How can anything cross the event horizon of a BH?

[Fiziqs]

Ok, although I had hoped to avoid having to ask a stupid question, it seems as though I'm just too dense to figure this one out on my own. Generally I can, with the help of Google, and the combined wisdom of the internet, deduce an answer. But either Google, the internet, or my brain has failed me this time.

So I'm going to ask a stupid question.

How can anything cross the event horizon of a black hole?

Of course I've found what I assume to be the accepted answer to this question on the internet, but it just doesn't make sense to me. If time slows down as an object approaches the event horizon of a black hole, to the point where the object never actually appears to cross said horizon. How can it ever cross it?

The only answer that I've been able to find, is that the object only appears to slow down from the perspective of an outside observer. From the objects own perspective nothing changes. It goes sailing right through the event horizon without barely noticing.

But, I always thought that the time differential between the observer and the object wasn't just one of perspective, but that time actually does slow down for the object. As in the twin paradox where one twin goes off in a FTL spaceship, and then returns months later to find that his twin is many years older. It wasn't that time simply appeared to slow down for the twin in the spaceship. But that time actually did slow down. For the twin in the spaceship, time did slow down relative to his brother. It wasn't just a matter of perspective. Time really, really does slow down.

If I then extrapolate this out to my black hole problem, time actually does slow down for the object approaching the event horizon of a black hole, relative to an outside observer. It's not just an illusion. If we compare clocks, one for the observer, and one for the object, we would find that indeed the object's clock has almost come to a complete stop. The object would percieve that it's still moving, and that its clock is still running normally, but it's not.

Just like the twin paradox, it's not just that time appears to slow down, it really does.

Ok, I'm smart enough to know that there's a flaw in my reasoning, but I don't know what it is. So can someone explain this to me in simple terms that I can understand.

I would greatly appreciate it, because this is bugging the heck out of me.

Thanks

 

[PAllen]

Time dilation just comes from different definitions of simultaneity between different observers, and there is complete symmetry: A thinks B's clock is slower, B thinks A's clock is slower (as long as they are just moving relative to each other). There is no way to say who is right. The twin situation results from diffferent routes between two events - each twin takes a different route through spacetime. Since they start and end together, the different route they took is not relative but objective; one route entailed more time than the other, and they both end up agreeing on this.

The way to resolve your confusion about the event horizon is to realize, e.g. the distant observer sees the infalling observer's 'watch' approach, but never reach 3pm. This is just a limitation on what they see - they can't see the rest of the infaller's history: from 3pm on, when they've crossed the event horizon.

A black hole analog of the twin situation would be for one person to go close the horizon and back to meet the distant twin. Now, the one who went near the horizon would objectively have aged less.

 

[pervect]

Of course I've found what I assume to be the accepted answer to this question on the internet, but it just doesn't make sense to me. If time slows down as an object approaches the event horizon of a black hole, to the point where the object never actually appears to cross said horizon. How can it ever cross it?

It sounds like you believe in absolute time, and that you think of time slowing down as similarly being absolute.

The short answer is that time is not absolute. In fact, it's common for A to think that B's clocks are running slow, while B thinks that A's clocks are running slow. This is the so-called twin paradox, and there's about a zillion threads on this already, probably two zillion by now. Anyway, while this behavior doesn't make any sense if one believes in absolute time, it's not that hard to grasp if one abandons the notion of absolute time and thinks of it as relative.

In the black hole case, because of the conventions of simultaneity, it appears to take an infinite time in the coordinate system of the obsever at infinity, but according to the clock on the object actually doing the falling, it takes only a finite time to cross the event horizon.

 

[Dale]

But, I always thought that the time differential between the observer and the object wasn't just one of perspective, but that time actually does slow down for the object. As in the twin paradox where one twin goes off in a FTL spaceship, and then returns months later to find that his twin is many years older. It wasn't that time simply appeared to slow down for the twin in the spaceship. But that time actually did slow down. For the twin in the spaceship, time did slow down relative to his brother. It wasn't just a matter of perspective. Time really, really does slow down.

When two clocks start out together, separate, and meet up later, the time elapsed on each clock is frame invariant. But when two clocks are spatially separated they each have to use some simultaneity convention to determine what the the time is on the distant clock. Because simultaneity is relative they may disagree on which is slower, etc.

This is what happens in the scenario you are describing with the black hole. As pervect and PAllen said, one observer uses a simultaneity convention that has the other observer never reaching the black hole. Meanwhile the other observer uses a simultaneity convention that goes right through the event horizon without any problem.

 

[James A. Putnam]

How can anything cross the event horizon of a black hole?

