Lensing in the Laboratory

By Mark Trodden | March 31, 2007 3:08 pm

Gravitational lensing – the phenomenon in which diverging light rays from a source are refocused due to the warping of space by a massive object – has in recent years evolved from a beautiful test of General Relativity to a precision tool with which to measure quantities of astrophysical and cosmological importance.

A particular example of this is the use of weak lensing of far away background galaxies by an intervening galactic cluster to allow the reconstruction of the two-dimensional mass profile of that cluster, and a measure of its total mass. The rough idea is represented in the figure below, which I took from NASA’s web page on lensing.

(A good place to find out more about the subject is on Joanne Cohn’s lensing page, and links therein.)

When this technique is applied to actual astrophysical images, one is often led to remarkable results, as, for example, in the discussion of dark matter in the bullet cluster, which seems to force us to take particulate dark matter even more seriously.

If one wants to get a more hands-on feeling or how lensing works – strong versus weak; aligned source and lens versus nonaligned; etc. – then there do exist a few options. One is to visit Pete Kernan’s Lens an Astrophysicist page, which allows one to submit the url of an image and then adjust the mass and position of a black hole, returning the (strongly) lensed image. I often use screen captures from this site in my colloquia, and here’s one of a recognizable character.

Last week, I spent a delightful few days visiting U.Penn, and interacting with their cosmology and particle physics groups. My host was Bhuvnesh Jain, who is an expert on lensing, and who told me about a second way = an innovative teaching tool that he and collaborators have developed to illustrate lensing. Bhuv and friends have set up a precision optics system, and had a couple of special optical lenses machined so that their effect is that of a black hole or a galactic halo. Their site presents many of these applications – for example, here’s a four image (ignoring the one at the center, that one typically doesn’t see) one, and an actual astrophysical version or comparison

It’s a wonderful way to allow students to play with the lenses and explore the circumstances under which different types of distortions occur. If you feel like building one yourself, they even tell you how to do it.

  • http://mollishka.blogspot.com mollishka

    Apparently you can get a similar lens (for much less money and effort!) by chopping/breaking a wine glass at the stem; the bottom glass distorts background sources much like an isothermal sphere gravitational lens distorts background sources.

  • http://countiblis.blogspot.com Count Iblis

    This is fascinating stuff. And much more is possible in principle. There is an old article (ApJ I think) in which it is shown that one can use the two radio images to do interferometry. The effective baseline would be huge and that would enable one to resolve tiny details of the quasar.

  • Pingback: pligg.com

  • http://www.astro.ucla.edu/~wright/cosmolog.htm Ned Wright
  • http://blogs.discovermagazine.com/cosmicvariance/mark/ Mark

    Thanks Ned (nice to hear from you!). These are great links. If readers don’t know Ned’s web site, you really should take a look at


    for one of the best educational cosmology sites you’ll ever find.

  • http://eskesthai.blogspot.com/2007/01/hubble-maps-cosmic-web-of-clumpy-dark.html Plato

    Ned’s work has indeed provided good information for those of us learning.


    The table top method would be like just drawing circles? See here for image and info.

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    Manual trackback:
    Relativistic Obesity
    “The obesity epidemic in America has reached relativistic proportions, with some overweight citizens gaining so much weight that they distort the space and time around them. As the photo below demonstrates…”

  • http://countiblis.blogspot.com Count Iblis

    Also, inflation theory describes the progress of the obesity epidemic pretty well

    Americans are not just getting fatter, they are ballooning to extremely obese proportions at an alarming rate.


  • Pingback: La Tierra a través de una lente gravitatoria « La Singularidad Desnuda


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Cosmic Variance

Random samplings from a universe of ideas.

About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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