Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jopie64 on 19/10/2013 15:50:45
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Thoughts about a black hole
What I know from a black hole is that it has an event horizon and a singularity.
I’ve been thinking about these concepts. The event horizon is the place from which the gravity escape velocity is greater then the speed of light, so supposedly nothing could get the speed to escape from it. The singularity however is the center point with (almost?) infinit energy and (almost?) no size.
Now concider this:
Alice and Bob are deciding to check the inner workings of a black hole, because they live near to it anyway. They draw straws and Bob loses so he has to take the space ship to the black hole while Alice stays watching what will happen to him. She wants to see what happens when Bob is going past the event horizon.
There he goes and Alice sees Bob getting closer and closer to the point of no return. She has to reajust her telescope a few times, because of the redshift of light emitted from the backlight of Bob’s spaceship. The light gets redder and redder and Bob’s spaceship is getting smaller and smaller, and he is traveling slower and slower so it seems. After days of watching, almost no light is coming from Bobs spaceship and he doest get any further while he has almost reached the point of no return. She draws the conclusion he will never reach the horizon and she will never see him back again.
Now Bob’s story:
Bob took a spaceship from which he could take a look outside both at the front and at the back. He also took a telescope to watch Alice for the last time while he is getting closer and closer to the point of no return. He also has to reajust his telescope a few times, because of the bright blue-shifted light emitted from Alices her telescope (she placed a laser on it so Bob could see where she is). Also (due to the gravitational field he is entering) he sees the universe around him develop quicker and quicker. Light is getting brighter and brighter.
Watching the other side, he can still see the event horizon being black and all, but it is getting smaller and smaller. Then, in an ultrabright flash, everything in the universe seems to happen at the same time (in fact it is happening really quick), and then… everything ends.
What I want to say is that my conclusion would be:
The event horizon is the same as the singularity.
The size of the event horizon is dependent on the reference frame from where you look at it. From close to the black hole it is smaller, because you are inside the gravitational field from it.
Is this conclusion correct? When no, where am I going wrong?
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Thoughts about a black hole
You might want to read this - http://exploringblackholes.com/
The singularity however is the center point with (almost?) infinit energy and (almost?) no size.
The energy is quite finite. It's the energy density that is infinite.
Now concider this:
Alice and Bob are deciding to check the inner workings of a black hole, because they live near to it anyway. They draw straws and Bob loses so he has to take the space ship to the black hole while Alice stays watching what will happen to him. She wants to see what happens when Bob is going past the event horizon.
Alice will never observe Bob to pass the event horizon. From her point of view he slows down as he approaches it and and while slowing down never crosses it.
There he goes and Alice sees Bob getting closer and closer to the point of no return. She has to reajust her telescope a few times, because of the redshift of light emitted from the backlight of Bob’s spaceship.
No adjustment is required for a telescope. She just has to use different detectors since the redshift keeps getting larger.
The light gets redder and redder and Bob’s spaceship is getting smaller and smaller, and he is traveling slower and slower so it seems.
The ship doesn't get smaller. What led you to believe that?
After days of watching, almost no light is coming from Bobs spaceship and he doest get any further while he has almost reached the point of no return. She draws the conclusion he will never reach the horizon and she will never see him back again.
Correct.
Now Bob’s story:
Bob took a spaceship from which he could take a look outside both at the front and at the back. He also took a telescope to watch Alice for the last time while he is getting closer and closer to the point of no return. He also has to reajust his telescope a few times, because of the bright blue-shifted light emitted from Alices her telescope (she placed a laser on it so Bob could see where she is). Also (due to the gravitational field he is entering) he sees the universe around him develop quicker and quicker. Light is getting brighter and brighter.
Watching the other side, he can still see the event horizon being black and all, but it is getting smaller and smaller. Then, in an ultrabright flash, everything in the universe seems to happen at the same time (in fact it is happening really quick), and then… everything ends.
No. That's not what Bob sees.
What I want to say is that my conclusion would be:
The event horizon is the same as the singularity.
Incorrect. The tidal forces experienced by someone at the event horizon are finite while at the singularity are infinite.
The size of the event horizon is dependent on the reference frame from where you look at it. From close to the black hole it is smaller, because you are inside the gravitational field from it.
That's incorrect.
Is this conclusion correct? When no, where am I going wrong?
