The Universe is made up of millions of galaxies. Also out
in space is background radiation from all the explosions and nuclear fusion
going on within stars and nebulae. And, of course, the universe has a whole lot
of empty space.
In fact, there is so much space even between single stars in a galaxy that it
is difficult to measure in miles or kilometers. Scientists use a distance
measurement in space that is expressed as the distance light will travel in the
course of a year - a light year. Light travels at a speed of about 186,000 miles
per second. That means that light can travel 7.5 times around the entire world
in just 1 second! In a year's time, light can travel six trillion miles
(6,000,000,000,000). It takes over 4 years for the light from the nearest star
to reach the Earth.
Now that we have the light year scale to use, we can
compare that to more understandable speeds and reference those speeds as the
time needed to arrive at a neighboring planet, star, or galaxy.
You know what if feels like in an automobile traveling 60 miles an hour. It
is not unusual for a jet to travel 10 times that speed - 600 miles an hour.
Therefore, a jet can go 10 miles in 1 minutes or 1/6 mile in 1 second. Now let's
compare that speedy jet to a rocket. A kind of average rocket speed is about 6
miles per second. Next, if we compare the rocket speed to the speed of light, we
find that light travels 31 thousands times faster!
So, finally, we can begin to grasp the size of the universe by comparing how
long it would take to travel from Earth to some of these distant objects if we
could travel by jet, rocket or sunbeam.
Travel Time from the Earth
17 years 8 months
10 years 10 months
5 years 5 months
8 years 10 months
74 uears 3 months
1 year 9 months
150 years 5 months
3 years 7 months
1 hr 11 min
318 years 6 months
7 years 7 months
2 hr 30 min
690 years 1 month
16 years 5 months
5 hr 25 min
4.8 million years
9.6 million years
Center of the Milky Way
2.2 million years
The table above is much more than a chart of travel time, it
represents a glimpse into the past. When you look into the night sky you are
looking into the history of the universe. The sunlight that shines on us is 8.5
minutes old when it reaches Earth. Sunlight reflected from Pluto takes 5.5 hours
to reach the astronomer's telescope. When the light of Sirius hits your eye,
those photons have been traveling for over 8 years through space. This means you
are seeing that star not as it is tonight but as it was over 8 years ago. And
most of the stars we see in the sky are hundreds or thousands of light years
away. The Andormeda galaxy, at a mere 2.2 million light years, is truly a next
door neighbor. All of the other galaxies are millions upon millions of light
years distant. And that's how big the universe is.
The numbers are only rough estimates and assume the celestial bodies are not
in motion for the sake of ease of calculation
Does the "Big Bang" mean that the Universe started out
as an extremely large supernova-like event?
Other than the fact that a supernova and the Big Bang are both
"explosions", there is little similarity. Cosmic
Mystery Tour at UIUC gives a definition of the Big Bang, and Timeline
of the Universe at NASA JPL talks about what scientists believe happened
in the time immediately afterward.
I read that, according to the data from the Boomerang experiment,
the Universe seems to be flat. Does this agree with General Relativity that
predicts a bent spacetime? After this discovery, which models of Big Bang
Their results do seem to imply that the Universe is flat, however this in
no way contradicts General Relativity, which allows for many different
geometries depending on the amount of matter and radiation in the Universe.
Please explain how fast the Universe is expanding.
It appears that the Universe is expanding at 80 km/sec/Mpc
(statistical error = 17 km/sec/Mpc), as calculated by the Hubble Space
Telescope's Key Project team (Mpc is megaparsec = 3.26 million light years).
What this means is that objects will, on the average, be moving away from us
at 80 km/sec for every megaparsec it is away from us. So another galaxy that
is 1 Mpc away will be moving away from us at about 80 km/sec, and one that
is 10 Mpc away will be moving at about 800 km/sec.
There's a good write-up at the MAP mission site entitled "How
Fast is the Universe Expanding?" MAP is scheduled to launch in the
fall of 2000. One of MAP's goals will be to accurately determine the Hubble
constant to better than 5% accuracy.
What is the speed at which our Galaxy is moving away from the
theoretical point from which the Universe is expanding? What is the speed at
which our solar system is moving around our galactic center?
A discussion on the "center of the universe" is here.
We're moving at 600 km/sec relative to the cosmic background. You can find a
good description of how we are moving through space at this NASA
Astronomy Picture of the Day page.
