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Joined: 2/27/2007
Msg: 60
Black!Page 2 of 9    (1, 2, 3, 4, 5, 6, 7, 8, 9)
And about observing a live performance, we cannot really be sure whether what we're looking at is still there. Some star we look at in the sky appears to be fine and doing its thing, but it could've burned out or gone supernova already. We wouldn't see that happen for years. The closest star to us is, I believe, 4 light years away, meaning we see it now as it was in 2003. If I remember correctly, it's not massive enough to collapse and then blow out its surrounding material, but, if it were, we would not suffer any consequences of this happening for 4 years. In fact, we wouldn't even know it'd happened.

Consider this. We don't even see ourselves in the present. We see ourselves as we were very very slightly in the past, but it's close enough to the present that we can work with it as a "current" observation. It'd be interesting to know exactly how far in the past our view of ourselves are. I'd bet it is not exactly the same for everyone and probably varies somewhat even within the same individual. First, the light bouncing off you to the mirror and back again (if that's how you're observing yourself) takes a small amount of time to make that trip, and then there's a larger (but still small) amount of time for the signal from the eye to get to the brain and then be processed into an image. I bet our view of ourselves is multiple hundredths of a second or up to something like of a tenth of a second outdated, not that that presents a real problem to us.
Joined: 2/27/2007
Msg: 62
Posted: 4/5/2007 1:25:32 PM
The example you give, while true, is not the reverse of the situation I described, which does not violate relativity because it doesn't require anything to exceed the speed of light.
Joined: 2/27/2007
Msg: 68
Posted: 4/6/2007 4:01:28 PM

You said that things can move away from you at faster than the speed of light. They cannot. Nothing can move faster than the speed of light under any circumstances in the universe.

If there was a train travinling east at the spped of light, and one travelling west at the speed of light, and you were on the east-bound train, the west-bound train would move away from you at the speed of light, not at twice the speed of light.

There are no circumstances where someone can observe movement faster than light.

But the rest of my original post on this topic explains that nothing actually IS moving faster than light.
Joined: 2/27/2007
Msg: 71
Posted: 4/6/2007 4:40:01 PM
I think you missed the word "nothing" in my previous post.
 zombies are chasing me
Joined: 4/5/2007
Msg: 74
Posted: 4/14/2007 3:21:38 AM
oOoOo good topic! :)

Chances are it's been answered. But meh here is my answer! :)

kk, so a black hole is called a hole because by definition it is. The sheer mass of the object(black hole) in such a small place causes it to burrow a "hole" in space. (possibly beyond?)

This effect is easiest to understand when illustrated on a 2 dimensional plane in a 3 dimensional environment.
Kinda like say having a rubber sheet held taunt at the edges and poked in the center (which causes it to distort into a funnel shape) -Now apply the same concept to a 3 dimensional space. :)

As for distribution of gravity on earth. No, it's not equal. There are places on earth's surface where you will weigh slightly different amounts. Because of the distribution of mass in our planet. The same is true for a neutron star. (Hypothetically such an object isn't likely to be evenly distributed density wise.

As for gravitational attraction on objects of different density. well, the denser the object is. The more focused it's gravitational field is (denser a sphere is the greater the pull is at it's surface.) But no matter the density, 2 objects of equal mass will generate an equal amount of gravitational force.

..and onto the last question:
If a object where to be pulled into a black hole.. It's direction of approach would not matter. A black hole is a 3 dimensional "hole". (Unless (technically) it ran into something orbiting the black hole such as the accretion disk or the "plume/jet thingy" -i forget the name of it)
Joined: 2/27/2007
Msg: 76
Posted: 4/14/2007 3:21:45 PM
The current evidence seems to support the theory that these jets are not made up of material coming back out of the black hole but rather that material that has not yet gone beyond the event horizon is being thrown outward by magnetic fields produced by quickly spinning black holes and/or their accretion disks.
Joined: 2/27/2007
Msg: 80
Posted: 4/16/2007 9:45:46 AM
What kind of magnetic field are you talking about? wouldn't the magnetic field pull it towards the black hole?

Depending on relative charges, you can get either attraction or repulsion. Think of two strong magnets held in each hand. When you try to force two positive poles together, they resist. The same happens when you try to force two negative poles together. And I know this may sound strange and seem to defy daily subjective experience, but when we physically interact with objects (anything from clapping your hands together to jumping in the air and being stopped by the ground) it is electromagnetic repulsion that insures we don't go through those objects.

