Making Sense of MOND, an Alternative Theory of Gravity

Galaxies are strange beasts: simply put, they rotate faster than they’re supposed to. At the speeds stars orbit the center, their centrifugal force (okay, not actually a real force) should overwhelm gravity and send them flying off into space.

One way to explain this is to assume the existence of a kind of matter we can’t see, which significantly raises the total mass of the galaxy and therefore its gravitational pull. This, called the dark matter theory, explains some phenomena but struggles with others, while something that doesn’t emit, reflect or even absorb any kind of electromagnetic radiation just seems a little weird to some. Another hypothesis, which also works in some cases but not all, is called Modified Newtonian Dynamics, or MOND for short.

Newton Spinning in His Grave

It may seem absurd for scientists in search of knowledge to make up stuff apparently at random. Both of these somewhat kooky theories, however, fit the data we have reasonably well. The same can be said for the double-helix structure of DNA, the Schrödinger equation and how atoms arrange themselves into benzene rings. All of these involved a level of creative thinking that must have seemed insane, to some and at the time, but they could all be proven eventually, and each kicked off an entirely new branch of science.

It’s already well-known that size matters: subatomic particles behave completely differently to volleyballs. It’s also true that the laws of physics change when something is going very fast or under the influence of a very strong gravitational field. It is therefore possible that making a “small” alteration to the math we have to describe gravity, which only becomes relevant on very large scales, can eliminate the need for something like dark matter to exist.

How It Works

The basic idea behind MOND is that, when it comes to large distances and low accelerations, star motion can be explained better by either changing the famous F=ma to F=ma2, or writing the gravitation equation F=Gm1m2/r2 as F=Gm1m2/r instead. This has since lead to a number of more detailed interpretations of MOND, both relativistic and non-relativistic.

There’s no inherent reason to assume that this is accurate, but it does help to explain a number of things that otherwise require dark matter. It’s also worth pointing out that we’ve only really tested our current theory of gravity within the solar system; for all we know, there could be space gremlins further out who like messing with the laws of physics.

Arguments for and Against

In our solar system, Mercury’s orbit is the smallest and fastest at only 88 days, while Pluto didn’t make it around the sun once in the 76 years between being discovered and fired from its job as a planet. This makes sense: the more gravity acts on an object, the faster it accelerates.

With entire galaxies, however, the outer rim rotates at the same speed as stars closer to the center. Dark matter can explain this, but only by assuming that it’s spread around fairly evenly instead of crowding around the core like visible matter. This is what you’d expect something with mass to do. MOND provides a much simpler, more elegant and more accurate explanation…but only for individual galaxies. Once you get to a system the size of a galaxy cluster, it breaks down and dark matter seems to be the winner.

There’s also the matter of the cosmic microwave radiation, a leftover from the Big Bang. This is generally uniform, but varies slightly in intensity and color temperature. These tiny fluctuations, with the help of some pretty impressive software, can be analyzed to produce a kind of sky map. The results are consistent with something up there that’s the same density dark matter is supposed to be. MOND offers no comparable answer.

Overall, in terms of the ability to make (correct) predictions, dark matter seems to be the stronger theory. Most cosmologists (who are obviously most concerned with very large-scale systems) regard modified Newtonian dynamics as a kind of mathematical curiosity. Several people are still doing work on it, though. It could just be that distance really does affect gravity in more ways than decreasing it at an inverse-square rate.

Related Information:
Dark Matter
More about the standard (but still unproven) way of explaining why galaxies seem to weigh more than they should.
Astronomy
A little about how astronomical research is conducted.

Dark Matter – Why Is Most of the Universe Invisible?

One of the biggest scientific mysteries of our time is why the galaxy doesn’t simply fall apart. Yeah, that’s right: we know that planets and indeed stars are kept in their orbits by gravity and not some kind of magical force. The problem is that, once you’ve added together the mass of the stars, interstellar gas, dust and all the objects we know about…there just isn’t enough gravity to go around. Not by a long shot: we should actually be seeing about 85% more stuff.

Sneaky Little Particles, Hiding all Over the Place

This is pretty much a certainty: between Newton and Einstein, we have a fair idea of how gravity works. Measuring the mass of a planet or asteroid, which determines how much gravitation it emits, is not very difficult. So, why doesn’t this number match the actual amount of gravity?

The most popular hypothesis among astronomers is that the “stuff” is in fact there, we just can’t observe it directly, hence the name dark matter. Even the name of this website is a kind of inside joke: there really is no such thing as a dark matter telescope.

