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Black Hole Time Dilation And Observation Of Accretion

  1. Mar 28, 2015 #1

    If time is infinitely dilated at the edge of the EH, how do we observe accretion?
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  3. Mar 28, 2015 #2


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    the accretion disk is outside the EH so we see most it just fine. Observations of events very close to the EH are observations in our frame of reference of events that actually happened a long time ago in our frame of reference. We know, however, that this is basically an optical illusion. That is, we don't believe that the infall of matter isn't happening just because our eyes tell us it hasn't happened yet.
  4. Mar 28, 2015 #3
    My guess from a quick read here
    is that the accretion disk is not the event horizon it is surounding the event horizon and therefore still able to emit light. it is the layer just before the point of no return. The site does mention another oddity tho and that is that space is being sucked into the black hole faster than the speed of light. How is that possible?
  5. Mar 28, 2015 #4


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    The site is wrong about that, or at least worded so carelessly that it might as well be wrong. Stuff like this is the reason for the Physics Forums rule requiring that references be to accepted sources as described in the rules.
  6. Mar 28, 2015 #5
    Oh my apologies i will have to refresh my memory as to accepted sites.
  7. Mar 28, 2015 #6


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    Accretion disks end at the lowest stable orbit - there are possible orbits even below that point (and escape is possible even below the lowest orbit), but those are unstable, and nothing orbits in exactly the right direction - material that is below the lowest stable orbit quickly falls into the black hole.
    The result: the inner edge of the accretion disk is well separated from the event horizon.
  8. Mar 28, 2015 #7
    There is no such thing as "time" by the way
  9. Mar 28, 2015 #8
    Time is what a clock measures? How can you say there is no such thing?
  10. Mar 29, 2015 #9


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    We never do observe objects passing the event horizon. The light from objects falling into a black hole gets redshifted until it can no longer be observed. The image of the object falling into the black hole gets redder and falls more slowly the more we observe it.

    But there's quite a bit of activity before the infalling objects get that far! As those above mentioned, the accretion disk itself is a fair distance from the black hole, and the matter that makes up the disk tends to be exceedingly hot, often so hot that it glows in x-ray wavelengths. There's also the fact that real black holes rotate, and rotating black holes are quite a bit more complex than non-rotating black holes. Specifically, the rotation of the black hole causes a sort of "bulge" to form just outside the event horizon and wider at the equator. This bulge is known as the ergosphere, and much of the matter that falls into the ergosphere tends to be ejected from the poles of the black hole at extraordinarily high energies. These jets can be tremendous for larger black holes. The supermassive black holes at the centers of galaxies can spawn jets that are larger in size than the galaxy they originate from. For example:
  11. Apr 12, 2015 #10

    My first post here. I am very much interested in understanding this.

    Because as of yet, I don't understand it. I have read the thread, but I found no explanation or justification to 'how a black hole grows'. Let's look at any active galaxy, maybe a quasar. So there is something huge happening in its center, around the black hole. I'm measuring the mass of the BH looking at the speed of the stars rotating around it. Say I am a magical human being with a lifespan of 1 billion year. So after 1 billion years I'm observing again this galaxy (quasar, whatever), I'm measuring its mass again. Will I find the same value? For me, from my point of reference, this BH has not grown 1 gram in one billion years? Because it seams that I cannot see anything 'actually passing the BH horizon', so for me this BH cannot grow, so I should find it has the same mass. For me, this conclusion is unacceptable (of course, as unacceptable as it is for me, it could be correct).

    I guess you can feel the frustration in my words. I would be grateful to anyone showing the weak point in this logic. Or a link to something I could read on the net, that would be great.

    Cheers, Virgil
  12. Apr 12, 2015 #11
    Over a billion years there is a good chance that you would see the the black hole capturing and consuming objects ranging from dust clouds to entire solar systems.
    When this happens the Black hole mass increases.
    It's thought to be probable that black holes can merge as well, so creating a more massive black hole.

    Although you won't actually see an object crossing the event horizon, it certainly can happen.
  13. Apr 12, 2015 #12


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    You'll measure a larger value.
    You cannot see the light, but the process still happens and you can still see that the mass of the black hole increased. It is not even relevant how it looks very close to the event horizon - for the mass measurement as you described it, it is sufficient if more mass falls into the accretion disk.
  14. Apr 12, 2015 #13
    You mean I can see/observe/measure matter passing the event horizon?

    But again, I read than me, as an external observer, I cannot see 'anything' passing the EH. So you mean, I cannot see the light but I can see heavy matter, baryons, passing the EH?
    Last edited by a moderator: Apr 12, 2015
  15. Apr 12, 2015 #14


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    It depends on what you call see/observe/measure. Can you see an electron? You can use tools that allow to be quite sure you have an electron at some place, but does that mean you "see" it? In the same way, you can be quite sure stuff fell into the black hole. Everything beyond that is philosophy and I won't discuss it.
  16. Apr 12, 2015 #15
    What you will observe is the light from the infalling object becoming increasingly redshifted until it is no longer detectable.
    That is a consequence of gravitational time dilation, an effect which is described by relativity.
    From your point of view the object just fades away, but for anyone unfortunate enough to be standing on the infalling object they will actually cross the horizon, and the matter of which they were composed is now (in some form), part of the black hole.
    Last edited: Apr 12, 2015
  17. Apr 12, 2015 #16
    But not for me, cause I can't see it happen!

    I know about light redshifted and gravitational time dilation, I wasn't talking about this.
    >>...but for anyone unfortunate enough to be standing on the infalling object... >> Sure, but I'm not talking about him, I'm talking about a person on Earth.

    I'm talking about measuring the mass of a black hole. It can be done looking at the velocities of objects rotating around it. This is not philosophy, this is physics. How can I 'measure' an increased mass of the BH since for me nothing passes its event horizon?
    Last edited by a moderator: Apr 12, 2015
  18. Apr 12, 2015 #17


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    Please don't make multiple posts in a row, you can edit your posts to add something.
    You also can't see my typing this post because the light will never reach you (it is blocked by the walls of the room I am in). Is there any doubt of it happening?

    The matter does pass the event horizon. You just never see light from this exact event, but you see every other effect (like the increased mass).
  19. Apr 12, 2015 #18


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    Do you only believe things happen when you see them happen? If the Sun sets and goes below your horizon, do you believe it's not there any more?

    If we see objects falling into a particular compact region, and the light from them redshifts until it can't be detected, and they never come out again, where do you think they went, if not inside the black hole?

    Just the way you said: look at orbital velocities of objects around it. The orbital velocity of an object at a given radius will be larger after something has fallen into the hole. You don't have to see the object fall below the event horizon to measure that.
  20. Apr 12, 2015 #19
    In your 'frame of reference' you won't see anything because once the object crosses the horizon no light or any other form of information is able to reach you.
    You will measure less and less photons as it approaches the horizon, and eventually so few that it may as well be zero.
    Maybe you will see one highly redshifted photon every few million years, that valiantly struggled to make it to your detector having started it's journey a little outside of the horizon.
    In the objects fame of reference though, it crossed the event horizon and any light or information of any kind it emits will never be seen by you.
    Last edited: Apr 12, 2015
  21. Apr 12, 2015 #20
    Sorry for the multiple posts. I'm just new and excited of being here.
    There is no law of physics forbidding you, from my system of reference, to type this. When I say 'see' please understand that I mean 'measuring' or 'understanding that it happens'. It is not 'see' as in using my eyes.
    It is physics forbidding matter, particles, anything, to pass the EH from my reference system. This is relevant.
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