From: Ronald Wong (ronwong@inreach.com)
Date: Tue Jun 01 2004 - 22:38:51 PDT
Message-Id: <l03102800bcdc17087c35@[209.209.18.97]> Date: Tue, 1 Jun 2004 22:38:51 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: re: Astro ?'s from a student
About a week ago, Treena asked:
>Is it true that gravitational energy waves can vary in length from a km
>to a few million miles?
Actually, the range over which gravitational wavelengths can vary is
considerably greater than that.
Consider the following:
A. Electric charges are surrounded by an electric field.
Accelerating an electric charge (by wiggling it back and
forth or moving it around in a circle) will disturb the
electric field, creating an electromagnetic wave that will
travel away from the accelerated charge at the speed of light.
B. Matter is surrounded by spacetime.
Accelerating an object (by wiggling it back and forth or
moving it around in a circle) will disturb spacetime,
creating an gravitational wave that will travel away from
the accelerated mass at the speed of light.
The speed of light is constant and is large in value. Because of this,
wavelength is inversely proportional to frequency by a very large constant.
Low frequencies produce very large wavelengths (a frequency of 1 Hz, for
instance, will produce a wavelength of 300,000,000 meters - about 2/3rds
the distance to the moon) and high frequencies do just the opposite (a
frequency of 300 000 000 Hz will produce a wavelength that proportionately
smaller, 1 m - a little longer than the distance from the tip of the
fingers of your outstretched hand to your nose).
This is true whether the oscillations involve an electric charge or matter.
So the range of possible wavelengths for both gravitational waves and EM
waves are the same and are even greater than the examples I've given - from
the smallest to the largest that is physically possible.
>This is based on his awareness of work from LISA & LIGO that I am not
>familiar with
LISA & LIGO are two large-scale projects designed to detect the presence of
gravitational waves using the principal of interference. LIGO is already up
and running and it involves two perpendicular arms that are each 4 km in
length. LISA is a project in the making and, if brought to fruition
(2009?), would consist of a triangle in space whose sides would be 5
million km in length.
There are similar, ongoing projects that are considerably smaller in scale.
Their underlying principle involve resonating masses. The objects - a
solid, metal cylinder or a sphere within a sphere - would fit in the back
of a small pick-up truck.
All the projects are trying to address an unfortunate fact of life/nature.
Namely, that the force of gravity and the field associated with it are VERY
weak.
A micro-coulomb of charge placed one meter from another micro-coulomb of
charge leads to an electrical force of 0.01 newtons acting on each of them.
This is a very small force - about 0.002 lbs.
In order for two identical masses separated by 1 meter to produce the same
0.01 newtons of gravitational force, their mass would have to be around 10
000 kg (around 11 tons EACH)!
Given how small gravitational effects are and how far we presently seem to
be from any source of gravitational waves, we are confronted with the fact
that only
1. the most massive bodies (like neutron stars or black holes - which
will distort spacetime the most and produce the biggest waves) orbiting
2. very close to one another (the orbital frequency will then be the
highest producing the smallest possible gravitational wavelength)
will distort spacetime in a manner that will produce gravitational waves
that we can detect - namely, those with a lot of energy and reasonably
small wavelength.
The last time I looked, the highest known gravitational frequency was 100
Hz (it could be higher by now). That corresponds to a wavelength of 3 000
km. Ideally, the detector should be 1, 1/2, or, if necessary, 1/4 of this
wavelength. All the current, operating detectors fall far short of this. As
a consequence, there is a low signal to noise ratio due to thermal effects
and mechanical vibrations (i.e. anything above absolute zero K and seismic
disturbances creates problems).
To get increase the signal to noise ratio, the small scale projects conduct
their experiments at near-absolute zero temperatures with the maximum
amount of mechanical isolation as possible.
For the large scale projects, the solution is to go into space (very cold
and extremely isolated) and approach (or exceed, in the case of LISA) as
much as possible the dimensions of the gravitational wavelength under
consideration. The latter increases the signal to noise ratio.
