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djdugan
2006-Dec-02, 04:04 AM
Why is it said that one could not travel at the speed of Light, yet if you fell into a massive black hole you would ???

antoniseb
2006-Dec-02, 04:33 AM
I expect that the people who say you would travel *at the speed of light* when you fall into a massive black hole are making an approximation. You wouldn't actually get to the speed of light on the way in.

Ken G
2006-Dec-02, 04:35 AM
What is meant by "you cannot travel at the speed of light" is that you cannot whizz past something else at a relative speed equal to the speed of light. Near a black hole event horizon, it is impossible for anything to "stand still" so that you could whizz past at faster than c. For objects that are spatially separated, like you falling in and me back on Earth, the very concept of speed is not so uniquely defined, and there need not be a "speed limit" there.

djdugan
2006-Dec-02, 04:53 AM
I believe it must be possible to travel at C, in that, in a Black Hole, Light can not escape, since C must be constant, then the rate of fall must be greater than C...
When it is said that there is not enough energy in the universe to propel an object to C, how does this justify what a black hole does to an object ???

So if I were to create the Gravitational field (just in front of me) of a super massive Black Hole I could attain C

And given that, since I would dimentionally, decrease in size, perpendicular to the direction of travel, I couldn't paste myself on some asteroid, because I couldn't hit anything ??? (being to smaller, perpendicular to direction of travel)

This is how I figure its done

Ken G
2006-Dec-02, 05:03 AM
When it is said that there is not enough energy in the universe to propel an object to C, how does this justify what a black hole does to an object ???Again, this statement misinterprets the meaning of the energy required to travel at c. Much of the universe has its distance from us increasing at a rate faster than c right now-- and it doesn't require any energy at all. Energy is a local concept, and it relates to one thing zooming past another in its vicinity.


So if I were to create the Gravitational field (just in front of me) of a super massive Black Hole I could attain C Relative to what?
And given that, since I would dimentionally, decrease in size, perpendicular to the direction of travel, I couldn't paste myself on some asteroid, because I couldn't hit anything ??? (being to smaller, perpendicular to direction of travel)
This is how I figure its done

Fraid you got that backward-- the shrinking is along the motion, not perpendicular to it. Not asteroids, dust particles, are gonna be a huge problem, I fear. I figure that's why "they" didn't do it.

djdugan
2006-Dec-02, 05:11 AM
If I travel in the z direction, I "shrink" in the x and y, true/false ?

If it true, and I'm zipping along at C, I'm thinking I'm incredibly small in the x and y (as in no profile at all)
Missing even dust particles ???

antoniseb
2006-Dec-02, 01:38 PM
If I travel in the z direction, I "shrink" in the x and y, true/false ?

False.

blueshift
2006-Dec-02, 09:04 PM
If one wishes to travel as light does then one must do as the competition does and one must look at it from both the particle view and the wave view.

From the particle view we must do as the competition does. The competition is the photon. The photon does not have any engines hooked up to itself and does not "reach" for speed c by accelerating to it. Photons free-float at speed c. Therefore, if we wish to travel at speed c we must do so as the photon does, without any effort that changes our motion. No machines..no rockets...nothing but ourselves at "rest" which means we can't use our legs either.

From the wave aspect things get even more challenging. A wave moves forward at speed c...backward at speed c....up at speed c...down at speed c...to the right at speed c...to the left at speed c and combinations of all of the above. Your body would have to be displaced 186000 miles after one second in all of those directions.

I am not saying that it is impossible. I am just saying that these are the challenges that must be met.

astromark
2006-Dec-03, 01:51 AM
Above are some very good reasons why you can not use a black holes gravity as a propulsion force. That same force would tear you to bits and, stopping might be some thing you can not do. Not to mention the fact that to create a black hole to propel your craft might be a we bit dangerous. I can not 'see' how a black hole could be controlled. I am not a force big enough. The energy requirements are just to big.

WaxRubiks
2006-Dec-03, 02:10 AM
the reason that nothing can travel faster than light is that light is the speed of em radiation if matter traveled at the speed of light then some part of that matter would have to travel faster than light as a lump of matter is an integrated information system with information traveling hither and thither thoughout it and in the direction of travel-that is my take on it at least. I dunno about black holes....

Maksutov
2006-Dec-03, 10:58 AM
Why is it said that one could not travel at the speed of Light, yet if you fell into a massive black hole you would ???Why not infinite mass?


http://img137.imageshack.us/img137/566/iconwink6tn.gif

Sean Clayden
2006-Dec-03, 01:02 PM
Light travels at the speed of C because it contains no physical properties.

What theoretically would happen to a solid object whilst travelling at c. Is it transferred into pure energy ? And would this transition to pure energy be reversible ?

Sean Clayden
2006-Dec-03, 01:08 PM
Why is it said that one could not travel at the speed of Light, yet if you fell into a massive black hole you would ???