See text above. It explains everything you want to know in addition to what I've described. Also there are some articles on this in the American Journal of Physics on the subject. Go to http://scitation.aip.org/content/aapt/journal/ajp and do a search. Let me know which ones you want to read.
These might be of interest to you. If so I can e-mail them to you if you wanted to PM your e-mail address to me.
Stellar sky as seen from the vicinity of a black hole by Joachim Schastok, Michael Soffel, and Hanns Ruder, Am. J. Phys., 55(4), Apr. (1987)
The observable universe inside a black hole by W.M. Stuckey, Am. J. Phys., 63(9), Sep. (1994)
Black hole physics illustrated in photon orbits by Haldan Cohn, Am. J. Phys., 45(3), Mar. (1977)
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Pete, can I question the fact that the energy inside a black hole (of any size) is finite in the theoretical limit of approaching a mathematical singularity. Consider the following simple facts described in a minimally mathematical way.
The potential energy released by a small particle falling under gravity is;
The mass of the particle x the gravitational field x the distance fallen.
This will be true inside the event horizon as well as outside. consider therefore the energy released by a particle on the outside of a large MASS collapsing simply under gravity inside an event horizon as it collapses from a radius r from the centre point (where the "singularity" will eventually form) to a radius of r/2 that is collapsing half way. During this period of the collapse the gravitational field that the small particle has increased as the inverse square of the distance fallen because the particle is being attracted by the large collapsing mass that the particle is outside of. ie
The gravitational force is proportional to
( the large Mass x the small mass/ the radius of the main mass(which is going from r to r/2)squared
this means that the energy being released is the integral of the gravitational force over the radius from r to r/2.
This is simply shown to be proportional to 1/r thad is as each time the distance collapse id reduced by a factor of 2 the energy releases by this collapse is greater than the previous collapse by half that is the energy released as the collpse towards a mathematical singularity happens the energy increases totally without limit so the energy inside a black hole collapsing to a mathematical singularity is infinite as well as the energy density!!
If you consider the collapse of a stellar mass hole towards the planck limit the energy released is plenty enough to create a universe as large and as complex as our own.
These particles will of course multiply greatly during the collapse by the creation of particle anti particle pairs in the increasingly violent collisions they will experience during the chaotic collapse process
All that we then need is a process to create the impression of space for the particles involved in this collapse. That is looking at it from the particles point of view and not from outside. for us to have created a new big bang!
Can anyone prove this simple analysis wrong?
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Consider the following simple facts described in a minimally mathematical way.
The potential energy released by a small particle falling under gravity is;
The mass of the particle x the gravitational field x the distance fallen.
While that’s true in Newtonian gravity it’s not that simple in general relativity. In any case the total energy of a particle moving in a static gravitational field is constant
This will be true inside the event horizon as well as outside.
I disagree. Inside the event horizon no observer can remain at rest. Therefore the observer will see a time varying gravitational field as he falls and that means things get complicated and the energy changes with time (I think).
consider therefore the energy released by a particle …
A particle does not release energy.
on the outside of a large MASS collapsing …
Now you’ve changed the problem away from a black hole. In any case outside the collapsing mass the field can be static and have the same field as if there was a black hole at the origin.
simply under gravity inside an event horizon…
You can’t treat the inside in the same way as you can the outside.
as it collapses from a radius r from the centre point (where the "singularity" will eventually form) to a radius of r/2 that is collapsing half way. During this period of the collapse the gravitational field that the small particle has increased as the inverse square of the distance fallen because the particle is being attracted by the large collapsing mass that the particle is outside of
What does it mean for a “particle has increased as the inverse square of the distance fallen”?
The gravitational force is proportional to
( the large Mass x the small mass/ the radius of the main mass(which is going from r to r/2)squared
Incorrect. In GR the gravitational force is more complicated than it is in Newtonian gravity. In GR the gravitational force is a function of the particle’s velocity. See http://home.comcast.net/~peter.m.brown/gr/grav_force.htm
this means that the energy being released is the integral of the gravitational force over the radius from r to r/2.
That does not follow from anything you’ve said above.
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Let me put the proposal in a different way
You must first remember that an the formation and crossing of an event horizon does not involve the suspension of any physical laws which are continuous from the point if view of the particles involved in the collapse. The gravitational field and field gradient will continue to increase as the collapse progresses towards the hypothetical singularity. Also I have no reason to believe that the law of the conservation of energy is not obeyed.
