Science Questions

Why can light not escape a black hole?

Sun, 6th Sep 2009

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Indrek Torilo asked:

If a difference between a star and a black hole is density of matter (black hole being a collapsed star) then why cannot light escape the gravity of a black hole but it can escape a gravity of a star? To my understanding:


1)When a star collapses it does not gain mass, it only becomes denser,

2)Gravity depends on mass, not density.


Chris -   The point he’s making is that a black hole is a collapsed star.  So, all the mass of the star ends up in the black hole.  So, if light can come out of the star in the first place, given that there’s no more mass now in the black hole when it’s collapsed, what’s changed that now light can’t get out?

Dave -   That’s right.  When you take a star and convert it into black hole, you actually normally lose an awful of mass.  It involves all sorts of explosions and lots of energy given off so that black hole normally weighs an awful less than the original star did, but that mass is much, much closer together - it’s much more dense.  The force of gravity even the Newtonian force of gravity is essentially proportional to the inverse square.  So, if you’re twice as far away from it, the force gets four times weaker.  So, if you take a star and squash all that mass very close together and then you stand on the surface of it, apart from being burned up and everything, you stand on the surface  of it then you’re going to be a lot closer, a lot more mass.  So the gravity is going to be much, much stronger.  And once you go into relativity and general relativity then that mass can bend space enough that light always gets bent around and it can never escape at all, ever.

Chris -   So, if the black hole blew up again and you took the same mass and put it back to something that was the original size of the star – so in other words, the density was low again then it would start to emit light again.

Dave -   Yeah then light could escape no problem.


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Indrek Torilo asked the Naked Scientists: Hi Naked Scientists,   I love your show and I have downloaded and listened to all the podcasts on your site back-to-back while I drive. Truly awesome! Anyways, I have a question now.   If a difference between a star and a black hole is density of matter (black hole being a collapsed star) then why cannot light escape the gravity of a black hole but it can escape a gravity of a star? To my understanding: 1)when a star collapses it does not gain mass, it only becomes denser, 2)gravity depends on mass, not density.   Kind regards,   Ints, from NSW, Australia What do you think? Indrek Torilo , Sun, 6th Sep 2009

Have a look at my comment in this thread regarding the effect of density and gravitational force: LeeE, Sun, 6th Sep 2009

First you need to understand where gravity comes from. It is the magic force that pulls every atom of matter towards every other atom of matter. Because nearly all the matter on planet Earth is beneath our feet, gravity pulls us down to the ground. But to answer your question there’s 3 other things you need to know about gravity:

1) Gravity is very weak, although it may not seem like it when you fall off your bike. If you pick up a mug of coffee you have beaten gravity, since gravity wants to hold the mug to the table. If you pick up a paperclip with the weakest toy magnet from a Christmas cracker that magnet has beaten all the gravity in all the world.

2) Gravity fades quickly with distance. When you are standing, the ground just beneath your feet is very close to you and pulls quite hard. But there isn’t much of it. Most of the rest of the world is also pulling you down but it’s miles away. On the other side of the world Australia is massive and is also pulling you down but it is nearly 8,000 miles away and only pulls you very weakly.

Because gravity is pulling at different rates from different places scientists like to average out the gravity to a place they call the Centre of Gravity. On Earth the Centre of Gravity is at the centre of the planet. Some gravity is closer. Some is further. Some stronger, some weaker. But it all averages out at about the centre.

3) Gravity is cumulative. One atom of matter has a tiny, tiny amount of gravity but all the atoms of the world have enough gravity to hold us to the ground.

Now.... say you are in an airplane going on your holidays. You can walk up and down in the plane and gravity is more or less normal. But imagine that while you’re flying some aliens come along and squash the planet Earth down to the size of a beach ball. So far as gravity in the plane goes you shouldn’t feel any difference. The World still has the same amount of atoms in it so it still has the same amount of gravity – it’s just all squashed into a really small size. And its Centre of Gravity is in the same place because the top of the planet has been squashed down and the bottom has been squashed up so everything still averages out at it’s centre.

But if you parachute down to ground things would be very different. Balancing on a planet Earth the size of a beach ball, now all the matter in the world is within two feet of you and will all be pulling on you really strongly. You’ve got all the gravity of the world concentrated into a small ball and, as a result, it would squash you flat!

So as you squeeze a planet smaller it still has the same amount of gravity over all but it is now concentrated into a smaler space, so it feels stronger if you get right up next to it.

If you squeezed the Earth even further, until it has a radius of about 12mm (or about an inch across) something strange would happen. Now all the matter of the World is squashed soooo close together and it’s gravity is soooo concentrated that gravity within the Earth becomes stronger than all the forces which normally push atoms apart and the world would start to squash itself down even more on it’s own. This size is indicated by what's known as the Schwarzschild Radius and if you squash any object down enough eventually you will reach it’s Schwarzschild Radius, and it will continue to collapse on it’s own. This is how Black Holes are made.