Here's an article on giving an idea of just how strong the jets that eject this matter are. Amazing.
Joined: 1/10/2007
Msg: 81
Posted: 4/16/2007 11:04:56 AM
A few points: First gravity actually isn't perfectly constant everywhere on Earth. The variances are utterly small and beyond unassisted detection but they are there. This would be true of all bodies.

While the term "hole" is not exact here it is also not as inaccurate as it might at first seem. Space itself is curved or bent by the presence of a physical object so there is a hole in a sense. Because a BH rotates up to several times a second, objects will be drawn into its accretion disk along its equator before entering the event horizon--the point where light can no longer escape because escape velocity exceeds the speed of light. This requires quantum mechanics to describe in detail as we are dealing with more then 3-dimentional space.

Also, not everything pulled-in by the BH actually ends-up there, much of it is thrown-off in polar quasars. Imagine trying to fill a dog dish with a fire hose. Also, black holes slowly evaporate over time, bleeding off subatomic particles a few per second. This is the Hawking Radiation that made Steven Hawking a household name. The time scale for this are measured in goggles of years (1o to the 1ooth power).

Goggle "Neil DeGrass Tyson" to find his website. He's an astrophysicist and curator of the Hayden Planetarium in NYC. He is well versed in describing these effects in layman terms.
Joined: 2/27/2007
Msg: 83
Posted: 4/16/2007 4:29:57 PM

Take the red shift for example. We can tell distant objects are traveling away from us by observing their spectral image. If the image is shifted toward the red end of the spectrum the object is traveling away, towards the blue and they are traveling closer. The problem is we are observing the light from objects that are 12 billion light years distant, so our information on those objects movement is 12 billion years old.

This is correct, but, in case anyone's wondering, red-shifted galaxies are unlikely to have begun moving towards us because the recession of such distant objects from us is mostly due to the expansion of the universe, not their movement through space.
 zombies are chasing me
Joined: 4/5/2007
Msg: 88
Posted: 4/17/2007 8:26:14 PM

For example.... If I fell off my chair here and I fell to the ground ,because of gravity, I would obviously not be falling at the speed of light to the floor LOL LMAO, ....right?? so I don't understand how you can equate Gravity with light. ??
lol ever heard of acceleration lmao

I always thought gravity was instantaneous but I admitingly have no idea. :) anyone have references?
Joined: 1/10/2007
Msg: 90
Posted: 4/17/2007 9:06:15 PM

So how can gravity move at light speed it by it's nature it operates differently?

Gravity as all elementary forces move at the speed of light in the form of gravitons (waves that perform similar to particles).
 zombies are chasing me
Joined: 4/5/2007
Msg: 93
Posted: 4/18/2007 8:41:52 AM
lol dude I didn't even mention terminal velocity...? (as it wasn't really relevant due to the distance involved) and I do grasp it btw.. A humans terminal velocity in the lower elevations of earths atmosphere is around~ 110-130 miles per hour when falling (if i remember correctly) this is due to air resistance
side note: acceleration due to gravity on earth is 9.8 meters per second, per second

but of course this is off topic lol :)

lol I know....But my point is that light is constant, there is no acceleration, thats my point, in gravity there is. So how can gravity move at light speed it by it's nature it operates differently?

while the speed of light is technically a constant the speed of light is not always the speed of a photon -like when traveling through glass or air. the light can be bent or slowed down cause it is being absorbed and re-emitted by atoms in the substance

just cause something operates differently doesn't mean it cant go the same speed

btw light can push things have you heard of a solar sail? its basically a big sail (obviously) that is used to ride the light being emitted from a source such as the sun. this can also work with laser beams.
didn't read the whole thing but i saw some references to faster than light propagation of gravity in there so those intersted may find it intersting

-ima go to sleep mow lol ive been up since yesterday lmao :p
Joined: 3/18/2007
Msg: 95
Posted: 4/19/2007 11:29:23 PM

If I was sucked into a black hole would I still be able to use my x-ray vision to see what I was missing out on back here?

Actually X-rays are emitted from the accretion disk around the black hole, energy (regardless of wavelength) is subject to the same gravitational attraction. As for seeing what's going on outside of the black hole, you wouldn't be able to. Time would have effectively stopped for you because of time dilation, while the rest of the universe would've witnessed your utter annihilation by tidal forces (which, by their frame of reference, would happen very quickly).
 zombies are chasing me
Joined: 4/5/2007
Msg: 96
Posted: 4/20/2007 1:47:40 AM
i thought it was the other way around.. one who gets close to a black hole would witness the universe outside accelerate while that person would seem to slow down from a outside perspective.