What if the energy driving and directing the Big Bang was God?
The Big Bang theory does not seem to account for the source of any
energy/matter. What if it derived from a God in the process of creating a
Universe and His method was the Big Bang?
Your question has come to NASA, and I'm sure you know that we
answer scientific questions, not religious ones. But this question has been
addressed by our sister site, Imagine
I have seen the "expanding Universe" pictured as the
surface of an inflating balloon. Using this image, can the direction back to
the origin (center of the ballon) of the Universe be determined? Are there
theories regarding what now occupies this point of origin? If there is
nothing there, can we look through it to the far side of the Universe?
The inflating balloon example uses a 2-dimensional surface (the
balloon's surface) to illustrate what is happening in 3 dimensions. The
"center" of the balloon is not on the surface, and so if there is
a "center" of the Universe, it is in the fourth dimension, not in
the observable 3-D space. But a better way to think of it is that the whole
Universe was at the center when the Big Bang happened (the balloon was
scrunched up into a very small space). One has to be careful not to stretch
one's analogies too far :-)
My astronomy teacher told our class that the Universe is overall
homogenous, and that the Universe is expanding. He also said that scientists
have trouble figuring out why the Universe is so uniform in all directions.
I also know that there are three major theories of the shape of the
Universe: spherical, Euclidean, and saddle Universe. If the shape is
spherical, is it possible that the reason the Universe looks the same when
we're looking to the north as it does when we're looking to the south is
because the light bends in such away that we're actually looking at the same
It's not that the Universe is so uniform that is the puzzle. It's
the fact that the remnant of the Big Bang (the cosmic microwave background)
is very uniform, yet galaxies developed very early in the Universe. Galaxies
are large non-uniformities in the mass of the Universe, and how matter got
so clumped from such a smooth origin is the puzzle. While it is not
impossible that light wraps around the Universe, there is no evidence that
is does (we don't see the same galaxy or quasar in different directions).
What is the approximate temperature of the Universe, and how can
it be calculated?
The entire Universe is filled with the remnants of the Big Bang,
in the form of photons (electromagnetic packets). They have cooled down to
about 2.7 Kelvin or 2.7 degrees above absolute zero (-270.7 degrees
Centigrade). So this is the temperature of space. It can be calculated from
the expansion of the Universe, and it has been measured.
You can learn more about the COBE mission that measured this here.
Drs. Eric Christian and Louis Barbier
The Universe gets colder because it's expanding, but it
can't get to 0 o K, can it?
Absolute zero cannot be obtained, and the approach to it will be
very, very slow.
How are the big bang and the law of conservation of energy
Since the Universe (as far as we can see) has a finite amount of
energy, all that is required is for that amount of energy to be present in
the Big Bang, and energy is conserved. There is no way to prove that this is
true, but it is a good working hypothesis.
How is cosmic nucleosynthesis and helium abundance related to the
Big Bang theory?
Most of the helium in the Universe was created about three
minutes after the Big Bang, when the temperature had cooled enough for nucleosynthesis
to take place. Current theory says that about 25% of the baryon mass would
have been helium after the Big Bang. Only 1 or 2% more helium has been
created in stars since then.
I read recently that there is a consensus among many top
scientists that the Universe is ever expanding and that there seems to be
proof of this now. I further understood that this discovery and agreement
among the scientists was the great discovery of 1998, perhaps of the
century. Sadly I can't find the article or related articles. Could you
possibly refer me to somewhere I could again access this information?
Science Magazine named the evidence (not proof) that the
Universe's expansion is accelerating as the "Breakthrough of the
Year" for 1998. You can get more information at http://www.lbl.gov/supernova/.
How do astronomers use Doppler shift to conclude that the
Universe is expanding?
There's a good explanation of the astronomical use of the Doppler
shift at our sister Web site, Imagine
Is it not true that, according to Relativity, light
traveling through gravitational fields will become red-shifted? And seeing
as we are learning that most of the Universe is dark matter, could not the
observed red shift be a factor of light having been affected by the mass of
this dark matter?
