The Orville Season Two - Thoughts? - Page 44
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  1. #431
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    Quote Originally Posted by tomBitonti View Post
    Is this correct?
    I think so. Freyar might correct me. But, light follows geodesics. From the event horizon inwards, there are no geodesics that ever carry you *farther away* from the singularity, iirc. They all spiral inwards to varying degrees. Each time you take a step towards the singularity, there is no going back out.

    There may be some exceptions, especially for black holes with high rates of spin. But I'm not near my notes at the moment to check.

    If one were held still, perhaps, but an infalling observer should still see objects which fell ahead of them. In the infalling frame, the light proceeds outwards.
    There isn't one single infalling frame for all things inside the event horizon. Each thing has its own rest frame. The space is decidedly *not* flat in there, so frames at different distances from the singularity are under acceleration relative to one another, and the acceleration difference will increase as you get closer to the singularity. And at the event horizon, you've already gotten to the point where light of things ahead of you has been red-shifted to infinity. I don't see why that would change when you cross the horizon.

    Again, maybe I am forgetting something, and I am not near my notes.

  2. #432
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    Quote Originally Posted by Umbran View Post
    I think so. Freyar might correct me. But, light follows geodesics. From the event horizon inwards, there are no geodesics that ever carry you *farther away* from the singularity, iirc. They all spiral inwards to varying degrees. Each time you take a step towards the singularity, there is no going back out.

    There may be some exceptions, especially for black holes with high rates of spin. But I'm not near my notes at the moment to check.

    There isn't one single infalling frame for all things inside the event horizon. Each thing has its own rest frame. The space is decidedly *not* flat in there, so frames at different distances from the singularity are under acceleration relative to one another, and the acceleration difference will increase as you get closer to the singularity. And at the event horizon, you've already gotten to the point where light of things ahead of you has been red-shifted to infinity. I don't see why that would change when you cross the horizon.

    Again, maybe I am forgetting something, and I am not near my notes.
    This particular question has bugged me for a while. I don't have a clear answer, just what seems to be necessary for consistency.

    Very definitely, there would be perceivable effects of the curvature; that would be tidal effects, and red and blue shifting of light.

    What seems necessary is that light can appear to proceed "outwards", but that is only within the frame of the infalling observer. From a frame which is still relative to the singularity, both the observer and the observed light are and must always move towards the singularity.

    That seems to be necessary for the transition across the event horizon to not have a dramatic effect. The alternative is a total disconnection (aside from gravitational effects) of anything inside the event horizon from anything closer to the singularity. Normal matter couldn't even exist, as it relies on particle exchange, and that would become impossible in an outward direction.

    Thx!
    TomB

  3. #433
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    Quote Originally Posted by tomBitonti View Post
    This particular question has bugged me for a while. I don't have a clear answer, just what seems to be necessary for consistency.
    Did a bit of quick reading from work...

    The whole idea of "you could fall in and nothing happens" as you approach a supermassive black hole, is not what it appears. It is really talking about your approach (like, you don't get sphagettified). And there's no discontinuity at the event horizon...

    However, after that, things get messy. Fast. It turns out that any choice of coordinate system runs into trouble, and you have to start talking about coordinate-invariant qualities. But then it becomes near impossible to speak about what happens to say, a human's body. You have funny things happen depending on whether you enter traveling in the direction of the hole's spin, or against the spin. Ugly.
    Last edited by Umbran; Thursday, 16th May, 2019 at 08:06 PM.

  4. #434
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    Given that they have control of both gravity and inertial, I tend to think of the ship as an independent and closed system.

    ... until that control fails.

  5. #435
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    Scot Grimes just won the lottery. Marrying Adrienne Palicki

    https://comicbook-com.cdn.ampproject...ied-wedding%2F

  6. #436
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    Quote Originally Posted by Aeson View Post
    Scot Grimes just won the lottery. Marrying Adrienne Palicki
    So, in a meta sense, he married his best friends ex-wife. What's just weird.
    Laugh Aeson, Dannyalcatraz laughed with this post

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    Still not as bad as if Kit Harrington had married Daenerys instead of Ygritte..

  8. #438
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    Quote Originally Posted by Umbran View Post
    Did a bit of quick reading from work...

    The whole idea of "you could fall in and nothing happens" as you approach a supermassive black hole, is not what it appears. It is really talking about your approach (like, you don't get sphagettified). And there's no discontinuity at the event horizon...

