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johnvv
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10 Aug 2017 19:27

Marko S. wrote:
Okay, I don't know where to post this, so I will post this question here.

This is related to eclipse coming in 21st of August. But, I have something interesting. I tried somewhere in previous month to advance time in Space Engine to the date when eclipse occurs. Then I would land in the America at the spot where it should be total eclipse. As I was waiting, I saw the moon just passing by the Sun not even touching it. Actually, it wasn't even close touching it. I also tried that recently but it didn't work.

Then, I tried that in Celestia. And, it actually showed the eclipse! I wasn't expecting it to be that accurate. And it also has that eclipse finder. But I was just advancing the time.

My question is: Does orbits in Space Engine need to be updated? I get it that it is still beta and that stuff, but I think it is easier just for Moon to get it's path right. This is more of a suggestion than some question for science and astronomy, but okay.

And also: Does the Moon looks like it travels from west to east during the eclipse because it travels slower than Sun on our sky? Because a lot of people have trouble answering this question. i think I got that right. Well, that's what I would answer if someone asked me.

the moon starts on the right side of the sun as the sun moves west
so from the moon is crossing from the right to the left
a short video ( created using Celestia and the NAIF SPICE kernels )
for the metro Detroit area , then the shadow  path for the country
https://youtu.be/yjxgMycfKnU
 
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Watsisname
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10 Aug 2017 21:28

Mr. Missed Her wrote:
Source of the post About black holes, is it possible there's not actually a singularity - no infinitely dense point - to black holes? From my understanding of relativity and time dilation, time gets slower and slower as the gravity's escape velocity grows closer and closer to c, meaning that nothing can pass through the event horizon by themselves.

Yes, it's totally possible (perhaps even expected) that in reality, black hole singularities are not actually infinitely small.  That everything is compressed to singularity is a prediction of general relativity.  Once the collapse has brought the matter within its own Schwarzschild radius, it cannot be supported by any pressure forces, and the only direction it can possibly move is further inward.  It is thought that some future theory of quantum gravity will lead to different predictions about the nature of the singularity, though this will not change the behavior of the rest of the black hole or its horizon.

The bit about time dilation's role in the collapse is a common confusion -- even the relativists first studying the collapse had difficulty with this.  The way it works is that, for the observer outside the black hole, time does come to a halt at the event horizon.  So according to them, the black hole actually has no interior!  All the matter is "smushed" into very thin shells that get closer and closer to the horizon.  (They also vanish from view, because the light is also being redshifted to blackness, which happens very quickly.)

But for the frame of reference of the material collapsing to form the black hole, or for someone falling into the black hole sometime later, time does not halt at the horizon.  The collapse proceeds basically as a free-fall all the way to singularity, and in a very short time according to clocks in this in-falling frame.  So if you jump into a black hole you do pass through the horizon without any hang-up, even though to someone far away they never see you reach the horizon.

Mr. Missed Her wrote:
Source of the post Also, inside of the event horizon, would time travel backwards? Because I believe that time travel to the past is extremely paradoxical and therefore impossible, I'm starting to doubt that even event horizons actually exist.

Not quite.  What happens is that time and one of the spatial directions switch roles.  The inward direction acts like the direction of the future, so once inside the horizon you are forced to move further inward.  To move outward would be equivalent to travelling into the past, which as you say, is quite forbidden!

A very good way to think about it is as if you are fish about to go over a waterfall.  At some point, the current flows faster than you can swim, so even if you try swimming upstream, you're still going farther down.  This point where you can no longer possibly escape is location of the event horizon, and the singularity is like the sharp rocks at the bottom.

Mr. Missed Her wrote:
Source of the post Now, I am quite possibly wrong, since my understanding of Einsteinian relativity and quantum mechanics is small. Sadly, no high school that I know of offers those courses. Although if my understanding is correct, there should be this interesting result: black holes have holes inside. They're bubbles!

