    perhaps some day we will figure out how to travel faster to light, and the metaphorical light will be capable of reaching us. Though thats also another really bizarre fact.
    
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unknown@sh.itjust.works ⁨5⁩ ⁨weeks⁩ ago
Hi Astrophysics,

I always wondered why they draw black holes like they do in that the accretion looks like it’s drawn in two planes. I would have thought it would have looked a bit more like a saturns rings? Or is it exactly like saturns rings but we see the whole ring bent round the top because a black hole bends the light around so we can see it? Or is it something else entirely that they are trying to depict here?

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- phoenixz@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  Black homes are so massive and heavy that they bend light (well, technically every piece of mass does, even yo momma). It’s bend so extreme that that accretion disk appears warped.
  
  A similar thing happens with neutron stars that can also bend light in such a way that you can actually see part of the back of the star (if you were able to see it anyway, it would be dark) as light that would be emitted from parts of the back would warp around to the front where you could see it
  
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- observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  Yep, you got it right. The accretion disk is actually really flat. Those images are produced in simulations that take into account the curved (and very complex) paths light takes in the vicinity of a black hole. These images really depend on the angle between the line of sight and the disk.
  
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- OrganicMustard@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
  The second one. The image is simulated as how an external observer would see it. It was firstly done for the Interstellar movie.
  
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  - Natanael@slrpnk.net ⁨5⁩ ⁨weeks⁩ ago
    The first simulated images were actually computed decades ago, but there’s of course a massive difference in detail
    
    cnrs.fr/…/first-ever-image-black-hole-cnrs-resear…
    
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    OrganicMustard@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
    Wow, that first one is so cool.
    
    Yeah, I misremembered the Interstellar paper that said it was the first simulation for a movie and thought it was the first image simulation ever. It’s even referencing the old one there.
    
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  - unknown@sh.itjust.works ⁨5⁩ ⁨weeks⁩ ago
    Cool, thought for a long time there it was just an artistic liberty; this is much cooler.
    
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- FrenziedFelidFanatic@yiffit.net ⁨5⁩ ⁨weeks⁩ ago
  
  Or is it exactly like saturns rings but we see the whole ring bent round the top because a black hole bends the light around so we can see it?
  
  Hit the nail pretty hard on the head there
  
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captain_aggravated@sh.itjust.works ⁨5⁩ ⁨weeks⁩ ago
Documentary watcher here. Isn’t the “photosphere” of the star defined by the visible surface? It is my understanding you could be “hundreds of millions of kilometers inside” the corona and not know it, but inside the photosphere?

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- observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  Yes, but red supergiants differ from the sun in that their photospheres are extremely dilute and don’t have a sharp transition to the corona. I don’t know the details of this particular star but take Betelgeuse as an example (it’s probably not particularly large for this catrgory), it’s radius is ~640 the sun’s per Wikipedia, which gives a volume of ~260 million that of the sun. But it is only x15 times as massive as the sun, so on average ~20 million times less dense.
  
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  - xenoclast@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
    Meaning a little less than half of that radius is way way way less than 20 million times less dense. That’s wild.
    
    I think about that a lot when looking a lot luminous gas clouds that are millions of lys across. We can only “see” it because all the photons coming from that region of space is concentrated in a tiny visible area.
    
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UnderpantsWeevil@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
I was under the impression that the time it would take you to get spaghettified would render the fear of such an experience irrelevant, as you’d be long dead of natural causes before then.

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- observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  In the case you are unlucky enough to encounter the black hole “heads on” and fall into it radially, the proper time timescale to spaghettification is the size of the event horizon divided by the speed of light. The most supermassive black holes will have a horizon of around one light day, so that’s what we’re working with, a matter of days. If you come in on the most tangential orbit possible though, I guess you’re buying some time but I’ve never heard that it’s supposed to take many years of proper time (I doubt that claim a little bit, but haven’t calculated myself).
  
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radicalautonomy@lemmy.world ⁨5⁩ ⁨weeks⁩ ago

they will inevitably fall towards the centre and get spaghettified at some point

Not before they witnessed the birth and death of thousands of civilizations! (I know they wouldn’t actually be able to witness them, not having the right equipment and being dead in due order, it’s just neat to think about relativity in that context. 😊)

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SanndyTheManndy@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
What really are gravitational waves? Are they like electromagnetic waves? Do they cause orbital decay? I have so many questions.

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- observantTrapezium@lemmy.ca ⁨5⁩ ⁨weeks⁩ ago
  They are quite similar to electromagnetic waves, but also quite different. They are produced by masses accelerating (just like EM waves are produced by charges accelerating), and indeed cause orbital decay. But this orbital decay is only important in relativistic systems (so the Earth, which is orbiting the sun at 0.0001 the speed of light, is not going to fall into the sun because of gravitational waves).
  
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  - psud@aussie.zone ⁨4⁩ ⁨weeks⁩ ago
    Surely they’re more like ocean waves; EM waves are electric and magnetic fields pulling each other up by their boot straps. Gravity waves are distortions in spacetime
    
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    observantTrapezium@lemmy.ca ⁨4⁩ ⁨weeks⁩ ago
    EM and gravitational waves are seen as analogous because as I wrote, they are produced by acceleration of charges and masses, respectively. The physics behind them is very different (described by Maxwell’s equations for EM and Einstein field equations for GW), but all systems that have waves in them (including sound in the air, waves on the surface of water etc.) can be approximated as linear for small perturbations, which means that they satisfy the wave equation at that regime.
    
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- SpacetimeMachine@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
  I highly suggest you look up PBS spacetime on YouTube. They have an incredible amount of very informative videos on black holes and gravitational waves. As well as pretty much any other astrophysics topic you can think of (and many you can’t!)
  
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- CheeseNoodle@lemmy.world ⁨5⁩ ⁨weeks⁩ ago
  Gravitational waves do cause orbital decay as the energy required to create them comes from the objects own momentumn.
  
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