Many of the problems and misunderstandings about black holes are caused by taking a view from the outside and looking in. The really important thing is what the individual particles are doing in their locality as the collapse progresses and for the sake of simplicity I am considering what is happening to a massive particle just inside the event horizon at the instant that it first forms where the increasing gravitational field is forcing the collapse to continue.
Next you must consider that collapse under gravity results in a release of potential energy into kinetic energy. This is the process by which stars are created and this process will continue inside the event horizon this will cause an increase in the temperature of the particles.
The particles will continue to interact and collide with each other exactly as before the event horizon forms. Gravitational effects on ordinary particle interactions will not happen until very much later in the collapse process. This takes a finite time.
The question then is how much energy is released and what will happen in the early stages of this collapse inside the event horizon
Even for a stellar mass black hole the initial conditions are well within the range of observable high energy physics experiments. Electron pair production is possible at temperatures before black hole formation and the strong "cooling" effect of this process has been suggested as a possible process for a certain type of core collapse supernova. core collapse processes are essential to create the sort of pressures needed for the creation of stellar mass black holes.
The creation of larger mass particles will happen inside event horizons and this will in effect aid the collapse process but will eventually by observed asymmetries create more matter. This will of course be unobservable outside the event horizon but well within practical measurement and theoretical modelling.
The question is how far will this go? My hypothesis is this process could release so much energy that it might be able to create a whole universe as big and complex as ours not a tiny universe with the mass of a star!
I feel that saying that "inside the event horizon matter collapses to a singularity in a finite time and so is not worth studying in detail" is as bad as saying "our universe will in time suffer the heat death and vanish and is therefore uninteresting".
I feel strongly that the serious study of normal physical processes occurring during the early stages of black hole collapse, inside an event horizon could be one very important route to new and innovative thinking on both cosmology and the theory of everything.
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I'd like to note that Einstein insisted that event horizons cannot form (in finite time) which is consistent with the prediction that an observer falling to an event horizon will see the external universe age to infinity by the time they reach the event horizon and an external observer will see an infalling object slow to a virtual stop outside of the event horizon. If the infalling object is reflective, you should be able to bounce light off of it forever... so long as it's visible outside of the event horizon.
Personally, I think physicists tend to spend too much effort on the math and not enough on designing the thought experiments their equations are to describe. I don't know how many times I've heard people say that "it only looks like" stuff doesn't fall to an event horizon. All of Relativity is about how the universe looks. It's all about light paths.
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... an external observer will see an infalling object slow to a virtual stop outside of the event horizon. If the infalling object is reflective, you should be able to bounce light off of it forever... so long as it's visible outside of the event horizon.
Well, you could try doing that, but you wouldn't see the results because any reflected light would be far longer wavelength than the visible spectrum on reflection. No doubt there's a formula that will tell you what return frequency to expect given a specified input frequency to a reflector at a certain distance above the event horizon of a BH of given mass...
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... an external observer will see an infalling object slow to a virtual stop outside of the event horizon. If the infalling object is reflective, you should be able to bounce light off of it forever... so long as it's visible outside of the event horizon.
Well, you could try doing that, but you wouldn't see the results because any reflected light would be far longer wavelength than the visible spectrum on reflection. No doubt there's a formula that will tell you what return frequency to expect given a specified input frequency to a reflector at a certain distance above the event horizon of a BH of given mass...
Shining light down onto a reflective surface falling into a black hole doesn't yield any wavelength shift in the reflected light except that due to the object's motion. The change in wavelength due to gravity is reversed when the reflected beam climbs back out of the gravity well.
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I'd like to note that Einstein insisted that event horizons cannot form ...
Please provide the reference to where he made this assertion. I'd like to read it. Thank you.
It's mentioned here (http://en.wikipedia.org/wiki/Black_hole#Formation_and_evolution) and in his 1939 paper, "On a Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses".
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There are causality problems with matter falling through an event horizon. An infalling observer will see the universe age infinitely before he passes through the event horizon. That's not possible if he has passed through the event horizon. If the infalling observer has a mirror and you shine a laser beam onto it and detect the reflected beam, it will never disappear.