The only answer that I've been able to find, is that the object only appears to slow down from the perspective of an outside observer. From the objects own perspective nothing changes. It goes sailing right through the event horizon without barely noticing.

But, I always thought that the time differential between the observer and the object wasn't just one of perspective, but that time actually does slow down for the object..

Very good. One thing that you can be certain of, no matter who puts forward their answer, only one event occurs.

If I then extrapolate this out to my black hole problem, time actually does slow down for the object approaching the event horizon of a black hole, relative to an outside observer. It's not just an illusion. If we compare clocks, one for the observer, and one for the object, we would find that indeed the object's clock has almost come to a complete stop. The object would percieve that it's still moving, and that its clock is still running normally, but it's not.

There are not more than a single event unless there are more than one universe.

Ok, I'm smart enough to know that there's a flaw in my reasoning, but I don't know what it is. So can someone explain this to me in simple terms that I can understand.

I would greatly appreciate it, because this is bugging the heck out of me.

Thanks

I didn't see a flaw. Yes there are different perspectives, but, the only one that matters is the one involved in the event. The traveler either enters or they do not. If a theory says they both do and do not, then, that theory is not empirically provable.

James

 

[Dale]

Luckily that is not what GR says

 

[James A. Putnam]

Clocks are not causes. Clocks can be any activity. One's clock can do whatever it's environmental conditions cause it to do, but, clocks do not cause what happens at a black hole either for the traveler or any observer, even if there should prove to be such a thing as a black hole. Relativity theory consists of multitudinous numbers of perspectives. However, emprical evidence has to do with the one reality. While it may be the case that boundaries cannot be held to be clearly delineated, one either crosses what appears to be a boundary or they do not. General Relativity concerns itself with local events. Different perspectives are local events. Those perspectives appear to include the whole universe, but, appearances do not create new universes. The one local event that matters is for the traveler who may be attempting to enter a black hole. They either do or they do not. Something that I have written before that remains pertinent: Neither space nor time are represented in physics equations. Distance and duration are represented. Both of these have to do with the activity of objects. Space and time cannot be shown to experience activity.

James

 

[Dale]

Clocks are not causes.

I don't see the relevance, but clocks can cause bombs to explode, ovens to preheat, people to wake up, and many other things.

Different perspectives are local events.

Different perspectives are different coordinate systems or reference frames.

 

[ZealScience]

Ok, although I had hoped to avoid having to ask a stupid question, it seems as though I'm just too dense to figure this one out on my own. Generally I can, with the help of Google, and the combined wisdom of the internet, deduce an answer. But either Google, the internet, or my brain has failed me this time.

So I'm going to ask a stupid question.

How can anything cross the event horizon of a black hole?

Of course I've found what I assume to be the accepted answer to this question on the internet, but it just doesn't make sense to me. If time slows down as an object approaches the event horizon of a black hole, to the point where the object never actually appears to cross said horizon. How can it ever cross it?

The only answer that I've been able to find, is that the object only appears to slow down from the perspective of an outside observer. From the objects own perspective nothing changes. It goes sailing right through the event horizon without barely noticing.

But, I always thought that the time differential between the observer and the object wasn't just one of perspective, but that time actually does slow down for the object. As in the twin paradox where one twin goes off in a FTL spaceship, and then returns months later to find that his twin is many years older. It wasn't that time simply appeared to slow down for the twin in the spaceship. But that time actually did slow down. For the twin in the spaceship, time did slow down relative to his brother. It wasn't just a matter of perspective. Time really, really does slow down.

If I then extrapolate this out to my black hole problem, time actually does slow down for the object approaching the event horizon of a black hole, relative to an outside observer. It's not just an illusion. If we compare clocks, one for the observer, and one for the object, we would find that indeed the object's clock has almost come to a complete stop. The object would percieve that it's still moving, and that its clock is still running normally, but it's not.

Just like the twin paradox, it's not just that time appears to slow down, it really does.

Ok, I'm smart enough to know that there's a flaw in my reasoning, but I don't know what it is. So can someone explain this to me in simple terms that I can understand.

I would greatly appreciate it, because this is bugging the heck out of me.

Thanks

I think you have a little bit misunderstanding the relativity. THe clock is slowing down to the perspective of the twin on earth, but what the space twin is seeing is that clock of the Earth twin is slowing down. We cannot simply say that "we would find something", for we are not defined as any reference frame. Basically, what relativity is saying is these measurements is only valid when we have reference frames. Although the gravitational time dilation is not relative, but it also depends on where you are, which means when you are with the spaceship then the time of the spaceship is the same as yours.

Sorry for possibility of misunderstanding of your question and misinterpretation of relativity, but that's what I've learnt.