-but yeah black hole = "no good for health" unless you want to get taller! :) (Right before getting very short [/humor])
Joined: 8/21/2005
Msg: 97
view profile
Posted: 4/20/2007 9:21:28 AM
I think a pertinant point would be this:
The speed of light is NOT constant. It is dependant upon the medium through which it passes. For most purposes, it SEEMS constant.

Since vacuum is a characteristic of space, and space/time is a characteristic of the Universe, it stands to reason that vacuum per se only exists within the universe.

The universe is a product of the Big Bang, and you could argue that the Big Bang and the antecedent singularity ARE the Universe, simply at the earliest stages of its life. My questions would be, "Does vacuum exist at this stage in the universe?" and "If not, and light exists, what medium does it move through? How does this influence its speed? How can space/time be measured at this point? If light is the fastest known speed, does the universe expand at the speed of light, can light leave the universe, or is light bound within it regardless of speed?" To this last question, I would argue that the universe is defined by the presence of light/electromagnetism, and space/time does not exist until illuminated. Simply, light travels faster than matter, and nothingness does not become vacuum until it is part of the universe. This means that the size of the universe could be calculated as a sphere with a radius of C times the age of the universe. This also means we could never leave the universe by travelling less than the speed of light, unless there's a back door to bypass space/time.

This is a bit off topic, but it leads me to wonder about the speed [and general nature] of electromagnetism outside of the normal vacuum of space, in environments such as singularities or black holes.
Joined: 2/27/2007
Msg: 99
Posted: 4/20/2007 12:35:22 PM
To address the last two posts:

In the very early universe, there was no light. The photons were "locked up" by what would later become atoms.

And think about the difference between the speed of gravity and its pull like this:
Imagine, say, a baseball floating in an area of otherwise empty space. Now (not that this would happen) let's say suddenly a star pops into existence one light year away from the baseball. The gravitational influence of the star would begin spreading at the speed of light. The baseball would not be pulled towards the star until the star had been there for a year, at which point it would begin to accelerate at a rate determined by mass of the star, but the baseball would never reach the speed of light. The star's area of influence expands at the speed of light, it does not cause matter within that area of influence to move at the speed of light.
Joined: 2/27/2007
Msg: 101
Posted: 4/20/2007 1:03:37 PM
Right, gravity or gravity's "force" do not "equal" the speed of light, but the speed of the expansion of a gravitational field or the transmission of changes within that field happen at the speed of light.
Joined: 2/27/2007
Msg: 103
Posted: 4/20/2007 1:51:28 PM
^^ The same principles apply to a black hole's gravitational field. Where I said "star" earlier, I could just as easily have said "black hole".
Joined: 3/26/2007
Msg: 105
Posted: 4/20/2007 11:12:56 PM
Someone on the first page pretty much nailed it with that long description about the Schwarzschild solution to the Einstein equation. The Schwarzschild solution is one of the only known exact solutions to the 10 nonlinear partial differential equations in the Einstein equation (10 because the stress-energy and Einstein tensors are symmetric in four dimensions).

Someone said black holes were first theorized back in the "1700s or 1800s" or something to that effect. This is partially true, although I can't quite remember the approximate date, but they did not accurately know the speed of light until Maxwell predicted it and experimentalists like Michelson measured it in the mid- to late-1800s. The term black hole was actually invented by John Archibald Wheeler in, I believe, 1967 or 68. He's the same guy who came up with the term 'wormhole' and the phrase, "black holes have no hair" in regards to the three and only three properties black holes have: mass, charge, and temperature.

Stephen Hawking on the other hand, is more concerned with things like black-hole thermodynamics (this is where temperature and thus entropy come in) and black hole evaporation. This is the theory someone mentioned he changed or changed his mind about. Hawking predicted that black holes can evaporate, over a time about a billion trillion times longer than the current age of the universe, for a solar mass black hole, but did not say that they themselves emit radiation. Instead, 'Hawking radiation' is caused by quantum mechanical effects in the vaccuum near the event horizon. As virtual particle-antiparticle pairs pop in and out of existence in the vaccuum (yes, it is allowed by Heisenberg's uncertainty relation), one of them occasionally crosses the event horizon and falls into the black hole. The mechanism of mass loss in the black hole is the capture of this now real virtual particle with negative energy, which causes the other particle which escapes to become real, thus creating the appearance of mass loss and its associated radiation. The smaller the black hole, the faster it evaporates.