It's not a bad idea, but the problem is that light is red-shifted
when it gains gravitational potential energy, or in other words, when it is
generated from deeper in a gravity well than it is observed (if it were
observed deeper in the well, it would be blue-shifted). In order for us to
observe a gravitationally induced red shift in all directions, we would have
to be at the minimum gravitational potential in the whole Universe. Most
astronomers don't beleive that our location is special (the center of the
Universe). If the red shift is due to the expansion of the Universe, the
same red-shifts are observed throughout the whole Universe. The classic
analogy is that galaxies are like points on a balloon. If the balloon
expands,from one point it appears as if all the other points are moving
In an infinite universe all points, in essence, are the
center, and light entering from any distant point enters the central
gravitational well from the point of view of the observer. No?
Nope. In an infinite universe, the gravitational potential is
flat (there is no preferred point). The local galaxy will cause a potential
well, but that blue-shifts light (the light "accelerates" towards
the galaxy). In order to get red shifts, you need a gravitational hill, or
in other words, you need to be in a low mass region with the density of
matter increasing in all directions. That's not what's observed.
I thought that relativity predicted the red-shifting of
light through high mass density? I think I'm seeing it in just the opposite
way, and maybe that's where my confusion lies. Let's say from our position
in the Milky Way, we are only able to detect a certain portion of the mass
that surrounds us. Light making it's way through that mass then becomes
blue-shifted? Is that what relativity and light/mass interaction predicts? I
thought it was the reverse.
It's not passing through matter that does anything to light. It's
changing gravitational potential energy. If light claws its way out of a
potential well, it loses energy and is red-shifted. If it falls down a well,
it gains energy and is blue-shifted. But if it comes out the other side, it
loses that energy again. Light that is generated near a neutron star, for
example, is red-shifted. But it is the gravitational potential that does it,
not the mass density. Another way to think about it is that the light
depends only upon the difference between the gravitational energy where the
light is emitted and where it is observed. What lies between doesn't matter
for this effect (although there are plenty of ways in which it does matter).
What would the rate of expansion of the surface area of a sphere
of light be, and what would it look like, if the sphere's radius was
increasing at a rate equal to the speed of light -- if you saw it first
right at the moment of the Big Bang and then after it had been expanding for
15 billion years? And what value of pi would you use in the calculation?
This question came to me when I was reading "A Brief History of
Time" by S. Hawking. He said on "....if a pulse of light is
emitted at a particular time at a particular point in space, then as time
goes on it will spread out as a sphere of light whose size and position are
independent fo the speed of the source. After one millionth of a second the
light will have spread out to form a sphere with a radius of 300 meters;
after two millionths of a second, the radius will be 600 meters; and so
on." I tried to calculate the increase in the size of the surface area
as its radius expanded at the speed of light, and I found that when I got to
the point in time where the surface area of the sphere was really big....15
billion years old...that the surface area was so big, that the actual value
of its size seemed to depend on which value of pi I used.
The surface of a sphere expanding at the speed of light is A
= 4/3 * pi * c * c * t * t, where c is the speed of light, and t is
the time since the light was emitted. There is only one value of pi.
The precision that you use on your calculator will affect the final
precision of your answer, but since the speed of light is only known to
about 8 significant figures; using more than that in your value of pi
doesn't get you anything.
Did the Universe expand faster than light? If not, why does
light, up to 12 billion years old, reach us only now?
The Universe did not expand faster than light. The Universe was
big enough 12 billion years ago that the light from some distant objects is
only getting to us now. That doesn't mean that the Universe was more than 12
billion light years wide 12 billion years ago. Because we're moving away
from the object, the light has had to catch up to us.
Since red shift is a measurement of how distant an object is, and
the speed of light limits the maximum red shift, does this mean that the
speed of light imposes a maximum size for the Universe?
The answer to your question is yes and no. The red shift is
really not a factor, but the speed of light does impose a maximum size on
the OBSERVABLE Universe. By this, I mean that if there is something furthur
away than the speed of light times the current age of the Universe, the
light will not have reached us yet, and so we can't know anything about it.
That doesn't mean that the Universe isn't larger than we can see, we just
can't prove whether it is or not.
Whether space ever ends is a hard question. There is a limit to
the space that we can see, because if there is stuff beyond 15 - 20 billion
light years (the age of the Universe) the light from there hasn't reached us
yet. So we don't know.
As the Universe expands, it cools. When the Universe gets really
cold, will the entire Universe become a Bose-Einstein Condensate (BEC) and
then collapse back to a singularity, closing the Universe?
