Are Black Holes Unfalsifiable and Therefore Not Science?

[Dmitry67]

How long, per an outside observer, does it take collapsing matter to develope an event horizon per the outside observer?

I repeat, your question IS NOT PHYSICAL

You can ask 'when I receive the signal from infalling observer' - but we both agree that these signals are infinitely redshifted

There is no 'when' in GR in curved spacetime for the events in different locations.

'How long' can be defined as 'proper time' for the same observer.

'How long per outside observer' can be defined as:
* proper time of some of the observers when thy finally meet - it is not the case here;
* time calculated based on the arrival of the signals
* time calculated based on some coordinate system. But it depends on the coordinate system and dos not have physical meaning (example: in twin paradox, when one spaceships stops and starts to go back, it has to 'switch' its coordinate system which leads to assymetry and resolves the twin paradox)

 

[Jonathan Scott]

I repeat, your question IS NOT PHYSICAL

You can ask 'when I receive the signal from infalling observer' - but we both agree that these signals are infinitely redshifted

There is no 'when' in GR in curved spacetime for the events in different locations.

'How long' can be defined as 'proper time' for the same observer.

'How long per outside observer' can be defined as:
* proper time of some of the observers when thy finally meet - it is not the case here;
* time calculated based on the arrival of the signals
* time calculated based on some coordinate system. But it depends on the coordinate system and dos not have physical meaning (example: in twin paradox, when one spaceships stops and starts to go back, it has to 'switch' its coordinate system which leads to assymetry and resolves the twin paradox)

I understand your point, but I reckon that's cheating. For example, time rates near a massive object vary with potential, but from the point of view of an observer using a specific background coordinate system (say isotropic) we can use that as a valid model of what "really" happens.

According to almost any coordinate system EXCEPT that of a free-falling observer falling with the material into the forming black hole, such events take an infinite time and therefore do not complete within the lifetime of the universe. From the point of view of a falling observer, things happen in a finite proper time, but any attempt to map that time back to external space-time gets very mixed up, as time and space have swapped roles.

We know from experience with ordinary levels of gravity that this "slowing" of proper time is a physical effect; if you go very close to a black hole and manage to get away again, a lot of extra time will have elapsed. On those grounds, it seems that the infinite delay should be considered an equally physical effect.

So what happens from the point of view of a falling observer and how can this be consistent with the external view? I don't know, and I've not seen any explanation I can believe.

 

[Dmitry67]

1
using a specific background coordinate system (say isotropic) we can use that as a valid model of what "really" happens.

2
According to almost any coordinate system EXCEPT that of a free-falling observer falling with the material into the forming black hole, such events take an infinite time and therefore do not complete within the lifetime of the universe. From the point of view of a falling observer, things happen in a finite proper time, but any attempt to map that time back to external space-time gets very mixed up, as time and space have swapped roles.

1
And by changing that 'specific' coordinate system we can change what 'really' happens :) It is well known that just by walking in different directions in your room you can change what "now" "really" happens in the Adromeda galaxy by several years :)

2
Then you are violating the very central idea of the General Relativity - that ALL coordinate systems are equal in rights. Observers point of view on Earth is not more 'valid' in any sense than the point of view of a free falling observer.

Finally, it is not only "except the free falling observer" but a wide class of falling systems. Not only freely falling: if you resist falling into a black hole, but your engines are not powerful enough, then you are not freely falling but still you reach the singularity in finite proper time.

 

[Phrak]

I repeat, your question IS NOT PHYSICAL

Yes, well, I'll be sure to let Misner, Thorne and Wheeler know your assessment of their NONSENSICAL use of the language.

 

[Dmitry67]

Any arguments except the names?

 

[Jonathan Scott]

1
And by changing that 'specific' coordinate system we can change what 'really' happens :) It is well known that just by walking in different directions in your room you can change what "now" "really" happens in the Adromeda galaxy by several years :)

I disagree; I was talking about sequences of events which may or may not happen. These are the same in all coordinate systems. They can be described consistently in anyone of them. Time, space and directions can of course vary.

2
Then you are violating the very central idea of the General Relativity - that ALL coordinate systems are equal in rights. Observers point of view on Earth is not more 'valid' in any sense than the point of view of a free falling observer.

There's no difference in validity within the range in which the coordinate system is valid, but if part of it corresponds to a non-existent region of the universe, that doesn't necessarily count as valid.