Mass causes spacetime to stretch. That stretch becomes infinite at the event horizon. I think black holes are infinitely deep (deepening without limit) and have yet to find a reason to disagree with Einstein.
Generally, when your physical model yields infinities or zeroes, it's wrong. There are lots of infinities and zeroes at the event horizon.
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Shining light down onto a reflective surface falling into a black hole doesn't yield any wavelength shift in the reflected light except that due to the object's motion. The change in wavelength due to gravity is reversed when the reflected beam climbs back out of the gravity well.
Sadly, I don't have the physics to calculate the result. However, I'm told that a remote viewer will see an infalling object both slow down and become increasingly red-shifted as it approaches the EH. It seems to me that the red-shifted light the remote observer sees reflected from the object will be from incident light (i.e. external illumination), as no light will illuminate it from the black hole itself. This suggests that, for whatever reason, the in-going blue-shift is exceeded by the outgoing red-shift, or the object would, as is often described (incorrectly, I'm told), appear to hover on the event horizon for ever without any red-shifting out of the visible spectrum.
But naturally I'm prepared to accept correction or clarification of these details by someone who can do the maths :)
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Let's please keep the discussion on topic, related to Black Holes and Event Horizons.
You are welcome to argue the topics, but personal quibbling doesn't help the discussion.
One - Liners, "XYZ is Wrong" should be backed up with additional discussion of the physics supporting your assertion.
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Shining light down onto a reflective surface falling into a black hole doesn't yield any wavelength shift in the reflected light except that due to the object's motion. The change in wavelength due to gravity is reversed when the reflected beam climbs back out of the gravity well.
Sadly, I don't have the physics to calculate the result. However, I'm told that a remote viewer will see an infalling object both slow down and become increasingly red-shifted as it approaches the EH. It seems to me that the red-shifted light the remote observer sees reflected from the object will be from incident light (i.e. external illumination), as no light will illuminate it from the black hole itself. This suggests that, for whatever reason, the in-going blue-shift is exceeded by the outgoing red-shift, or the object would, as is often described (incorrectly, I'm told), appear to hover on the event horizon for ever without any red-shifting out of the visible spectrum.
But naturally I'm prepared to accept correction or clarification of these details by someone who can do the maths :)
The object or observer approaching an event horizon (either falling in or hanging from a rope or suspended by thrusters) will slow down. I used the example of shining light down into the gravity well and reflecting it back up because the wavelength shift due to gravity (blue shift going down and redshift coming up) is reversed so losing contact with the infalling object/observer due to redshift isn't an issue.
The infalling observer will not only be seen to slow to a halt just outside the event horizon, they will see the entire universe age infinitely. Why people would think what happens is somehow different from what's observable is something I've never understood.
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To get back to the original question properly now things have been sorted out.
Firstly let me say is that descriptions of effects caused at and around black holes are mostly approximately correct in the good popular science books but they are expressed badly deliberately to create the biggest gee whizz science effect and make them look very mysterious. What the books do not express well is some of the true relationships of space and time and demonstrate the scale.
Precisely what happens inside the event horizon of a black hole is not observable from the outside. However it is partially analysable theoretically although some aspects of this are still being discussed and alternative theories suggested.
Firstly let me state that in most models nothing much happens as the event horizon is crossed if you are small enough, all that changes is your relationship with the rest of the universe, that is you cannot go back out and cannot send messages out.
Next let me say that there are no infinities here that is the gravitational fields and gradients are not infinite it is just that the escape velocity has reached the velocity of light. Red shifts out and blue shifts in do occur but are not infinite and this is not the hypothetical "singularity" which is right at the centre of the (Schwarzschild) black hole and occupies no space at all.
The event horizon has a real size ranging form about two or three miles for the smallest stellar mass black holes to about the size of the orbit of the planet Neptune for the very largest ones at the centre of very big elliptical galaxies.
Now let me define this term "singularity".
From a mathematical point of view it is a point of zero size in all directions that is not just very small but genuinely zero.
However physically it is different. To all but a few physicists it is very clear that something of absolutely zero size makes no sense, it is just something we do not really understand (yet) and there is no clear agreement about how to solve this problem but "singularity" is what we have decided to call it. it is a bit like on old maps where in unknown territory they often wrote "here be dragons"!
