alt Hacker NewsOne 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).
Replies
in what sense. the black hole is a result of the interaction of light with the curavture of space time. this is not modelled by newtonian gravity (or physics for that matter) at all. light is assumed to not interact with gravity in that framework in any way.
>> Newtonian gravity calculations are quite spot on
Which is why the concept existed long long before any talk of relativity or time dilation, back before anyone knew the name Einstein. (He was like 4yo when this came out)
From American Physical Society: "the true “father” of the black hole concept was a humble 18th century English rector named John Michell–a man so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later."
https://www.aps.org/apsnews/2009/11/1783-john-michell-black-...
Could you provide a bit more detail? I don't quite see how one would define the event horizon for instance.
Newtonian gravity gives infinite time to fall into black hole?
I thought you need at least special relativity for time dilution.
[dead]
One significant difference in interpretation that layfolks do consider important is that it's possible to exit from within the event horizon of a black hole in Newtonian gravity (without exceeding the speed of light) whereas in general relativity that is not possible.
In relativity the event horizon really is a point of no return, whereas in Newtonian physics it's the point at which escape velocity is equal to the speed of light. The escape velocity on the surface of the Earth is 11 km/s, but that doesn't mean that I as a human can't escape the surface of the Earth momentarily just by jumping and I certainly don't jump 11 km/s or even 1 km/s for that matter.
Similarly in Newtonian gravity it would be possible for an object to enter the event horizon of a black hole, and then "jump" back out at a much lower speed than the speed of light and then get sucked back beyond the event horizon, and you can in principle do this over and over again so long as you had some sufficiently powerful source of thrust.
In general relativity this is in principle not possible.