Universe Today

"Relativity theory" usually refers to Einstein's theory of relativity, which is actually two separate theories, special relativity and general relativity (though the former is a special case of the latter – hence its name!); however, Newtonian mechanics is also a relativity theory!

The "relativity" in relativity theory refers to a principle (or class of principles); namely, the principle of relativity. In a nutshell, principles of relativity are assumptions about the 'sameness' of theories in physics; for example, that the laws of nature which a theory of physics describes are the same no matter who is measuring them, nor when they are measured.

Another way of saying this is that the laws of nature have some symmetry (a technical term) … and in a very deep result (called Noether's theorem, named after Emmy Noether; it's actually her first theorem) every symmetry has a corresponding conservation law. So, for example, if the laws of nature are symmetric with respect to time (i.e. it doesn't matter when you measure them, you get the same laws) then energy is conserved ('energy', here, has a specific meaning).

In special relativity, the relativity principle is that the laws of nature are the same in all inertial frames of reference (in simple words, an inertial frame of reference is one that is not accelerating) … if you are moving, relative to me, at a constant velocity (and neither of us is experiencing any acceleration), we will find that the laws of nature are the same.

In general relativity, the laws of nature are the same, period (i.e. no matter who, when, where, or how they are determined, the laws will be the same).

Of course, there's more to both special relativity and general relativity than just statements of the relativity principles they assume … just as there's more to Newtonian mechanics that the same relativity principle which special relativity assumes (e.g. Newtonian mechanics assumes absolute time).

It's astonishing just how wide-ranging the results from these simple relativity principles are, in Einstein's two theories (think of E = mc², for example) … and how counter-intuitive some of them are too (e.g. the twins paradox, the relativity of simultaneity).

There's lots of material on the internet on relativity theory (but do be careful, some sites are just crackpot nonsense, and some are downright anti-science); two examples of good summaries/intros: Special Relativity (from SLAC), and Relativity Tutorial (Ned Wright, UCLA).

There are lots and lots and lots of Universe Today articles which include at least a mention of relativity theory; here is a sample: Small Engine for the Big Job of Testing Theory of Relativity, and Pulsars Confirm Einstein's Theories.

Likewise, relativity theory figures prominently in many Astronomy Cast episodes; here is a sample: Interstellar Travel, and How to Be Taken Seriously By Scientists.

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The positron is a lepton, the antimatter counterpart to the electron. It has the same mass and spin as the electron, is a fermion just like the electron, but its charge is the opposite (+1, vs -1).

The positron was predicted by Paul Dirac, in 1928 (actually, 'predicted' is a bit too strong, 'hypothesized' would be a more accurate word), and discovered by Carl Anderson, in 1932 (it was Anderson who gave it its name; he won the Nobel Prize in physics, in 1936, for this discovery). Curiously, the positron may have been detected earlier, but was not recognized.

The positron was the first kind of antimatter discovered, and was not found in the debris of particle collisions in a particle collider (like almost every other kind of anti-particle), but in a cloud chamber with lead plates (collisions between cosmic ray particles and lead nuclei in the plates produced positrons).

Positrons, like electrons, are stable … until they meet their matter counterparts! When that happens, the pair annihilates, producing two (or more) gamma rays; if two, they have an energy of 511 keV each (if the electron-positron pair has an extremely high energy, the annihilation may produce other particles, such as D mesons, or a W⁺/W^- pair).

Because the 511 keV 'annihilation line' is quite distinctive, it plays a key role in gamma ray astronomy. Positrons can be created by a wide variety of astrophysically important mechanisms, from cosmic ray collisions to radioactive decay of certain nuclides to pair-production from gamma rays (and more). Perhaps the most exciting, though still only hypothetical, mechanisms involve cold dark (non-baryonic) matter (CDM), in the chain of reactions from an annihilation, for example.

Positrons are a component of cosmic rays, and their energy spectrum can be used to test models of cosmic ray production and transport; there's been much excitement recently over observations suggesting an excess of positrons in certain energy ranges (beyond what standard models would expect) … the signal of some kind of CDM perhaps? This recent CERN Courier article gives a good overview …

… and there's also the Universe Today articles Astrophysics Satellite Detects Dark Matter Clue?, and Solved: Mystery of Gamma Ray Distribution in the Milky Way. Other Universe Today stories featuring positrons include Positron Drive: Fill 'er Up For Pluto, The Search for Positronium, and Is a Nearby Object in Space Beaming Cosmic Rays at Earth?