When the Universe has cooled down to less than a Kelvin, BEC
might form locally (inside former stars and planets), but there is no known
force that is stronger than gravity across the vast distances that the
Universe will have spread. So if gravity doesn't close the Universe, nothing
we currently know of will do it.
How often do supernovae occur? What devices do we have to
pinpoint these as they occur (or rather as light and particle emissions
There is about 1 supernova per century in our Galaxy. There
is a Supernova Cosmology Project
to search for supernova run by Saul Perlmutter at LBL.
How many galaxies are close to our galaxy? What is the farthest
galaxy seen from Earth?
Our galaxy (the Milky Way) is one of at least 17 galaxies that
are called the "Local Group". There are probably other galaxies in
the Local Group that we haven't seen yet (blocked by gas or the rest of the
Milky Way). The Andromeda galaxy (M31) and the Milky Way are the two largest
galaxies in this group. They are all within about two million light years of
Do all galaxies appear in a cartwheel '2D'-like format or can
they have many arms which extend from the center in a 3D fashion?
There are roughly three types of galaxies: elliptical, spiral,
Elliptical galaxies are just relatively uniform balls (although not
necessarily spherical) of stars.
Spiral galaxies, like our Milky Way, are typically flat disks (plates)
with a bulge at the center. The cartwheel arms form in the disk, and so
are '2D' not '3D'. The thickness of the arms in the Milky Way is a few
hundred light years, but the disk is 100,000 light years in diameter.
Irregular galaxies are ones that aren't elliptical or spiral, like the
Greater and Lesser Magellanic Clouds.
What exists between galaxies? Is it just a vacuum of space, or do
they all butt up to one another?
There is lots of space between galaxies. The nearest galaxy close
in size to the Milky Way is Andromeda, and that is 1,600,000 light years
away, although there are smaller ones closer (like the Magellanic Clouds).
They do not butt up against each other, although they can collide.
What is the velocity the Milky Way or Local Cluster moves from
the origin of the Big Bang, and does this speed affect our time? For
instance, would time move much faster if the galaxies/Universe ceased
The Milky Way appears to be moving at 600 km/sec relative to the
primordial background radiation (the remnant of the Big Bang). For more
information on this, you can check the Astronomy
Picture of the Day archives.
This speed is much less than the speed of light, however, so the effect
on time is negligible.
How can you tell where our galaxy ends and another begins?
Galaxies are large groups of stars that are held together by
their mutual gravitational attraction. Although they are very large (the
Milky Way galaxy that the Sun is in is 100,000 light years across) the
distance between galaxies is even larger (the Andromeda galaxy is 2 million
light years away). There is a lot of nothing between the galaxies. So it is
easy to tell where one ends (when there are no more stars) and where the
next begins (where the stars start again).
For more information on galaxies, you can check our sister site,
The reason it is dark in space actually has to do with the fact
that the Universe we can see is finite (has limits), either finite
in size or age. They are essentially the same thing because the finite age
of the Universe (15 - 20 Billion years) means that light from stars furthur
away than 15 to 20 Billion light years hasn't reached us yet. So the
Universe looks to be 15 to 20 Billion light years in radius, even
if it's bigger.
The fact that the sky is dark is known as Olber's paradox. If the
Universe was infinite, there would be a star in every direction, and the sky
would be uniformly bright. Instead the stars and light are spread out enough
that it is dark.
I am confused. If the Universe is 10 to 20 billion years old, and
black holes are the end of a star cycle taking maybe 10 billion years, how
is it possible for quasars, which need, or are a form of, black hole, to be
14 billion light years from Earth. Wouldn't the quasar's rays still be
traveling to Earth's atmosphere?
Let's say the farthest quasar from Earth was formed at the exact same
time its host black hole was created, and that the black hole's former star
self was formed a second after the Big Bang. If the star took 10 billion
years to become a black hole, wouldn't that mean that the farthest quasars
are only 10 billion light years away? This, I realize, is not possible,
since I understand it took at least 1 billion years after the Big Bang for
the first stars to form. And it would take a star the size of our Sun 10
billion years from creation to become a black hole. How is it possible? Is
my understanding wrong?
