Why "even light"?

[Steve81]

50 minutes ago, Genady said:

I think that this demonstration is wrong for a number of reasons:

1. Blanket breaks. Spacetime does not.

2. The depression in the blanket is caused by weight of the object. A black hole is caused by its size.

3. The depression is caused not by the object's gravity, but by the Earth's gravity.

4. The blanket responds to the weight due to its elasticity. The spacetime does not.

...

It’s obviously not a perfect representation. It’s a blanket for crying out loud! But I’ve used that to get the point across in the past.
 

Re: the black hole, that depends on what you mean by size. It’s a matter of mass.
 

Edited August 11, 2023 by Steve81

[Genady]

5 minutes ago, Steve81 said:

Re: the black hole, that depends on what you mean by size. It’s a matter of mass.

Consider a collapsing star of mass M, not a black hole. As it collapses, its size changes, but the mass stays the same. When its radius becomes smaller than 2GM/c², it becomes a black hole. With the same mass.

7 minutes ago, Steve81 said:

to get the point across in the past.

Did a correct point get across?

[Steve81]

5 minutes ago, Genady said:

Consider a collapsing star of mass M, not a black hole. As it collapses, its size changes, but the mass stays the same. When its radius becomes smaller than 2GM/c², it becomes a black hole. With the same mass.

Did a correct point get across?

Perhaps not 😂 But is it still not a consequence of the gravity, and therefore the point essentially remains the same, with a little added info?

Edited August 11, 2023 by Steve81

[Genady]

1 hour ago, Steve81 said:

Perhaps not 😂 But is it still not a consequence of the gravity, and therefore the point essentially remains the same, with a little added info?

Which point? What consequence of gravity?

What is the difference between this demonstration and a demonstration of a weight hanging on a spring? A heavier weight stretches the spring more. What does the blanket demonstration demonstrate that the simple spring does not? 

You put heavier and heavier weights on the spring, and at some point, the spring snaps. Does this demonstrate a black hole? How?

[Steve81]

1 hour ago, Genady said:

Which point? What consequence of gravity?

What is the difference between this demonstration and a demonstration of a weight hanging on a spring? A heavier weight stretches the spring more. What does the blanket demonstration demonstrate that the simple spring does not? 

You put heavier and heavier weights on the spring, and at some point, the spring snaps. Does this demonstrate a black hole? How?

The spring can work to demonstrate the pull of gravity, but the blanket, at least IMHO, is a better visual to represent how gravity curves spacetime. With a large enough weight, the blanket really starts to wrap around the weight (pulling in adjacent objects too, to demonstrate that aspect of gravity). If you disagree, or don’t see the merit, that’s fine. Other people have understood my meaning, and I accept that I won’t please everybody.

Edited August 11, 2023 by Steve81

[phyti]

If light cannot escape a black hole, how can gravitational effects radiate into space at light speed?

 

[Genady]

29 minutes ago, phyti said:

If light cannot escape a black hole, how can gravitational effects radiate into space at light speed?

 

What gravitational effect radiates from black hole at light speed?

1 hour ago, Steve81 said:

With a large enough weight, the blanket really starts to wrap around the weight (pulling in adjacent objects too, to demonstrate that aspect of gravity).

I can agree that it demonstrates that aspect of gravity. My 'displeasure' is that it does not demonstrate black hole.

[Steve81]

28 minutes ago, Genady said:

I can agree that it demonstrates that aspect of gravity. My 'displeasure' is that it does not demonstrate black hole.

It takes a mild leap, if we’re imagining our blanket as an analogue for spacetime, to imagine a weight/gravitational effect so strong that the blanket wraps completely around the weight. The curvature is such that anything trapped inside can’t get out.

[Genady]

23 minutes ago, Steve81 said:

It takes a mild leap, if we’re imagining our blanket as an analogue for spacetime, to imagine a weight/gravitational effect so strong that the blanket wraps completely around the weight. The curvature is such that anything trapped inside can’t get out.

The blanket will not wrap completely around the weight. It will wrap around its lower half and will go straight up from the equator, leaving the upper half open. So, nothing gets trapped inside.

[Steve81]

16 minutes ago, Genady said:

The blanket will not wrap completely around the weight. It will wrap around its lower half and will go straight up from the equator, leaving the upper half open. So, nothing gets trapped inside.

Hence why I said we need to make a mild leap. It’s still just a blanket. 😆

Edited August 11, 2023 by Steve81

[Genady]

30 minutes ago, Steve81 said:

Hence why I said we need to make a mild leap. It’s still just a blanket. 😆

So, you lie to poor children? 😄

[Steve81]

Just now, Genady said:

So, you lie to poor children? 😄

Actually it was a civil engineer I worked with at my previous job 😛

[studiot]

5 minutes ago, Genady said:

So, you lie to poor children? 😄

Yes indeed, but are you not doing the same ?

 

I clearly remember having hammered into me "You cam't take the square root of a negative number"

Then some years later I learned about imaginary and complex numbers.

 

You keep saying 'even light...etc'.

 

But you should be concentrating on the 'black hole'.

Remember that the original black holes - invented by the mid victorians - were introduced as the nearest thing man could make to a 'black body'.

