Astrophysicist Neil DeGrasse Tyson. Death by Black Hole

[jowjowman]

Listen to Neil DeGrasse Tyson, author of Death by Black Hole and Other Cosmic Quandaries.
Tells the entertaining story of what it's like to be sucked into a black hole.

http://fora.tv/2008/02/19/Neil_DeGrasse_Tyson_Death_by_Black_Hole"

 

[humanino]

https://www.youtube.com/watch?v=YotBtibsuh0

http://video.google.com/videoplay?docid=6473201666872695446

 

[mgb_phys]

Thank you - I've never heard this guy before, great speaker.

 

[physics girl phd]

I've always said the LHC would be much more interesting than global warming. Can't wait til they fix it up again and turn it on FULL power.

 

[Garth]

I've always said the LHC would be much more interesting than global warming. Can't wait til they fix it up again and turn it on FULL power.

Somebody has done it already - in their garage!

Garth

 

[Crazy Tosser]

!
No you don't get squeezed, just like when you are traveling at relativistic speeds you don't get shrinked... But outer observers will see your width get smaller... And you will probably also see your feet as not as wide.

Unless I understand relativity wrong, which I don't think I do.

 

[mgb_phys]

I think the main concern is the tidal forces, if the attraction at your feet is a few kN greater than the attraction at your head bad things are going to happen - in any frame of reference!

 

[humanino]

Unless I understand relativity wrong, which I don't think I do.

No offense I hope, but it is likely that this guy Neil understand it better. The word is "spaghettification".

 

[Crazy Tosser]

No offense I hope, but it is likely that this guy Neil understand it better. The word is "spaghettification".

Well, I don't think that just because he is on TV, he necessarily understands it better than me, allright?

Now, from your frame of reference (it's hard to talk about FoR's on a molecular level, but let's assume that the center of your brain will be your frame of reference), the space and time immediately around your frame is always the same. Now, all objects "below" you (closer to the black hole) might appear smaller in width, and objects "higher" than you (farther from the black hole) might appear bigger in width, however you will not FEEL anything, because from the frames of reference of the molecules in your nerve cells, nothing is happening directly to THEM, so it is pointless to say that there will be any "compressing force". It's like looking at everything around you through a lens, nothing really gets distorted or displaced.

 

[mgb_phys]

You are correct for a point source.
However if your head is accelerating at 1000m/s² and your feet are accelrating at 1010m/s² then one second later you are going to look very long, silly and dead, even in your own frame of reference.

Well, I don't think that just because he is on TV, he necessarily understands it better than me, allright?

He has a Ba from Harvard, an MA from Texas and a PhD from Columbia

 

[Stingray]

Now, from your frame of reference (it's hard to talk about FoR's on a molecular level, but let's assume that the center of your brain will be your frame of reference), the space and time immediately around your frame is always the same. Now, all objects "below" you (closer to the black hole) might appear smaller in width, and objects "higher" than you (farther from the black hole) might appear bigger in width, however you will not FEEL anything, because from the frames of reference of the molecules in your nerve cells, nothing is happening directly to THEM, so it is pointless to say that there will be any "compressing force". It's like looking at everything around you through a lens, nothing really gets distorted or displaced.

This is not a coordinate effect. Solid objects falling into black holes will experience significant internal stresses. That can be discussed without any mention of reference frames if you like. It's qualitatively the same in Newtonian gravity. Tidal forces pull on one axis and squish the other two. It's just that that effect is rather negligible on earth.

 

[Crazy Tosser]

This is not a coordinate effect. Solid objects falling into black holes will experience significant internal stresses. That can be discussed without any mention of reference frames if you like. It's qualitatively the same in Newtonian gravity. Tidal forces pull on one axis and squish the other two. It's just that that effect is rather negligible on earth.

I agree that they experience internal stress because the black hole attracts everything to it's center, so your left and right shoulders are going to experience acceleration towards each other. However, this internal stress doesn't occur because "space shrinks" or as he put it:

"Space you occupy right here [arms up] is larger than space you occupy right here [arms down]" [4:10]

All I wanted to point up is that relative "shrinkage" of space does not in any way cause the "squeeze" effect that he talks about in [4:27], but the fact that the gravitational forces are not parallel. And you definitely do not get "extruded through the fabric of space" [4:32]. Basically, despite all his BAs, MAs, and PhDs he needs a better understanding of what space really is. It's not something absolute and physical that can squeeze you or interact with you, it's simply what you exist in and what, to a certain degree, controls your motion.

 

[Stingray]

Basically, despite all his BAs, MAs, and PhDs he needs a better understanding of what space is really is. It's not something absolute and physical that can squeeze you or interact with you, it's simply what you exist in. No object's space-time can somehow influence it relative to itself.

While I'll agree that his explanation is imprecise, he's just trying to give a basic idea. There's much much worse that's been said in the name of "popular science."