Finally, it is not only "except the free falling observer" but a wide class of falling systems. Not only freely falling: if you resist falling into a black hole, but your engines are not powerful enough, then you are not freely falling but still you reach the singularity in finite proper time.

True, which is why I said "almost".

 

[Dmitry67]

I disagree; I was talking about sequences of events which may or may not happen. These are the same in all coordinate systems. They can be described consistently in anyone of them. Time, space and directions can of course vary.

No, because some observers cross the horizon

Let me explain my point of view in a different way.

"you-now" is a point in spacetime. There are infinitely many spacetime trajectories crossing that line. Each line represents some observer.

Some spacelines never hit the BH: for example, "you-remaining-on-EARTH". Some crosses the horizon: "you-decided-to-go-into-the-BH"

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

But if at least in some coordinate systems crossing "you-now" BH is formed, how can you say "it will never form"?

 

[Jonathan Scott]

No, because some observers cross the horizon

Let me explain my point of view in a different way.

"you-now" is a point in spacetime. There are infinitely many spacetime trajectories crossing that line. Each line represents some observer.

Some spacelines never hit the BH: for example, "you-remaining-on-EARTH". Some crosses the horizon: "you-decided-to-go-into-the-BH"

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

But if at least in some coordinate systems crossing "you-now" BH is formed, how can you say "it will never form"?

The region within the falling observer coordinate system which corresponds to falling past the horizon has no equivalent in the universe, as the time coordinate within the more conventional coordinate systems of its junction with the universe is "at infinity" (and the time coordinate on the other side is space-like).

Basically, as the observer falls, we see his clocks slowing to a halt, although he doesn't notice. This is just like the "stasis chamber" of science fiction. From the point of view of the observer, he doesn't know that he's been frozen in time, and if he is rescued and brought back to life, no time has elapsed. Even though from his point of view everything was normal up to the point where he was frozen, that doesn't necessarily mean that one can extrapolate to the assumption that he will be unfrozen again!

Some people insist that this is "not a problem", and insist that mathematical arguments or numeric simulations have shown that event horizons can form in finite time and things can fall inside them. I am personally yet to be convinced. Of course, in standard black hole theory, once collapse has started, it is inevitable, so the effect is the same anyway as a black hole.

 

[Dmitry67]

There is nothing to be convinced or not. Just look at the diagrams with the lighcones (I provided 4 links). What happens inside and around the BH is very simple. An attempt to think it terms of 'time dilations; leads to many misconceptions.

 

[Jonathan Scott]

There is nothing to be convinced or not. Just look at the diagrams with the lighcones (I provided 4 links). What happens inside and around the BH is very simple. An attempt to think it terms of 'time dilations; leads to many misconceptions.

I'm quite familiar with various different coordinate systems. If you take the falling observer's point of view there does not appear to be any problem with passing the event horizon (and hitting the singularity) in finite time. However, as I explained with the "stasis" example, that does not necessarily mean it's actually possible.

If you consider paths which fall very close to the horizon but then turn round and return, you see that the closer they go, the longer they take. This means that in the limit, as observed externally, the falling observer takes infinite time to cross the horizon, and could still in theory be rescued at any time during the entire future of the universe! So when do they actually fall in?

I find myself somewhat frustrated with books which show that in the falling observer's coordinate system, the horizon coordinate is passed in finite proper time and therefore conclude that the fact that this takes an infinite time as seen by an external observer is somehow just an irrelevant illusion. This isn't just a matter of different a point of view; it's a contradiction, and requires more than hand-waving to explain it.

 

[Dmitry67]

This isn't just a matter of different a point of view; it's a contradiction, and requires more than hand-waving to explain it.

I don't see any contradictions. The problem is that many people are trying to 'map' falling observer time to 'external' time, thinking in terms of

t' = x * t

where x is some variable. Obviously, you get into a problem when x becomes 0 or infinite. But who garanteed you that there is ONE 'river' of time and all times can be 'mapped' into each other?

Talking about the handwaving, what is NOT explained by the spacetime diagrams I provided? Let's talk about the physical things (what is observed, when signals arrive, etc) and avoid non-physical questions (when I am here on Earth, has black hole already formed? etc)

 

[Dmitry67]

If you consider paths which fall very close to the horizon but then turn round and return, you see that the closer they go, the longer they take. This means that in the limit, as observed externally, the falling observer takes infinite time to cross the horizon, and could still in theory be rescued at any time during the entire future of the universe! So when do they actually fall in?