No one has ever found so much as a cupful of the stuff. We don’t have a clue what it consists of, though some kind of quark soup seems like a strong possibility. Since we don’t know, we might as well call all the matter we can’t observe through radio waves, visible light or other kinds of radiation “dark”.

Detecting Dark Matter

Just about all we understand about dark matter is that it has mass. A little bit of detective work therefore allows astronomers to see its effects. In particular, scientists had suspected for decades, and finally proved it in 1979, that light passing near a massive object is deflected somewhat like the lens of a refractor telescope does:

lens
A red galaxy directly between us and a blue galaxy causes light from the blue one to appear as a ring.

(A true geek will tell you that the light is still traveling in a straight line; it’s the structure of space itself that gets bent. The more you know…) This means that tiny distortions in the way we see distant astronomical objects give us clues about how much dark matter may lie along the path traveled by its light.

Additional observations tally with these gravitational lensing experiments. Dark matter is one of the easiest ways to explain the shape and size of far-off galaxies: if there weren’t some extra substance of unknown composition in there, they would look very differently, rotating much more slowly or being less crowded. To put this into perspective, our sun orbits the center of the milky way at about 500,000 m.p.h, yet still takes 230 million years to go around once.

Perhaps the most visually interesting evidence for the existence of dark matter comes from observing collisions between and within galaxies. Galaxies are, of course, almost completely made up of empty space, but when stars are forced into relatively close proximity, they do have an effect on one another.

A colliding galaxy; observations show that there are large masses of dark matter near the green gas cloud formed by the collision.

As it turns out, dark matter basically ignores the ordinary kind except when it comes to gravity. Intriguingly, however, it seems that it does somehow interact with itself, occasionally making large clumps of invisible material form and separate from what we can see. These can be detected, including by the methods mentioned above.

A Good Concept, But Far from Certain

While dark matter seems plausible enough for a ton of money and effort to be spent on looking for it, the hypothesis (which is what you call a scientific explanation that agrees with the available evidence, but hasn’t been adequately tested through experiment) is not universally accepted. Some observations can be interpreted in favor of its being true, others seem to argue against this.

What we can conclude is the following: either our theories of astrophysics aren’t 100% complete yet, or something really interesting exists out there. Whatever science fiction has told you about dark matter, the truth is probably stranger. If so, there’s a whole new field of knowledge out there waiting to be discovered.

Related Information:
Deep Space Telescope
A little more about the instruments used to search out cosmological phenomena.
MOND
One of the theories that tries to explain gravitational anomalies without exotic forms of matter.

Orbital Telescopes: The Cutting Edge of Observational Astronomy

It is perhaps an astronomy nerd’s greatest dream – telescopes viewing space from space itself. These scopes are launched from the surface and orbit the Earth as satellites. Up there, their sight is unobstructed by any atmospheric disturbance, meaning pollution, weather and normal air turbulence, and they can therefore observe points in the sky in great detail. Getting above the atmosphere has another advantage, too: many wavelengths of radiation are simply blocked by the earth’s ionosphere, ozone layer and air, meaning the only way to “see” some kinds of non-visible light is from the vacuum of space.

What makes things even more exciting is that NASA and other organizations don’t spend millions on sending a cheap scope you can buy at any supermarket into the sky. Space telescopes are constructed with the most advanced mirror and imaging technology available.

Perhaps the king of these scopes is the Hubble. Loads of fanfare accompanied the launching of this scope and it has actually caused a minor revolution in the astronomy world. Since 1990 when the scope was placed into orbit by the space shuttle Discovery, the Hubble has produced a cornucopia of images that have been published around the world, such as its beautiful close up of the Eagle Nebula:

Eagle Nebula

This didn’t occur without a setback or two – within weeks of it going online, a tiny flaw was discovered in its mirror, which led to another shuttle being dispatched to give the scope a set of “glasses” to correct its vision.

The Hubble has allowed scientists to peer farther into the galaxy than ever before. Its images of really deep-space objects have shown us galaxies billions of light years away, which has helped answer questions about the formation and the overall composition in the universe.

Penny-Wise, Pound-Foolish

While this is very impressive, space telescopes remain controversial. They are very, very expensive to build and maintain and new ground-based technologies are making these telescope less competitive. Saying that the money should rather be spent at home is short-sighted, though. Sometimes, the only way to get the benefits of science is to actually invest in it, and technologies ranging from cellphones to MRI machines would never have been invented if it weren’t for discoveries made by astronomers.

Related Information:
Dark Matter
Mysterious and difficult to detect, dark matter is one of the scientific puzzles space telescopes may help to solve.
Refractor
Many amateur telescopes are of the refractor type and not intended to work in vacuum; learn more about them here.

