# What cosmological event could snuff out the sun without destroying Earth?

1. Sep 25, 2012

### CCWilson

No. The premise of the story is that the Earth is slung out of solar orbit and loses its energy source, and with around five years lead time, a number of huge underground cities are built using mostly geothermal energy to generate electricity and keep our species going.

2. Sep 25, 2012

### Staff: Mentor

Tidal effects scale with M/r^3, orbital influence scales with M/r^2 - if we fix the orbital influence (enough to kick earth out of the solar system), a bigger mass reduces tidal effects. We know that our sun influences the path of earth in a significant way (we orbit it), so a black hole with 3 solar masses in a distance of about 1 AU (and a velocity of the order of 30km/s) could kick us out, while its tidal influence is smaller than the moon's. Completely negligible compared to the influence on the orbit.

Last edited: Sep 25, 2012
3. Sep 25, 2012

### CCWilson

mfb, what about the effects of the change in direction on the Earth, both on an observer sitting in a lawn chair and on the tides? In other words, if you're jogging along holding a pail of water and a cat carrier and then make a sharp right turn, the water sloshes up on the outside of the pail, and the cat is thrown toward the side of the carrier to the outside of your turn. I also wonder whether there would be geological effects that would predispose to earthquakes. Obviously the magnitude of those effects would depend on whether the Earth was whipsawed around the black hole or simply pulled into a slightly different path.

4. Sep 26, 2012

### Staff: Mentor

Those are all tidal effects. If the moon does not tilt your cat, a black hole in 1 AU distance would not do that either. Similar with earthquakes.

If the black hole comes closer, it can have significant tidal effects, but they are not required to remove earth from its orbit.

5. Sep 26, 2012

### CCWilson

Beg to differ. Tidal effects occur because of the differential gravitational forces of the moon on the oceans on the near side (greater) vs the oceans on the far side (lesser). That's why the moon has more tidal effect than the sun - because the diameter of the Earth - 13,000 km - is a significant percentage of the distance from moon to Earth (400,000 km) and a very small percentage of the distance from Sun to Earth (150,000,000 km) - so the difference in gravitational attraction from near to far side is greater for the moon, even though the Sun is much more massive than the moon.

But we're not talking here about a difference in gravitational attraction between two sides of the Earth. Our planet is being slung out of its orbit not by tidal effects but by the gravitational attraction of the black hole to the Earth overall. I agree that the tidal effects wouldn't necessarily be large, but the centrifugal/centripetal force from the change in direction would. I suspect that earthquake generating effects would be small, but the water in our oceans would, I think, be sloshed around, resulting in high tides, and I'll bet a human would feel those carnival-ride effects.

6. Sep 26, 2012

### Staff: Mentor

You cannot feel an homogeneous gravitational field - one of the fundamental rules in General Relativity. You can feel the different gravitational attraction at different points, and those are tidal effects. Without those tidal effects, earth could accelerate with whatever value in any coordinates - it does not matter, as all objects get the same acceleration.

7. Sep 26, 2012

### CCWilson

I may be coming around to your opinion on this - but it's a brain twister. Thinking about the Earth in relation to the solar system, if the Earth got whipped around slingshot fashion, you'd think an observer on Earth would feel the change in direction. But I shouldn't think of the Earth in relation to the solar system, but more in relation to spacetime. The black hole would massively deform spacetime, and the Earth and everything on it would simply be along for the ride, floating along where the gravitational forces tell it to go. The only gravitational effects we would feel would be those of the Earth, as usual.

I'm still not sure of this, however. Bears further contemplation, especially since the gravitational fields are changing.

8. Sep 27, 2012

### onomatomanic

The key concept here is "being in free-fall". You do not feel any gravitational forces with respect to which you are simply falling. That includes the Sun and the Moon, and, for someone in orbit, the Earth as well. Analogously, it includes the Black Hole. The only reason we do feel the gravitational force of Earth is that, not being in orbit, we aren't able to fall - because the pesky surface gets in the way, sooner or later (hopefully sooner, for practical reasons).