Why would you be travelling at c whilst falling into a black hole ?

Just because light cannot "escape" from a black hole doesn't mean that you would be travelling a c when you entered it.

I believe light cannot escape as it has nothing to reflect back off. Imagine shining a torch into a bottomless hole.

Ken G
2006-Dec-03, 02:07 PM
I believe light cannot escape as it has nothing to reflect back off. Imagine shining a torch into a bottomless hole.

Even light travelling "outward" cannot escape a black hole, essentially because space and time get so monkeyed with that "outward" turns into "the past", or at least I've heard it explained that way. I don't know how the concept of "the past" applies to a photon, but I think the idea is not that the photon has nothing to reflect off, it's that none of the directions lead out. It's the one situation where the old joke "you can't get there from here" is actually true. The speed of the photon is not what matters, it's the geometry of spacetime itself.

publius
2006-Dec-03, 07:47 PM
Even light travelling "outward" cannot escape a black hole, essentially because space and time get so monkeyed with that "outward" turns into "the past", or at least I've heard it explained that way. I don't know how the concept of "the past" applies to a photon, but I think the idea is not that the photon has nothing to reflect off, it's that none of the directions lead out. It's the one situation where the old joke "you can't get there from here" is actually true. The speed of the photon is not what matters, it's the geometry of spacetime itself.

Ken,

You can see how this sort of works with Schwarzschild. Consider the time and radial metric factors, g_00 = 1 - R/r, and g_11 = 1/(1 - R/r), where R is the Schwarschild radius. When r = R, g_00 goes to zero and g_11 blows up.

But look at what happens for r < R. g_00 and g_11 both become negative. That means space and time "flip" roles inside the horizon, as the 't' contribution to s becomes negative and the r contribution becomes positive. Inside the horizon "time points" in the radial direction, and our "direction of time" outside become a spatial direcition inside.

So once you fall in (in your own proper time, of course), you are then "compelled" to move to the singularity at r = 0. All world lines converge there. Not just geodesics, but all possible time-like and null world lines go there. And the funny thing is those world lines just "end" there, whatever that means.

-Richard

Michael Noonan
2006-Dec-03, 08:45 PM
What if the singularity is not a black hole but just a constriction in the width of the galaxy. I was thinking along the lines of a bounce back tube initially but it could equally be like a four sided pyramid where each of the corners is either inflow or outflow.

Certainly one type of black hole has matter in it because a star collapses but what if a galaxy sized black hole has been emptied out but due to the weight of holding time and gravity together just squeezes things down somewhat?

Cheers

publius
2006-Dec-03, 09:22 PM
What if the singularity is not a black hole but just a constriction in the width of the galaxy. I was thinking along the lines of a bounce back tube initially but it could equally be like a four sided pyramid where each of the corners is either inflow or outflow.

Certainly one type of black hole has matter in it because a star collapses but what if a galaxy sized black hole has been emptied out but due to the weight of holding time and gravity together just squeezes things down somewhat?

Cheers

Michael,

No offense intended, but I have no idea what you're asking or even talking about in general.

-Richard

Squashed
2006-Dec-04, 04:12 PM
Ken,

You can see how this sort of works with Schwarzschild. Consider the time and radial metric factors, g_00 = 1 - R/r, and g_11 = 1/(1 - R/r), where R is the Schwarschild radius. When r = R, g_00 goes to zero and g_11 blows up.

But look at what happens for r < R. g_00 and g_11 both become negative. That means space and time "flip" roles inside the horizon, as the 't' contribution to s becomes negative and the r contribution becomes positive. Inside the horizon "time points" in the radial direction, and our "direction of time" outside become a spatial direcition inside.

So once you fall in (in your own proper time, of course), you are then "compelled" to move to the singularity at r = 0. All world lines converge there. Not just geodesics, but all possible time-like and null world lines go there. And the funny thing is those world lines just "end" there, whatever that means.

-Richard

Publius,

Does space or spacetime fall into a blackhole?

If space falls into a blackhole then an object could exceed the speed of light whilst falling into the blackhole because the speed of space is straight-fowardly additive to the local speed which means that objects can exceed the speed of light from a distant perspective whilst using the local ruler of the distant observer.

Ken G
2006-Dec-04, 04:41 PM
And the funny thing is those world lines just "end" there, whatever that means.



Yeah, really. Here's a Q&A I won't bother to pose: it takes a finite time to fall into a black hole singularity (for the faller). Then what? It would seem that there are things outside of science, not just on the "cause" side of things, but also on the "effect" side. (Of course, everything inside the event horizon might be argued as being outside of science, so my question is just for those who like to ponder quasi-scientific extensions inside the event horizon.)

publius
2006-Dec-05, 06:15 AM
Yeah, really. Here's a Q&A I won't bother to pose: it takes a finite time to fall into a black hole singularity (for the faller). Then what? It would seem that there are things outside of science, not just on the "cause" side of things, but also on the "effect" side. (Of course, everything inside the event horizon might be argued as being outside of science, so my question is just for those who like to ponder quasi-scientific extensions inside the event horizon.)