    However, after that, things get messy. Fast. It turns out that any choice of coordinate system runs into trouble, and you have to start talking about coordinate-invariant qualities. But then it becomes near impossible to speak about what happens to say, a human's body. You have funny things happen depending on whether you enter traveling in the direction of the hole's spin, or against the spin. Ugly.
    This seems to be on point:

    https://web.stanford.edu/~oas/SI/SRG...ect10_2007.pdf

    Some main points are this, from page 3, regarding a non-rotating case:

    This reflects the fact that for the in falling object, it is always in a freely falling frame, an ever shrinking IRF. As the object passes the horizon, nothing significant happens. It doesnt even notice that it has passed.
    That "IRF" in "ever shrinking IRF" is short for "Inertial Reference Frame". Close to the infalling observer, spacetime is still flat: An infalling observer will still see their feet, and locally physics will work as expected.

    For the spinning case, page 4 changes the metric, and has:

    So our map, so far, looks like any other flat spacetime IRF however here thismust be seen as a patchwork of small local frames. We will examine a series of pointsand rays to find how to represent the shell observer and Schrodinger (far away) coordinates within this map.
    So locally we are still OK, but the global picture is a lot more complicated.

    Off topic: There is an interesting bit towards the end (page 8) which talks about difficulties of working out quantum gravity:

    To explore fluctuations greater than the Planck mass, or equivalently to measure distances shorter than the Planck length Rp, neither general relativity nor quantum field theory can be used alone. We can also get a sense of the energy of the fluctuation and the time limit by going backto the uncertainty relation.

    Again, to describe fundamental particles with an energy of the scale of the Planck energy or time intervals less than the Planck time requires a theory of quantum gravity. We can take this time limit as the approximate limit in exploring the initial conditions of the big bang. What this argument suggests is that we can push the separate theories of general relativity and quantum field theory (quantum mechanics united with special relativity) back to a time of about 10-42 s or so. Prior to this time physics is governed by the unknown theory of quantum gravity. If a theory of quantum gravity is developed, the hope is that it will describe the initial conditions of the universe and answer all questions about its development. Hence, such a theory would be enormously powerful. Over the past 50 years it has become clear that such a theory is not going to be easily developed.

    What are the difficulties in uniting these two powerful theories? There are several different ways to point out the conflicts. First, it is clear that general relativity needs to have some modification on the extremely small, high energy scale. At the center of black holes and the beginning of the universe, the theory calls for a singularity. This singularity is a point of infinite spacetime curvature and energy density. Such singularities are mathematically unacceptable. However in low curvature, low energy, regions the general theory is an accurate theory. Quantum field theory (the unification of quantum mechanics and special relativity) on the other hand is an accurate theory on short distance, moderately high energy, scales. On the large scale, low energy scale, quantum mechanics transitions to classical mechanics. A transition which is not entirely well understood. There is much work today on this transition regime between quantum mechanics and classical physics.Another problem in bringing together these two theories is the question of what exactly is being quantized. To discuss quantization, first consider classical electromagnetic theory and its quantized form quantum electrodynamics. The process of quantizing the electromagnetic theory replaces the notion of an electromagnetic wave with particles (quanta) which mediate the electric and magnetic forces. The photon is the quanta of EM radiation. Classically it is electromagnetic waves (or electric and magnetic fields) which mediate the forces. This is what is being quantized.(The process is more complicated then simply replacing waves with particles but there is nospace to discuss the details). For general relativity what is to be quantized? Recall that the Einstein equation gives the metric solution for a particular distribution of energy. The metric is the field which mediates the gravitational force (we are drawing an analogy to electricity and magnetism, again, there is no gravitational force but the curvature of spacetime). So the quantity to quantize is the metric itself. Or, put another way, spacetime itself must be quantized! This is surely a strange requirement. What does it mean to replace spacetime with quanta (gravitons) which mediate the gravitational force? What do these particles propagate through, since there is no longer a continuum of spacetime? Tying this together with quantum mechanics which employs time and space as parameters to describe the wave functions of the quanta here being space and time itself. It is somewhat self-referential and causes difficulties in even beginning to construct a theory. Quantum theory relies on a spacetime background, however here we are doing away with such a concept and replacing it with discrete particles.
    Thx!
    TomB

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