Unfortunately, the very size of the detector creates it's own problems. But
the engineers and scientists working on the project are confident they'll
solve all the problems.
>
>How long is the transient, poles of gamma radiation being shot off by
>neutron stars?
I don't understand the question so I can't answer it.
>
>If two black holes were to hit each other, would they collide,
Yes. And it would be quite an event. A tremendous amount of energy would be
released in a very short period of time (the equivalent of about 10% of the
total mass). All of it would be in the form of a gravitational wave
(remember: this involves a pair of black holes so there won't be any EM
waves radiating away from them) and would create the gravitational wave
equivalent of a sonic boom.
>If a super massive bh and a regular bh is getting sucked in, then the
>smaller one will spin around the larger until absorbed is his view.
Orbiting pairs of black holes (whether super massive or not) do a very good
job of disturbing spacetime. This takes energy. The gravitational waves
produced as a result of the disturbance carries this energy away from the
orbiting bodies. In spacetime, this loss of energy comes at the expense of
their motion and they spiral in towards each other over time.
>Also, why do most galaxies, with bulges at least, have black holes at the
>center? Hard to understand he says, how they come to be, why not at edge?
I'm going to respond to these two questions in the reverse order:
You need a star with more than ten times the mass of our sun to create a
black hole. The way stars form in a galaxy makes this somewhat rare but we
are finding them around us. The nearest one is only 1600 light years away
in the constellation Sagittarius. We recently found one in the halo of our
galaxy so I wouldn't be surprised if one was found at the edge of our
galaxy. This opinion doesn't mean one CAN/WILL be found at the edge so I'll
have toleave it to someone else to answer this question because I just
don't know.
What I can tell you is that the black holes that we find in the center of
most - if not all (the jury is still out on this one) - galaxies (no matter
what their shape) are totally different from the ones we find around us.
The ones found in the center of a galaxy are referred to as a massive black
holes (or "super" massive black holes). Unlike the black holes around us
that are typically less than ten times the sun's mass, the one at the
center of a galaxy is a million to a billion times more massive than our
sun.
Current models of a galaxy's history suggest that far more stars appear to
form in the center of a galaxy than elsewhere. Through gravitational
attraction or collisions, many of these stars merge into a single, massive
object and that leads to the creation of a massive black hole in the center
of the galaxy. This is why "most galaxies...have [massive] black holes at
the center".
>- do we revolve around the super massive black hole at the center of our
>galaxy... doomed to be sucked in
Well, all the evidence points to the fact that we ARE orbiting a massive
black hole in the center of our galaxy.
Fortunately, it seems to have gobbled up all the gas and debris surrounding
it.
As a result, we are said to live in a "normal" galaxy. This saves us from
having to deal with the extreme conditions that exist in "active" galaxies
like quasars, Seyfert variables, and blazars. In these galaxies, more
energy is being given off by the core of the galaxy than the sum of all the
stars it contains. This comes about because, unlike a normal galaxy, there
is still a large amount of material surrounding the active galaxy's massive
black hole and it's constantly being drawn into the hole, leading to the
creation of tremendous amount of EM radiation.
>... doomed to be sucked in
Given our distance from the center of our galaxy, I wouldn't worry too much
about being "sucked in".
Before that happens we'll have other, more catastrophic things to worry about:
A. The sun will have evolved into a red giant - consuming our planet
in the process and, at around the same time,...
B. we will be colliding with our neighboring galaxy, the Andromeda galaxy.
It's going to slam into us in about 3 billion years and, after another
billion years passes, what's left will be in the form of an elliptical
galaxy (or so the models predict).
Now THERE's something to stay up all night long and worry about.
On that note, I leave you.
Sweet dreams.
ron
This archive was generated by hypermail 2.1.3 : Mon Aug 02 2004 - 12:05:38 PDT