PDA

View Full Version : Is spaghettification really possible?



tommac
2009-Mar-18, 02:26 PM
Is spaghettification really possible? My random thought of the day driving into work was about spagettification. It seems to make sense that at one point the gravitational difference between my legs and my head are so great that I would be stretch or rather ripped apart. However ... my counter thought was that there is also a great time dilution countering the gravitational force. So the clock that I strapped to my foot is running MUCH slower than the Clock that I hold above my head. Right?

If my feet are running slower ( and eventually time stops ) ... I should see the clock at my feet stop. Then shouldnt that not only counter spagettification but actually cause pancakification?

phunk
2009-Mar-18, 03:40 PM
I haven't done the math yet to back this up, but I'm fairly sure the gravitational gradient is much larger than the time gradient.

nauthiz
2009-Mar-18, 03:54 PM
Qualitatively, the fact that high tide happens when the moon is overhead rather than a bit below the horizon seems like a rather large mark against pancakification.

Phunk's right, though, this is something that would be best figured out by grinding through the math. I don't know enough to do that, but I'm inclined to place my trust in the people who do. They all seem to be pretty unanimous on spaghettification being possible.

tommac
2009-Mar-18, 05:18 PM
Yes but the moon over the tides has very little effect on time dilation right?

Also at the EH space is compressed to 0 thickness. So at one point you would see your feet as being flat ( thus pancaked ) and stuck in time.

nauthiz
2009-Mar-18, 05:28 PM
Also at the EH space is compressed to 0 thickness.

Come again?

grant hutchison
2009-Mar-18, 05:56 PM
Also at the EH space is compressed to 0 thickness. So at one point you would see your feet as being flat ( thus pancaked ) and stuck in time.You're confusing the view of a distant stationary observer (the "Schwarzschild observer") with a local observer. Under GR a local observer (falling or stationary) doesn't experience any such thing.

Grant Hutchison

tommac
2009-Mar-18, 06:02 PM
You're confusing the view of a distant stationary observer (the "Schwarzschild observer") with a local observer. Under GR a local observer (falling or stationary) doesn't experience any such thing.

Grant Hutchison

Grant, I am not confusing. The point here is that the head of a person falling feet first into an EH is the external observer ... so the head is watching the feet get pancakafied. Note that the head and the feet although connected have differnt levels of time dilation.

So as I fall into a EH whose gravtiational pull is strong enough to cause spagettification in the first place would also cause to have a different warpage of space-time at my head than my feet.

tommac
2009-Mar-18, 06:04 PM
Come again?

At the EH of a black hole space compresses to infinity ( in the direction of gravity ) and time stops. As my feet get close to the BH the density of my feet will approach infinity.

korjik
2009-Mar-18, 06:11 PM
Time stops for a distant observer. Six feet from an event horizon is not distant. At that point you must do the math to get the correct answer. All you are doing is hand waving while making bad assumptions.

This is most definitely a situation where you cannot make assumptions about what is going to happen. You must run the numbers, cause the numbers could get very wierd.

grant hutchison
2009-Mar-18, 06:18 PM
Grant, I am not confusing.You are. You're mixing Schwarzschild coordinates with local coordinates. There's no "compressed space" for a local observer, and no extreme gradient of time dilation.

Grant Hutchison

nauthiz
2009-Mar-18, 06:29 PM
At the EH of a black hole space compresses to infinity ( in the direction of gravity ) and time stops.

It sounds like you might be taking a common analogy designed to illustrate aspects of relativity to people who don't have a working familiarity with the concept of spacetime a bit too literally.

Possibly a better way to look at the deformation of spacetime that happens around a gravitational object is that the coordinate system is deformed in a way that causes the time axis to point in some "spatial direction". That is, in the absence of any other factors, moving forward in time (which you always have to do) also moves you toward massive objects. In this case, the event horizon of a black hole is just a surface beyond which there are no possible spatial translations you can make which will counteract the rate at which moving foward in time also moves you toward the black hole's center of mass.

(Of course, that's also an analogy and shouldn't be taken too literally, either.)


Also, there isn't really any such thing as time "stopping." Time is a dimension. The idea of a dimension moving or stopping is meaningless except as a form of figurative language. Sort of like saying that something tastes red.

publius
2009-Mar-18, 07:12 PM
Is spaghettification really possible?

Yes.

While the math of getting there is quite complex, lots of tensor fun, the end result is quite simple for radial free fallers as well as stationary observers. The local tidal tensor is simply Newton. 2GM/r^3 * diag(-2, 1, 1) in spherical coordinates. That gives you the differential acceleration in a local neighborhood about the local frame as measured by the local ruler and clock. That gives you the local force you must exert to keep something stationary with you as fall, again according to the local coordinates. It becomes arbitrarily large as r --> 0, stretching in the radial direction and compressing in the tangential directions.

In the coordinates of an external observer, things look different, but that's the picture of the local observer.

-Richard

tommac
2009-Mar-18, 07:34 PM
You are. You're mixing Schwarzschild coordinates with local coordinates. There's no "compressed space" for a local observer, and no extreme gradient of time dilation.

