Naked Science Forum
General Science => Question of the Week => Topic started by: jamest on 01/12/2023 15:48:20
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Listener David wants to know:
"What are black holes made of?"
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Mass and/or energy (any kind): but concentrated into such a small radius that the escape velocity exceeds the speed of light.
The most common way that black holes form today is when a massive star (>10 times the mass of the Sun) explodes as a supernova. The remnant left behind will be a black hole.
Less massive stars can form a dense neutron star; if two neutron stars later collide, they could also turn into a black hole.
The James Webb Space Telescope has revealed large active galaxies, soon after the Big Bang. These are believed to have supermassive black holes at the centre. Theoreticians are inventing new mechanisms that would allow these supermassive black holes to form in the hot, dense early universe, without first forming stars.
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Good Work @ Evan.
@ James...i know You are Not allowed to respond.
I Understand how things work in here.
& I'm Glad QOTW is back on track.
Good Job!
ps - Once David's question has been answered, I'd like to know a lil bit more about Light/Escape Velocity/Gravity n so on.
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Hi Zero. Throw a ball up in the air and what happens? it goes so high and then falls down again. Next fire a high powered rifle vertically and the bullet will go much higher but fall down again. When one reaches sufficient speed a projectile will not fall down and it will escape the earth's gravitational attraction, this is called the escape velocity and from memory(dodgy!) it is about 23000miles per hour for our planet. As a mass, planet or otherwise, becomes greater so also does the escape velocity increase and in the extreme case of a black hole the escape velocity exceeds 186000miles per second, ie the speed of light, and no light can escape.
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Thanks @Paul!
Short, Simple & Sweet.
: )
But what I'm Curious about is...
If Escape Velocity is above 186,000 miles/sec, does a Single Photon start to get Stretched?
ps - Vesc = seems Variable, depending upon your launch site location, isn't it?
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Zer0, the escape velocity is determined by the gravity experienced at the launch site, and most likely by the topology of the mass responsible for the gravitation. As regards what happens to photons that cannot escape the event horizon of a black hole I can't tell you as I don't know. A member such as Halc or Origin should be able to help.
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What are black holes made of?
Black holes are essentially places, not objects. The interior of a black hole is just more spacetime, just like outside, except more curved. If you fell into a large black hole, you'd notice no particular change as you crossed the event horizon. You could go on with your day as usual. All events inside a black hole are causally in the future of events outside, so there would be no way to affect anything outside.
The spacetime curvature would result in a tidal effect which gets stronger the closer to the singularity you get. For small black holes and other dense objects (such as a neutron star), the tidal effects would be fatal as you get anywhere close to it, let alone cross some event horizon. Hence my reference to a large black hole above where the tidal effects are not so noticeable even after crossing the event horizon.
If Escape Velocity is above 186,000 miles/sec, does a Single Photon start to get Stretched?
It really makes no sense to talk about any velocity over c. There is no escape velocity from a black hole, not even greater than c.There is no meaning to a photon getting stretched. Sure, it has a wavelength, but that is frame dependent, and photons and everything else would behave pretty normal at (and beyond) the event horizon, per the first postulate of special relativity which says that the laws of physics are the same in any inertial frame, and spacetime at an event horizon is locally Minkowskian (flat), so the physics of inertial frames very much does apply there.
Zer0, the escape velocity is determined by the gravity experienced at the launch site,
Escape velocity is a function not of the gravity experienced (how much you weigh there), but rather the relative gravitational potential. So for instance, escape velocity of Earth is about 11.2 km/sec, but from the surface of Uranus you might weigh about 10% less but the escape velocity there is 21.4 km/sec, nearly twice that of Earth. The difference is that the gravitational potential on Uranus is lower than it is on Earth, assuming each is the sole object being escaped respectively.
Escape velocity has little to do with black holes since the concept is meaningless for one. There is no escape, at any speed and any acceleration. It would literally be trying to travel to the past. You just can't go that way.
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Thanks Y'all!
Glad i asked, orelse i would have Never known.
: )
Perhaps David got Alot more than they asked for, so be it.
(lol)
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does a Single Photon start to get Stretched?
If you have a dense object like a neutron star or a white dwarf star, and fire a photon (or a laser beam) with a known wavelength outwards, someone at a great distance would measure it with a longer wavelength than when it was emitted.
- You could say "The photon wavelength was stretched"
- This is sometimes called "Einstein shift", since it was predicted by Einstein
- This was observed in 1954 with a white dwarf star
If you have a less dense object like the Earth, this gravitational redshift is much less
- However, it was measured in the 1959 Pound?Rebka experiment (https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment), using gamma rays
For black holes, photons released within the event horizon will never be seen outside it
- However, if you emitted a laser beam of known wavelength at (say) 10x the radius of the event horizon, a distant observer would see it significantly red-shifted
- However, if you emitted the laser beam at (say) 5x the radius of the event horizon, a distant observer would see it even more red-shifted
- So you could say "as you approach the event horizon of a black hole, a Single Photon (or laser beam) starts to get Stretched"...
See: https://en.wikipedia.org/wiki/Gravitational_redshift#Astronomical_observations
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And thank you, Halc, for correcting my error concerning gravitational potential. All my life I have dealt with volts and amps and gravitation really never crossed my path so I never analysed it. What you say makes perfect sense: if we were dealing with electrostatic attraction/repulsion it is the potential that would be of interest.
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& Thanks Evan for Answering it with Clarity.
All responses Appreciated!