Yeah, I don't know of any High School that would teach these subjects, but if you go through a physics program at college or university you'll get some much deeper knowledge about them.  There is also lots of material available freely online. :)
 
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midtskogen
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10 Aug 2017 22:05

Watsisname wrote:
Source of the post The way it works is that, for the observer outside the black hole, time does come to a halt at the event horizon.  So according to them, the black hole actually has no interior!  All the matter is "smushed" into very thin shells that get closer and closer to the horizon.  (They also vanish from view, because the light is also being redshifted to blackness, which happens very quickly.)

Is it also correct to say that the gravity waves of black hole mergers also never quite stop to ring, just that the frequency very quickly drops towards zero and the signal becomes undetectable?
NIL DIFFICILE VOLENTI
 
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Watsisname
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11 Aug 2017 00:47

After the merger it's actually not the frequency that drops, but the amplitude.  A good model for it is that the event horizon oscillates as an under-damped harmonic oscillator, so the wave amplitude dies as an exponential as the horizon sheds off any irregularities and settles down into the shape described by the Kerr metric.

So, yeah, in a sense they never really stop ringing, but the amplitude drops toward zero very quickly.

I think you're imagining though that the waves should be redshifted down as they climb out the gravity well.  This is true, but the effect doesn't change much over time (after the merger, the gravity well that they're climbing out of is basically the same).  The more important change in frequency is due to the orbital period decreasing during the in-spiral phase.
 
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midtskogen
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11 Aug 2017 01:32

Yeah, but I was more thinking about what it would take for the time dilation effect to catch up with the decreasing period during the in-spiral.
NIL DIFFICILE VOLENTI
 
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the photo guy
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15 Aug 2017 01:42

Hey! another question from me.

does a black hole have an escape velocity  that's faster then light? or does a black hole actually have acceleration due to gravity. EX: earth = 9.807 m/s2 that's faster then light?
if its the escape velocity then I'm having a hard time knowing why a small black hole manages to stop light and then pull it in at an extremely short time. 
if its acceleration then does that mean once you have entered the event horizon you will be traveling faster then light?   
HI! I would like to say that I'm just a friendly teen who loves talking and helping people out :) Space engine is truly a wonderful program and the website is also very cool! Many great people here. I hope to make some good friends here.
 
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FastFourierTransform
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15 Aug 2017 03:04

the photo guy wrote:
Source of the post earth = 9.807 m/s2 that's faster then light?

Acceleration has little to do with speed. In a classical sense you could say that light is not accelerating at all but has the highest speed possible for example. I could be accelerating as crazy but having zero speed in this instant. Don't confuse both ideas.
The escape velocity in the surface of the event horizon is indeed the speed of light but this idea has nothing to do with the acceleration at that point (in fact talking about acceleration here can be very very tricky; maybe Watsisname comes with a usefull answer in that sense).
With all of this I want to say that the question:
the photo guy wrote:
Source of the post 9.807 m/s2 that's faster then light?

Has no meaning. Is like saying does the number of potatoes in this bag is hotter than a sunny day? those are different quantities, and because of that you can't compare them.
 
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Watsisname
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15 Aug 2017 03:40

the photo guy, the better choice is escape velocity.  For any black hole, the escape velocity at the event horizon is equal to the speed of light.  In fact, if you set the escape velocity of an object to be equal to the speed of light, you'll end up with the formula for the size of a black hole:

Escape velocity is [math]

Set that equal to c, then solve for r, and we get [math]

which is exactly the formula for the size of the event horizon, or "Schwarzschild Radius", of a black hole.  Actually, this is a coincidence.  It used Newtonian gravity, where we really should be using general relativity, but it happens to give the right answer in this case.


The problem with using acceleration for this is that acceleration and velocity are incompatible.  They have different units.  So there is no sense in which an acceleration is faster or slower than light (or any other velocity that you choose).  You just can't compare them.