Ken,

Something additional to ponder about this is the following. The "cold dust" collapse model of a spherical dust collapsing to a singularity under its own gravity is actually the same metric, the same space-time solution as one of the general Big Bang metrics, just *time-reversed*. Well, to be clear, the *interior solution* of that collapsing matter is the same, not the vacuum outside.

The world lines of all the collapsing particles end in the former, but they *begin* at the latter. The only difference is are initial/boundary conditions that have the one "explode" while the other "implode".

When it's stated (correctly if the metric actually corresponds to reality) that time began at the Big Bang singularity, that is just the time-reversed version of time (and space) "ending" at the singularity in the collapse model.

So asking what happens "after" you hit the singularity is really the same as asking what happened "before" the Big Bang. :)

And I'll add again, that if Abhas Mitra is right, the question of what happens after you hit the singularity will be moot, because a singularity can't form. He says the cold collapse model is flawed, wrote some papers about it and gained some converts. His purported proof hinges on a "null surface" (whatever that actually means) cannot form if the infalling matter has any kinetic energy at all. That is, the collapsing matter would have to be at absolute zero, and radiate away all its binding energy released as it collapsed. That can't happen in the real world, and so the cold dust model fails, or so he says. None of the "high priests" of GR have yet formally responded.

So for fun, I'm gonna bet Mitra is right, and singularities don't form, we just have "ECOs". :)

-Richard

Orby
2006-Dec-05, 10:29 AM
It is a good read here, with choice cuts from boths sides of the event horizon.

I would love to ask whether laughter can escape from a black hole, and if so, does it go hither and thither before colliding with a spacetime, and would there be detectable traces afterward? but there is already too much quasiness.

It's a privilege, guys.

Squashed
2006-Dec-05, 04:48 PM
...I would love to ask whether laughter can escape from a black hole, and if so, does it go hither and thither before colliding with a spacetime, and would there be detectable traces afterward?
...

Yes, laughter can escape from a blackhole but unfortunately it is redshifted so much that by the time we can detect it the laughter has changed into a ridiculing sneer. If the universe lasts long enough that sneer will change into a sob, then a whimper and finally silence.

Orby
2006-Dec-05, 06:06 PM
Orby had several messages from those wanting to down Pierre in flames or point him elsewhere. He refrained from editing traces in post #21 to mirth. That post was an attempt to cop to his style of entry. And acknowledge the fine community he discovered but abused. Crawler. This is his first instant post.

publius
2006-Dec-06, 12:14 AM
Publius,

Does space or spacetime fall into a blackhole?

If space falls into a blackhole then an object could exceed the speed of light whilst falling into the blackhole because the speed of space is straight-fowardly additive to the local speed which means that objects can exceed the speed of light from a distant perspective whilst using the local ruler of the distant observer.

Squashed,

In trying to describe various types of space-time that come up in relativity, you'll often see people talk about space "moving", or even rotating when frame dragging is afoot. That's really just a sort of poetic language trying to paint a word picture. The idea is to convey what inertial paths (geodesics) are doing.

For example, you drop something into a Kerr black hole with no tangential velocity, it will start rotating, because of the gravitomagnetic field, while it would just be a straight radial free fall in Schwarzschild. Many people will describe that as space being "dragged" around, revolving around with the black hole.

But, really, space and time are just "there" and it's the stuff in it (matter and radiation) that does the moving. At least for static metrics. With time varying metrics, like the Big Bang expansion solutions, you have space "expanding" and all that.

You might like to think of "fabric" that is stretching there, but still, that's just metaphorical language to describe what the space-time geometry is doing.

-Richard

Ken G
2006-Dec-06, 07:17 AM
So asking what happens "after" you hit the singularity is really the same as asking what happened "before" the Big Bang. Yes, that's what I was noting as guided by your remarks. One is an effect without a cause, the other is a cause without an effect. I see these as clear examples of seeing to the "boundary" of the scientific approach itself. Van Rijn, on the "Ann Coulter" thread in General Science, did not see it that way.


So for fun, I'm gonna bet Mitra is right, and singularities don't form, we just have "ECOs".

But what is the observable difference outside the event horizon?

publius
2006-Dec-06, 07:40 AM
But what is the observable difference outside the event horizon?

