# Why "even light"?

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

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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.

Nice! I suspect I’ll still have to use the blanket for a simple explanation of the curvature caused by gravity, but that additional info is fascinating, and a good addition to the explanation.

1 hour ago, studiot said:

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.

Just picked it up on Amazon.

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17 hours 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?

It's a common misconception that you'd need a "higher" dimension (four spatial dimensions) for a a 3D universe to "curve into".  All you need is a universe that does not adhere to Euclidean rules of geometry, but is rather non-Euclidean.

While non-Euclidean geometry is often modeled as a plane projected onto a surface of a sphere or other 3D object, that's not what is "really" happening. This is just a way for us to more easily visualize Non-Euclidean geometry.

Here is an animation I did that gives a way to visualize the trampoline example in 3D:

That being said, we do live in a 4D universe. Space-time is three spatial and one time dimension.

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14 minutes ago, Janus said:

Here is an animation I did that gives a way to visualize the trampoline example in 3D:

That being said, we do live in a 4D universe. Space-time is three spatial and one time dimension.

Seems a great deal like my blanket and weights idea 😀

Edited by Steve81
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LIGO detects gravitational waves, since 2016.

Gravitational waves are very low frequencies < 7000 Hz, well below the visible range.

Thus the waves are detectable while the black hole is not visible.

The statement ‘even light can’t escape’ is referring to the range detectable by humans.

Pound-Rebka did an experiment demonstrating light (em radiation) gains energy moving in the direction of a g-field, and loses energy moving in the opposite direction. This is in agreement with gravitational red shift.

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31 minutes ago, Janus said:

It's a common misconception that you'd need a "higher" dimension (four spatial dimensions) for a a 3D universe to "curve into".  All you need is a universe that does not adhere to Euclidean rules of geometry, but is rather non-Euclidean.

While non-Euclidean geometry is often modeled as a plane projected onto a surface of a sphere or other 3D object, that's not what is "really" happening. This is just a way for us to more easily visualize Non-Euclidean geometry.

Here is an animation I did that gives a way to visualize the trampoline example in 3D:

That being said, we do live in a 4D universe. Space-time is three spatial and one time dimension.

18 minutes ago, Steve81 said:

Seems a great deal like my blanket and weights idea 😀

Please be aware that I was referring very specifically to the trampoline analogy when I talked about an extra dimension. The dimension is in no way "higher" than the others by the way, as they are all supposedly equivalent. For the trampoline the extra dimension is required.

It is worth noting at this point that gravity is a distributed function or effect with a possibly different value at every point in the space concerned.
This is why in the Newtonian model it is called a "body force" as opposed to the line segment vectors of contact forces.

It is possible to modify the trampoline model by introducing a variable stiffness function, and perhaps extending it to 3 dimensions like a sponge, which would then produce the effects you show at the expense of considerable mathematical complexity. This could reproduce the animation shown.

This is reminiscent of the efforts scientists made in the past to preserve the heliocentric model of the solar system by introducing cycle upon cycle to account for the observed movements.

None of this gets away from the problem I stated earlier. The trampoline model, being a purely mechanical force model, requires that extra dimension.
It is well known that the vector product in 2 dimensions automatically takes into the third known dimension.
But there is no known corresponding effect in 3 dimensions that requires a 4th to operate. Rotations and other vector products are 'closed ' is three dimensions and only produce all their known effect within that 3D space.

That is why, in my view, the trampoline model is counterproductive.

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10 minutes ago, studiot said:

That is why, in my view, the trampoline model is counterproductive.

While imperfect, the trampoline model can be demonstrated to an elementary school student. Don’t let perfection be the enemy of good. This is the kind of stuff that will get kids interested in science in the first place, and want to learn more.

Edited by Steve81
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35 minutes ago, phyti said:

LIGO detects gravitational waves, since 2016.

Gravitational waves are very low frequencies < 7000 Hz, well below the visible range.

Thus the waves are detectable while the black hole is not visible.

The statement ‘even light can’t escape’ is referring to the range detectable by humans.

Pound-Rebka did an experiment demonstrating light (em radiation) gains energy moving in the direction of a g-field, and loses energy moving in the opposite direction. This is in agreement with gravitational red shift.

The detected gravitational waves were produced when two stars, black holes or not, collided. The waves radiated from the collision area. They did not radiate from a black hole. After the collision process has ended and resulting black hole has settled, no gravitational waves come out of there.

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56 minutes ago, Steve81 said:

Seems a great deal like my blanket and weights idea 😀

I think it's better to stick to "rubber sheet" instead of blanket, because the latter doesn't suggest as well that stretching is involved. If there's no stretching, there's only extrinsic curvature, and the analogy isn't as good.

I think you could probably even demonstrate intrinsic curvature using a stretched rubber sheet without requiring extra dimensions.

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11 minutes ago, Steve81 said:

This is the kind of stuff that will get kids interested in science in the first place, and want to learn more.

