3 ms·

Once you know speed of light is the maximum possible speed, you can try to guess what happens when you have enough mass to make escape velocity higher than the

Once you know speed of light is the maximum possible speed, you can try to guess what happens when you have enough mass to make escape velocity higher than the speed of light (it is not possible, therefore you can't escape).

jfengel 3y ago
This doesn't assume that light is the maximum possible speed. But it does show that you couldn't use light to see such an object. Relativity makes black holes much more "final" than this original concept, but he was definitely thinking in the right direction.
tzs 3y ago
I've never liked the "escape velocity >= c" explanation of why you can't get out a black hole. The escape velocity for a body at a given point is the velocity needed to escape from that point ballistically. If you don't limit yourself to entirely ballistic trajectories you can escape without ever reaching that point's escape velocity. To explain why nothing, not even things on non-ballistic trajectories, cannot get out of black holes you've got to turn to weird headache inducing general relativity stuff.
- at_a_remove 3y ago
  You are correct. It's a fortunate coincidence and nothing more. It's much easier to think of the gradual warping of spacetime around a black hole so that more and more directions are down. East? No, that's really east and down. Left? Nope, you thought you were going left but it is left and down. Less and less "up" is available to you until you are on a knife edge where you can make an orbit (assuming a non-rotating black hole, no angular momentum) around the event horizon with a photon. Beyond that, everything is some flavor of down.
- 29athrowaway 3y ago
  In 1700s?
  - tzs 3y ago
    Yup. Those in the 1700s who wrote about stars with so much mass that light cannot escape would have known that their calculations only applied to ballistic trajectories. They would have believed that non-ballistic objects such as rockets would be able to escape because all they had was Newtonian physics. That when mass becomes high enough that light can't escape nothing else can escape either doesn't pop out of the math until you have general relativity.
- martyvis 3y ago
  That's right. The Earth's escape velocity is just over 11km/s or 4000km/h. If you have a rocket that only has enough force to propel you at say 100km/h away from the earth you can go anywhere you want in the universe - provided you keep applying that force through thrust. If you stop the rocket you will be subject to gravitational force to bring you back (or until the gravitational pull from other bodies of becomes dominant)
  - thfuran 3y ago
    >If you have a rocket that only has enough force to propel you at say 100km/h away from the earth you can go anywhere you want in the universe Unless the rocket collapses into a black hole under the sheer mass of all that fuel.
  - albumen 3y ago
    Nitpick: 11km/s escape velocity for earth is ~40,000km/h (not 4,000)
- marcosdumay 3y ago
  Well, if you go with Newton's explanation for light, it's quite obvious that it can only follow ballistic trajectories. One would certainly not expect elementary particles to function like rockets. But yeah, nobody could come with a description of a black hole anything like the modern one before 1900. Anything older is certain to be very different.
  - jahnu 3y ago
    We don’t need to assume a small rocket strapped to a particle ;) Imagine accelerating an electron in some sort of magnetic field, in series. In a Newtonian system any speed is achievable. GR forbids this though.
    - naasking 3y ago
      > GR forbids this though. Does it though? https://youtu.be/9-jIplX6Wjw?si=xbNYKT9YN13VyJrV https://youtu.be/9-jIplX6Wjw?si=xbNYKT9YN13VyJrV
pdonis 3y ago
> enough mass It's not a question of enough mass. It's a question of being compact enough. There is no minimum mass for a black hole, but the smaller the mass, the smaller the surface area of the horizon (the 2-sphere that the mass has to collapse inside to form the hole).
wyldfire 3y ago
But wouldn't you have to conclude / assume that light is affected by gravity? Was that well-known at that time?
- deleted 3y ago
  [deleted]
- tsimionescu 3y ago
  Newton introduced the corpuscular theory of light, in modern terms saying that light is made of tiny particles. Once you have that, it becomes natural to think that light is probably attracted by gravity just like everything else is. The wave theory of light came quite a bit later, in Maxwell's time.
archgoon 3y ago
> Once you know speed of light is the maximum possible speed But he didn't postulate this at all. He simply postulated (as per Newton) that light is made of particles with mass, which could thus be affected by gravity. Since they are moving at a finite speed, the light particles would be slowed down by gravity, and eventually completely stopped. He further proposed, based on this model, that the mass of an object could potentially be determined by determining how much slower the particles were moving. This doesn't work. Light doesn't really get slowed down by gravity, and you definitely don't have any slow light that was created from stars trillions of miles away. It's a cool idea, but doesn't work with how light actually propagates.