So as a star collapses it's gravity becomes more concentrated so it collapses further still. Most physicists believe that it will therefore keep on collapsing indefinitely, becoming infinitely small. The gravity as you get up really close to it will therefore increase indefinitely and so it’s gravity will become infinitely large at its infinitely small surface. There will therefore be a region around it where even light will not escape and this is called it’s ‘Event Horizon’. The event horizon therefore indicates a region where the escape velocity needed to escape the gravity of the object is greater than the speed of light. It does not indicate the actual size of the black hole which is, as I said before, infinitely small.

If our Sun suddenly collapsed into a black hole it would not affect the orbit of the planets in our solar system because it’s overall gravity would not be affected.

Incidentally, some ‘light’ can escape a black hole and is known as Hawkin Radiation, which can cause a black hole to slowly ‘evaporate’.

This is my understanding of the relationship between black holes, gravity and light. I am not a physicist and I am sure other members will find some holes in my explanation. But I believe it is broadly speaking correct.
John Chapman, Mon, 7th Sep 2009

A simple to use calculator is available here to show the characteristics of blackholes syhprum, Mon, 7th Sep 2009

That long post of yours (above) has confused me. Is the "gravity" to which you refer 'gravitational force', 'gravitational field', 'gravitational potential' or just plain mass? Why not use the conventional terms in an explanation? It would make it far easier to follow and to comment  and to see whether your model is an improvement on the existing one. lyner, Tue, 8th Sep 2009

Ooh.  Shiny. jpetruccelli, Tue, 8th Sep 2009


I don't bleedin know. What do you think this is, a science forum?

Well, it was supposed to be a layman's explanation, although it probably reveals the entire extent of my knowledge about black holes. I'm not sure that I appreciate the difference between the three but 'gravity is weak' refers to what I might call it's force, 'gravity fades' I suppose refers to it's field and 'gravity being cumulative' could be gravitational force or potential. I don't really know.

The explanation is one that was used in a science fiction book I once read who's two main characters were a physicist and a black hole! The novel was written for a general readership, although the author did have a PhD in physics. It made good sense to me at the time although the book probably did a much better job of explaining it than me.

I jumped in with a rushed answer before one of you experts could write a technical post. I hoped my explanation was a good approximation of the principles involved. Is there anything wrong with it? Please try and keep your reply not too technical. Ta. John Chapman, Tue, 8th Sep 2009

Your numbered points 1,2,3 are spot on.
I think that, when you use the word gravity, you are really talking about the force that something with mass produces on something else with mass - that is really the gravitational field (at least the field is the force on one kg ).
When you're talking of the gravity getting more concentrated it's really the mass that is getting more concentrated.
When you are below the surface of the Earth, for instance, all the 'shells' outside you produce a zero field - pulling in different directions and actually cancelling out. So you experience less and less weight force as you go down because its only the bits underneath that are attracting you. (Always, as you say, effectively towards the Centre of Mass).
IF, however, you are dealing with a dense object like a black hole (or your golf ball sized Earth), the Mass is so concentrated that you can get a lot nearer the CM before you are inside any 'shell'. So the gravity field gets higher and higher (the so-called inverse square law: halve the distance and you get four times the field). At any given large distance, you couldn't tell the difference, though.

For any massive object to become a BH, its Mass needs to be concentrated into within the Swartzchild radius which 'that link' will calculate for you.
I don't think it's nitpicking to draw the distinction between the Mass and its effect. It helps in the end. lyner, Wed, 9th Sep 2009

Thank you SC.

Your comments are always appreciated and, when I understand them, highly educational!
John Chapman, Thu, 10th Sep 2009

Here is a question;

   How close would a 96 ton object have to be to a 234 ton objest in space before their gravitational fields pull them together. G-man, Sat, 12th Sep 2009

If they are stationary and free floating, surely any distance. As far as I know, although gravity weakens quickly with distance, it never actually disappears. This is what leads some astrophysicists to believe that the whole universe will eventually come crashing in on itself in a 'Big Crunch' (although I believe that the velocity with which the universe is expanding following the Big Bang and the amount of dark matter in the universe has lead many physicists to accept that the Big Crunch will never happens and we will keep expanding indefinately until the whole universe becomes cold and dark).

By the way, welcome to the site, G-man.
John Chapman, Sat, 12th Sep 2009

  HI; and thanks for the welcome!