Looking for more? The Astronomy Cast episode Antimatter is well worth a listen.

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If a particle contains quarks, and is held together by the strong force, it is a hadron. As far as we know, only two combinations of quarks – and thus two kinds of hadron – are possible: three quarks, which is called a baryon (e.g. proton, neutron), and a quark and anti-quark, which is called a meson (e.g. pion). The name is derived from the ancient Greek word hadros, meaning thick.

Perhaps the best known use of the word hadron is in the Large Hadron Collider, which aims to smash protons (a kind of hadron, remember) together at very high energies; it is located near Geneva, and straddles the French-Swiss border.

The quarks with comprise a hadron determine its properties; for example, the electric charge of a hadron is an integer multiple of the electron's charge (-2, -1, 0, 1, or 2), depending on the quarks which comprise it (so the proton's charge is +1, as it is made up of two up quarks, with +2/3 charge each, and one down, with -1/3 charge). A hadron's mass has, however, little to do with the masses of the quarks of which it is composed; rather, most of the mass arises from the energy associated with the strong interaction (mediated by gluons) which keeps the quarks together in the hadron.

Just as an atom may become excited – one of its electrons moves into a higher energy state – so hadrons may become excited too; these states are called resonances, and there are hundreds and hundreds of them. In atoms, excited states decay electromagnetically (usually by emission of a photon); in hadrons, by the strong force (also sometimes called the strong nuclear force, or the color force); resonances have extremely short half-lives.

At a sufficiently high temperature and pressure, hadrons may cease to exist as independent particles; matter in this form would be a quark-gluon plasma, analogous to an ordinary plasma … the color force would play the role of electromagnetism (and the range of distinct phenomena extremely rich). Study of these phenomena is built on the theory of quantum chromodynamics, the strong interaction counterpart to quantum electrodynamics (USQCD is one collaboration involved in one kind of QCD research).

Aside from stories about the LHC (e.g. Particles Injected into Large Hadron Collider, Will the Large Hadron Collider Destroy the Earth?), Universe Today articles on hadrons include Forget Neutron Stars, Quark Stars Might be the Densest Bodies in the Universe, and the light-hearted The Particle Zoo: Collecting Your Own Subatomic Particles.

Two Astronomy Cast episodes are worth a listen, re hadrons, Nucleosynthesis: Elements from Stars, and The Search for the Theory of Everything.

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The mass of the electron, or the electron's mass, written as m_e, is 9.109 382 15(45) x 10^-31 kg. This is the "CODATA recommended value". It was published in March 2007, and is referred to as the 2006 CODATA recommended value.

Some background: CODATA stands for Committee on Data for Science and Technology. Per NIST (the US National Institute for Standards and Technology), "CODATA was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. It seeks to improve the compilation, critical evaluation, storage, and retrieval of data of importance to science and technology. The CODATA Task Group on Fundamental Constants was established in 1969. Its purpose is to periodically provide the international scientific and technological communities with an internationally accepted set of values of the fundamental physical constants and closely related conversion factors for use worldwide. The first such CODATA set was dated 1973, the second 1986, the third 1998, the fourth 2002, and the fifth (the current set) 2006."

The mass of the electron is one of the fundamental physical constants, so called because they are widespread in theories of physics, and because they are widely used in the application of those theories to other branches of science and to practical uses (such as engineering). Four of the other fundamental physical constants are c (speed of light in a vacuum), e (the charge of the electron), h (Plank's constant), and α (fine structure constant).

The method used for measuring m_e is to measure the Rydberg constant (R_∞) and calculate m_e from it ( m_e = 2R_∞h/(cα² ); the Rydberg constant is, in the words of the paper (by Peter J. Mohr, Barry N. Taylor, David B. Newell) in which the 2006 CODATA recommended values were published "can be accurately determined by comparing measured resonant frequencies of transitions in hydrogen (H) and deuterium (D) to the theoretical expressions for the energy level differences in which it is a multiplicative factor." For more details, refer to the paper itself.

Given that it is a fundamental physical constant, no surprise that Universe Today has some articles on it! For example Are the Laws of Nature the Same Everywhere in the Universe, and Fermilab putting the Squeeze on Higgs Boson.

Here are two Astronomy Cast episodes in which the electron mass figures prominently Electromagnetism, and Energy Levels and Spectra.