Do you have evidence to support this?

42 minutes ago, phyti said:

The statement ‘even light can’t escape’ is referring to the range detectable by humans.

No, it is referring to all light and to everything else.

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Do you have evidence to support this?

Are you seriously demanding evidence that performing science demonstrations / experiments for kids will tend to increase their interest in science? Buzz off.

8 minutes ago, md65536 said:

I think it's better to stick to "rubber sheet" instead of blanket, because the latter doesn't suggest as well that stretching is involved. If there's no stretching, there's only extrinsic curvature, and the analogy isn't as good.

I think you could probably even demonstrate intrinsic curvature using a stretched rubber sheet without requiring extra dimensions.

Thanks for the thought!

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1 minute ago, Steve81 said:

Are you seriously demanding evidence that performing science demonstrations / experiments for kids will tend to increase their interest in science? Buzz off.

Yes. I think that such demonstration, as well as visiting art exhibits, music concerts, ballet performances, travel to different countries, etc., broaden their horizons. I don't think that they specifically increase their interest in science.

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Yes. I think that such demonstration, as well as visiting art exhibits, music concerts, ballet performances, travel to different countries, etc., broaden their horizons. I don't think that they specifically increase their interest in science.

On an individual basis, perhaps not. In the aggregate, some kids are going to find science experiments pretty cool, and want to learn more. If that’s something you can’t agree with, clearly you don’t know many kids.

Edited by Steve81
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2 minutes ago, Steve81 said:

On an individual basis, perhaps not. In the aggregate, some kids are going to find science experiments pretty cool, and want to learn more.

Agree. For this statement, I will not ask for evidence.

PS. I've replied before seeing you last statement. But have answered it anyway.

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50 minutes ago, Steve81 said:

While imperfect, the trampoline model can be demonstrated to an elementary school student. Don’t let perfection be the enemy of good. This is the kind of stuff that will get kids interested in science in the first place, and want to learn more.

Not at all sure who your target audience is here.

Are you asking 'kids' to think in the abstract, when they have not encountered enough concrete examples to compare with ?

The most motivational thing I saw was starting Physics with a bunch of other 12/13 year olds in a coeducational school.

The (lady) teacher's topic was 'Forces'.

Please pause for a moment and ask yourself how you would approach this for a class of such kids ?

before you look at the spoiler.

Spoiler

The teacher produced an ordinary spring balance for weighing sacks of spuds or suitcases.
She passed this around and invited a class competition to see who could pull it out to the greatest reading.

Then she started to tell us about forces.

I'm sure you can imagine the competition amongst the boys to 'show off'

Edited by studiot
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16 minutes ago, studiot said:

Not at all sure who your target audience is here.

Are you asking 'kids' to think in the abstract, when they have not encountered enough concrete examples to compare with ?

The most motivational thing I saw was starting Physics with a bunch of other 12/13 year olds in a coeducational school.

The (lady) teacher's topic was 'Forces'.

Please pause for a moment and ask yourself how you would approach this for a class of such kids ?

before you look at the spoiler.

Reveal hidden contents

The teacher produced an ordinary spring balance for weighing sacks of spuds or suitcases.
She passed this around and invited a class competition to see who could pull it out to the greatest reading.

Then she started to tell us about forces.

I'm sure you can imagine the competition amongst the boys to 'show off'

Target audience is basically anyone without a background in science, but interested in learning something new. I'm not a teacher by trade of course, though my wife is. See my avatar for more clarification on how I prefer to operate 😛

Nice experiment btw!

Agree. For this statement, I will not ask for evidence.

PS. I've replied before seeing you last statement. But have answered it anyway.

My apologies for getting snippy, but it's irritating when someone seems to be disingenuous with their replies. I never said every child would be interested.

Edited by Steve81
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1 hour ago, Steve81 said:

Are you seriously demanding evidence that performing science demonstrations / experiments for kids will tend to increase their interest in science? Buzz off.

Hey! What the hell? You don't get to talk to people that way here. Civility is our #1 rule.

And FYI, lots of pop-sci mangling of explanations have been very detrimental. You argue that it makes science more accessible to kids, but many popular science articles are extremely misleading. I think it's MUCH better to teach correct science, even if it's harder, than to dumb it down to the point where it's causing more problems (like the rubber sheet analogy).

1 hour ago, Steve81 said:

On an individual basis, perhaps not. In the aggregate, some kids are going to find science experiments pretty cool, and want to learn more. If that’s something you can’t agree with, clearly you don’t know many kids.

Please don't use Strawman arguments here. Nobody, NOBODY claimed kids don't find experiments cool, and I think you know that.

57 minutes ago, Steve81 said:

My apologies for getting snippy, but it's irritating when someone seems to be disingenuous with their replies. I never said every child would be interested.