I have to pack up now but I hope to come back later and clarify a few more details.
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The object or observer approaching an event horizon (either falling in or hanging from a rope or suspended by thrusters) will slow down.
From the viewpoint of a distant observer the infalling observer appears to slow.
I used the example of shining light down into the gravity well and reflecting it back up because the wavelength shift due to gravity (blue shift going down and redshift coming up) is reversed so losing contact with the infalling object/observer due to redshift isn't an issue.
So what makes the torchlight different from the other incident light that reflects off the infalling object when the torch is off, and which does become increasingly red-shifted as it returns?
The infalling observer will not only be seen to slow to a halt just outside the event horizon, they will see the entire universe age infinitely. Why people would think what happens is somehow different from what's observable is something I've never understood.
It's because there are two quite different viewpoints. The infalling observer only slows from the viewpoint of a distant observer - the light is delayed and red-shifted on its way back from the BH. But he (the infaller) is actually accelerating into the BH away from the incident light from the rest of the universe - it can never catch up with him once he passes the EH (which he doesn't even notice, assuming the BH is big enough for the tidal forces at the EH not to tear him apart). In a sense, to the distant observer, he leaves his image behind at the EH. This article (http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html) explains better than I can.
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The object or observer approaching an event horizon (either falling in or hanging from a rope or suspended by thrusters) will slow down.
From the viewpoint of a distant observer the infalling observer appears to slow.
In Relativity there is no difference between what appears to happen and what does happen. Clocks in motion or under acceleration (or gravity) not only appear to slow, they really do. Objects in motion or acceleration don't merely appear to shrink, they really do. Objects falling into a black hole don't only appear to slow to a virtual halt, they really do. Relativity doesn't just describe the appearance of reality.
Relativity defines the relationship between the time experienced by the infalling observer when falling into a black hole compared to a distant, inertial observer. The relationship is exponential and by the time the infalling observer's time is up, an infinite amount of time will have passed for the external observer. This time dilation isn't an illusion.
I used the example of shining light down into the gravity well and reflecting it back up because the wavelength shift due to gravity (blue shift going down and redshift coming up) is reversed so losing contact with the infalling object/observer due to redshift isn't an issue.
So what makes the torchlight different from the other incident light that reflects off the infalling object when the torch is off, and which does become increasingly red-shifted as it returns?
Drop a mirror into a black hole and it will be observed to slow toward a halt. Shine a beam of light down into the gravity well to bounce off the mirror and the light returned will have no frequency shift due to Relativity, only due to its motion downward, which will impart red shift. The light beam passing down into the gravity well will shorten in wavelength and then, after being reflected, it will lengthen again as it travels back up out toward our distant observer. This won't eliminate all the redshift since the redshift will increase as the infalling mirror is observed to accelerate downward but will then be cancelled as the Relativistic effects cause the mirror to slow.
The infalling observer will not only be seen to slow to a halt just outside the event horizon, they will see the entire universe age infinitely. Why people would think what happens is somehow different from what's observable is something I've never understood.
It's because there are two quite different viewpoints. The infalling observer only slows from the viewpoint of a distant observer - the light is delayed and red-shifted on its way back from the BH. But he (the infaller) is actually accelerating into the BH away from the incident light from the rest of the universe - it can never catch up with him once he passes the EH (which he doesn't even notice, assuming the BH is big enough for the tidal forces at the EH not to tear him apart). In a sense, to the distant observer, he leaves his image behind at the EH. This article (http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html) explains better than I can.
It's true that the two viewpoints will be different, but I'm presuming we're assuming Relativity is correct in our discussion, here. At least I am. However, for Relativity to hold in this case, the assumptions it's based on must also hold: the speed, c, must be the same for all observers; gravity and acceleration are equivalent; and all observations in all frames of reference are causally consistent. It's literally not possible to use General Relativity to yield a result that violates one of these assumptions.
This isn't a minor point. For anything to fall to an event horizon requires violation of causality, invalidating Relativity and the foundation of all science.
Going back to the falling mirror model, the contemporary interpretation of black holes requires that there would be a point when the light beam will stop being reflected because the mirror would have (in "real" reality) fallen through the event horizon but the mirror would still be visible (in the illusory reality of Relativity). At this point, causality fails and Relativity is no longer valid. If matter can fall to an event horizon (or for event horizons to form in finite time) then Relativity is wrong. While this might be the case it is an absolute certainty that Relativity does not (cannot) yield this outcome.