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Particles made up of three quarks are called baryons; the two best known baryons are the proton (made up of two up quarks and one down) and the neutron (two down quarks and one up). Together with the mesons – particles comprised of a quark and an antiquark – baryons form the hadrons (you've heard of hadrons, they're part of the name of the world's most powerful particle collider, the Large Hadron Collider, the LHC).

Because they're made up of quarks, baryons 'feel' the strong force (or strong nuclear force as it is also called), which is mediated by gluons. The other kind of particle which makes up ordinary matter is leptons, which are not – as far as we know – made up of anything (and as they do not contain quarks, they do not participate in the strong interaction … which is another way of saying they do not experience the strong force); the electron is one kind of lepton. Baryons and leptons are fermions, so obey the Pauli exclusion principle (which, among other things, says that there can be no more than one fermion in a particular quantum state at any time … and ultimately why you do not fall through your chair).

In the kinds of environments we are familiar with in everyday life, the only stable baryon is the proton; in the environment of the nuclei of most atoms, the neutron is also stable (and in the extreme environment of a neutron star too); there are, however, hundreds of different kinds of unstable baryons.

One big, open question in cosmology is how baryons were formed – baryogenesis – and why are there essentially no anti-baryons in the universe. For every baryon, there is a corresponding anti-baryon … there is, for example, the anti-proton, the anti-baryon counterpart to the proton, made up of two up anti-quarks and one down anti-quark. So if there were equal numbers of baryons and anti-baryons to start with, how come there are almost none of the latter today?

Astronomers often use the term 'baryonic matter', to refer to ordinary matter; it's a bit of a misnomer, because it includes electrons (which are leptons) … and it generally excludes neutrinos (and anti-neutrinos), which are also leptons! Perhaps a better term might be matter which interacts via electromagnetism (i.e. feels the electromagnetic force), but that's a bit of a mouthful. Non-baryonic matter is what (cold) dark matter (CDM) is composed of; CDM does not interact electromagnetically.

The Particle Data Group maintains summary tables of the properties of all known baryons. A relatively new area of research in astrophysics (and cosmology) is baryon acoustic oscillations (BAO); read more about it at this Los Alamos National Laboratory website …

… and in the Universe Today article New Search for Dark Energy Goes Back in Time. Other Universe Today stories featuring baryons explicitly include Is Dark Matter Made Up of Sterile Neutrinos?, and Astronomers on Supernova High Alert.

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To some extent, 'parallel universe' is self-referential … there are parallel meanings of the very term! The two most often found in science-based websites (like Universe Today) are multi-verse, or multiverse (the universe we can see is but one of many universes), and the many-worlds interpretation of quantum physics (most often associated with Hugh Everett).

Cosmologist Max Tegmark (currently at MIT) has a neat classification scheme for pigeon-holing most parallel universe ideas that have at least some relationship to physics (as we know it today).

The most straight-forward kind of parallel universe(s) is one(s) just like the one we can see, but beyond the (cosmic) horizon … space is flat, and infinite, and the laws of physics (as we know them today) are the same, everywhere.

Similar, but different in some key ways, are parallel universes which developed out of inflation bubbles; these have the same (or very similar) physics to what applies in the universe we can see, except that the initial values (e.g. fine-structure constant) and perhaps number of dimensions may differ. The Inflationary Multiverse ideas of Standford University's Andrei Linde are perhaps the best known example of this type. Parallel universes at this level tie in naturally to the (strong) anthropic principle.

Tegmark's third class (he calls them Levels; this is Level 3) is the many-worlds of quantum physics. I'm sure you, dear reader, are familiar with poor old Schrödinger's cat, whose half-alive and half-dead status is … troubling. In the many-worlds interpretation, the universe splits into two equal – and parallel – parts; in one, the radioactive material decays, and the cat dies; in the other, it does not, and the cat lives.

Level 4 contains truly weird parallel universes, ones which differ from the others by having fundamentally different laws of physics.

Operating somewhat in parallel are two other parallel universe concepts, cyclic universes (the parallelism is in time), and brane cosmology (a fallout from M-theory, in which the universe we can see is confined to just one brane, but interacts with other universes via gravity, which is not restricted to 'our' brane).

As you might expect, much, if not most, of this has been attacked for not being science (for example, how could you ever falsify any of these ideas?), but at least for some parallel universe ideas, observational tests may be possible. Perhaps the best known such test is the WMAP cold spot … one claim is that this is the imprint on 'our' universe of a parallel universe, via quantum entanglement (the most recent analyses, however, suggest that the cold spot is not qualitatively different from others, which have more prosaic explanations What! No Parallel Universe? Cosmic Cold Spot Just Data Artifact is a Universe Today story on just this).

