Hubble's Universe Unfiltered

  • June 3, 2014

    Visual "Proof" of General Relativity

    by Frank Summers

    In a previous blog post, "Einstein's Crazy Idea", I discussed how Einstein' s theory of general relativity is a reinterpretation of gravity. Newton's original idea of gravity visualized it as a force between massive objects. Einstein instead surmised that the presence of mass warps space, and that curved space-time produces the motions we attribute to gravity. Earth's orbit around the Sun is either a curved path through flat space (Newton) or a straight path through curved space (Einstein).

    Both ideas of gravity produce the same observed motions for most cases. But there are a number of situations, generally involving very strong gravitational effects, where general relativity explains phenomena that gravitational forces get slightly wrong. The differences are often subtle and take quite a lot of explanation to appreciate. However, one example is visually obvious: gravitational lensing.

    The above image of galaxy cluster Abell 1689 is a prime example of gravitational lensing (click the image to see a larger version). Throughout the image are numerous small arcs, streaks, and strange-looking objects. Most of these are relatively normal galaxies (a few really are just strange-looking objects), whose images have been stretched and twisted by the galaxy cluster and general relativity.

    The combined mass of the thousands of galaxies in the cluster (and their associated dark matter) heavily distorts the space-time around the cluster. Light from more distant galaxies passes through that warped space. The images of those distant galaxies become distorted as if they were being seen through an odd-shaped glass lens. In fact, the physics of light redirection using gravity is entirely analogous to that using lenses. It is the optics of complex lenses, but using mass instead of glass.

    Newton's gravity can not produce such gravitational lensing. Well, to be complete, a gravitational force could produce half of the lensing effect of general relativity, but only if one assumes that photons (i.e., particles of light) have mass. Modern physics considers photons to be massless particles, and hence gravitational lensing does not exist in Newton's version of gravity, only in Einstein's general relativity.

    For that reason, I like to say that pictures of gravitational lensing are visual "proof" of general relativity. You don't have to delve into the astronomy, physics, or complex mathematics -- just examine the image. Such distortions arise from general relativity.

    Now, the visual distortions may be easy to spot, but that's not to say that these images are easy to interpret. Just the opposite is true. I'll provide some examples of the complexities of understanding gravitational lensing in my next blog post.

    [NOTE: This post is the second in a series of four, and is a slightly modified version of the same post on the Frontier Fields blog.]

  • May 27, 2014

    Einstein's Crazy Idea

    by Frank Summers

    General relativity is just plain weird.

    The basic idea of gravity we are taught in school comes from Isaac Newton’s “Principia” in 1687. Gravity is a force exerted by objects with mass. The greater the mass, the greater the gravitational force. The larger the distance between objects, the lesser the force (it decreases with the square of the distance). The gravity of the Sun pulls on Earth and holds it, along with the other planets, asteroids, comets, etc., in orbit.

    Not so, according to Albert Einstein in 1916. He came up with a completely new, and quite radical, alternative explanation.

    Einstein’s crazy idea is that the presence of mass warps the fabric of space around it. Then, that warped space controls the motion of other masses nearby. Newton’s idea of a gravitational force is thus replaced with four-dimensional space-time geometry. Planets orbiting around stars, and stars traveling through galaxies — these are space-time distortions moving within other space-time distortions. As one famous description puts it: mass tells space how to warp, while warped space tells mass how to move. Yeah, weird.

    On the face of it, Isaac and Albert are just describing the same phenomenon from two different points of view: the former sees a force, while the latter sees geometric distortions. And, since the algebraic equations of the gravitational force are so, so, so, so, so very much simpler than the tensor calculus of general relativity, why go to all the relativistic trouble?

    The answer is that there are certain situations, generally involving very large masses, where Newton’s gravity is demonstrably wrong. The most famous of these is the precession of the perihelion of Mercury.

    The orbit of Mercury is not fixed in space. Each time Mercury orbits the Sun, its orbit rotates by a minuscule amount. The position when Mercury is closest to the Sun, called perihelion, is used to measure this orbit rotation, called precession. While Newton’s gravity predicts a precession of the perihelion of Mercury, the measured value is significantly higher. This mismatch between prediction and observation is resolved by Einstein’s general relativity in that the warping of space at such a close distance to the Sun produces a slightly stronger precession than gravitational force.

    The other famous demonstration of general relativity is the bending of light as it passes a massive object. Light rays also have their paths changed by passing through warped space. A total solar eclipse on May 29, 1919, served to test this effect. During the eclipse, astronomers could see stars whose light had passed close to the Sun. Their apparent position on the sky would be shifted from their normal position due to passage through the warped space around the Sun. By observing the precise positions of such stars both before and during the eclipse, astronomers measured the effects of general relativity. (See the image accompanying this post.)

    Those 1919 observations did much to confirm that this crazy idea of general relativity reflected the reality of the universe. We now have many tests of general relativity. Most are subtle and require significant explanation. However, there is one that is visually striking, and which is critical to the scientific underpinnings of some important (and very cool) Hubble observations. I’ll address that in my next blog post.

    [NOTE: This post is the first in a series of four, and is a slightly modifed version of the same post on the Frontier Fields blog.]