You need to get used to people asking for clarity and citations. In science, we want to use the correct terminology, definitions, measurements, and calculations always. Many times, responses are digging deeper into what you claimed so we can either agree or disagree with you. Saying nothing is like tacit agreement, so you may get pushback on some assertions until we better understand what you're saying.

We attack ideas here, but never people. This is scientific rigor, or what passes for it on a free science discussion forum moderated by volunteers.

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23 minutes ago, Phi for All said:

Please don't use Strawman arguments here. Nobody, NOBODY claimed kids don't find experiments cool, and I think you know that.

Did you read his post? He's asking for evidence to support a pretty easy to read statement that anyone with a basic grasp of the English language can understand. That's being deliberately obtuse, and if that's the kind of behavior you wish to defend as a moderator, I'm done here.

Edited by Steve81
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1 minute ago, Steve81 said:

Did you read his post? He's asking for evidence to support a pretty easy to read statement that anyone with a basic grasp of the English language can understand. That's being deliberately obtuse, and if that's the kind of behavior your wish to defend as a moderator, I'm done here.

I did read Genady's post. He asked for references for a statement you made. Why do you find that odd? You claimed that the trampoline model for gravity is the kind of thing that will get young students interested, and Genady asked for maybe a link to a study or article where some rigor was put into the statement. Do you think it's bad to be skeptical of things we seem to take for granted?

There's no evidence that he's being "deliberately obtuse", and plenty of evidence that he doesn't necessarily agree with you. He might be asking for a citation for your statement in order to get you to think harder about it or question it.

Instead of guessing at people's motivations, perhaps just answering their questions might be a good strategy. You may think your statement is easy to understand, but you thought of it, so it makes perfect sense to you. Genady has a very good grasp of English (and a few other languages), so I'm sure they'll be happy to discuss this with you if you can avoid disparagement.

FWIW, moderators here don't usually moderate threads they're involved in. All my statements have been made as a member.

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17 minutes ago, Phi for All said:

I did read Genady's post. He asked for references for a statement you made. Why do you find that odd? You claimed that the trampoline model for gravity is the kind of thing that will get young students interested, and Genady asked for maybe a link to a study or article where some rigor was put into the statement. Do you think it's bad to be skeptical of things we seem to take for granted?

He wasn't questioning the specific experiment; if he was, he did so poorly by his choice of text to quote. In any case, his follow-up indicated, he questioned that science experiments at all would get kids "interested". All this because of a faulty, and utterly ridiculous inference that I meant every child would be interested in science as a result of observing science experiments.

Quote

FWIW, moderators here don't usually moderate threads they're involved in. All my statements have been made as a member.

A difference without a distinction, sir.

49 minutes ago, Phi for All said:

And FYI, lots of pop-sci mangling of explanations have been very detrimental. You argue that it makes science more accessible to kids, but many popular science articles are extremely misleading. I think it's MUCH better to teach correct science, even if it's harder, than to dumb it down to the point where it's causing more problems (like the rubber sheet analogy).

That's one opinion, but in the real world we teach children things that are grossly oversimplified all the time, just to instill a basic understanding of a topic. Just how accurate do you think elementary school history lessons are, exactly?

Edited by Steve81
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Can this latest exchange be split off this thread, please? I am asking as the OP of the thread and as one who is referred as "he" in this exchange. It is OT anyway, isn't it?

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2 hours ago, Steve81 said:

This is the kind of stuff that will get kids interested in science in the first place, and want to learn more.

The only reason I didn't ask for a citation was that @Genady beat me to it. I wouldn't be surprised if it was true but it sounds like one of those things people say because everyone else says it. Sort of like "If you make the work environment more fun your workers will be more productive!" Except that is not true.

After all, how do you know that kids get interested because of fun experiments, rather than kids interested in science already will like the fun experiments?

If you don't like being questioned about your assertions you are going to have a rough go of it here.

Edited by zapatos
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7 minutes ago, zapatos said:

The only reason I didn't ask for a citation was that @Genady beat me to it. I wouldn't be surprised if it was true but it sounds like one of those things people say because everyone else says it. Sort of like "If you make the work environment more fun your workers will be more productive!" Except that is not true.

If you don't like being questioned about your assertions you are going to have a rough go of it here.

Do you accept my revised statement that Genady agreed with as self evident, or would you still prefer I try and find a study someone did to prove my point?

I don't mind being questioned / challenged, but it seems ridiculous to ask for evidence of such a statement. You can disagree, that's fine. But I'm not spending hours trying to dig out a research paper to prove something so basic on a forum.

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1 minute ago, Steve81 said:

Do you accept my revised statement that Genady agreed with as self evident, or would you still prefer I try and find a study someone did to prove my point?

I'd like to see a study please.

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3 minutes ago, zapatos said:

I'd like to see a study please.

There's a handy tool, it's called Google. Find it yourself. If you find evidence to the contrary, feel free to post it, and I will retract my statement.

Edited by Steve81

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