Why Refractor Telescopes Are so Popular Among Skygazers

A refractor telescope is a type of optical scope used by both scientists and hobbyists. Essentially, they work by a series of lenses, somewhat like those you’ll find in eyeglasses, that focuses and intensifies light entering through one end. This gives a clear, enlarged image at the eyepiece.

These types of scopes are most commonly used by amateur skygazers to learn and have fun while admiring the beautiful heavens. Because they don’t use mirrors in the way reflecting telescopes do, they don’t require any difficult collimation (alignment) procedures that will prevent images from getting into focus until done correctly.

For this reason, they are great for inexperienced users and many beginners’ scopes are based on the refracting principle. Whatever level of scope you’re interested in, there a few basic parts that can be found in any refractor telescope.

Anatomy of a Telescope

Looking at one of these, the first thing you’ll notice is of course the main scope itself. On top of this is mounted what’s known as the finder, which is just a less powerful telescope that helps you locate the general area of objects in the sky and therefore makes aiming easier. Finally, the stand can be of two varieties.

The most common is a stand that allows the scope to move horizontally and vertically, called an altitude-azimuth or alt-az base. The other kind is known as an equatorial mount, which has to be aligned to the earth’s North Pole. The advantage of this type of scope is that they can track objects across the sky with a simple adjustment. Because the earth rotates, stars and planets will slowly drift out of view; an equatorial mount just makes it easier to adjust for this.

A refractor telescope does have a couple of drawbacks, most of which become more noticeable as the size of the scope increases. For the typical home user, though, the images they see with their refractor should be plenty clear enough, while the simplicity of these scopes is a major point in their favor.

Related Information:
Astronomical
More about how and for what telescopes are used.
Celestron
Though they mostly manufacture reflecting instead of refracting scopes, this company sells some nice beginner’s telescopes.

Why Astronomers Love Deep Space Telescopes and Their Pictures

Our galaxy, the Milky Way, contains about 300 billion stars, giving you plenty to look at in our own neighborhood. Many astronomers just aren’t satisfied with this, though, and insist on seeing as far out as technology allows.

Deep space telescopes are a specialized type of equipment that is meant to look beyond our galaxy to the outer reaches of the observable universe. There are untold numbers of galaxies and formations outside of our galaxy waiting to be seen and observed, meaning that each increase in viewing power has an exponential effect on the types of phenomena that can be studied.

Pay to Play

These scopes are well beyond the grasp of the average hobbyist. While you can construct a very large telescope on your own with a little ingenuity, homebuilt scopes will never compare to the instruments available to scientists, universities, and institutions like NASA. The most famous and powerful of these deep space telescopes is the Hubble. Having been in orbit around the Earth for nearly twenty years now, the Hubble has taken incredible pictures of stars being born in addition to photographing a mind-boggling number of galaxies.

Like other deep space telescopes, the Hubble has an amazing sensitivity to light and great resolving power. Much of this relies on computer processing of images. By the time light from such distant objects reaches the earth, even the best telescope isn’t capable of displaying an image the human eye can discern.

Deep space observations have been invaluable to cosmologists, helping them to explain how galaxies are formed and how the universe has evolved since the Big Bang. Some galaxies, like our own, are shaped roughly like spirals, while others have contours more like an ellipse. Studying distant galaxies also increases our chances of one day observing rare, exotic astronomical phenomena: supermassive black holes, gamma ray bursts and perhaps, one day, even evidence of alien civilizations.

Related Information:
Space Telescope
Obtaining clear images of distant galaxies often requires a telescope that lives above our planet’s atmosphere.
Celestron
Celestron telescopes aren’t capable of seeing quite so far, but remain a great choice for hobbyist astronomers.

An Overview of Celestron’s Wide Range of Telescopes

Celestron Telescope is one of the premier manufacturers of high-quality scopes that will enable you to explore and appreciate the night sky, regardless of your age and experience level. Whatever you’re hoping to see, and even if you’re just starting out with astronomy, you can find exciting products from Celestron that take advantages of some of the best available technologies and construction methods.

Stargazing Made Easy

With each model, the company strives to provide a lightweight and ergonomic design that makes observing the stars easy and comfortable. Many of the company’s scopes feature electronic gadgets that you can use to observe whatever celestial features you’d like to see without having to aim manually. These typically have well over 10,000 sets of coordinates pre-programmed, including those of planets, Messier objects like nebulas and well-known stars.

After calibrating the scope to your position on earth, you can experience the joy and awe of seeing numerous features that are invisible to the naked eye with the push of a few buttons. You won’t have to worry about struggling to find a star, or wondering if you’ve got the right spot; the machine does almost all the work for you. If this sounds like your kind of astronomy, you might want to take a look at their NexStar SE range.