  • October 10, 2013

    The Naming of Comets

    by Frank Summers


    The poem below is an adaptation of T. S. Eliot's poem "The Naming of Cats" from "Old Possum's Book of Practical Cats." This book served as the basis for the musical "Cats." Unless you are an Eliot or broadway afficianado, the verse below will make immeasurably more sense if you read the original first. That work is under copyright, and can not be printed here, but can be found elsewhere on the internet. Notably, the "Look Inside" feature includes the entire poem as the first work in this version of the book (at the time of writing). Also, the performance of the poem in the film version of "Cats" is available as a YouTube video.



    The Naming of Comets
    by F. J. Summers

    The naming of comets is a scientific matter,
    It isn't just one of your late night games;
    You may think at first I'm from a mad alma mater
    When I tell you, a comet must have three different names.

    First of all, there's the name that the public uses daily,
    Such as Lovejoy, Kohoutek, West or McNaught,
    The first name was Halley, or in America "hailey,"
    The name of the discoverer, or so you've been taught.

    There are compound names to handle the muddle,
    When two or more observers have made their claims,
    Such as Hale-Bopp, Shoemaker-Levy, or else Tempel-Tuttle,
    But most of them sensible astronomer surnames.

    But I tell you, a comet needs a name that's particular,
    A name that's specific, for a catalog, not fun,
    Else how can the solar wind push its ion tail perpendicular,
    Or spread out its dust tail when it passes the Sun?

    Of names of this kind, let's take C/2012 S1,
    Where a "P/" says periodic, but a "C/" says its not,
    Codes for the year, the half-month and the count and you're done,
    Now a name that belongs to only one comet you've got.

    But above and beyond there's still more naming jargon,
    And those are the names from a scientific source,
    With orbit and origin thrown into the bargain,
    These groups of comets are found in research discourse.

    When you notice a comet with a profound eccentricity,
    It's a "New" comet, I tell you, from the far Cloud of Oort,
    And a Kuiper Belt comet can achieve synchronicity,
    With the comets, with the orbits, with the periods that are short.
    The families of Jupiter,
    Encke and Chiron,
    Halley and External, cometary class names.



    Since poetic license, and adherence to T. S. Eliot's original form, can sometimes confuse a scientific message, here's a prose interpretation of the comet naming verses.

    The comet names most people know about are the ones honoring people, telescopes, or institutions. For example, Comet Lovejoy was discovered on November 27, 2011, by Terry Lovejoy. Comet Catalina-NEAT was found by both the Catalina Sky Survey and the Near-Earth Asteroid Tracking program (NEAT). However, Comet Halley (pronounced "hal-ee" or "hall-ee," but definitely not "hail-ee") is unusual in that Edmund Halley did not discover the comet, but predicted its return. Only a few other comets are named after the people who calculated their orbits. Details of who gets credit can be found in the IAU Comet-naming Guidelines.

    Using common names gets rather confusing in the era of space missions and dedicated observing programs. NEAT has discovered dozens of comets, while the Solar and Heliospheric Observatory (SOHO) has discovered a couple thousand. Keeping track of those requires a more catalog-style name. The scientific designation includes the type of comet and when it was discovered such that Comet Lovejoy is also known as "C/2011 W3."

    The two most common prefixes are "P/" for a periodic comet that completes an orbit in less than 200 years, and "C/" for a comet only seen during one passage by the Sun and not expected to return for a long time. Comet Lovejoy has an orbital period of about 622 years, and gets the "C/" prefix.

    The year of discovery is straightforward, but the letter following it is not. The twenty-four half-months of the year are labeled from A to Y, skipping "I" because it can be easily confused with the number one. Comet Lovejoy's discovery date of November 27, 2011, gets the designation of "2011 W."

    Finally, the "3" at the end of Comet Lovejoy's designation simply says that it was the third comet discovery assigned during that half-month period. The full naming description, including other prefixes and some special cases, can be found in the Cometary Designation System resolution.

    The third "name" referenced in the poem above is that of the various classes in which astronomers have grouped comets. The traditional split has been into long-period comets (more than 200 years to complete an orbit) that come from the Oort Cloud and short-period comets (less than 200 years) that derive from the Kuiper Belt. In recent years, classification is based on an orbital characteristic called the "Tisserand parameter," but the two main groups still remain.

    Within the short- and long-period groups, several sub-groupings are defined, often named after a proto-type. Comet Halley is a long-period comet that has been captured into a short-period orbit (75.3 years) and is the standard for the "Halley-type" comets. Comet ISON appears to be on its first pass from the Oort Cloud through the inner solar system and is called a "New" comet. The orbits of short-period comets can be dominated by interactions with Jupiter (Jupiter family), or exist outside (Chiron-type) or inside (Encke-type) that region. A description of comet classes can be found in this 1996 paper on "Comet Taxonomy," though these definitions do adjust and new ones can be added over the years.

    Whew! That's a lot of comet naming details just to explain a somewhat whimsical poem. However, I do very much enjoy combining the scientific and the literary in this fashion. In any case, this poem serves as a great way to test the size of the Venn diagram intersection of astronomy enthusiasts and musical lovers.