What Is a Black Hole?

 

 What Is a Black Hole? 

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

 How Big Are Black Holes?

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

 The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

How Do Black Holes Form?

Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the galaxy they are in.

If Black Holes Are "Black," How Do Scientists Know They Are There?

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth?

Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

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Does light have mass?

Does light have mass?

 

The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes".

Light is composed of photons, so we could ask if the photon has mass.  The answer is then definitely "no": the photon is a massless particle.  According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits.  Even before it was known that light is composed of photons, it was known that light carries momentum and will exert pressure on a surface.  This is not evidence that it has mass since momentum can exist without mass.

Sometimes people like to say that the photon does have mass because a photon has energy E = hf where h is Planck's constant and f is the frequency of the photon.  Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc².  They also say that a photon has momentum, and momentum p is related to mass m by p = mv.  What they are talking about is "relativistic mass", an old concept that can cause confusion. Relativistic mass is a measure of the energy E of a particle, which changes with velocity.  By convention, relativistic mass is not usually called the mass of a particle in contemporary physics so, at least semantically, it is wrong to say the photon has mass in this way.  But you can say that the photon has relativistic mass if you really want to.  In modern terminology the mass of an object is its invariant mass, which is zero for a photon.

If we now return to the question "Does light have mass?", this can be taken to mean different things if the light is moving freely or trapped in a container.  The definition of the invariant mass of an object is m = sqrt{E²/c⁴ - p²/c²}.  By this definition a beam of light is massless like the photons it is composed of.  However, if light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in both directions symmetrically in the box, then the total momentum is zero in the box's frame of reference but the energy is not.  Therefore the light adds a small contribution to the mass of the box.  This could be measured--in principle at least--either by the greater force required to accelerate the box, or by an increase in its gravitational pull.  You might say that the light in the box has mass, but it would be more correct to say that the light contributes to the total mass of the box of light.  You should not use this to justify the statement that light has mass in general.

Part of this discussion is only concerned with semantics.  It might be thought that it would be better to regard the mass of the photons to be their (nonzero) relativistic mass, as opposed to their (zero) invariant mass.  We could then consistently talk about the light having mass independently of whether or not it is contained.  If relativistic mass is used for all objects, then mass is conserved and the mass of an object is the sum of the masses of its parts.  However, modern usage defines mass as the invariant mass of an object mainly because the invariant mass is more useful when doing any kind of calculation.  In this case mass is not conserved and the mass of an object is not the sum of the masses of its parts.  Thus, the mass of a box of light is more than the mass of the box and the sum of the masses of the photons (the latter being zero).  Relativistic mass is equivalent to energy, which is why relativistic mass is not a commonly used term nowadays.  In the modern view "mass" is not equivalent to energy; mass is just that part of the energy of a body which is not kinetic energy.  Mass is independent of velocity whereas energy is not.

Let's try to phrase this another way.  What is the meaning of the equation E=mc²?  You can interpret it to mean that energy is the same thing as mass except for a conversion factor equal to the square of the speed of light.  Then wherever there is mass there is energy and wherever there is energy there is mass.  In that case photons have mass, but we call it relativistic mass.  Another way to use Einstein's equation would be to keep mass and energy as separate and use it as an equation which applies when mass is converted to energy or energy is converted to mass--usually in nuclear reactions.  The mass is then independent of velocity and is closer to the old Newtonian concept.  In that case, only the total of energy and mass would be conserved, but it seems better to try to keep the conservation of energy.  The interpretation most widely used is a compromise in which mass is invariant and always has energy so that total energy is conserved but kinetic energy and radiation does not have mass.  The distinction is purely a matter of semantic convention.

Sometimes people ask "If light has no mass how can it be deflected by the gravity of a star?".  One answer is that all particles, including photons, move along geodesics in general relativity and the path they follow is independent of their mass.  The deflection of starlight by the sun was first measured by Arthur Eddington in 1919.  The result was consistent with the predictions of general relativity and inconsistent with the newtonian theory.  Another answer is that the light has energy and momentum which couples to gravity.  The energy-momentum 4-vector of a particle, rather than its mass, is the gravitational analogue of electric charge.  (The corresponding analogue of electric current is the energy-momentum stress tensor which appears in the gravitational field equations of general relativity.)  A massless particle can have energy E and momentum p because mass is related to these by the equation m² = E²/c⁴ - p²/c², which is zero for a photon because E = pc for massless radiation.  The energy and momentum of light also generates curvature of spacetime, so general relativity predicts that light will attract objects gravitationally.  This effect is far too weak to have yet been measured.  The gravitational effect of photons does not have any cosmological effects either (except perhaps in the first instant after the Big Bang).  And there seem to be far too few with too little energy to make any noticeable contribution to dark matter.

