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rustyhancock 2 hours ago [-]
One of the most remarkable aspects of physics is that Newtonian gravity calculations are quite spot on for all the parts of black holes layfolk consider important.
1. Both predict the same critical radius, now called the Schwarzschild radius.
2. Both predict a central singularity with formally infinite density in idealised collapse.
3. Both predict exactly the same finite time to fall into the singularity under equivalent idealised conditions.
4. Both give the same equation for radial free fall, using different definitions of time.
What GR added was meaning to these things (and of course precise calculations outside of very idealised scenarios and symmetric conditions).
ant6n 2 hours ago [-]
Newtonian gravity gives infinite time to fall into black hole?
I thought you need at least special relativity for time dilution.
rustyhancock 1 hours ago [-]
Both have finite time falling in to the singularity. If you use a point test particle Newton and GR agree on that finite time.
That's proper time I.e. the infalling particles wristwatch time.
In GR, external observers would never see the infalling particle cross the event horizon but that's a coordinate artifact. If using our wrist watch time for that particle.
gradus_ad 30 minutes ago [-]
But observers can see a black hole grow over time, so does that mean they observe growth without consumption?
rustyhancock 9 minutes ago [-]
Were past what I could confidently answer (if I'm wrong hopefully someone corrects me) but I'm not sure I follow, if the BH grows it would encompass what we see as nearly smeared over it's horizon.
There's also surprising results around what the external gravitation field appears to be for an object.
I.e. no matter how mass is distributed behind the event horizon externally it looks like the spherically symmetric Schwartzchild metric.
That has implications for what will emit gravitational waves.
Eithway, smeared over the horizon but never crossed and crossed the horizon externally look the same from a gravity perspective.
ziiinq 2 hours ago [-]
[dead]
myrmidon 2 hours ago [-]
What I find really interesting is that black holes don't really "require" infinite density; the mass density inside the event horizon never exceeds neutron star levels, and becomes in fact lower the heavier the black hole gets (=> event horizon enclosed volume grows faster than mass).
Borg3 1 hours ago [-]
For large blackholes of course. And this actually makes other interesting property. Our universe is in fact giant blackhole with our own event horizon outside it. This also support cyclic big bang theory.
Once entire universe will collapse to singularity, the smallest possible particles shatters, leaving pure energy. Pure energy doesnt have "static" mass so no gravity => kaboom, we have new iteration. Theory of multiverse (multiple black holes aka universes) is out of scope here because we will never be able to escape ours.
adrianN 33 minutes ago [-]
It is my understanding that energy produces a gravitational field regardless of the form it takes.
nkrisc 20 minutes ago [-]
What is “pure” energy?
fuzzfactor 22 minutes ago [-]
Nothing beats a close-up.
spottedmarley 2 hours ago [-]
First you'll need a camera that can detect mathematical equations.
JohnKemeny 2 hours ago [-]
Is it just me, or could they have added a, say, picture of a black hole?
cogogo 51 minutes ago [-]
I thought I was the only one. But there is a really good video linked at the bottom of the page that has the two images and a summary of the work to build them.
I also loved the analogies of reading the date of a coin in LA from NYC or seeing a tangerine on the moon. But they kept talking about how surprisingly small black holes are. I wanted that described in relative terms. Maybe I just did not read carefully enough but are they the size of a pinhead? The solar system? I know I can look this up.
bcraven 2 hours ago [-]
It's the one in your head that you picture, as not many have been taken:
The EHT is a remarkable achievement in datagathering and collaboration. But I always sort of wonder, a bit of doubt if you will. seeing as the process(simplified nearly to the point of absurdity) is that of building a image generator based off what we think a black hole looks like then feeding it very vague spectral data and getting it to generate an image. isn't that a huge confirmation bias. That is. of course the black hole looks like simulated ones, that is how the generator was programmed to make them.
GreenLightGo 1 hours ago [-]
Why does it look like it's moving to me?
lifestyleguru 2 hours ago [-]
I remember times when the best available photo of Pluto looked like this.
2 hours ago [-]
ButlerianJihad 2 hours ago [-]
This is a transcript of a 60-minute podcast. Would you like that TIFF in Zmodem format? Or they could read it out in base64?
monster_truck 2 hours ago [-]
I'd use a camera, personally
jareklupinski 2 hours ago [-]
make sure you set the exposure to a negative value
1. Both predict the same critical radius, now called the Schwarzschild radius.
2. Both predict a central singularity with formally infinite density in idealised collapse.
3. Both predict exactly the same finite time to fall into the singularity under equivalent idealised conditions.
4. Both give the same equation for radial free fall, using different definitions of time.
What GR added was meaning to these things (and of course precise calculations outside of very idealised scenarios and symmetric conditions).
I thought you need at least special relativity for time dilution.
That's proper time I.e. the infalling particles wristwatch time.
In GR, external observers would never see the infalling particle cross the event horizon but that's a coordinate artifact. If using our wrist watch time for that particle.
There's also surprising results around what the external gravitation field appears to be for an object.
I.e. no matter how mass is distributed behind the event horizon externally it looks like the spherically symmetric Schwartzchild metric.
That has implications for what will emit gravitational waves.
Eithway, smeared over the horizon but never crossed and crossed the horizon externally look the same from a gravity perspective.
I also loved the analogies of reading the date of a coin in LA from NYC or seeing a tangerine on the moon. But they kept talking about how surprisingly small black holes are. I wanted that described in relative terms. Maybe I just did not read carefully enough but are they the size of a pinhead? The solar system? I know I can look this up.
https://www.si.edu/newsdesk/releases/astronomers-reveal-firs...