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Deep Space Picture
From Hubble Telescope

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

Destination Jet
(600 mi/hr)
(25,000 mi/hr)
(186,000 mi/sec)
Moon 16.5 days 9.4 hr 1.2 sec
Sun 17 years 8 months 4 months 8.5 min
Mercury 10 years 10 months 3 months 5 min
Venus 5 years 5 months 1.5 months 2.5 min
Mars 8 years 10 months 2.5 months 4 min
Jupiter 74 uears 3 months 1 year 9 months 35 min
Saturn 150 years 5 months 3 years 7 months 1 hr 11 min
Uranus 318 years 6 months 7 years 7 months 2 hr 30 min
Pluto 690 years 1 month 16 years 5 months 5 hr 25 min
Alpha Centauri 4.8 million years 114,155.2 years 4.2 years
Sirius 9.6 million years 228,310.4 years 8.4 years
Pleiades Cluster ---------- ---------- 400 years
Crab Nebula ---------- ---------- 4000 years
Center of the Milky Way ------------ ---------- 38,000 years
Andromeda Galaxy ------------ ---------- 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


Where is Space?
Big Bang Explosion
Age of the Universe
Flat Universe
Boomerang Experiment
How Fast is the Universe Expanding?
What is the Universe Expanding Into?
Speed of Our Galaxy
Big Bang and Religion
How Will the Universe End?
Center of the Universe?
Uniformity of the Universe
Temperature of the Universe
Conservation of Energy and Big Bang
Big Bang and Nucleosynthesis
Expansion of the Universe
Using the Doppler Effect to Determine Expansion of the Universe
Expansion of a Sphere of Light
Universe Expands Faster Than Light?
Maximum Size of the Universe?
Does Space End?
When the Universe Cools
Searching for Supernovae
How Are Galaxies Born?
Distance to Galaxies
Number of Galaxies in the Universe
Types of Galaxies
What's Between the Galaxies?
Does Speed of Galaxy Affect Time?
Where Does Galaxy End?
Why is it Dark in Space?
Gamma Rays
Quasars, Black Holes, and the Evolution of the Universe
What is a Black Hole?
Where Does the Matter Go Inside a Black Hole?
Black Hole at the Center of the Galaxy?
Black Hole and the Solar System
Black Holes and the Speed of Light
Black Hole Event Horizon
How Do Gravitons Escape from Black Holes?
Black Hole to Another Dimension?
Black Hole Out of Space/Time?
Matter in Space
More on the Density of Matter in Space
Matter in Space and the Strong Force
Center of Mass of the Solar System
Dark Matter
Friction in Space
Current in a Vacuum
Does Heat Travel Through a Vacuum?
Gravity Waves
Is Light Affected By Gravity?
Keeping a Wormhole Open?
What Does a Wormhole Look Like?
What About Wormholes and Time Travel?
Time Travel
Looking Back in Time
How Does Light Travel Through Space to Earth?
Speed of Light in Deep Space
Spots That Have Never Seen Light?
Inches in a Light Year
Faster Than Speed of Light?
Light-Powered Travel?
Inflating a Balloon in Space
Radio Signals in Space
Will a Simple Magnet Work in Space?
Will a Compass Work in Space?
Propulsion With Van Allen Belts?
Shuttle Standing Still?


  1. Where is Space?

    I have a question that is simple, yet to me quite complex; perhaps you can help: Where is space?

    The question is very simple, and so is the answer: Space is everywhere.

    Dr. Louis Barbier

  2. Big Bang Explosion

    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.

    Dr. Eric Christian and Beth Jacob

  3. Age of the Universe

    Has the age of the Universe been pinned down yet? Is it still thought to be between 10-20 billion years old?

    This is one of the main questions being addressed by the Hubble Telescope team. Check out their Web page here.

    Dr. Louis Barbier

  4. Flat Universe

    I just read that the Universe is flat. How does this relate to the thickness of the Universe?

    "Flat" refers to the expansion of the Universe, not the size or shape. There's a good explanation of the "flat Universe" at

    Dr. Eric Christian and Beth Jacob

  5. Boomerang Experiment

    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 will survive?

    I think you should go to the source, check out the BOOMERANG home page.

    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.

    Dr. Louis Barbier

  6. How Fast is the Universe Expanding?

    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.

    Beth Jacob and Dr. Eric Christian

  7. What is the Universe Expanding Into?

    My understanding of the Universe is that it is constantly expanding in all directions -- the Big Bang theory. If this is true then what is it expanding into? How is the Universe creating more space?

    This question was covered by Imagine the Universe! and by Scientific American.

    Dr. Eric Christian

  8. Speed of Our Galaxy

    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.

    Dr. Eric Christian and Beth Jacob

  9. Big Bang and Religion

    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 the Universe!.

    Beth Jacob

  10. How Will the Universe End?

    What will be the end (death) of the Universe?

    Please see our sister Web site, Imagine the Universe! for a good discussion of this.

    Dr. Louis Barbier

  11. Center of the Universe?

    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 :-)

    Dr. Eric Christian

  12. Uniformity of the Universe

    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 thing?

    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).

    Dr. Eric Christian

  13. Temperature of the Universe

    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.

