Are Black Holes Unfalsifiable and Therefore Not Science?

[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?