9. Sep 27, 2012

### Staff: Mentor

How about the fact that life could not exist without the sun, no matter how far down you dig? How do you grow crops? How do you raise lifestock? How do you create an environment for humans? Waste disposal? Potable water? Breathable air? I assume the seas and all sea life were abandoned. Is this some silly Noah's Ark scenario?

Animals? Plants? How many humans did you plan to take underground out of all of the billions?

Medicine? Hospitals and medical care?

Even assuming some life is possible, what's the quality of life and how do you accomplish it?

10. Sep 27, 2012

### CCWilson

Evo, that's what the novel is all about. I've worked out most of the issues you raise. If you're interested, scan this thread, unless its silliness puts you off your feed.

11. Sep 27, 2012

### Staff: Mentor

What about 3km to find Desulforudis audaxviator?
Well, and an artificial environment might work as well.

12. Sep 27, 2012

I think that an extra solar object of sufficient mass is the best bet: Aliens are a silly way of implementing "Deus ex machina"

13. Sep 27, 2012

### JohnRC

If you can afford to have a lead-in time of about 7-10 years while the Earth was slowly being ejected from the solar system, then the scenario that would least affect humanity would be a fly-by of some sort (from an object within or outside the solar system) that gently accelerated Earth into a rather more eccentric orbit, leading to gravitational whip effect ejecting it from the solar system when its path took it close to Jupiter on the first orbital pass-by.

This scenario would have the additional advantage that you would be free to do whatever you wanted with the moon -- destroy it, remove it, or leave it.

14. Oct 11, 2012

### FalseVaccum89

Actually, this is not accurate; unless one is near the event horizon of a black hole, one experiences the exact same level of spacetime curvature (thus the same level of gravitaitonal force) as with a normal body of comparable mass. A black hole should have the *exact* same effect as a star with comparable mass, unless it were to collide with something.

15. Oct 11, 2012

### Drakkith

Staff Emeritus
He's talking about our Sun capturing the black holes in an orbit. Since the Sun is much less massive than any known black hole, such a thing is not possible.

16. Oct 11, 2012

### onomatomanic

^ Exactly. And the post mfb's was in reply to specified that this is a "massive black hole", so we don't even have to rely on its conforming to the known/predicted mass range. No capture is possible between the two stellar-mass objects without the assistance of at least one other in that range.

17. Oct 13, 2012

### Staff: Mentor

Thanks Drakkith and onomatomanic.

To quantify the ability of our solar system to catch massive objects:
Assume that a massive, slow-moving object (10km/s far away) approaches our solar system. Assume that its mass is small compared to the mass of sun (the reason will become clear later). How can we capture it? Gravitational interaction with a planet. It has to dump enough energy to fall below escape velocity - and it has to do so in a single interaction, as two significant interactions with planets in a single pass through the solar system are extremely unlikely.
The best geometry is a head-on approach to a massive, fast-moving planet, with a very near miss: With the approximation that all objects are point-masses, the planet will be shot in the opposite direction, getting a velocity kick of twice the relative velocity. Real planets have a finite size, which can limit the maximal momentum transfer to lower values, but I will neglect this issue here.

We need massive, fast-moving planets close to the sun... I will begin with Jupiter here and consider Mercury afterwards:

Jupiter orbital velocity 13km/s
Escape velocity (solar system) at its distance: sqrt(2)*13km/s = 18.38km/s
Velocity of incoming object at Jupiter orbit: sqrt(10^2+18.38^2) km/s = 20.93km/s

Relative velocity: 20.92km/s+13km/s = 33.92km/s.
=> Jupiter velocity change 2*33.92km/s = 67.85km/s
Required velocity change of incoming object: (20.93-18.38)km/s=2.54km/s.

=> maximal mass of incoming object: 67.85/2.54 = 26.7 Jupiter masses = 0.026 solar masses.