    Dr. Eric Christian

  14. Conservation of Energy and Big Bang

    How are the big bang and the law of conservation of energy compatible?

    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.

    Dr. Eric Christian

  15. Big Bang and Nucleosynthesis

    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.

    Dr. Eric Christian

  16. Expansion of the Universe

    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

    Dr. Eric Christian

  17. Using the Doppler Effect to Determine Expansion of the Universe

    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 the Universe!

    Beth Jacob

    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 away.

    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).

    Dr. Eric Christian

  18. Expansion of a Sphere of Light

    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.

    Dr. Eric Christian

  19. Universe Expands Faster Than Light?

    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.

    Dr. Eric Christian

  20. Maximum Size of the Universe?

    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.

    Dr. Eric Christian

  21. Does Space End?

    Does space ever end?

    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.

    Dr. Eric Christian

  22. When the Universe Cools

    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.

    Dr. Eric Christian and Beth Jacob

  23. Supernovae

    Do you know of any Web sites with good information about supernovae explosions?

    The "Imagine the Universe!" Web site has a good write-up on supernovae (

    Beth Jacob

  24. Searching for Supernovae

    How often do supernovae occur? What devices do we have to pinpoint these as they occur (or rather as light and particle emissions reach Earth)?

    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.

    Dr. Louis Barbier

  25. How Are Galaxies Born?

    How are galaxies born?

    This isn't our specialty, but you might want to check out the Imagine the Universe! page on this subject.

    Beth Jacob

  26. Distance to Galaxies

    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 us.

    The farthest galaxy seen is about 13 billion light years away. You can see the press release at

    Dr. Eric Christian

  27. Number of Galaxies in the Universe

    What is an estimate of the number of galaxies in the observable Universe?

    Current estimates are that there are billions to a few tens of billions of galaxies in the observable Universe.

    Dr. Eric Christian

  28. Types of Galaxies

    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, and irregular.


    • 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.


    Dr. Eric Christian

  29. What's Between the Galaxies?

    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.

    Dr. Eric Christian

  30. Does Speed of Galaxy Affect Time?

    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 movement?

    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.

    Dr. Eric Christian

  31. Where Does Galaxy End?

    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, StarChild (

    Dr. Eric Christian

  32. Why is it Dark in Space?

    Why is it dark in space?

    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.

    There's also the simple explanation at our sister site, Imagine the Universe!

    Dr. Eric Christian and Beth Jacob

  33. Gamma Rays

    Can you recommend one or more good basic gamma ray web sites?

    Here are some web sites I know of that will provide you with information on gamma rays:


    I hope you find these sites helpful.

    Laura McDonald -- "Guest Answerer"
    Energetic Gamma Ray Experiment Telescope (EGRET)
    Compton Gamma Ray Observatory (CGRO)

  34. Quasars

    Is there such a thing as a quasar?

    Yes, quasars exist. You can find out more about them and similar objects at our sister site, Imagine the Universe!

    Dr. Eric Christian

  35. Quasars, Black Holes, and the Evolution of the Universe

    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 cycles.

    Dr. Eric Christian

  36. What is a Black Hole?

    What exactly is a black hole?

    Actually, the topic of your question is well covered by our sister learning center, Imagine the Universe!.

    Dr. Eric Christian

  37. Where Does the Matter Go Inside a Black Hole?

    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 Imagine the Universe! Your question is answered there.

    Beth Jacob

  38. Black Hole at the Center of the Galaxy?

    I have heard that black holes have been found in the centers of other spiral galaxies besides our own. Does the Milky Way have one?

    Probably. You can read more about this at our sister site Imagine the Universe!

    Beth Jacob

  39. Black Hole and the Solar System

    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 difference.

    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 dwarf.

    Dr. Eric Christian

  40. Black Holes and the Speed of Light

    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 it.

    Two different things.

    Dr. Eric Christian

  41. Black Hole Event Horizon

    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.

    Dr. Eric Christian

  42. How Do Gravitons Escape from a Black Hole?

    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.

    Drs. Louis Barbier and Demos Kazanas

  43. Black Hole to Another Dimension?

    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.

    Black holes in the news:

    Dr. Eric Christian

  44. Black Hole Out of Space/Time?

    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.

    Dr. Eric Christian

  45. Matter in Space

    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 well.

    Dr. Eric Christian

  46. More on the Density of Matter in Space

    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 friction.

    Dr. Louis Barbier

  47. Matter in Space and the Strong Force

    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.

    Dr. Eric Christian

  48. Center of Mass of the Solar System

    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: where I think you'll find what you want. Hope this helps.

    Dr. Eric Christian

  49. Dark Matter

    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! (

    Dr. Eric Christian

  50. Friction in Space

    Does friction exist in deep space?

    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.

    Dr. Eric Christian

  51. Current in a 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.

    Dr. Louis Barbier

  52. Does Heat Travel Through a Vacuum?

    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.

    Dr. Eric Christian

  53. Gravity Waves

    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:

    Dr. Eric Christian

  54. Is Light Affected By Gravity?

    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.

    Dr. Eric Christian

  55. Wormholes

    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.

    Drs. Eric Christian and Louis Barbier

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