Looking Back
in Time

A Cosmic Speed Limit

The velocity of light plays a central role is astronomy and in physics. According to the Einstein's Theory of Relativity, nothing in our universe can exceed the velocity of light; thus, it is a kind of cosmic speed limit against which all other velocities may be measured. More generally, light is part of the electromagnetic spectrum, which includes infrared radiation, radio waves, gamma rays, X-rays, ultraviolet radiation, and so on. All of these are a form of light; they just have energies that differ from the visible light that our eyes can see. Thus, these forms of electromagnetic radiation all travel at the speed of light too.

The Speed of Light is Constant

Furthermore, contrary to normal intuition, the Theory of Relativity tells us that light always travels at the same speed relative to some observer, no matter what the relative motion of the observer. Thus, light emitted from a moving airplane does not travel with the speed of light plus the speed of the airplane, it travels with the "speed of light", no matter what the speed of the airplane! Although this seems strange, it has been confirmed in many experiments. These experiments show that it is our "common sense" that is wrong in this case!

To be precise, what we usually call the "speed of light" is really the speed of light in a vacuum (the absence of matter). In reality, the speed of light depends on the material that light moves through. Thus, for example, light moves slower in glass than in air, and in both cases the speed is less than in a vacuum. However, the density of matter between the stars is sufficiently low that the actual speed of light through most of interstellar space is essentially the speed it would have through a vacuum, so we don't make much error by ignoring the difference.

Looking Back in Time

Because light travels at a large but finite speed, it takes time for light to cover large distances. Thus, when we see the light of very distant objects in the universe, we are actually seeing light emitted from them a long time ago: we see them literally as they were in the distant past.


Figure: Supernova 1987a (the bright star at the lower right) and the Tarantula Nebula of the Large Magellanic Cloud. Source: Anglo-Australian Telescope photograph by David Malin; Copyright by Anglo-Australian Telescope Board

For example, Supernova 1987a occurred in a "nearby" galaxy called the Large Magellanic Cloud (adjacent figure). Its light was observed on Earth in 1987, but the distance to the Large Magellanic Cloud is about 190,000 light years. Thus, we normally say that Supernova 1987a occurred in 1987, but it really happened about 190,000 years earlier; only in 1987 did the light of the explosion reach the Earth! If we want to know what the Large Magellanic Cloud looks like "now", we will have to wait 190,000 years.

In comparison, the Sun is only about 8 light-minutes away. So the light we see from the Sun represents what the Sun looked like 8 minutes ago, and we must wait another 8 minutes to see what it looks like "now".

The Most Distant Objects Observed

The most distant things that astronomers can see are about 18,000,000,000 light years away. Thus, the light that we presently see from these objects began its journey to us about 18 billion years ago. Since that is close to the estimated age of the Universe, this light is a kind of "fossil record" of the Universe not long after its birth! Thus the observation of very distant objects is in a very real sense equivalent to looking backwards in time.

Until recently, the most distant objects were quasars. Now however, examination of the Hubble Deep Field has revealed galaxies that may be further away than the most distant quasars (Ref). The image adjacent left shows what may be the most distant object yet observed. It is the faint red smudge at the tip of the arrow, and appears to be a galaxy further away than any quasar (Ref). If the indirect method of estimating their distance is reliable, at least six galaxies (including the one in the adjacent image) may be so far away that we are seeing them when the Universe was less than 1 billion years old. If so, this implies that the formation of galaxies started relatively soon after the big bang.

A Technique for Identifying Distant Galaxies

The distant galaxy identification technique used here relies on the Hubble Law that more distant galaxies will be more redshifted. By looking at distant galaxies using different filters to emphasize light of different wavelengths, and then comparing the results for unknown galaxies with a control group of galaxies for which the distance is already known, it is argued that statistically one can determine distances to the most distant galaxies. By statistical, we mean that there could be error in the determination for any one galaxy, but for a set of galaxies the results will be reliable, on average. The technique is illustrated in the following figure.

Here four filters have been used on the Hubble Space Telescope to emphasize progressively shorter wavelength light from right to left in the four images. The galaxy indicated by the arrow is only seen easily in the near IR region of the spectrum, indicating that it has a very large redshift and therefore is distant.


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