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# Being Mister Fantastic

Look at yourself in your bathroom mirror. Why, you look younger than your age!

And the farther you are from the mirror, the younger you look. If the mirror were really really far … remember the scene in “2001: A Space Odyssey” in which David Bowman sees himself as a fetus in the womb? Yup.

As you stand in your bathroom and you look down, the skin of your feet looks noticeably younger than that of your hands. Wait–bend forward and touch your feet. Do they still look younger?

Now walk to the kitchen and open the refrigerator. See that head of lettuce? The leaves are long—they reach halfway to the back of the shelf. The edges at the front look brown and wrinkly, but farther back the lettuce still look fresh green. If you stare at the lettuce long enough, you’ll see the wrinkles slowly move toward the back until they cover the entire thing.

It gets weirder. When you wave your arm straight out in front of you, it seems to drag in space, like sea grass bending in the waves. The hand eventually catches up, but with a delay.

Next, you walk across the living room. When you get to the other side, you turn around and you look back. You notice that your feet didn’t move at all. It’s like they were stuck to the floor where you were standing before. Your entire body is stretched diagonally from here to other side of the room, as if you were pulling a trick worth of the Fantastic Four. Soon, your feet start moving, too, and eventually they catch up with you.

None of this is science fiction, Mister Fantastic. We see things because light travels from those things to our eyes. But light moves at a finite speed. The farther the object is, the longer it takes for light to reach you. So everything we see, we see with a delay. It is a very small delay—10 nanoseconds for something that’s ten feet away. For practical purposes, that’s too tiny for us to observe with the naked eye. But it’s just a matter of degrees.

If light happened to be a lot slower—for example if it took one second to cover one meter—the weirdness would become obvious. (An example, which I mentioned in my previous post about how space used to be red, is that if you were immersed in a medium that suddenly transitions from opaque to transparent–for example from ice to water–you would see a spherical wall surrounding you and expanding away from you in all directions, at the speed of light.)

To imagine a world with slow light does not require bending the laws of physics too much. Light already travels at different speeds in different media (water, air, glass), and in all those cases it goes slower in the medium than it does when it propagates through empty space. In certain very controlled situations, in fact, physicists can slow light down to the point that it becomes virtually still.

Empty space is also known as the vacuum. And Einstein’s famous rule is this: nothing can travel faster than the speed of light in the vacuum. In principle, though, nothing prevents material things from moving faster in a medium than light moves in the same medium.

In practice, it’s rare for light to slow down so much or for matter to move so fast that matter can overtake light. But in some cases it does happen: this effect it is responsible for the blue glow of the cooling pools where nuclear waste is held. Thus, you could think of a medium that slows light so much that light becomes slower than your own motion.

So here’s my tip for virtual reality geeks: develop an immersive system in which you can adjust the speed of light and see what happens to your body and the world around it. The results could be even trippier than “2001.”

The Starchild image is a screenshot from a copyrighted film, and the copyright for it is most likely owned by Warner Brothers. The image is reproduced here as a web-resolution screenshot for critical commentary and discussion of the film; its use qualifies as fair use under United States copyright law.

About the Author: Davide Castelvecchi is a freelance science writer based in Rome and a contributing editor for Scientific American magazine. Follow on Twitter @dcastelvecchi.

The views expressed are those of the author and are not necessarily those of Scientific American.

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1. 1. ImproperUsername 1:27 pm 07/17/2011

Wow man! You just like, blew my mind! Like, man, I was an art student in the 60′s again.

2. 2. ImproperUsername 1:29 pm 07/17/2011

If there were a Google+ button with this blog post, I would click it.

3. 3. hobsonlane 6:15 pm 07/17/2011

You lost me somewhere between your blending of the psychological affects of image resolution on self-image, with a thought experiment (or LSD trip) into the effects of the finite speed of light, followed by optical illusions caused by the finite speed of electro-chemical reactions in the optical nerve and the psychological “feed-forward” signals that affect our perception of the position of our body parts. This article reduces my respect for the scientific nature of SA. I never thought you’d pander to the fantastic fantasy of the comic-book crowd, forgoing serious analytical discussion.