Other Universe Today stories on parallel universes include If We Live in a Multiverse, How Many Are There?, Warp Drives Probably Impossible After All, and Book Review: Parallel Worlds.

Astronomy Cast has several episodes which include mention of parallel universes, but the best two are Multiple Big Bangs, and Entanglement.

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Sometimes, in astronomy, the name of a thing describes it well; a parabolic mirror is, indeed, a mirror which has the shape of a parabola (an example of a name that does not describe itself well? How about Mare Nectaris, "Sea of Nectar"!). Actually, it's a circular paraboloid, the 3D shape you get by rotating a parabola (which is 2D) around its axis.

The main part of the standard astronomical reflecting telescope – the primary mirror – is a parabolic mirror. So too is the dish of most radio telescopes, from the Lovell telescope at Jodrell Bank, to the telescopes in the Very Large Array; note that the dish in the Arecibo Observatory is not a parabolic mirror (it's a spherical one). Focusing x-ray telescopes, such as Chandra and XMM-Newton, also use nested parabolic mirrors … followed by nested hyperbolic mirrors.

Why a parabolic shape? Because mirrors of this shape reflect the light (UV, IR, microwaves, radio) from distant objects onto a point, the focus of the parabola. This was known in ancient Greece, but the first telescope to incorporate a parabolic mirror wasn't made until 1673 (by Robert Hooke, based on a design by James Gregory; the reflecting telescope Newton built used a spherical mirror). Parabolic mirrors do not suffer from spherical aberration (spherical mirrors cannot focus all incoming, on-axis, light onto a point), nor chromatic aberration (single lens refracting telescopes focus light of different colors at different points), so are the best kind of primary mirror for a simple telescope (however, off-axis sources will suffer from coma).

The Metropolitan State College of Denver has a cool animation of how a parabolic mirror focuses a plane wave train onto a point (the focus).

Universe Today has many articles on the use of parabolic mirrors in telescopes; for example Kid's Telescope, Cassegrain Telescope, Where Did the Modern Telescope Come From?, Nano-Engineered Liquid Mirror Telescopes, A Pristine View of the Universe … from the Moon, Largest Mirror in Space Under Development, and 8.4 Metre Mirror Installed on Huge Binoculars.

Telescopes, the Next Level is an excellent Astronomy Cast episode, containing material on parabolic mirrors.

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Vitaly Ginzburg, a Russian physicist and Nobel laureate, died yesterday of cardiac arrest. He was 93 years old. Ginzburg shared the 2003 Nobel Prize in physics for his work on superconductors, but contributed to many other fields of study, including quantum theory, astrophysics, radio-astronomy and diffusion of cosmic radiation in the Earth's atmosphere. In addition, he is known for his contributions to the development of the Russian hydrogen bomb in the 1950s, for which he received the Stalin Prize. Read more…

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You watch something (some distance from you) move … its direction changes … that's angular motion. In other words, as measured from a fixed point (or axis), the angular motion of an object is the change in direction of the line (of sight) to the object; the angle swept by the line. Notice that if the distance to the object changes but the direction doesn't, then there is no angular motion (though there is radial motion).

Standing on the surface of the Earth (and not moving, relative to the hills, valleys, etc), you see the Sun rise, move across the sky, and set. Ditto the Moon … and the stars, and the planets, and satellites like the ISS, and … "moving across the sky" simply means the direction of the Sun (the line from you to the Sun) changes, so that motion is angular motion.

Because it involves changes in angle, angular motion is measured in terms of degrees per second (or hour) … or radians per minute, or arcseconds per year, or … i.e. an angle per a unit of time.

Well, that's one particular kind of angular motion, angular velocity (strictly we need to add a direction, to make it a velocity; in which way is the angle changing, due East perhaps?). There's also angular acceleration, which is just like linear acceleration except that what the "per second per second" (or, perhaps, "per year per year") refers to is an angle, not a length (or distance).

As the Earth turns on its axis once a day, and as a circle has 2π radians, the angular motion of the stars and the Sun is 2π rads/day, right? Well, close, but no cigar … the Earth also revolves around the Sun, so from one day to the next it has moved approximately 1/365-th of a complete circle, and as the Earth's rotation is in the same direction as its orbit, the angular motion of the stars is a little bit less than 2π rads/day (it's actually 2π radians per sidereal day!).