Black holes are well-accepted scientifically now. They are not some plot device concoted by Hollywood. There is a growing body of observational evidence for these objects in binary systems with companion stars and in the centres of almost every galaxy we look at, including our own. When matter falls into a black hole, it releases gravitational potential energy equivalent to roughly 40-50% of it's mass-energy. By comparison, Hiroshima was obliterated by the conversion of ~1% of 30kg or so of uranium mas to energy, so this is a very energetic phenomenon. This is the most reasonable explanation of active galaxies and quasars, supergiant stars orbiting unseen companions which give off large amounts of x-rays, and the incredibly rapid proper motions of stars in the galactic nucleus which suggest a mass of several million suns packed into a very small region.

Also, why would it not be reasonable to assume these objects exist? There is clearly evidence for white dwarf stars, the hot ash remnants of small- to medium-mass stars, which are supported by electron degeneracy pressure (it was called fermionic exclusion pressure in this thread - electrons are fermions (spin +/- 1/2)). Also, since the discovery of pulsars (rotating neutron stars), there has been evidence of objects formed in more dramatic events called supernovae, which are supported by neutron degeneracy pressure (neutrons are also fermions). What then comes after neutron degeneracy? Some though quark degenracy at one point, as quarks are fermions (which is why neutrons are too), but this idea has lost favour in the scientific community. So what is to stop an object from collapsing if there is no outward radiation pressure, like in a main-sequence star? There seems to be nothing, and so the object collapses forever. String theory would have us believe it collapses to the size of a string but we'll have to wait to see on that one.

This is where the term singularity comes from. Essentially, it's like having a plot of something like 1/r^2 and as you approach zero it shoots up faster and faster. When you "get" to zero, you have a singularity there because, as we like to say in physics, the function "blows up" when you divide by zero. The singularity of a black hole is where time ends for all observers. The event horizon is where all futures in spacetime end at the singularity. The photon sphere is where the star's last light was emitted as it collapsed (you could actually see the back of your head here if you looked horizontally). Since photons go the speed of light in any frame of reference, they're still happily chugging away, though in incredibly warped spacetime. An observer would see the black hole (only by the backround light it blocked and bent around itself) rushing at them at almost the speed of light.

Anyways, it's late and I'm sure I've made some spelling errors, but I thought I would add my bit to this discussion. In cause you were wondering, I'm just finishing my third year of my undergrad degree in physics and I'm particularly interested in classical and quantum field theory and general relativity.

Joined: 3/26/2007
Msg: 107
Posted: 4/21/2007 11:13:13 AM
Couldn't have said it better Jerry.

Can you maybe briefly explain that because I'm not sure I'm getting your drift here. You did mention that light speed is constant ....right?. But with gravity , we have to take into account that there is an acceleration that must take place for something to move faster.

No matter how you look at light, in the classical or quantum sense, it always goes the speed of light. There is no acceleration. Classically, light is an electromagnetic effect caused by accelerating charges. No matter how you accelerate the charges, they will have time-varying electric fields. Faraday's Law (one of Maxwell's equations) says that a changing electric field induces a magnetic field. But if there is a rate of change OF the rate of change of the electric field, this creates a magnetic field which also varies in time. The process continues ad infinitum and this is what we call an electromagnetic wave. The constants of the wave equation for an electromagnetic wave dictate how fast this goes. This speed is

1/(root(mu_0*epsilon_0)) where mu_0 is the permeability of free space and epsilon_0 is the permitivitty of free space. This fraction comes out to be the speed of light in a vacuum.

In the quantum case, a photon is a masless boson which is created by the electromagnetic interatcion. It makes sense that a massless particle would automatically travel at the speed of light.

