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Nasa’s Roman Mission Will Hunt for Primordial Black Holes

nasa.gov

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Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#41

Post by dotancohen »

ibeforee wrote: post #39
exe34 wrote: post #32

Earlier quoted context omitted.

If you sit on a merry-go-round, and spin it very fast, you feel the "centrifugal force" trying to keep you in an inertial frame. That's because you're having to hold on to the ride. If you're in a spacecraft in orbit around Earth, you don't feel the force keeping you in a circular motion, because both you and the craft are experiencing the same force. The worse thing about going past a blackhole would be tidal forces…

I was wondering that actually if the hole goes through your leg does it leave a hole or does your whole body get sucked in. I imagine it is the whole body unless you are travelling really fast at the time. Like near speed of light. Because the gravity outside the event horizon will still be crazy strong going out for several km (earth is a good comparison in the gravity is still fairly strong about 6000km from the ce…

Black holes warp the spacetime around them, so your idea of distance is isn't really valid. Also the idea of time is warped as well, so the black hole doesn't quite just pass through your leg as you expect.

Nor does the black hole "suck" anything in.

What would happen from your perspective if such a black hole were to pass you at high velocity would be the same as if you were to pass the black hole at high velocity. From your perspective, you would begin orbiting that object, carried along with it. So would all the matter near you. But it would be matter, not objects, as the tidal forces would spaghettify all objects very quickly.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#42

Post by sandworm101 »

tsimionescu wrote: post #29
sandworm101 wrote: post #14

Earlier quoted context omitted.

The tidal forces would be no different than using jupiter for an assist. You would still have the probe pass by at several thousand, several tens of thousands of kilometers. A close-in gravity assist may look better on paper, but the practicalities and speeds of such a thing are risky. One small error and the mission would be over quick, launched out at a radically incorrect trajectory.

Isn't the whole advantage that you could get much closer to the center of mass of the black hole compared to a planet, thus gaining much more kinetic energy per unit thrust, but also risking higher tidal forces?

On paper yes, but doing so also reduces the time for the burn. Probes have very low-thrust engines. Even during a jupiter slingshot they barely have time to accellerate much on thier own. Often they do not bother, relying totally upon the grav assist to accellerate. The danger too of a closer approach is that something gets miscalculated. Get too close and an inevitable tiny misalignment will throw you onto a wild unwanted trajectory. There is no gps out there. Knowing exactly where and how a probe is moving isnt easy.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#43

Post by perihelions »

api wrote: post #2

Primordial black holes are a fascinating topic. One of my favorite hypotheses is that "planet nine," a large possibly 1-5 Earth mass planet suggested by some orbital models to exist beyond Neptune and Pluto in the far outer solar system, may be a primordial black hole. If such a thing existed it'd be a black hole about the size of a billiard ball and would be extremely hard to detect. It would not emit Hawking radiat…

- "In that case you'd see X-rays, gamma rays, etc., but maybe only briefly."

One speculative possibility is that we could find a continuous gamma annihilation signal from its dark matter halo,

https://arxiv.org/abs/1909.11090

https://hn.algolia.com/?query=What%20If%20Planet%209%20Is%20...

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#44

Post by perihelions »

HeatrayEnjoyer wrote: post #35
messe wrote: post #26

Earlier quoted context omitted.

It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to r…

But you are still burning the same amount of chemical energy...

It's super un-intuitive, but you're indirectly harnessing the gravitational potential energy of lowering your propellant into a gravity well and leaving it there. The overall orbital energy gain of the spacecraft can exceed the chemical energy of the fuel.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#45

Post by perihelions »

dotancohen wrote: post #40
messe wrote: post #26

Earlier quoted context omitted.

It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to r…

The phenomenon described is not due to the Oberth effect. What would be happening would be that the human in the spacecraft, and the spacecraft, are accelerating at exactly the same rate because they are accelerating due to gravity. Thus, the human feels no pressure accelerating him (no outside force acting upon him) from e.g. his seat. And his internal organs feel no pressure accelerating them from each other. They…

Your parent comment is the textbook definition of the Oberth Effect phenomenon. I think you've misread something.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#46

Post by dotancohen »

perihelions wrote: post #45
dotancohen wrote: post #40

Earlier quoted context omitted.

The phenomenon described is not due to the Oberth effect. What would be happening would be that the human in the spacecraft, and the spacecraft, are accelerating at exactly the same rate because they are accelerating due to gravity. Thus, the human feels no pressure accelerating him (no outside force acting upon him) from e.g. his seat. And his internal organs feel no pressure accelerating them from each other. They…

Your parent comment is the textbook definition of the Oberth Effect phenomenon. I think you've misread something.

I mean to say that the Oberth effect is one phenomenon, and that the lack of a feeling of acceleration is another effect.

The Oberth effect itself is not responsible for the lack of feeling of acceleration during the assist.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#47

Post by pavel_lishin »

ibeforee wrote: post #39
exe34 wrote: post #32

Earlier quoted context omitted.

If you sit on a merry-go-round, and spin it very fast, you feel the "centrifugal force" trying to keep you in an inertial frame. That's because you're having to hold on to the ride. If you're in a spacecraft in orbit around Earth, you don't feel the force keeping you in a circular motion, because both you and the craft are experiencing the same force. The worse thing about going past a blackhole would be tidal forces…

I was wondering that actually if the hole goes through your leg does it leave a hole or does your whole body get sucked in. I imagine it is the whole body unless you are travelling really fast at the time. Like near speed of light. Because the gravity outside the event horizon will still be crazy strong going out for several km (earth is a good comparison in the gravity is still fairly strong about 6000km from the ce…

Depends on the size of the hole. If it's small enough, all you might get is one long bruise due to tidal/gravity effects, without losing a single atom of your body

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#48

Post by api »

woopsn wrote: post #15

Fascinating mission. I don't understand how microlensing events resulting from primordial black holes can be differentiated even statistically, given that they "can't be formed by any known physical process", but in any case it's exciting to discover there's much more of something up there than we knew. Even if they are "just" rogue planets.

We have a variety of ways to make insanely precise measurements of light to the point of counting photons. I’m guessing that factors into being able to detect microlensing.

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#49

Post by jebarker »

Tostino wrote: post #12
Loughla wrote: post #11

Earlier quoted context omitted.

I feel like if you were close enough for a gravity assist, wouldn't the tidal forces just tear the probe apart?

Not necessarily. If you replaced the sun with an equivalent mass black hole, none of the orbits of the planets would change.

I'd like to see a simulation of what that would look like. I mean, if by magic the sun was replaced with an equivalent mass black hole in an instant would anything be visible from earth before the inevitable freeze?

Re: Nasa’s Roman Mission Will Hunt for Primordial Black Holes

#50

Post by lxgr »

perihelions wrote: post #44
HeatrayEnjoyer wrote: post #35

Earlier quoted context omitted.

But you are still burning the same amount of chemical energy...

It's super un-intuitive, but you're indirectly harnessing the gravitational potential energy of lowering your propellant into a gravity well and leaving it there. The overall orbital energy gain of the spacecraft can exceed the chemical energy of the fuel.

Wow, that's an amazing intuitive explanation of "where the extra energy comes from" in an Oberth effect burn that I haven't heard before. Thank you!
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