The assumption that you've got wrong is that it takes 10 billion
years for a star to become a black hole. Only extremely massive stars
explode in supernova and become black holes. Our Sun will end up as a white
dwarf, and stars more massive than the Sun can end up as neutron stars. Only
stars with mass more than 10 times our Sun's will become black holes, and
these stars burn brighter and die sooner. Their lifetime is only a few
hundred million years, not 10 billion. Also, quasars and giant black holes
at the center of galaxies may form from stellar and gas collisions, and may
not require that the stars that started them go through their entire life
I read somewhere that after time and matter were sucked into the
center of the black hole, they became a part of the singularity amd ceased
to exist as we know space and time. Is it possible that time/light/matter is
then "stored" in the center of the black hole?
There's a good page on black holes, including the singularity, at
the Universe! Your question is answered there.
Would the solar system, if still remaining after the Sun goes
supernova, be eventually pulled spiralling into the black hole that's in the
center of the Milky Way?
You've got two common misperceptions here. A black hole doesn't
have any more gravitational attraction than the star or whatever that formed
it. If the Sun instantaneously turned into a black hole, the Earth and all
the other planets would orbit just the same with no change in gravity.
Because black holes are much more concentrated, however, you can get closer
to them than you could to the Sun and get to a region where the force of
gravity is much higher. But at the distance of the Earth there is no
The other common misperception is that the Sun is going to go supernova.
Only the largest of stars can go supernova. Our Sun is much too small. It
will eventually expand to a red giant, and then contract down to a white
From what I understand, if something could travel at the speed of
light, it's mass would reach an enormous size, and it would appear to be
nothing (since as things approach the speed of light they become thiner and
thiner until they appear to be nothing). This is similar to the description
of a black hole, something with an enormous mass that we are unable to see.
Is there any connection between these? That is, black holes could be objects
or planets that have reached the speed of light. This would explain their
huge mass and the fact that we cannot see them.
They are not really related. If an object is moving at close to
the speed of light, it will appear to have a very high density (more mass
and thinner), but it will not be a black hole. It will appear normal to
something moving along at the same speed. A black hole has enough rest mass
(mass in the reference frame where it is at rest) that light cannot escape
If the center of the event horizon sphere surrounding a black
hole is a point of zero dimension, and the velocity of light is absolutely
constant, the sphere should be absolutely perfect. If a photon originates on
a radius of the event horizon sphere, half way out to the
"surface" of the sphere, would it not (with its head start) be
able to travel beyond the surface of the sphere? Would it continue on out
only to be drawn back toward center from further out? If that happened, it
would have established a single point on another and bigger event horizon
sphere. If a huge number of photons were being created at all distances from
center, might not the event horizon of the black hole be a very fuzzy thing?
The event horizon of a black hole is not "fuzzy". A
photon that starts an infinitesmal distance inside the event horizon never
makes it past the horizon. It does not get a little further before it is
dragged back. This is because space is warped, and you can't think about it
the way you normally think about gravity.
If gravitons are quanta of the gravitational field, and mediate
the gravitational force, how do gravitons escape from black holes? I
understand that regions of strong gravitational fields can cause production
of particles (in much the same way as strong E-fields can cause
electron-positron pair production). Is this the mechanism by which gravitons
are mediated? That is, they are not exchanged between the actual masses
doing the gravitating, but by the space which they distort?
Thank you for your inquiry. Your question is one of the
questions asked most frequently at relativity courses in graduate school.
A black hole is a region of space-time which curves back in upon itself -
presumably because of the collapse of a massive star to a size below the
Schwarzschild radius. Photons or other particles don't escape from
"inside" a black hole, because the extreme curvature doesn't allow
any "escape trajectories". Imagine particles moving along the
inner surface of a ball, and trying to get outside the ball - they never
could. (A black hole is not spherical like a ball however.)
To better answer your question, let me first move away from gravitons,
which we do not understand very well, to photons, which are better
understood. It is known that black holes can have charge, and therefore a
static electric field. The question is, how does this field escape the
horizon? In that, I should remind you that the horizon is the surface
through which one cannot send any signals. Now, signals (i.e. photons)
involve time changing fields. Indeed no such signal can come out of the
horizon. However, the electrostatic field of a charge does not convey any
signal; it can therefore escape from the black hole. In technical parlance
it is a space-like object, and those can cut across horizons. Or
differently, in the language of excitations of the electromagnetic field
(the photons)- there are transverse and longitudinal photons. The transverse
ones are those we see, and travel at the speed of light. The longitudinal
ones are those which "are there" in space, they do not convey any
signal and are the static fields of the charges.