Many kinds of angular motion, in astronomy, have special names; for example, the angular motion of stars with respect to distant quasars (actually the fixed celestial coordinate system) is proper motion; the tiny ellipses (relatively) nearby stars seem to complete every year is parallax; and there's precession, nutation, … and even the anomalous advance of the perihelion (of Mercury)! This last one is actually one component of a precession, but it played an important role in the history of physics (the first test the then new theory of general relativity passed); by the way, it's only about 43" (" = arcseconds) per century.

Wellesley College's Phyllis Fleming has a 100-level concise intro to angular motion.

Some of the many Universe Today stories which involve angular motion are Globular Clusters Sort their Stars, and Does a Boomerang Work in Space?

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We all have things that keep us up at night, as we try to solve the problems in our lives. But just think of the poor physicists: They are trying to solve the problems of the Universe! At a recent physics conference at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, a panel of scientists were asked what questions in physics kept them awake at night. Here are their answers:
Read more…

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If you measure temperature relative to absolute zero, the temperature is an absolute temperature; absolute zero is 0.

The most widely used absolute temperature scale is the Kelvin, symbolized with a capital K, which uses Celsius-scaled degrees (there's another one, the Rankine, which is related to the Fahrenheit scale). We write temperatures in kelvins without the degree symbol; absolute zero is 0 K.

Another name for absolute temperature is thermodynamic temperature. Why? Because absolute temperate is directly related to thermodynamics; in fact it is the Zeroth Law of Thermodynamics that leads to a (formal) definition of (thermodynamic) temperature.

The Kelvin is one of the International System of Units (SI) base units (there are seven of these), and is defined with reference to the triple point of water ("The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water" is the 1967/8 definition; the current one – adopted in 2005 – expands on this to take account of isotropic variations).

Why is it called the Kelvin? Because William Thompson – Lord Kelvin – was the first to describe an absolute temperature scale, in a paper he wrote in 1848; he also estimated absolute zero was -273 ^oC – pretty close, eh?

Project Skymath has a nice introduction to absolute temperature.

Some Universe Today material you may find interesting: Absolute Zero, Coldest Temperature Ever Created, and Planck First Light.

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Well, actually, the people who invented the first successful imaging technology using a digital sensor, called a CCD (Charge-Coupled Device), have been awarded the Nobel Prize in Physics. In 1969 Willard S. Boyle and George E. Smith came up with the idea "from their own heads," Smith said, and CCDs revolutionized photography, as light could now be captured electronically instead of on film, and became an irreplaceable tool in astronomy, providing new possibilities to visualize the previously unseen. The device also made it possible for amateur astronomers to rival the professionals in terms of quality astrophotography. CCD technology is also used in many medical applications, e.g. imaging the inside of the human body, both for diagnostics and for microsurgery. Sharing the prize with Boyle and Smith is Charles K. Kao, who in 1966 made a discovery that led to a breakthrough in fiber optics.

Both achievements helped shape the foundations of today’s networked societies.

Read more about the prize here.

Listen to the call where Smith learned he had been awarded the Nobel Prize in Physics.

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The nucleus of an atom consists of protons and neutrons (except if it's ordinary hydrogen, that has just one proton); the nucleus of an anti-atom consists of antiprotons and antineutrons (except if it's 'ordinary' anti-hydrogen). Yep, that's right, the antiproton is the antimatter (or anti-matter) counterpart of the proton.

The proton has a positive charge, so the antiproton has a negative one; the proton is composed of two up quarks and one down quark, so the antiproton is composed of two anti-up quarks and one anti-down one; BUT the antiproton and proton have the same spin and isospin (1/2), and the same mass (938 MeV/c²).

Who got the Nobel Prize for Physics in 1959? Emilio Gino Segrè and Owen Chamberlain. And what did they get it for? Discovering the antiproton! In 1955.

The mass of the antiproton has been measured to ten significant figures, and it's the same as the mass of the proton. This is important, because it puts constraints on theories of why the universe we observe is made of matter (and not equal parts of matter and antimatter).

Antiprotons have been used in colliders (they collide with protons) to produce single top quarks (rather than top-antitop pairs), and many other things too (they may turn out to be more effective in treating some cancers than protons).