Economical Options

This may all sound a bit too advanced for many people and the company sells many beginner’s models as well. The FirstScope reflecting scope and the AstroMaster series are lines of telescopes that offer a comparable level of optical quality, but are very simple to use and don’t come with any digital widgets.

Whatever your level of skill, which scope will be right for you depends a lot on your location and your desired experience. For novices, this probably means getting to know the basics of finding their way around the night sky. Old hands, on the other hand, will probably want to explore certain celestial features in more detail. The galaxy holds a treasure trove of beautiful sights. Whatever your specific requirements, Celestron Telescope probably has a telescope that would work for you.

Related Information:
Space Telescope
Space-based telescopes can see objects no terrestrial scope can hope to make out.
Refractor
One of the two most popular kinds of telescope are called refractors because of their optical design.

Old But Not Forgotten: The Astroscan Telescope

An Astroscan telescope is probably one of the simplest and most convenient around – unless you’re serious about performance, at least. Great for beginners and kids, this design avoids any complex adjusters and stands, allowing you to start skygazing without any prior knowledge whatsoever. The telescope, because of its minimal part count, is also very inexpensive compared to some other tabletop models, though in terms of performance it doesn’t compare at all well to options like the Orion Starblast.

What Makes an Astroscan Unique?

An Astroscan telescope, like the somewhat similar Dobsonians, utilizes a reflective design that allows for plenty of light-gathering ability. A curved objective mirror concentrates incoming light, sending it to a secondary mirror from where it travels to the eyepiece. The most unique feature is that this mirror is housed in a kind of oversized globe attached to a short light tube, making it look like anything but a telescope.

One thing that makes an Astroscan so easy to use is its mount: instead of being perched on top of a tripod, it simply lies loosely in a kind of cradle – just grasp it and pull to change where it’s pointing. This simple system works well enough considering this compact telescope’s wide field of view and relatively low magnification, but would cause serious problems with a more powerful scope. If what you need is an easy-to-use scope that doesn’t need to be able to spot fainter objects, perhaps as a present for a younger child, an Astroscan is a solid choice.

The mirror is also permanently aligned so there’s no need for the difficult collimation process that is an unfortunate necessity with most reflective telescopes. This, too, comes with a caveat: since the mirrors are aligned at the time of production, they will probably focus well enough to see, but not to the level of perfection demanded by serious astronomy enthusiasts. Still, many of us have fond memories of the Astroscan, which won several design awards for its innovative features.

Related Information:
Deep Space
Read more about telescopes designed to look way, way beyond the solar system.
Refractor
A very different kind of telescope that’s also suitable for beginners.

Astronomical Telescopes: The Link Between Astronomy and Telescopes

In contrast to sciences like physics and biology, astronomy is usually based on observation rather than experiments. You can’t, after all, move a star to a more convenient location. This means that astronomers’ instruments have to be as precise as humanly possible.

Astronomical telescopes have been in existence since the beginning of the seventeenth century. Originally a device for use on ships, turning them skywards quickly proved that the universe was more complex than we originally thought. Today, several types of telescope are available, from lightweight, portable scopes made for the home user to massive installations costing millions of dollars.

Shedding Some Light on the Subject

Optical scopes – that is, those that work with visible light – are the most common and best known. Their purpose is simply to display the best image possible at the highest magnification for viewing or photography. They come in three basic varieties, all of which can be found among affordable models intended for the hobbyist. The simplest kind is called a refracting scope, which uses a series of lenses to enhance the view of the user. A reflecting telescope uses a series of curved mirrors instead, which means that they have both advantages and drawbacks compared to refractors. A catadioptric design uses a combination of the two methods, usually in a very compact device.

Examples of each of these types can be found both in huge observatory-based scopes and in smaller models you can set up in your back yard. In addition, different telescopes come with a variety of bases and stands that help you with finding and tracking objects in the night sky. The most common of these is a standard tripod base that lets you rotate the scope vertically and horizontally until it’s aimed where it should be. Equatorial bases are an alternative. These align the scope to the axis of the earth’s rotation, making it easier to track objects for long periods. This is especially useful when it comes to astrophotography.

Optical telescopes are the only type a casual skygazer will typically use. Professional researchers, however, have a wider array of tools at their disposal, including radio and energetic particle telescopes. Each kind helps to reveal more information about bodies such as nebulas and planets – as it turns out, there’s still a whole lot waiting to be discovered up there.

Related Information:
Refractors
Learn more about this common type of telescope.
Astroscan
A basic type of scope intended for use by complete newbies.