Now at some level of approximation one could substitute gravitons for
photons. the ones that correspond to the longitudinal part are those
providing the static Newtonian (far away) fields of black holes (and all
other objects). The ones producing gravitational radiation require
"shaking" the gravitational lines of force and those come from
outside the horizon.
Is it true that black holes are a gate to another dimension?
There is no scientific evidence that black holes lead to another
dimension. Even if they did, you would be torn into your component atoms by
tidal forces long before you actually got through the black hole. It does
make for interesting science fiction stories, however.
What happens when you're not in space/time? Or, more precisely,
if you go through a black hole is there a separate dimension on the other
side? If there is, you continue to exist. If there isn't, you cease to
exist. Though NASA can't necessarily send an intelligence probe through a
black hole (too much radiation, too far away, no funding, and probably no
response from the probe) what would happen if you rotate something 360 +
1/infinity degrees? Since a black hole is an infinite curve it's probably
the same thing.
We don't know exactly what would happen if you weren't in
space/time, since we have no observations of any place that is outside of
space/time. If you traveled into a black hole, the tidal forces would tear
you into component atoms before you got to the Schwarzschild radius. After
that, your mass would be added to the singularity, and that's it. There is
no evidence that you would come out somewhere else (no signs of white holes,
for example). Rotating something by 360 + 1/infinity degrees just gets you
immeasurably little past 360 degrees, it does nothing special.
I have come across conflicting information. I have read there is
no matter at all in space, and also that most of the gas in space is
hydrogen and helium. I'm guessing that the hydrogen and helium are found in
space within galaxies, and no matter is between galaxies (what about dark
matter?). Please explain!
There is matter spread all through the Universe, it is just
spread very, very, very, very thin. The average density of gas in our Milky
Way galaxy is about one atom per cubic centimeter. This is a much better
vacuum than is obtained in a laboratory, but when integrated over the
Galaxy, comes out to quite a lot of mass. This gas is mostly hydrogen
(~90%), and helium (~9%), and less than one percent everything else. The gas
between galaxies is even thinner, but there is probably something there (it
hasn't been measured, though). The amount of dark matter is very much in
question. From the effects of its gravity, it appears that dark matter is
associated with galaxies, but extends further than the visible matter
(stars). From the dynamics of galaxies in galactic clusters and
superclusters, it looks like there is dark matter between the galaxies as
It seems to me that with space travel, the speed of a spacecraft
would be limited by the matter in space due to friction. Is this true?
The density of matter in our Galaxy is about 1 particle/cm3
(in the disk, with the halo being less dense). The density of matter in
intergalactic space (between galaxies) is about 2 x 10-31 gm/cm3,
mainly hydrogen. At these densities, I don't think one has to worry about
What does it mean to say that stars burn the same throughout the
Universe? Aren't some hotter than others? Isn't it true that most of the
Universe is composed of plasma, i.e. an unstructured mixture of quarks and
electrons WITHOUT a strong force? This was at least the case just after the
Big Bang. So the strong force is/was different at different places and
velocities. If I'm way off base, please direct me to a good book.
Stars get their energy by fusing lighter elements into heavier
elements. There are no free quarks in stars; they have electrons and nuclei
right through. Saying that stars burn the same throughout the Universe means
that observations of stellar temperatures and the elements present are
consistent no matter where we look.
Most of the Universe IS a plasma, but a plasma is a gas of electrons and
atomic nuclei, not quarks. Current theory says that you can't have a quark
soup except at very high temperatures, such as right after the Big Bang. But
even then, the strong force was present (as were gravity, electromagnetism,
and the weak nuclear force). There are NO observations that indicate that
the four basic forces of nature aren't constant throughout the Universe.
I'm not sure of any popular book that goes into this stuff, maybe one of
Stephen Hawking's. Sorry.
I have recently heard about a Dr. Theodore Landsheidt from
germany that did some research on plotting the center of mass of our solar
system. He said that the center of mass revolves around the Sun and can move
as far as one solar radius from the actual surface. can you tell me where i
can find more info about this and/or where i can find a recent diagram
plotting the path the center of mass follows around the Sun?
All that is really needed in order to plot the center of mass of
the solar system is to know the weights and positions of the planets.
