# Earth and Black hole

1. Feb 19, 2008

### pchalla90

What would happen if the whole solar system were to be absorbed by a nearby black hole in an instant? would people on earth realize anything different, because the relative distances between all the celestial bodies would remain constant because it was all absorbed at the same time?

black holes are dense enough to absorb light. so would the light coming from the sun cease to reach the earth? would gravity be the same between the organisms on the surface of the earth and the earth? would the earth still be going around the sun in the same manner?

this is just for fun. it's not schoolwork. just general curiosity.

2. Feb 19, 2008

### Wallace

Black holes produce enormous tidal forces as you get near to the event horizon. A tidal force is something that acts differently on different parts of a body, in the case of a black hole say you approached feet first, your feet would be accelerated towards the centre more rapidly than your head with unpleasant consequences! I think it was Steven Hawking who termed the phrase 'spaghettification' to describe this process!

So in a word, yes we would certainly notice if we were plunged into a black hole!

3. Feb 19, 2008

### Chronos

The good news is we would know the black hole was coming for years before it ate the solar system. The bad news is we would also know there is nothing we could do about it.

4. Feb 20, 2008

### xantox

Not necessarily enormous: depending on the mass of the black hole, the tidal effects at the horizon may become lower than those found at earth's surface.

Last edited: Feb 20, 2008
5. Feb 20, 2008

### George Jones

Staff Emeritus
Maybe not, and if we do notice, it won't be for long!

A rough calculation gives that about 1/3 of a second elapses between when discomfort is first felt and when the singularity is hit. This time is independent of the size of the black hole.

Quantification of this: If the mass of a black is greater than about 50,000 solar masses, then discomfort is first felt inside the horizon, not at the horizon or outside the horizon.

I haven't run the numbers for when tidal forces at the horizon equal tidal forces at the Earth's surface, but this happens for a black hole that has substantially more mass than 50,000 solar masses.

6. Feb 20, 2008

### pchalla90

Does that 1/3 of a second mean the time between when we initially enter the black hole and when we feel the "spaghettification"? I'm assuming that's when you get stretched out to be taller than you should be (if you entered feet or head first). I'm also assuming it's like taking a picture of a human and simply extending it in one direction. Is it like that or am I completely off?

7. Feb 20, 2008

### xantox

It's the time between it starts hurting (tidal acceleration exceeding 1G), and the time the singularity is hit. There is both stretching in the z direction, and crushing in the x and y directions.

Last edited: Feb 20, 2008
8. Feb 20, 2008

### pchalla90

Singularity? Is that when the body assumes its normal shape again? If that's true then there's 1/3 of a second of discomfort and then it's back to normal?

9. Feb 20, 2008

### xantox

The singularity is the "center" of the black hole, which we cannot say much about. There's 1/3 second of both disconfort and complete destruction, and then we don't know.

10. Feb 20, 2008

### pchalla90

ah. what a pleasant ending.

thanks to all who replied.

11. Feb 20, 2008

### xantox

I got around 50,000,000 solar masses to match a free fall tidal gradient on a human body at Earth's surface. This should allow for about 8 minutes of comfortable vacations inside the black hole.

Last edited: Feb 20, 2008