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Why is the sky blue?

Why is the sky blue?

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.

The white light from the sun is a mixture of all colours of the rainbow.  This was demonstrated by Isaac Newton, who used a prism to separate the different colours and so form a spectrum.  The colours of light are distinguished by their different wavelengths.  The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 nm, with orange, yellow, green, blue and indigo between.  The three different types of colour receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our colour vision.

Tyndall Effect

The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.  This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in.  From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank.  The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses.
This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering—after Lord Rayleigh, who studied it in more detail a few years later.  He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.  It follows that blue light is scattered more than red light by a factor of (700/400)⁴ ~= 10.

Dust or Molecules?

Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere.  Even today, people sometimes incorrectly say that this is the case.  Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering.  The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment.  He was even able to use the calculation as a further verification of Avogadro's number when compared with observation.  The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

Why not violet?

If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength.  The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light.  Our eyes are also less sensitive to violet.  That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue.  The rest of the answer to this puzzle lies in the way our vision works.  We have three types of colour receptors, or cones, in our retina.  They are called red, blue and green because they respond most strongly to light at those wavelengths.  As they are stimulated in different proportions, our visual system constructs the colours we see.

Response curves for the three types of cone in the human eye

When we look up at the sky, the red cones respond to the small amount of scattered red light, but also less strongly to orange and yellow wavelengths.  The green cones respond to yellow and the more strongly scattered green and green-blue wavelengths.  The blue cones are stimulated by colours near blue wavelengths, which are very strongly scattered.  If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colours appear blue with an added red tinge.  The net effect is that the red and green cones are stimulated about equally by the light from the sky, while the blue is stimulated more strongly.  This combination accounts for the pale sky blue colour.  It may not be a coincidence that our vision is adjusted to see the sky as a pure hue.  We have evolved to fit in with our environment; and the ability to separate natural colours most clearly is probably a survival advantage.

A multicoloured sunset over the Firth of Forth in Scotland.

Sunsets

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away.  If the air is polluted with small particles, natural or otherwise, the sunset will be more red.  Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers.  The sky around the sun is seen reddened, as well as the light coming directly from the sun.  This is because all light is scattered relatively well through small angles—but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours.

A blue haze over the mountains of Les Vosges in France.

Blue Haze and Blue Moon

Clouds and dust haze appear white because they consist of particles larger than the wavelengths of light, which scatter all wavelengths equally (Mie scattering).  But sometimes there might be other particles in the air that are much smaller.  Some mountainous regions are famous for their blue haze.  Aerosols of terpenes from the vegetation react with ozone in the atmosphere to form small particles about 200 nm across, and these particles scatter the blue light.  A forest fire or volcanic eruption may occasionally fill the atmosphere with fine particles of 500—800 nm across, being the right size to scatter red light.  This gives the opposite to the usual Tyndall effect, and may cause the moon to have a blue tinge since the red light has been scattered out.  This is a very rare phenomenon, occurring literally once in a blue moon.

Opalescence

The Tyndall effect is responsible for some other blue coloration's in nature: such as blue eyes, the opalescence of some gem stones, and the colour in the blue jay's wing.  The colours can vary according to the size of the scattering particles.  When a fluid is near its critical temperature and pressure, tiny density fluctuations are responsible for a blue coloration known as critical opalescence.  People have also copied these natural effects by making ornamental glasses impregnated with particles, to give the glass a blue sheen.  But not all blue colouring in nature is caused by scattering.  Light under the sea is blue because water absorbs longer wavelength of light through distances over about 20 metres.  When viewed from the beach, the sea is also blue because it reflects the sky, of course.  Some birds and butterflies get their blue colorations by diffraction effects.

Why is the Mars sky red?

Images sent back from the Viking Mars landers in 1977 and from Pathfinder in 1997 showed a red sky seen from the Martian surface.  This was due to red iron-rich dusts thrown up in the dust storms occurring from time to time on Mars.  The colour of the Mars sky will change according to weather conditions.  It should be blue when there have been no recent storms, but it will be darker than the earth's daytime sky because of Mars' thinner atmosphere.

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Dark Energy, Dark Matter

 

Dark Energy, Dark Matter

In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

What Is Dark Energy?

Universe Dark Energy-1 Expanding Universe

This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.

NASA/STSci/Ann Feild

More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe. 

Dark Matter Core Defies Explanation

This image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The result could present a challenge to basic theories of dark matter.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10¹²⁰ times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.

What Is Dark Matter?

Abell 2744: Pandora's Cluster Revealed

One of the most complicated and dramatic collisions between galaxy clusters ever seen is captured in this new composite image of Abell 2744. The blue shows a map of the total mass concentration (mostly dark matter).

By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHOs". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).

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