4. 4. jtdwyer 7:48 pm 07/17/2011

5. 5. Glendon Mellow 9:28 pm 07/17/2011

Amazing. So fun to think about.

We need an Xbox game, stat.

6. 6. @dcastelvecchi 9:51 pm 07/17/2011

hobsonlane,

I appreciate you taking the time to comment but I disagree about the lack of scientific interest. The kind of thought experiment I suggest is, I believe, a useful exercise for reasons that I plan to discuss in a future post. Briefly, it can help you visualize the universe.

Also: I am suggesting that someone develop a new type of virtual reality. SciAm since its inception has been a magazine about technology (which includes software) as much as about science.

And last but not least, please note that this is a blog, not an article. Blogs are forums for informing but also musing, entertaining, and opining.

7. 7. kronski 10:58 pm 07/17/2011

I like this idea of computing a simulation where light moves slowly (or you move quickly relative to light). Unfortunately, much of the algorithmic tractability of interactive 3D rendering relies on the fact that in all the media we’re interested in, light moves in a straight line. There are simple transformations (Snell’s law) for when the medium changes as in when there are pools or containers of water and glass, but for the most part this is true.

In order to compute the color of a pixel in ray-tracing you compute the color that reaches the rendering plane at position (x,y). In standard ray-tracing, you draw a ray from where your eye is to this position and then “trace” its path until you reach a surface, then you look for light sources and compute the relevant color based on the relevant angles to these sources and the properties of the surface. This can all done with standard linear algebra and efficient, ray-intersection tests with three-dimensional primitives are well known.

To do ray-tracing when relativistic effects are present, you will not only have to trace the path of your light ray in space, but *also* in time! Computationally, you will need to keep track not just of the current three-dimensional positions of the different objects that make up your scene, but the four-dimensional shapes that they carve out in spacetime for the time periods of interest and then compute intersections with these.

This is a cool idea, but certainly a nontrivial task to achieve rendering at interactive frame-rates!

Great post!

8. 8. Torbjörn Larsson, OM 10:02 am 07/18/2011

Good idea! Also a defeater of the “were you there” idiocy of people who doesn’t know first thing about real physics and how we make actual observation.

Another trippy trip is to gank up the speed of yourself compared to light. VR simulations that include all the visible effects in relativity is on the web. (Effects such as the actual Terrell rotation one sees, not the gedanken experiment, now folk physics, “shrinking” of simplified relativity.) Recommended!

9. 9. @dcastelvecchi 2:26 pm 07/18/2011

Yes! I remember seeing videos like the one you’re referring to, perhaps it was this one?

and I also thought I might blog about those too, some day.

10. 10. Alex Wild 5:14 pm 07/19/2011

You don’t need a virtual reality machine to replicate this effect.

Just look at any video taken by a dSLR as it pans quickly. The “rolling shutter” effect results from a video feed coming line-by-line off the sensor.

11. 11. @dcastelvecchi 11:59 am 07/20/2011

The rolling shutter is pretty cool but if I am not mistaken it scans the field of view top to bottom. The slow light effect scans the world in concentric spheres in 3D.

12. 12. fnori 12:30 pm 07/22/2011

Very interesting article. Indeed, I agree that it would be interesting to develop an immersive system in which you could adjust the speed of light and see what happens to your body and the world around it.

13. 13. jgrosay 8:05 am 07/23/2011

Well, light is both a wave and a particle, the wave doesn’t travel in vacuum, but propagates in itself, the particle that constitutes light. Maybe it’s not possible to reach speeds faster that the speed of light propagating in itself because some inherent feature of the space-time continuum, not because a characteristic of light; at last, when a body accelerates and reaches speeds closer to the speed of light,its mass increases towards an infinite mass, and its internal time flow becomes close to stuck in reference to the time flow outside;surpassing the speed of light would mean a mass bigger than infinite,and perhaps a negative time flow, an apparent impossible.

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