That said, spacetime certainly does interact with matter. That's really the whole point of relativity. There are absolute properties associated with a particular geometry, and these are what induce stresses in falling bodies.

 

[Ivan Seeking]

I agree that they experience internal stress because the black hole attracts everything to it's center, so your left and right shoulders are going to experience acceleration towards each other. However, this internal stress doesn't occur because "space shrinks" or as he put it:

"Space you occupy right here [arms up] is larger than space you occupy right here [arms down]" [4:10]

All I wanted to point up is that relative "shrinkage" of space does not in any way cause the "squeeze" effect that he talks about in [4:27], but the fact that the gravitational forces are not parallel. And you definitely do not get "extruded through the fabric of space" [4:32]. Basically, despite all his BAs, MAs, and PhDs he needs a better understanding of what space really is. It's not something absolute and physical that can squeeze you or interact with you, it's simply what you exist in and what, to a certain degree, controls your motion.

What is gravity?

 

[humanino]

Well, I don't think that just because he is on TV, he necessarily understands it better than me, allright?

All right, that's true. Except that, in the case at hand, I'm quite sure

get information before stating you know

 

[humanino]

What is gravity?

According to my online directory

- graveness: a manner that is serious and solemn
- a solemn and dignified feeling

According to some guy named Richard Feynman, it's also an interaction resulting from the exchange of massless spin-2 particles.
But this guy is dead.

 

[Ivan Seeking]

According to some guy named Richard Feynman, it's also an interaction resulting from the exchange of massless spin-2 particles.

Show me one and I'll buy you a beer.

 

[humanino]

Show me one and I'll buy you a beer.

I'm not sure [itex]g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}[/itex] with [itex]g[/itex] the metric, [itex]\eta[/itex] the Minkowski metric and [itex]h[/itex] the graviton field will do since

• this does not allow coupling to fermions
• this probably does not qualify as "shown"

so I'll try

found in this web page

 

[Crazy Tosser]

What is gravity?

That said, spacetime certainly does interact with matter. That's really the whole point of relativity. There are absolute properties associated with a particular geometry, and these are what induce stresses in falling bodies.

Equivalence of gravity and acceleration was explained by Einstein in General Relativity. From that equivalence he was able to explain to explain gravity as a distortion of our world's geometry in a 4-Dimensional space-time.

From any object's frame of reference, it is at absolute at rest. That is the most basic postulate of relativity. That means that space-time curvature immediately at and around the object, from that object's frame of reference is zero, because the shortest path it's taking through space-time is "straight up" - through time. The illusion that space-time somehow interacts with matter is simply due to the fact that you are not a "rigid body", but are made up of millions upon millions of tiny "rigid" bodies - atoms (I will not say molecules, because sooner or later the intermolecular forces will be overpowered by black hole's gravity). From the frame of reference of each atom in your body, the others are taking the shortest paths in space-time and overall, relative to each other, they are all moving apart from each other "vertically" - what we call the vertical tidal forces, and are moving closer to each other "horizontally" - horizontal tidal forces. These relative movements have absolutely nothing to do with the "space-time fabric" (I doubt there is such a thing) itself somehow interacting with objects.

On a finishing note, I'd like to say that, humanino, accepting every word he says as the absolute truth just because Wikipedia said blah blah and blah about him, doesn't speak positively about your ability to think logically.

 

[Stingray]

From any object's frame of reference, it is at absolute at rest. That is the most basic postulate of relativity. That means that space-time curvature immediately at and around the object, from that object's frame of reference is zero, because the shortest path it's taking through space-time is "straight up" - through time.

Your first statement follows trivially from definitions. It's not a postulate of relativity. Your second claim is not correct at all. Curvature (measured by the Riemann tensor, for example) means something very different from what you think. It cannot be transformed away in any reference frame. If it's nonzero in one frame, it's nonzero in all of them. The reverse is also true.

Different parts of an object also do not usually move on geodesics. In some sense, they "try" to, and deviations require stresses to be developed. I do not see how something capable of dismembering you is not an interaction. Even discounting internal tidal forces, gravity has plenty of observable consequences. What makes (say) electromagnetic effects any different?

 

[tribdog]

picture a funnel. If I shove you into the funnel you are going to feel squeezed no matter what else is going on.

 

[Ivan Seeking]

so I'll try

found in this web page

Wow, Feynman was right!

 

[humanino]

accepting every word he says as the absolute truth just because Wikipedia said

Sometimes you are right, yes : that's not why I agree with him. In fact, first I looked at one of his talks. It was informative and entertaining. So I looked at another of his talk, and it was again of very good quality. Then I looked at his biography. Once again, it is a well known and elementary fact that tidal forces will tear your body apart if you fall in a black hole.

 

[humanino]

Wow, Feynman was right!

Surely, you must be joking Ivan !

 

[Ivan Seeking]

Surely, you must be joking Ivan !

Hey, I was just following your lead. What is the meaning of it all?