I can ask you a similar question - without black holes.

Spaceship flies toward the Andromeda at very high speed. Then it turns back and returns back to Earth.

The trip took only few years measured by the clock on the spaceship, while on Earth it took millions. So while the austranaut on the spaceship aged only few years, his twin brother on Earth had died a long time ago.

The question you are asking "when actually (on the spaceship clock) the brother on Earth died?" Do you agree that this question is not physical?

 

[chronon]

But who garanteed you that there is ONE 'river' of time and all times can be 'mapped' into each other?

Unfortunately most people tend to think that the different times can be mapped into each other. For instance in the webpage mentioned earlier in the thread http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html#evaporate it says

I won't experience that cataclysm myself, though; I'll be through the horizon, leaving only my light behind.

implying that the light he leaves behind is seen after he has crossed the horizon

 

[Jonathan Scott]

Unfortunately most people tend to think that the different times can be mapped into each other. For instance in the webpage mentioned earlier in the thread http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html#evaporate it says implying that the light he leaves behind is seen after he has crossed the horizon

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe (although of course nothing we know is strong or fast enough to do so)?

I reckon that makes it reasonable to say that he hasn't yet fallen in.

 

[chronon]

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe .

No, after a certain amount of time has passed it will be impossible for light to catch up with him before he crosses the event horizon, and so it would definitely be impossible to send a rescue mission (which would travel slower than light). On the other hand, if he had powerful enough rockets, then he could at decide to turn round at any time before he has crossed the horizon.

 

[Dmitry67]

oops, chronon was faster...

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe (although of course nothing we know is strong or fast enough to do so)?

I reckon that makes it reasonable to say that he hasn't yet fallen in.

No, I don't agree.

for me, the point of no return is at the event horizon.

for you, when I am too close to the horizon it is too late to decide to flight to me to save me: when you approach the BH trying to 'save' me you see how I 'unfreeze' and sink deeper and deeper BEFORE you approach

To simplify, let's say that a simple signal from you can save me: if you send a signal 'please return, I forgive you :)' and I receive it I turn around and return. But if I am too deep inside the black hole then it would take a while for the light signal to cover the distance to the black hole, and it would be too late!

Will you see the universe end?
If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically-- that I'd see the universe end in a spectacular flash as I went through the horizon. This isn't the case, though. What an external observer sees depends on what light does after I emit it. What I see, however, depends on what light does before it gets to me. And there's no way that light from future events far away can get to me. Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time.

observer falling into the BH will not see how the Universe ends, and even won't see any signals sent too late!

 

[Jonathan Scott]

No, after a certain amount of time has passed it will be impossible for light to catch up with him before he crosses the event horizon, and so it would definitely be impossible to send a rescue mission (which would travel slower than light). On the other hand, if he had powerful enough rockets, then he could at decide to turn round at any time before he has crossed the horizon.

I don't remember anything about a time limit from when I studied this area (which was admittedly long ago); I thought that it was simply necessary to get closer and closer to the speed of light to catch up the later you started. Can you quote a specific reference, please?

 

[Dmitry67]

Just take one of the diagrams and draw the worldlines there.

 

[Jonathan Scott]

Just take one of the diagrams and draw the worldlines there.

That's an example of what I mean by handwaving.

What I'd like to see is a bit of maths showing that there's a limit incoming light cone beyond which signals cannot reach the falling observer's geodesic. I don't remember having seen that calculation, but I should be able to work that out myself; it would just be easier if someone could point me to it.

 

[Ich]

What I'd like to see is a bit of maths showing that there's a limit incoming light cone beyond which signals cannot reach the falling observer's geodesic.

Look https://www.physicsforums.com/showthread.php?p=2417483#post2417483". There's a finite redshift at the EH, disproving the "he sees the future of the universe" thing. This could also be a starting point for your calculations.

 

[Chronos]

An observer inside an event horizon would suffer from reverse redshift. He/she would 'see' the universe age at infinite speed according to GR. Draw the light cone.

 

[Dmitry67]

observer inside the horizon reaches signularity in finite time. You mean, he will see everything before he hits singularity? this is not true and I am ready to draw a lightcone.

 

[Chronos]

My error, you are correct. Time will gradually speed up as observer approaches the singularity.

 

[Phrak]

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

Would you then argue just as animatedly against claims the existence of black holes?