But light  - in the form of IR and microwave - certainly escapes from these. Indeed it has a temperature that can be measured.

So perhaps the 'even light' was meant to distingiuish those original black holes from the populist term coined much later .

 

As to lying to children, saying even light cannot escape,  is not strictly true, just a populist presentation that is nearly true.

For a more accurate, but still childlike, exposition see here

https://www.universetoday.com/152547/we-knew-black-holes-have-a-temperature-it-turns-out-they-also-have-a-pressure/

 

1 hour ago, Steve81 said:

Hence why I said we need to make a mild leap. It’s still just a blanket. 😆

We have had several threads here discussing how poor and inaccurate the trampolite / stretched blanket analogy is for GR.

[Steve81]

22 minutes ago, studiot said:

We have had several threads here discussing how poor and inaccurate the trampolite / stretched blanket analogy is for GR.

Would you mind pointing me in the right direction? I did a search, but there's a great many responses to the query. Also how inaccurate are we talking? Not good enough for a simple visual demonstration inaccurate?

Edited August 11, 2023 by Steve81

[Genady]

39 minutes ago, studiot said:

I clearly remember having hammered into me "You cam't take the square root of a negative number"

Then some years later I learned about imaginary and complex numbers.

Right. I would put in a small print, "as long as we're talking about the black holes of classical general relativity".

 

19 minutes ago, Steve81 said:

Not good enough

Consider this fact for example: the curvature is such that you would NEVER see anything falling into the black hole. You would see the things moving slower and slower and slower as they get closer and closer to the BH, never crossing the event horizon. Can the blanket demonstration demonstrate this?

[Steve81]

6 minutes ago, Genady said:

Consider this fact for example: the curvature is such that you would NEVER see anything falling into the black hole. You would see the things moving slower and slower and slower as they get closer and closer to the BH, never crossing the event horizon. Can the blanket demonstration demonstrate this?

No, of course not; on the other hand, I'm not attempting to demonstrate/communicate the full complexity of the black hole either. I'm trying to explain why "even" light can't escape, since the "simple" definition of gravity as a force shouldn't apply to massless particles. I've watched Interstellar, so I'm familiar with the time dilation effects 😆

Edited August 11, 2023 by Steve81

[studiot]

30 minutes ago, Steve81 said:

Would you mind pointing me in the right direction? I did a search, but there's a great many responses to the query. Also how inaccurate are we talking? Not good enough for a simple visual demonstration inaccurate?

Simply put the trampoline is a 2D membrane that is distorted in the third dimension by a force acting in that dimension.

That's all fine and dandy, but we live in a 3D universe - as far as we can tell there is no 4th dimension - but scaling up the 'trampoline' would require a 4th dim.

 

There are proper astrophysicists here that can offer multicolour explanations @Janus?

 

 

[Steve81]

6 minutes ago, studiot said:

Simply put the trampoline is a 2D membrane that is distorted in the third dimension by a force acting in that dimension.

That's all fine and dandy, but we live in a 3D universe - as far as we can tell there is no 4th dimension - but scaling up the 'trampoline' would require a 4th dim.

 

There are proper astrophysicists here that can offer multicolour explanations @Janus?

 

 

FWIW, what I'd be looking for is a superior, but understandable by "average" people like myself, way to demonstrate why escaping from a black hole is...problematic. Knowledge is power, and I'm always ready to learn. On the other hand, if one can't effectively share that knowledge with others, it loses value.

Edited August 11, 2023 by Steve81

[studiot]

18 minutes ago, Steve81 said:

FWIW, what I'd be looking for is a superior, but understandable by "average" people like myself, way to demonstrate why escaping from a black hole is...problematic. Knowledge is power, but if you can't effectively share it with others, it loses value.

Here is some useful beginning information from this book which makes good bedtime reading.

Peter Collier didn't understand it either so he taught himself from the ground up and wrote a book about it.

hint Click on the image to get the full size

 

[Steve81]

Thanks! I’ll check it out when I get home. I’d give you an upvote, but I ran out of reactions to give! Apparently this site has an issue with positive feedback  😆

[Genady]

1 hour ago, Steve81 said:

what I'd be looking for is a superior, but understandable by "average" people like myself, way to demonstrate why escaping from a black hole is...problematic.

Here is how the masters explained it, from Misner, Thorne, Wheeler, Gravitation, p. 823.

For clarification, r=0 is the BH center, r=2M is the BH event horizon.

[StringJunky]

20 minutes ago, Genady said:

Here is how the masters explained it, from Misner, Thorne, Wheeler, Gravitation, p. 823.

For clarification, r=0 is the BH center, r=2M is the BH event horizon.

So, one can't get out of a BH because time-reversal is not allowed? is that the basic upshot of that?

Edited August 11, 2023 by StringJunky

[Genady]

1 minute ago, StringJunky said:

So, one can't get out of a BH because time-reversal is not allowed?

Correct.

[StringJunky]

1 minute ago, Genady said:

Correct.

Is this principle universal and not just confined to BHs? I mean is time-reversal not allowed, full-stop?

Edited August 11, 2023 by StringJunky

[Genady]

Just now, StringJunky said:

Is this principle universal and not just confined to BHs?

You mean about the time reversal? If so, yes. It is a universal causality relation between events.