Sloan Digital Sky Survey

Swannie writes: "There's a story in today's Chicago Tribune about a joint project that Fermi Lab is taking on with Rensselaer Polytechnic Institute in New York. The goal is to produce a 3D map of the universe using a really big digital camera, and a really creative way to add "depth" to the image. The article has some decent technical details for a newspaper, including a pretty picture." Update: 03/12 15:44 GMT by M : The blurb is in error. A particular scientist from Rensselaer is mentioned in the article, but Rensselaer isn't part of the project as an institution.

Comments

How to view a 4D object in 3D

[Azahar]

This is a straight paste from an old email.

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I promised that I would tell you how to visualise a four dimensional object.

Imagine a picture in a book that shows a scene viewed through an arch. The picture is two dimensional (2D) but it creates the illusion of what the scene would look like viewed in 3D. If we want to find what is around the corner then we need a different picture viewed from a different angle. If we have enough pictures then we can create a movie of what it would look like to walk past the arch and view the scene as we walked (or flew because they are pictures we are not limited by such mundane things as gravity).

We can use a similar technique to visualise a 4D object. By the way, dimensions come in different sorts. Our three spatial dimensions are bidirectional but time is monodirectional and we can only travel through time in one direction. Those tiny rolled up dimensions that are produced by string theory are just little pockets that could make a grain of salt vanish from existence (as we knew it, it would still exist where we had no way of seeing or measuring it) . My fourth dimension is quite clearly a bidrectional spatial dimension.

Imagine a cube on a pedestal. The cube is perfect and all one colour and is four dimensional. Start with the 3D cube and imagine if it went back the same distance as the length of its sides and the other cubes stretched out behind it like cream on milk. You have just seen the image through the arch. Move around a little and the cubes stretch behind the one on the pedestal no matter which angle you view it from, even from directly above or below.

In reality the 'other' cubes would be directly behind the 3D cube and so invisible but I was only after a way to visualise or perhaps conceptualise the fourth dimension. When I consider objects that are different colours and shapes along their oopth (I have invented this word, length, width, depth and now we have oopth). When objects change through their oopth then they become much more interesting and much harder to visualise. Once you can do it then the relationship of time to space is immediate and necessary. A suitable object to conceptualise is a person changing from a baby to a child to an adult to a public servant.

This is all rather hard and you can forget it if you like but it will change your view of everything if you can master it.
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