jfengel 3y ago

This doesn't assume that light is the maximum possible speed. But it does show that you couldn't use light to see such an object. Relativity makes black holes much more "final" than this original concept, but he was definitely thinking in the right direction.

tzs 3y ago

I've never liked the "escape velocity >= c" explanation of why you can't get out a black hole. The escape velocity for a body at a given point is the velocity needed to escape from that point ballistically. If you don't limit yourself to entirely ballistic trajectories you can escape without ever reaching that point's escape velocity. To explain why nothing, not even things on non-ballistic trajectories, cannot get out of black holes you've got to turn to weird headache inducing general relativity stuff.

at_a_remove 3y ago
You are correct. It's a fortunate coincidence and nothing more. It's much easier to think of the gradual warping of spacetime around a black hole so that more and more directions are down. East? No, that's really east and down. Left? Nope, you thought you were going left but it is left and down. Less and less "up" is available to you until you are on a knife edge where you can make an orbit (assuming a non-rotating black hole, no angular momentum) around the event horizon with a photon. Beyond that, everything is some flavor of down.
29athrowaway 3y ago
In 1700s?
- tzs 3y ago
  Yup. Those in the 1700s who wrote about stars with so much mass that light cannot escape would have known that their calculations only applied to ballistic trajectories. They would have believed that non-ballistic objects such as rockets would be able to escape because all they had was Newtonian physics. That when mass becomes high enough that light can't escape nothing else can escape either doesn't pop out of the math until you have general relativity.
martyvis 3y ago
That's right. The Earth's escape velocity is just over 11km/s or 4000km/h. If you have a rocket that only has enough force to propel you at say 100km/h away from the earth you can go anywhere you want in the universe - provided you keep applying that force through thrust. If you stop the rocket you will be subject to gravitational force to bring you back (or until the gravitational pull from other bodies of becomes dominant)
- thfuran 3y ago
  >If you have a rocket that only has enough force to propel you at say 100km/h away from the earth you can go anywhere you want in the universe Unless the rocket collapses into a black hole under the sheer mass of all that fuel.
- albumen 3y ago
  Nitpick: 11km/s escape velocity for earth is ~40,000km/h (not 4,000)
marcosdumay 3y ago
Well, if you go with Newton's explanation for light, it's quite obvious that it can only follow ballistic trajectories. One would certainly not expect elementary particles to function like rockets. But yeah, nobody could come with a description of a black hole anything like the modern one before 1900. Anything older is certain to be very different.
- jahnu 3y ago
  We don’t need to assume a small rocket strapped to a particle ;) Imagine accelerating an electron in some sort of magnetic field, in series. In a Newtonian system any speed is achievable. GR forbids this though.
  - naasking 3y ago
    > GR forbids this though. Does it though? https://youtu.be/9-jIplX6Wjw?si=xbNYKT9YN13VyJrV https://youtu.be/9-jIplX6Wjw?si=xbNYKT9YN13VyJrV

pdonis 3y ago

> enough mass It's not a question of enough mass. It's a question of being compact enough. There is no minimum mass for a black hole, but the smaller the mass, the smaller the surface area of the horizon (the 2-sphere that the mass has to collapse inside to form the hole).

wyldfire 3y ago

But wouldn't you have to conclude / assume that light is affected by gravity? Was that well-known at that time?

deleted 3y ago
[deleted]
tsimionescu 3y ago
Newton introduced the corpuscular theory of light, in modern terms saying that light is made of tiny particles. Once you have that, it becomes natural to think that light is probably attracted by gravity just like everything else is. The wave theory of light came quite a bit later, in Maxwell's time.

archgoon 3y ago

> Once you know speed of light is the maximum possible speed But he didn't postulate this at all. He simply postulated (as per Newton) that light is made of particles with mass, which could thus be affected by gravity. Since they are moving at a finite speed, the light particles would be slowed down by gravity, and eventually completely stopped. He further proposed, based on this model, that the mass of an object could potentially be determined by determining how much slower the particles were moving. This doesn't work. Light doesn't really get slowed down by gravity, and you definitely don't have any slow light that was created from stars trillions of miles away. It's a cool idea, but doesn't work with how light actually propagates.