But, I'll ignore these problems for now the same way Einstein did, by modelling a black hole, with an event horizon, that exists at time = zero and remains unchanged for all of time.
The model for black holes I was taught has spacetime changing at the event horizon where the one of the space dimensions rotates so that all space dimensions are tangent to the surface of the event horizon. The time dimension rotates to be radial with "future" pointing toward the singularity.
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Android, what you say is true for someone looking from outside of what is going on. This is in fact largely irrelevant for understanding what is really going on. What is most important is what particles and objects close together in a state of free fall are experiencing. you must remember that with the exception of the existence and effects of extreme gravity gradients locally everything always looks perfectly normal with no significant changes in local space and time except of course for the effects of normal physical changes like heating of gases due to compression etc.
I agree that looking a long way away the observable universe will change and if it changes on observable timescales it will be possible to understand what is going on.
It annoys me that people are obsessed by what is seen by outside observers as objects get close to an event horizon but do not bother to think what is happening to the particles themselves because that is where a real innovation may be found.
OK I accept that this is all that we, as outside observers may see. However just think, when we see a red hot poker cooling down do we marvel at the fact that it "never" reaches room temperature? Why then should we marvel at an observable red shift as things approach an event horizon. In reality the image would fade form visibility or detectability quite quickly and the extreme events described are not really observable.
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From the viewpoint of a distant observer the infalling observer appears to slow.
In Relativity there is no difference between what appears to happen and what does happen. Clocks in motion or under acceleration (or gravity) not only appear to slow, they really do. Objects in motion or acceleration don't merely appear to shrink, they really do. Objects falling into a black hole don't only appear to slow to a virtual halt, they really do. Relativity doesn't just describe the appearance of reality.
Yes, that's true; a black hole is an interesting instance because the distant observer can never interact with the infaller.
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... Why then should we marvel at an observable red shift as things approach an event horizon. In reality the image would fade form visibility or detectability quite quickly and the extreme events described are not really observable.
Yes; popular science descriptions have sometimes given the impression that the image of the infaller would be observed lingering for ever at the event horizon, without saying that, in fact, the image would quickly vanish into the extreme IR and fade to indetectability.
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Whether matter would redshift beyond visibility is irrelevant to the requirements of Relativity. Because this issue is somewhat off-topic I've created a different thread, "Why do people think Relativity predicts event horizons?"
I've tried to present the simplest argument I can for why the contemporary view is wrong but I'll be happy to address any additional issues individually.
The more I think about this the more clear it becomes that Einstein was right. And, even if he wasn't, it's still not possible to use Relativity to demonstrate the existence of event horizons since doing so requires violating at least one fundamental assumption of Relativity (that observations from all reference frames are equally valid).
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In Relativity there is no difference between what appears to happen and what does happen.
That’s incorrect. There is a clear distinction between what is observed and what is reckoned. E.g. when an object is approaching us there are two effects going on which determine what we ‘see’ and what we ‘reckon’– The former is defined according to what is observer actually sees with nothing taken into account while the second is defined according to what we know to be the case by both calculation and observation. For example; when an object approaches us then we see things happening faster than what we know to be true. The Doppler effect causes this. The second is what we know to be the case from what theory tells us. I.e. what is the rate that photon emitter is emitting photons when we account for Doppler shift. That’s why in the authors of the text Exploring Black Holes uses the term reckon for what actually happens as opposed to what appears to happen.
E.g. see how the term reckon is used here
http://astro.cornell.edu/academics/courses/a290/lectures/A2290_36%20(Free%20Fall).pdf
Clocks in motion or under acceleration (or gravity) not only appear to slow, they really do. Objects in motion or acceleration don't merely appear to shrink, they really do. Obj
But they appear to run slow for two reasons, (1) time dilation and (2) Doppler effect. If time dilation didn’t exist in nature then the rates at which moving clocks run would still appear to run at a rate which would depend on (1) the speed of the object and (2) the direction the object is moving in.
Etc.
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The event horizon and the singularity are two different things. The event horizon is when light can no longer escape the black hole's gravity and the singularity is theoretically an point in the center of infinite destiny and is infinitely small. it is still theoretical though.