Antiprotons are one kind of cosmic ray particle. There's a lot of interest in studying them, because they might tell us about the nature of dark matter (some kinds of hypothetical dark matter particles might decay into antiprotons, and when primordial black holes evaporate – if they exist – they might produce antiprotons). So far, antiproton cosmic ray research points to all the antiproton cosmic rays we observe being produced by collisions of high energy cosmic ray protons with the nuclei of atoms (or ions) in the interstellar medium. AMS (Alpha Magnetic Spectrometer experiment) would continue this research, but it has yet to fly (even though it's been ready for quite a while).

Searching for Antimatter in Antarctica, Antimatter, and Repaired too Late? Tevatron May Beat LHC in Hunt for Higgs Boson; just three Universe Today articles with more on antiprotons.

More from Astronomy Cast: Antimatter.

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What is 0 gravity? We normally know it as an environment with no gravity. However, this definition is actually unscientific. Gravity actually exists anywhere two quantities of matter exist side by side. Also, gravitational fields exert themselves several miles beyond the massive celestial objects that produce them. So even though astronauts are in orbit seem to be floating free of gravity they actually are not. This is further proven by the fact that a space shuttle is in orbit at all. Its path is still dictated by the force of the Earth's gravitational field.

So what is actually going on when astronauts are in 0 gravity? It can more accurately be called weightlessness or free fall. This is acutually the result of Einstein's Physics. Free fall is the effects of weightlessness created when an object falls solely under the force of gravity. The best way to explain this is Eistein's elevator illustration. Say that you were in an elevator that was suddenly falling from a great height. Since its is an enclosed space you would not be subject to factors like air resistance. So in your own frame of reference it would seem that you are actually floating free of gravity. Zero gravity would only exist as your personal perception in the elevator.

This is what happen with freefall. Back to the space shuttle. What happens is that space shuttle is travelling with constant acceleration similar to that of gravity. This creates the 0 g effect for astronauts. Another interesting thing about the state of weightlessness is the effects that it has on the human body after a long period of time. It has been observed that astronauts who spend a considerable amount of time in space lose bone mass and atrophy of certain muscles. This is because the human body was adapted to exist under the influence of gravity, providing density to bones, and exerting consistent stress on muscles even when at rest.

This has proven to be a major challenge for scientist trying to plan long term missions such as to the Moon and Mars. However, there are many methods of artificial gravity now being considered to counter the problem. One solution is to rotate a space craft using centripetal acceleration to produce gravity. This is a sure solution but there are concern about disorientation caused by a much smaller radius than the earth called the Coriolis effect. However this can be ameliorated by a larger craft. The other solution is to have a spacecraft constantly accelerate at the same rate as Earth's gravitational constant. This can work but there are no propulsion systems that can yet do this efficiently.

If you enjoyed this article there are several others on Universe Today that you will be sure to enjoy. There is a great article with links on articles explaining gravity. There is also a great article about artificial gravity.

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What keeps us from floating off into space? Why does something I drop fall to the ground? Kids are famous for asking questions like this, which usually cause parents to mumble something about gravity or tell them they will learn it when they get older. Here are a number of resources that can answer some of those questions.

Kidipede explains what gravity is on the Earth and in the universe.

How Stuff Works has a number of experiments for kids regarding the laws of gravity.

Science Experiments  offers a simple science experiment about gravity for kids, including a video clip showing how to do the experiment.

The USGS has a simple definition of what gravity is.

This site explains that the reason things do not fall off the Earth is because of gravity.

Physics 4 Kids has information on gravity for children. It also covers the topics of planetary gravity and the Moon. Additionally, the site has other links to different resources.

Spaghetti Box Kids has an experiment that teaches kids about density and gravity. The project involves making miniature hot air balloons.

About.com offers information on Sir Isaac Newton and tells about his work regarding gravity and his three laws.

Teacher Tech has an entire lesson plan mapped out around Sir Isaac Newton. It teaches about Newton and his three laws of motion. Additionally, it has a quiz for students and two science experiments involving gravity and motion.

Science Monster makes learning about gravity fun and easy. In addition to providing easy to understand definitions of gravity and intertia, the website has a game you can play that further reinforces the concepts.

This is a video clip from NASA showing how important gravity is in our everyday lives. It also has links to other video clips from NASA. This material is rated for grades 5 through 12 according to NASA.

Kids Konnect  has links to a variety of sources related to gravity including NASA. The site also has a number of links to information about Sir Isaac Newton who is famous for his work regarding gravity.

Universe Today has articles on planets for kids and Solar System projects for kids.

If you are looking for more information, check out Kids Astronomy and Primary Games.

Astronomy Cast has an episode on gravitational waves.

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