Grant Hutchison

I may be wrong but not confused. I am considering my head a distant observer of my feet ... what i am understanding from you is that my head is not a distant enough observer for there to be much of a difference in time dilation even very close to the EH.

tommac
2009-Mar-18, 07:40 PM
Also, there isn't really any such thing as time "stopping." Time is a dimension. The idea of a dimension moving or stopping is meaningless except as a form of figurative language. Sort of like saying that something tastes red.

Agreed ... but the external observer views time as stopped at the EH of a black hole. A local observer would feel time was running normally.

tommac
2009-Mar-18, 07:46 PM
Richard,

My question comes down to ... is there a difference in space-time between EH + 0ft and EH + 6ft. Is the 6ft difference enough for an observer to see relativistic changes happen at his/her feet? Is 6 ft enough to be considered a distant observer when observing the EH.




Yes.

While the math of getting there is quite complex, lots of tensor fun, the end result is quite simple for radial free fallers as well as stationary observers. The local tidal tensor is simply Newton. 2GM/r^3 * diag(-2, 1, 1) in spherical coordinates. That gives you the differential acceleration in a local neighborhood about the local frame as measured by the local ruler and clock. That gives you the local force you must exert to keep something stationary with you as fall, again according to the local coordinates. It becomes arbitrarily large as r --> 0, stretching in the radial direction and compressing in the tangential directions.

In the coordinates of an external observer, things look different, but that's the picture of the local observer.

-Richard

astromark
2009-Mar-18, 08:20 PM
I do know this is just an exercise in samatics...after all no such observation is or could ever be possible. The cold hard fact is that yes as you ascend oops thats descend into a black hole lots of weird stuff happens. As you get near to that event horizon your feet will accelerate away from you. This sounds very nasty and is against my better judgment to attempt or test. It makes sense that these things might occur, but to argue the manner of witch and how it might look is a nonsense you must agree. Better to wast your mental efforts on finding that Higgs Boson particle and understanding it and how we might achieve warp speed...:)Doing the maths to work out what you could never test is just as silly a... ?
Is a gnome in a green suit a leprechaun.? there he goes.....

novaderrik
2009-Mar-18, 09:11 PM
mmmm... Spaghetti...

mmm.. pancakes...

can we now talk about lasagnafication or chips and salsafication?

grant hutchison
2009-Mar-18, 10:40 PM
I may be wrong but not confused.Honest: you're confused. And you'll certainly stay confused as long as you think you're not confused, although this does not imply that you will become less confused once you know you're confused. :)
All this stuff about your feet turning into pancakes and being "stopped in time" is very clearly coming from trying to apply one set of coordinates to a situation in which they simply do not apply.
As Richard says, there is a different metric for a "free-faller from infinity", which you can derive from the Schwarzschild metric of a "stationary observer at infinity". When you do that, you can plot the worldlines of two separate free-falling observers, one behind the other, and derive their relative acceleration as they would measure it locally: that gives you the familiar tidal stretch between (say) your head and your feet, and it takes into account the full spacetime curvature. You can also draw the path that light takes when emitted by the leading object and received by the trailing object, and notice that light has no problem getting from one to the other as the free-fallers cross the horizon: there's no infinite gravitational time-dilation at the horizon for a free-faller.

Grant Hutchison

tommac
2009-Mar-18, 11:06 PM
Forget about feet for a second.

Lets take a 1 light year long pole falling into a BH. There is a clock on one end of the pole and a clock on the other end of the pole. Do those two clocks stay in sync as the pole falls into the BH? Or does one end of the pole get effected more from relativistic warping of the black hole?





Honest: you're confused. And you'll certainly stay confused as long as you think you're not confused, although this does not imply that you will become less confused once you know you're confused. :)
All this stuff about your feet turning into pancakes and being "stopped in time" is very clearly coming from trying to apply one set of coordinates to a situation in which they simply do not apply.
As Richard says, there is a different metric for a "free-faller from infinity", which you can derive from the Schwarzschild metric of a "stationary observer at infinity". When you do that, you can plot the worldlines of two separate free-falling observers, one behind the other, and derive their relative acceleration as they would measure it locally: that gives you the familiar tidal stretch between (say) your head and your feet, and it takes into account the full spacetime curvature. You can also draw the path that light takes when emitted by the leading object and received by the trailing object, and notice that light has no problem getting from one to the other as the free-fallers cross the horizon: there's no infinite gravitational time-dilation at the horizon for a free-faller.

Grant Hutchison

grant hutchison
2009-Mar-18, 11:34 PM
Forget about feet for a second.

Lets take a 1 light year long pole falling into a BH.What's the difference, apart from this new scenario being utterly physically implausible?

Grant Hutchison

mugaliens
2009-Mar-19, 12:16 AM
As you and your body approach the event horizon, you do not witness time slowing down, as would the distant observer witness. Instead, you witness the extreme gravitational gradient which pulls about a billion lbs in force in difference between your lower and upper extremities, not only spaghettifying your flesh before you reach the EH, but as your molecules get closer, ripping those apart. Just before the EH, your atoms are ripped apart, too.

Spaghettification?

Yeah, baby - to the extreme.

