Timeline for Why can't light escape from a black hole?
Current License: CC BY-SA 3.0
9 events
| when toggle format | what | by | license | comment | |
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| Jan 17, 2019 at 14:39 | comment | added | brentonstrine | @RobJeffries it's not an 'argument' just a helpful way of thinking about it for someone who is learning about black holes. I'm trying to think of another way to explain this which is both simple for a beginner to understand and doesn't have misleading implications, but I can't think of one--if you can I'm sure many people would benefit from a new answer to this question. | |
| Jan 16, 2019 at 18:15 | comment | added | ProfRob | The argument in terms of escape velocity is deeply incorrect, since it seems to allow for escape from a black hole if a force is continuously applied - which isn't in fact possible. | |
| Nov 1, 2013 at 14:28 | comment | added | jmarina | @brentonstrine the free velocity you mention you get from a gravity assist: www2.jpl.nasa.gov/basics/grav/primer.php the orbital velocity of the Earth is about 30km/sec, it varies a bit down to 29kps because the orbit is not an exact circle, we are actually closer to the sun in northern winter and farther by about a million km in summer. nssdc.gsfc.nasa.gov/planetary/factsheet | |
| Oct 31, 2013 at 8:11 | comment | added | brentonstrine | Ah, I see. The escape velocity is factoring in the "free" velocity you get from gravitational pull if going back down (but angled enough to miss the planet). Is that right? | |
| Oct 31, 2013 at 5:37 | comment | added | Jason Goemaat | @jmarina is kinda right but the explanation is more interesting. Escape velocity actually decreases with distance from the body you are trying to escape from. For instance at 9,000 km up the escape velocity is about 7.1 km/s. The reason is that if you are going that speed aimed to just miss the Earth then you would pick up extra speed from falling towards it. And while the escape velocity from the sun at the sun's surface is 617.5 km/s, at Earth's orbit it is only 42.1 km/s. | |
| Oct 30, 2013 at 16:29 | comment | added | brentonstrine | jmarina, that's interesting, I haven't heard that. Would you mind providing a link with more info or the name of the effect you're describing? | |
| Oct 30, 2013 at 9:20 | comment | added | jmarina | escape velocity for Earth is 11.2kps; however, this applies to thrown objects; you have to throw a rock at 11.2kps (ignoring atmospheric drag) in order for it to leave the Earth and not fall back; however, if your rock has an engine that can apply thrust, it can leave the Earth at a much lesser speed. The longer it is able to apply thrust, the slower it can go when leaving. | |
| Oct 29, 2013 at 18:10 | history | edited | brentonstrine | CC BY-SA 3.0 |
Adding images and links, making "kips" explanation clearer.
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| Oct 29, 2013 at 17:29 | history | answered | brentonstrine | CC BY-SA 3.0 |