 

[humanino]

I was trying to remember what Dirac told Feynman when they met. Something like "I have an equation by my name. Do you have one ?". Anybody remembers ?

What is the meaning of it all?

I'm sure you are not young enough to have forgotten. It is 42.

 

[George Jones]

I was trying to remember what Dirac told Feynman when they met. Something like "I have an equation by my name. Do you have one ?". Anybody remembers ?

I think you mean an interation in which the normally irrepressible Feynman was so in awe of meeting Dirac that Feynman was tongue-tied, and the normally taciturn Dirac had to try and draw Feynman out.

http://books.google.ca/books?id=WuO7FO44NyoC&pg=PA535&lpg=PA535&dq=dirac+feynman+sat&source=web&ots=heH35kyqks&sig=Kkk2bKXZF9YNRa-aWXQiy3Ta3-8&hl=en&sa=X&oi=book_result&resnum=1&ct=result

 

[humanino]

I think you mean an interation in which the normally irrepressible Feynman was so in awe of meeting Dirac that Feynman was tongue-tied, and the normally taciturn Dirac had to try and draw Feynman out.

http://books.google.ca/books?id=WuO7FO44NyoC&pg=PA535&lpg=PA535&dq=dirac+feynman+sat&source=web&ots=heH35kyqks&sig=Kkk2bKXZF9YNRa-aWXQiy3Ta3-8&hl=en&sa=X&oi=book_result&resnum=1&ct=result

Thanks George !

 

[Crazy Tosser]

picture a funnel. If I shove you into the funnel you are going to feel squeezed no matter what else is going on.

Your first statement follows trivially from definitions. It's not a postulate of relativity. Your second claim is not correct at all. Curvature (measured by the Riemann tensor, for example) means something very different from what you think. It cannot be transformed away in any reference frame. If it's nonzero in one frame, it's nonzero in all of them. The reverse is also true.

Different parts of an object also do not usually move on geodesics. In some sense, they "try" to, and deviations require stresses to be developed. I do not see how something capable of dismembering you is not an interaction. Even discounting internal tidal forces, gravity has plenty of observable consequences. What makes (say) electromagnetic effects any different?

Maybe I was a bit confusing and long on some parts, maybe some of my definitions differ from yours. So I will try to summarize and forget about this thread.

Relativistically, any object occupies the same amount of space relative to itself. Just because it occupies a different amount of space relative to other objects doesn't make it "compressed".

Curved space-time (which is not a funnel, but distorted geometry of our world) makes particles of an object accelerate into each other. That creates pressure. However, space-time itself doesn't "wrap" around anything or "squeeze" anything.

Voila.
~Tosser

 

[Ivan Seeking]

What force vectors are operating on a body due to the center of mass of the bh?

 

[humanino]

I will try to summarize and forget about this thread.

Forget this thread all you want, I don't want other people to visit PF and read wrong scientific facts on it.

any object occupies the same amount of space relative to itself

Locally yes, you are right. But your body does not occupy a single point in space. Just think that the gravitational field becomes so intense, that your feet accelerate differently from your head ! That should be scary enough. Eventually, according to classical relativity, even atoms and nucleons are torn apart.

Voila.

Voila rien, c'est d'la merde.

 

[Ivan Seeking]

I'm sure you are not young enough to have forgotten. It is 42.

Darn! I was thinking of 43. Oh well.

Since you have shown that there is at least one Graviton, you will have to come over if you want your beer.

 

[humanino]

Since you have shown that there is at least one Graviton, you will have to come over if you want your beer.

Or organize the next PF gathering at my place

Thank you for the invitation ! Since I always want beer, does it mean that, unless I organize this PF gathering in a reasonable amount of time, I must come or I will contradict myself ?

 

[Crazy Tosser]

Forget this thread all you want, I don't want other people to visit PF and read wrong scientific facts on it.
Locally yes, you are right. But your body does not occupy a single point in space. Just think that the gravitational field becomes so intense, that your feet accelerate differently from your head ! That should be scary enough. Eventually, according to classical relativity, even atoms and nucleons are torn apart.
Voila rien, c'est d'la merde.

It's ok. Famous "astrophysicists" talking about things they do not know or understand on TV is much scarier.

The concern I was addressing is not the tidal forces, but once again (sigh) statements he made between [4:10] and [4:40] - that an object "occupies a different amount of space" and that causes it to be "squeezed through the "fabric" (ain't quotes big enough or red enough) of space". If you agree with that statement, it's your problem, General Relativity doesn't.

 

[Stingray]

I don't see anything wrong with talking about being "squeezed through the fabric of spacetime." It's colorful language, but he wasn't giving a technical talk. Even if he was, there really isn't anything wrong with it. There's no contradiction with GR.

The bit about occupying different amounts of space makes a lot less sense, so I would perhaps agree with that criticism. Your attempted corrections don't really improve the situation, however.