Ken,

And I should have added, an event horizon does not form, either. If you had something form an event horizon (at least this type of horizon), then a singularity would form, because "all paths would have to go a point and end", or something like that. :)

So, in Mitra's ECO theory, an event horizon does not form, but something very close to it. You've got a surface that is so time dilated and redshifted that it is very, very close to a black hole. The current "cold dust" collapse model places a limit on the minimum radius of a spherical object (relative to the Schwarzschild radius), and a maximum surface redshift. Ie, something denser than that couldn't resist collapse. I think that is either 2 or 3 Sch. radii, or surface redshift factor of 2 or 3 or something in there.

Most "that's got to be a black hole" declarations are based on that limit. Mitra's ECO theory removes that limit, so stuff can still appear to disappear without showing an "impact flash". And I'll stress that Mitra did all this purely in GR, and says the proper solutions of the EFE *preclude the collapse and event horizon formation*.

And we probably can't tell the difference from here on earth (yet). Mitra's ECO would be radiating, but every so slowly, and every so redshifted. I think it would be radio, but very weak. Close up, you could probably measure it.

-Richard

Ken G
2006-Dec-06, 08:19 AM
Perhaps the distinction is mostly philosophical, if it is so hard to distinguish observationally. For those who get worked up about the philosophy of science and truth, that's a big deal, but I am more in the "use it if it works" camp.

Squashed
2006-Dec-06, 06:23 PM
Squashed,

In trying to describe various types of space-time that come up in relativity, you'll often see people talk about space "moving", or even rotating when frame dragging is afoot. That's really just a sort of poetic language trying to paint a word picture. The idea is to convey what inertial paths (geodesics) are doing.

For example, you drop something into a Kerr black hole with no tangential velocity, it will start rotating, because of the gravitomagnetic field, while it would just be a straight radial free fall in Schwarzschild. Many people will describe that as space being "dragged" around, revolving around with the black hole.

But, really, space and time are just "there" and it's the stuff in it (matter and radiation) that does the moving. At least for static metrics. With time varying metrics, like the Big Bang expansion solutions, you have space "expanding" and all that.

You might like to think of "fabric" that is stretching there, but still, that's just metaphorical language to describe what the space-time geometry is doing.

-Richard

publius,

When spacetime is dragged or expands it can allow the speed of light to vary, according to a distant observer using their local ruler, and so when space expands it creates a situation where light seems to be moving faster than "c".

A hypothetical case is where expansion enlarges the distance between two distantly separated objects as if one were moving at "c" with respect to the other object and so if someone pointed a flashlight at the other object the photons would appear to the other object to be traveling at zero speed whereas if the flashlight were pointed in the opposite direction away from the other object then the photons would be traveling at 2*c.

You indicate that these situations are the result of the geodesics but if geodesics are just plain inertial paths then how can these inertial paths increase velocities and still be inertial?

publius
2006-Dec-06, 07:00 PM
You indicate that these situations are the result of the geodesics but if geodesics are just plain inertial paths then how can these inertial paths increase velocities and still be inertial?

This is the whole point of General Relativity. Inertial paths are determined by the geometry of space-time. When space-time has curvature (in your coordinates, which can be due to both real gravity and the psuedo gravity of accelerating reference frames) inertial paths are not straight lines, not constant velocity.

In General Relativity, "inertial" means following geodesics, which are determined by the local space-time. Proper acceleration, due to a real force, is deviation from those geodesics. Using the elegant (and terse) language of GR, we can still write Newton's 2nd law, as F = m*a, but 'a' becomes a rather complicated expression that is the measure of the deviation from the geodesic per unit proper time.

Remember some of our threads here about the Equivalence Principle? Standing here on earth, feeling a force, we are the ones who have proper acceleration, not free-falling objects.


-Richard

publius
2006-Dec-06, 10:39 PM
Perhaps the distinction is mostly philosophical, if it is so hard to distinguish observationally. For those who get worked up about the philosophy of science and truth, that's a big deal, but I am more in the "use it if it works" camp.
Ken,

"Use if it works" sounds of the same vein as Feynman's "Shut up and calculate" school. :) Me, I guess all in all, I'm somewhere in between that and getting all worked up. And it varies with the particular subject. I would like to know "ultimate, deep truth", but I'm not going to worry myself to death with it, as it may be unknowable anyway.

Oh, and Squashed brought up something about black hole magnetic fields in the recent "black hole" thread in Q&A here that is possible observational difference. An ECO could have a very large "instrinsic" B field and they call it a "MECO" then for Magnetospheric Eternall Collapsing Object. :)

The MECO proponents think they've observed the central black hole candidate in some quasar having a large intrinsic B field much greater than a charged Kerr hole could have (and they say they've ruled out field from the accretion disc, but I don't know enough about that to make any judgements).

Anyway, this might be the best observational way to try to tell the difference.

-Richard

Grey
2006-Dec-07, 05:59 PM
"Use if it works" sounds of the same vein as Feynman's "Shut up and calculate" school. :)As an aside, it looks as though Feynman never said that (http://www.aip.org/pt/vol-57/iss-5/p10.html).