I just read that the Commander of the spaceshudle had to fire the thrusters to close the last two cm's to the spacestation. G-man, Sat, 12th Sep 2009

My opinion :)

think of it as a question of distance. When you look at earth as you stand on it you will feel one G, right? At least I know I do so. (or more lately due to unforeseen circumstances (obesity:) Only Joking.. )

But if you went to Earths exact 'middle' all those 'gravitational forces' would equal each other out. That means that in the middle of the earth if it kept its size as usual you weight 'nothing' at all. Consider now shrinking Earth to somewhere around five to two cm (sorry don't remember the exact measurement/circumference there) What you do then is to break a limit for how much mass inside a certain radius that is allowed to interact (two way communication) within our universe. If we leave the hypothetical discussion of what Hawking radiation might be out of this for the moment you have now succeeded in creating a Black Hole from our poor Earth. If you were left where you were standing before the Earth shrunk you would still feel that one G. If you on the other tentacle followed its downsizing :) you would now be ripped to smithereens by its 'tidal forces' as its same 'total invariant mass' now would occupy such a small place in SpaceTime.

It's like all particles have a gravitational 'influence' on everything else and when you change their density and compresses it under the ‘Schwarzchild radius’ that a Black hole demands (defined as that ‘radius for a given mass where, if that mass could be compressed to fit within that radius, no force could stop it from continuing to collapse’ into a zero size hole which then would be the beginning of a new Black Hole) Then the mass becomes 'infinite' and your black hole will be there. In fact it seems to be a balance between mass and distance that will create a black hole.

Another example of this principle is " The Planck length is related to Planck energy by the uncertainty principle. At this scale, the concepts of size and distance break down, as quantum indeterminacy becomes virtually absolute. Because the Compton wavelength is roughly equal to the Schwarzschild radius of a black hole at the Planck scale, a photon with sufficient energy to probe this realm would yield no information whatsoever. Any photon energetic enough to precisely measure a Planck-sized object could actually create a particle of that dimension, but it would be massive enough to immediately become a black hole (a.k.a Planck particle), thus completely distorting that region of space, and swallowing the photon. "

In this case we are discussing 'energy' instead of 'invariant mass' but the energy needed for the transformation to happen would, as I understand it, need to be already 'infinite' for it to 'transform'. One of the criteria people use for defining a Black Hole is that light will have no way out from its EV (event horizon)and only 'travel' in one direction, to its core. You can see Black Holes as 'mathematical infinities' and, just as those, coming in different sizes. Thats why I speak about distance versus mass as defining when it happens. It's a strange concept I agree.

As for if 'pure energy/radiation' could transform into a Black Hole inside SpaceTime I believe that idea to be a violation of the idea that 'invariant mass' accelerating never can become a Black Hole, no matter its velocity, as long as light can be sent out from it.

And a singularity differs from a star in that, as its mass becomes 'infinite' its size will also 'disappear' into infinities becoming impossible to define, as I have understood it. For us that means that a black holes mass might be 'size less' even though the space surrounding it will 'expand' as seen from a observer inside the EV. yor_on, Sun, 4th Oct 2009

Mass is inversely related to density. \rho= M/V where V is for volume and M is for mass, and \rho makes density. Imagine we could tamper with the sun. Imagine we decided to expand the sun a million times; we might end up with what could be described as a thick gas of particles, whose gravitational pull was less than that of the compactified sun we began with.

Now imagine we decided to add the mass of a million averagelike stars together with our sun, but not expanding the struture of the sun, we would be trying to squeeze matter into the free spaces - this high density means an increase of gravtational pull. The more mass you add to a given space in he vacuum increases the gravitational influence of the object in question.

Remember, mass (and thus gravity) is inversely related to te density of a system. Mr. Scientist, Sun, 4th Oct 2009

Why are my lines scored out above lol? Mr. Scientist, Sun, 4th Oct 2009

If you try to model this action with maths and take relativity phenomena into account you will find that it can not happen. Acceleration contains time as a member. Gravity affects time. So gravity affects gravity and is therefore self limiting. It can not progress to a singularity. Vern, Sun, 4th Oct 2009

You probably clicked the strike through button. It is just to the right of the underline button which is to the right of the italics button which is to the right of the bold button.  It is just above the smiley button. Vern, Sun, 4th Oct 2009

Oh :) Mr. Scientist, Mon, 5th Oct 2009

Nice explanation Vern, please include me as a member of the 'no singularity club' syhprum, Mon, 5th Oct 2009

I'm not the originator of the notion; I came across a study by a group of university students who were trying to model Black Hole creation; they decided that it was not possible. Now I can't find the reference. Vern, Mon, 5th Oct 2009

You know, M-Theory, last time i read can actually rid quantum theory of singular regions in space and time. Mr. Scientist, Mon, 5th Oct 2009

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