Also, the equivalence principle in general relativity says that depending on your frame of reference, you could view gravity as an acceleration in one frame or vice versa. Say you jump out of a plane and neglect air resistance. You will, in your frame of reference, be weightless. In the frame of the ground, you are accelerating towards the Earth (or the Earth towards you) at 9.81 m/s/s. The same goes for astronauts on the space shuttle. They are in a gravitational field and are constantly plummeting towards the Earth, but since they revolve so quickly around the Earth, their altitude does not change and they appear weightless in their frame of reference.
Joined: 2/27/2007
Msg: 108
Posted: 4/21/2007 7:12:46 PM
And just to add to that, weightless is not the same as massless. Just making sure nobody gets confused and thinks those astronauts could instantly accelerate to the speed of light.
Joined: 3/26/2007
Msg: 110
Posted: 4/21/2007 9:47:00 PM
All good questions.

Here, you have to look at it from the frame of one ship. If they are moving directly away from each other, which ship you consider doesn't matter. In one frame, the other ship will appear to be moving away at high speed, but no matter how fast either ships go, this speed will never exceed c. This comes out of the Lorentz velocity transformations which tell you how one velocity looks in another frame. I'd say this is where special relativity gets the most confusing, mathematically speaking.

If the ships move apart on an angle, you can redfine your coordinate system so things move in only one dimension of space. That's the great thing about special reltivity. Remember, no one frame of reference is more right than another. We could say they move apart at 120 degrees in one frame, or redefine the coodinates such that they only move apart along the line of sight of either ship. In this case, the new speeds can be calculated and then used in the Lorentz transform equations to find the relative speeds. No matter how many ships you consider, you have to find the "lay of the land" so to speak from each one, because each frame of reference has a different point of view. This is true whether ships are going towards or away from each other.

Another important thing to consider, and this is a fundamental postulate of relativity, is that the speed of light is the same in ALL frames of reference. If a ship is going 0.9999999c and send out a pulse of light in all directions, that sphere of influence will be a sphere for ANY observer.

You can calculate time dilation effects quite simply on any decent calculator. The equation is

t = (1/(root(1-(v/c)^2)))*t_0 ,

where v is the ships speed relative to you, c is the speed of light, and t_0 is the "proper time" or the time in your rest frame. The bracketed term (1/(root(1-(v/c)^2))) is usually called gamma to make calculations less messy. This is the fraction which determines time dilation and length contraction effects.

You asked what the time dilation would be for a ship going 0.5c past you was. Say the you measure 10 seconds on your watch. To the guy on the ship, that same 10 seconds actually takes t = (1/(root(1-(0.5)^2)))*10s = 11.5 seconds.

The effect is more drastic at higher speeds. In the same scenario, t = (1/(root(1-(0.99)^2)))*10s = 70.9 seconds. Relative to the observer on the ship, it takes over a minute of his time for your watch to read 10 seconds!

As for the doppler effect, I'm pretty sure that if you put in speeds of 2c, you would get a complex (imaginary) shift in wavelength. When calculating reletivistic doppler shifting, use the relative speed of the source, which you can again find using the Lorentz transformations. In general, the faster the source moves toward you, the bluer its light appears. The faster it moves away, the redder it's light appears.

One last note. It is perfectly acceptable for you to be at rest and observe two ships moving away from each other at high speeds. Say there is a space station some ways away in your rest frame and two ships leave, each going 0.75c. Relative to YOU, the ships are moving apart at 1.5c. When you consider the frame of one ship or another, you again have to consider relativistic effects.
Joined: 2/27/2007
Msg: 111
Posted: 4/22/2007 12:28:57 AM
To specifically address this:

"Wouldn't c be exceeded if 2 objects were moving away from each other on a straight line at anything faster than .5 c each
That is to say when viewed from each other? Or relative to each other?
Wouldn't the other object dissappear, relatively speaking?"

The objects would not disappear from view from either viewpoint. Light coming from an object moving away from you inherits none of that object's momentum, so it doesn't have to "overcome" that .5c, though it would be red shifted. Also, let's say there is a lighted dot moving from side to side on that object. It seems to me that it would appear to move more slowly than if the object were not moving away from you.
Joined: 3/26/2007
Msg: 112
Posted: 4/22/2007 5:36:38 AM
Ah touché.

That's what I was alluding to when I mentioned the sphere-shaped pulse of light coming from a ship. No matter how you look at it, you'll always see light coming from it. You said it better though lol.

And yes, you can't give momentum to something which doesn't have momentum based on its mass and speed, like light. Light does have momentum from quantum mechanical effects though.
Joined: 2/5/2007
Msg: 113
Posted: 4/23/2007 12:37:28 PM
It is a spherical hole, think of it as the void in the center of a block of swiss cheese, the same in all directons, as opposed to a hole through a slice of swiss cheese.
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