Mercury: Orbital velocity 47.87km/s, maximal mass of incoming object 317 mercury masses = 0.055 Jupiter masses.

As you can see, Jupiter's mass dominates the results - even with earth in a mercury orbit, 317 earth masses would be about one Jupiter mass (and not 26.7).

This gives 26.7 Jupiter masses = 0.026 solar masses as an upper limit for any reasonable capturing process in the solar system.

What happens if we take the finite size of the objects into account? Objects with 26.7 Jupiter masses are brown dwarfs, with a size similar to Jupiter.
Escape velocity scales with sqrt(M/r), at twice the Jupiter radius (closest possible flyby without touching) this corresponds to 60km/s*sqrt(26.7/2)=220km/s. Based on an initial relative velocity of 33.92km/s, the velocity at closest approach is sqrt(220^2+33.92^2)km/s=222.6km/s.

Calculate the (minimal) eccentricity:
$e=\sqrt{1+\frac{2\epsilon h^2}{\mu^2}}$ with ε=1/2 (33.92km/s)^2, h=4*(jupiter radius)*(222.6km/s) and μ=G*26.7*(jupiter mass)
=> e=1.175
This gives a maximal deflection of 2.04 or 117° - in other words, only ~85% of the maximal velocity change can be used and the maximal mass is even lower. And a minimal separation of 2 Jupiter radii is not possible anyway - the brown dwarf is extremely dense, its Roche limit for Jupiter will be significantly larger.

I would expect ~15 Jupiter masses as a more reasonable number.

18. Oct 13, 2012

### CCWilson

How about a relatively slow moving black hole of the mass of the sun? Could that be captured into a binary?

19. Oct 14, 2012

### Drakkith

Staff Emeritus
Doesn't matter. The Sun, and most everything else in the solar system, would fall towards a black hole, as it falls towards the Sun, with both the Sun and the BH gaining velocity the whole way and being flung out after closest approach. Once away from each other their relative velocity would be similar to what it was before the encounter. Stellar mass black holes are also not the mass of the Sun, but on the order of around 3+ solar masses. There is no known way for a black hole to form with just 1 solar mass.

20. Oct 14, 2012

### Ryan_m_b

Staff Emeritus
At first I wondered about a larger black hole shrinking over time to one this size but the universe isn't old enough by far to allow for that type of time scale.

To the OP how about a very small black hole falling into the sun? Over time the sun will shrink and dim as it falls in (though I'm not sure how the exact process will go). As it's sci fi you don't have to explain exactly how this small black hole was formed, you could even mention it baffled scientists but they've got bigger things to deal with now.

21. Oct 14, 2012

### Staff: Mentor

If we allow very improbable events, there are two small loopholes:

- let a brown/red dwarf scratch the surface of sun, capturing ~2*10^(-4) brown/red dwarf masses, and let it get very close to a planet afterwards (probably Jupiter) to give it some angular momentum. It will end in an extremely eccentric orbit, but bound in the solar system. This needs additional perturbations to get some stable system afterwards, but at least it is possible.
The same would be possible with a black hole of << 1 solar mass, but unless there are primordial black holes with that mass they do not exist.

- the sun could probably perform a similar capturing mechanism around a black hole, swallowing a planet afterwards to get some angular momentum. This event would ruin the whole solar system, some planets would fly away and the others would get completely new orbits afterwards.

22. Oct 14, 2012

### CCWilson

I understand that a black hole formed by the collapse of a massive star would be at least 2 1/2 solar masses in size - and that's the size of the black hole in my story. However, don't some physicists believe that some black holes may have formed near the big bang by some mechanisms that we don't fully understand that could indeed give us smaller black holes currently?

23. Oct 14, 2012

### Drakkith

Staff Emeritus
I've never heard of that, but I'm not an astrophysicist or cosmologist, so I really can't say for certain.

24. Oct 15, 2012