# Astrophysicist Neil DeGrasse Tyson. Death by Black Hole

1. Jan 7, 2009

### 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

2. Jan 7, 2009

3. Jan 7, 2009

### mgb_phys

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

4. Jan 7, 2009

### 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.

5. Jan 7, 2009

### Garth

Somebody has done it already - in their garage!

Garth

6. Jan 7, 2009

### 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.

7. Jan 7, 2009

### 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!

8. Jan 7, 2009

### humanino

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

9. Jan 7, 2009

### Crazy Tosser

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.

10. Jan 7, 2009

### 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.

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

11. Jan 7, 2009

### Stingray

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.

12. Jan 7, 2009

### Crazy Tosser

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.

Last edited: Jan 7, 2009
13. Jan 7, 2009

### Stingray

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.

14. Jan 7, 2009

### Ivan Seeking

Staff Emeritus
What is gravity?

15. Jan 8, 2009

### humanino

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

get information before stating you know

16. Jan 8, 2009

### humanino

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

17. Jan 8, 2009

### Ivan Seeking

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

18. Jan 8, 2009

### humanino

I'm not sure $g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}$ with $g$ the metric, $\eta$ the Minkowski metric and $h$ 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

19. Jan 8, 2009

### Crazy Tosser

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.

20. Jan 8, 2009

### Stingray

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?