Astronomical illusions that look faster than light

  • Image credit: ESO/M. Kornmesser
Date:3 May 2017 Tags:, ,

There are astronomical illusions that seem to be moving faster than light when we view them through a telescope. How is this possible?

By Avery Thompson

Nothing can travel faster than light. This is an innate law of the universe which can never be broken. But if you’re not careful when looking through a telescope, you could be fooled into thinking that this universal law is being violated.

Take a look at this image below. It’s a series of observations made by the National Radio Astronomy Observatory (NRAO) in the 1990s, a jet of hot gas being fired from a quasar. In this image, you can see one green blob of gas moving over about a six-year period. The scale on the bottom indicates that the green blob traveled about 35 light-years over those six years.

Jet of hot gas being fired from a quasar

How is this possible? The theory of relativity clearly prohibits moving faster than light, and yet here is an image of a gas jet doing exactly that. Maybe the bottom scale is wrong? Nope. We know how far away the galaxy is and we can calculate these distances very accurately.

It turns out that this image is a type of optical illusion arising from the fact that space is 3-dimensional, while our images are not. The gas jet in the photo is actually not headed sideways like it appears. Instead, it’s headed almost directly toward us.

In this video by Sixty Symbols, Professor Mike Merrifield looks at another example of this phenomenon:

This strange phenomenon—called apparent superluminal motion—works because of the limited speed of light. If a jet of hot gas is moving toward us at nearly the speed of light, the light that’s traveling toward us is coming from a place much closer than we think it is and arrives much sooner than we expect it to.

The result is that light that was emitted over a span of decades might all arrive on Earth within a span of only a few years. The jet is essentially racing behind the light it emits, compacting several decades of light into a much shorter timeframe. When we look at a jet like the one above, we see 35 light-years of distance covered in six years, but in reality the jet traveled much farther and took much longer to do it.

Even in astronomy, your eyes can deceive you.

Source: Sixty Symbols



This article about astronomical illusions was originally written for and published by Popular Mechanics USA.

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