Everything else will only have a small effect. I suggest you look at:
http://www.sunspot.noao.edu/PR/answerbook/gravity-2.html#q52 where I think
you'll find what you want. Hope this helps.
It's my understanding that 90% of the atoms in the Universe are
hydrogen, 9% are helium. The remaining 1% are the heavier elements (made by
stars, supernovas, etc). I wonder how 90% of the mass of the Universe can be
"invisible" - unless the visible, measurable Universe accounts for
just 10%. Another Web site told me that that was indeed the case. (The
supposed percentage of invisible stuff changes each week, but it's still
pretty high.) They said that those percentages only concern what's visible.
I don't see how this can be, even if it's true that neutrinos have a bit
of mass. How come only 10% of the Universe has been manipulated by gravity
so as to have the present outcome - galaxies, etc.?
I'm afraid that I can't answer the "How come?"
part of your question, because we don't know why the Universe is the way it
is. But the observations seem to indicate that only about 10% (I agree that
the numbers are variable in time) of the mass of the Universe is composed of
baryons (protons and neutrons, i.e. atoms).
Whether the rest of the mass is neutrinos or something stranger (superstrings,
neutralinos, WIMPs, or whatever) we don't know. But all of these objects
have mass, and so they do interact gravitationally. That is how we know
something is there: when you look at the gravitational field of galaxies,
galactic clusters, and galactic superclusters, it appears that there is a
lot more mass than can be explained by what we see in the stars, and the
larger the length scale, the bigger the discrepency. This so-called Dark
Matter is one of the big puzzles of Astrophysics. There is more information
on Dark Matter at our sister Web site, Imagine the Universe! (http://imagine.gsfc.nasa.gov).
Yes, when two surfaces rub together in outer space, there will be
friction. Friction is a surface effect and doesn't depend upon there being
air. There is also a force like air resistance from the very sparse gas in
space, but it will be very, very small, since space is a very good vacuum.
Is electron flow through a vacuum considered current?
Yes it is. Within our own heliosphere there are currents (of
electrons and also of protons) flowing toward and away from the Sun. These
currents generate magnetic fields, just as currents flowing through wires
generate magnetic fields around the wires.
Does heat travel through a vacuum, and if so how? If not, how
does the Sun heat the Earth?
Heat travels through a vacuum by infrared radiation (light with a
longer wavelength than the human eye can see). The Sun (and anything warm)
is constantly emitting infrared, and the Earth absorbs it and turns the
energy into atomic and molecular motion, or heat.
What are gravity waves? I understand that someone won the Nobel
Prize for discovering gravity waves. Gravity can be illustrated by taking a
heavy object and placing it on an elastic fabric; the more the fabric
curves, the stronger the gravity. But where do the waves come in?
Gravity waves are ripples on the elastic fabric of space. If you
hit or ring that heavy object, a ripple will move outward. This is a gravity
wave, and it travels at the speed of light. Hulse and Taylor won the Nobel
prize in 1993 for discovering a binary pulsar whose period was slowing down
exactly as predicted if the pair was losing energy by giving off gravity
waves. You can find more information at: http://nobel.sdsc.edu/laureates/physics-1993-press.html
Is light affected by gravity? If so, how can the speed of light
be constant? Wouldn't the light coming off of the Sun be slower than the
light we make here? If not, why doesn't light escape a black hole?
Yes, light is affected by gravity, but not in its speed. General
Relativity (our best guess as to how the Universe works) gives two
effects of gravity on light. It can bend light (which includes effects such
as gravitational lensing), and it can change the energy of light. But it
changes the energy by shifting the frequency of the light (gravitational
redshift) not by changing light speed. Gravity bends light by warping space
so that what the light beam sees as "straight" is not straight to
an outside observer. The speed of light is still constant.
Are wormholes in space real, or are they only science fiction?
Wormholes are allowed to exist in the math of "General
Relativity", which is our best description of the Universe. Assuming
that general relativity is correct, there may be wormholes. But no one has
any idea how they would be created, and there is no evidence for anything
like a wormhole in the observed Universe.
However, many experts in the field of gravitation and general relativity
have done a lot of work on them, including Stephen Hawking and Kip Thorne. A
good book on this subject is Black holes and Timewarps, Einstein's
Outrageous Legacy by Kip Thorne.
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