Jeff Root
2009-Mar-19, 05:33 AM
If you are at the event horizon of a black hole, and alive, then you
are falling, and the black hole is a big one. If the black hole was not
big you would not be alive because you would be torn apart by the
steep gravity gradient.

If the gravity gradient is not steep enough to tear you apart, then
it is nowhere near steep enough to cause a large difference in clock
tick rates between your head and your feet. Conversely, if there is
a large difference in clock tick rates between your head and your
feet, it is because your head and your feet are rapidly moving away
from each other due to the extreme gravity gradient.

-- Jeff, in Minneapolis

Jeff Root
2009-Mar-19, 05:48 AM
On the other hand, if you are just above the event horizon of a black
hole, and not falling, then you are dead, because the force needed to
keep you above the event horizon is more than enough to turn you
into a thin layer of chunky salsa on the floor of whatever is keeping
you from falling.

-- Jeff, in Minneapolis

tommac
2009-Mar-19, 06:08 AM
As you and your body approach the event horizon, you do not witness time slowing down, as would the distant observer witness. Instead, you witness the extreme gravitational gradient which pulls about a billion lbs in force in difference between your lower and upper extremities, not only spaghettifying your flesh before you reach the EH, but as your molecules get closer, ripping those apart. Just before the EH, your atoms are ripped apart, too.

Spaghettification?

Yeah, baby - to the extreme.

OK ... but how about the relativistic effects? Like difference in time dilation ... say between at the EH and x above the EH .... if a ridgid body has a difference in time dilations how does that work?

tommac
2009-Mar-19, 06:09 AM
If you are at the event horizon of a black hole, and alive, then you
are falling, and the black hole is a big one. If the black hole was not
big you would not be alive because you would be torn apart by the
steep gravity gradient.

If the gravity gradient is not steep enough to tear you apart, then
it is nowhere near steep enough to cause a large difference in clock
tick rates between your head and your feet. Conversely, if there is
a large difference in clock tick rates between your head and your
feet, it is because your head and your feet are rapidly moving away
from each other due to the extreme gravity gradient.

-- Jeff, in Minneapolis


Great answer! Thank you. This explains what I was trying to ask.

m74z00219
2009-Mar-19, 06:42 AM
Ok, what I think I get from this interesting discussion is that I will not (as a free-faller) notice any interesting time dilation effects. Especially, not for supermassive blacks holes because the gradient won't be as steep.

So, as the free-faller, you would see the EH, wouldn't you? From the free-faller's perspective, would you be "ok" until you hit the singularity? Or, I suppose, be torn up somewhere within the blackhole?

An interesting thing that I remember reading is that, if I were a stationery observer of some poor sap falling into a black hole, that I would never seem him (or any other object) cross the EH. I would just see his emitted light red-shifted to infinity as he approached the EH.

m74

grant hutchison
2009-Mar-19, 01:10 PM
So, as the free-faller, you would see the EH, wouldn't you?The event horizon isn't really an object you can see. As a faller, you'd see a patch of black sky ahead, which when you crossed the event horizon would occupy a region just 90 degrees across in your forward view, rather than the 180 degrees you might expect if you were passing through some sort of membrane. The dark patch would be outlined by the images of stars which had been displaced into your forward view by relativistic aberration.
Tidal forces ramp to infinite as your distance from the singularity declines to zero, so there would always come a point at which you'd be torn apart, either inside or outside the event horizon, depending on the black hole's mass.

Grant Hutchison

alainprice
2009-Mar-19, 03:38 PM
Mugaliens: As has been discussed in the last 4-5 posts, it is possible to cross an event horizon alive(and fairly well). So no, the event horizon doesn't rip atoms apart.

Tommac: you are confused. Grant is being concise and you are unable to respond in a direct manner to his comments.

M74: nice summary. I agree emtirely with your assessment.

The EH is a singularity only in certain reference frames. Let yourself fall into a BH and this singularity disappears. Time doesn't stop at the EH for the free-faller. There is no Unruh radiation. You can see your feet just fine as we must deal with gradients, instead of absolute speeds.

tommac
2009-Mar-20, 10:44 PM
What's the difference, apart from this new scenario being utterly physically implausible?

Grant Hutchison

OK ... lets go in another direction. Hawkings Radiation ... VPs get created and are split due to relativistic differences. Right?

So there is different space-time warpage for each of the particles right?

So if you were able to ride one of the particles it would look like the other one was warped ... in fact although the were born together they will age differently ...

Starfury
2009-Mar-21, 03:48 PM
tommac, please see my post in your previous thread "Free falling into a black hole" with the link to a video featuring Dr. Neil Tyson, who explains, in excruciating detail, that spaghettification is very real.

astromark
2009-Mar-24, 08:15 AM
The very space I occupy is being accelerated in the direction of the black hole itself. At some point my long since dead corps is being torn apart by just that force.

To argue the mechanics of that and in the manor of it is as nuts as ya can be.... that will not stop you or should it. My opinion is just that.

" Is spageghettifacation really possible ?" YES, it must be. The reference frame of accelerating mater into the very hart of a black hole... a lengthy description of events alleged to is not conformable, logical as it might be.