For a black hole, you could with Newton's Laws calculate the acceleration due to gravity at its event horizon, and the answer will vary depending on the mass of the hole.  For a larger black hole, you'll get a smaller acceleration due to gravity at its horizon.  But, this is misleading.  It implies you could hover at the horizon of a sufficiently massive black hole -- or even hover at any distance inside of the black hole if your thrusters are strong enough.  That implication is wrong.  It actually takes infinite acceleration to hover at the horizon, and no matter how large your acceleration is, you cannot hover inside the horizon.  You get swept into the singularity. 

Similarly, we should be careful about interpreting "escape velocity" at the event horizon.  It might imply that a light ray emitted from there would just barely escape to infinity, like a rocket launched at the escape velocity from a planet.  Or it may imply a light ray emitted from just inside the horizon would escape a little bit outside of it, and then fall back in.  These implications are wrong.  A ray of light emitted outward exactly at the horizon will simply be stuck there, as if it is climbing up a downward-moving escalator at the exact same speed that the escalator is moving.  And a light ray emitted outward from just inside will be immediately pulled further inward, just like a fish trying (and failing) to swim up a waterfall. 

Some might also think that you would be suddenly able to see the singularity (or whatever it is) deep inside the black hole, after you've fallen through the event horizon.  But you'll never see it.  Even when the singularity is mere inches in front of you (and assuming you haven't been torn apart by tidal forces yet), it is still invisible.  That's because all light is being pulled into the singularity -- none is leaving from it.

The reason these intuitions fail are because there are based on Newtonian experience, whereas what's really happening involves a general relativistic treatment of the space-time.  In a very real sense, a black hole in general relativity is a stronger attractor than anything in Newtonian gravity.  The behavior of things near or inside black holes is less like the orbits of satellites around planets, and more like fish caught in a current about to go over a waterfall.  Space itself is dragging things in, and inside the horizon it is completely overwhelming, while far away it is quite escapable.


Hopefully this "escalator" or "waterfall of space" analogy helps remove some of the mystery of black holes for you and answers your last questions. But if it is still confusing, feel free to ask more!  

And the last thing I want to say is that the speed of light does not get violated in a black hole.  Everything that falls in is moving at or less than the speed of light through the space.  But the space itself is flowing into the singularity faster than light, like the river going over a waterfall.  This might seem unsettling (and in fact this is a little bit of a simplification for how it really works -- the more correct explanation again requires general relativity and a description of space-time curvature), but the helpful answer is that special relativity places a limit on how fast things can move through space (speed of light), while general relativity allows the space itself to move, dragging things along with it, and the motion of space isn't limited by the speed of light.
 
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the photo guy
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15 Aug 2017 09:55

Watsisname wrote:
Source of the post Similarly, we should be careful about interpreting "escape velocity" at the event horizon.  It might imply that a light ray emitted from there would just barely escape to infinity, like a rocket launched at the escape velocity from a planet.  Or it may imply a light ray emitted from just inside the horizon would escape a little bit outside of it, and then fall back in.  These implications are wrong.  A ray of light emitted outward exactly at the horizon will simply be stuck there, as if it is climbing up a downward-moving escalator at the exact same speed that the escalator is moving.  And a light ray emitted outward from just inside will be immediately pulled further inward, just like a fish trying (and failing) to swim up a waterfall.

This part helps a lot! thank you! :) I was implying just like what you said: "the escape velocity causes the light ray to just barley escape to infinity" this raised my question for smaller back holes and I starting wondering if acceleration was in any way involved, but it turns out this implication are wrong!  :lol: your escalator and waterfall thing makes perfect sense to me!
like always, thank you wats! :D
Last edited by the photo guy on 15 Aug 2017 17:44, edited 1 time in total.
HI! I would like to say that I'm just a friendly teen who loves talking and helping people out :) Space engine is truly a wonderful program and the website is also very cool! Many great people here. I hope to make some good friends here.
 
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Watsisname
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15 Aug 2017 16:41

FastFourierTransform wrote:
Source of the post does the number of potatoes in this bag is hotter than a sunny day?

Scientists just don't know!
 
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spaceguy
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16 Aug 2017 13:31

Why doesn't Io have mountainous volcanoes?
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