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If there ever is a popular contest for "The Best-loved Scientific Instrument", the astronomical telescope will likely be one of the top contenders. Most people have seen or even used a telescope before. However, not too many have noticed that it is also a time machine. Here we will briefly discuss the basic principles of the astronomical telescope, and we will draw some examples of how astronomers use this time machine to study the history of universe.

Whatever the design of a telescope(1), it must have a main objective that collects signals from a large area and concentrates them into a point, called the focus. The objective could be a lens or a mirror, or it can even be made of some other materials. The larger the area of the objective, the more signals it collects. It is usually not difficult to make the objective large; what's difficult is to accurately focus the signals. The common optical telescope makes use of either refraction by a lens or reflection by a mirror to channel incoming light to the focus (Fig. 1, 2). The human eyes are therefore small telescopes, and astronomical telescopes are nothing but artificial eyes. When you look through a telescope, you effectively enlarge your iris to the size of the objective and soak up all the light gathered. The diameter of human iris is typically 7mm. Therefore, we can easily calculate the amplification of light intensity through a telescope - we simply take the square of the ratio between the diameter of the objective and 7mm. We can uncover much information about distant stars after their feeble light is intensified.

(Fig.1 The objective concentrates incoming light to a point called the focus.)

(Fig. 2 One of my favourite telescopes - a refractor with six inch aperture.)

People often ask, "How far can one see with this telescope?" or "How many times will this telescope magnify images?" Well, both questions are wrong. The main function of an astronomical telescope is to gather light. In principle, no matter how far an object is, some fractions of its light would enter the telescope as long as it is in the direction of the objective and not obstructed by some other objects. Whether the gathered light is intense enough for us to see depends not only on the size of the telescope, but also on the brightness of the object and atmospheric conditions. If the object we want to see is too dim, we cannot see it even if it is very close. For example, we can compare Pluto with M31, the spiral galaxy(2) in the constellation Andromeda (Fig. 3). Pluto is a member of our solar system, and its distance from Earth is about six billion km. Pluto does not emit visible light itself, and the feeble light we receive from it is the reflected sunlight from its surface. Even if we use the largest telescope on Earth, we cannot observe Pluto with much clarity; we could not even see it with naked eyes. On the other hand, in clear autumn nights, M31 is readily visible to human eyes without the help of a telescope, even though it is 2.2 million light years away(3), much farther from us than Pluto. The reason for the disparity is that M31 being a system of hundreds of billions of stars, is intrinsically very much brighter than Pluto. It is the most distant object that human beings can see with naked eyes! Take another example. Probably nobody can see a candle from 10km away, but we can all see the sun clearly (Fig. 4), even though it is 150 million km from us! Similarly, the effective magnification of an image depends more on the intrinsic brightness of the object and the atmospheric conditions than on the telescope itself. In fact, many astronomical objects subtend large angular sizes, and so there is no need for much magnification at all. For example, M31 is three times the angular size of Moon; it is a large object in the sky. We won't be able to see the entire galaxy if we magnify its image.

(Fig. 3: Pluto (left) and the Andromeda Galaxy M31 (right). Pluto is much closer to us than M31, and yet it cannot be seen with naked eyes on Earth. The Pluto photo was downloaded from HST/NASA website http://hubblesite.org/newscenter/, and the M31 photo was taken by Delphi Kwok using a 90mm telescope. Pluto photo courtesy NASA/STSCI. )

(Fig. 4 The closest star to Earth: Sun. The average distance between Earth and Sun is 150 million km. I used a small telescope with an aperture of only 7cm, together with a special filter to take these photos.
Never observe the sun directly without proper solar filters!)

Since the main function of the objective is to collect and focus incoming light, any materials that can bend light can be used to make it. Amazingly, giant telescopes exist in the universe naturally, and they are not even made of any material substance. The trick is to make use of gravitational force to bend light(4). One famous example is shown in Fig. 5A, in which the yellow patches form the galaxy cluster 0024+1654 (just a name). A galaxy cluster is a collection of galaxies, each of which typically contains about 100 billion stars, and the gravitational field in the vicinity of a galaxy cluster is obviously very strong. This particular cluster is about 5 billion light years away from us, and its gravitational force distorts the light from an even farther away galaxy to form a gravitational lens(5). The bluish-white galaxy shown in Fig. 5A is actually about 10 billion light years away, and we would not be able to see it without the help of this Nature's "telescope" to intensify its image. We can even observe the structure of this far away galaxy, resolving down to regions of about 300 light years in size! However, the "optical quality" of this "telescope" is far from being ideal. It distorts and makes five copies of the image. Another similar example is shown in Fig. 5B.

It is easy to spot Orion Nebula M42 (Fig. 6) in a clear winter night. This nebula, a cloud made of interstellar gas and dust, is approximately five thousand light years from us. In human scales, a distance of five thousand light years is perhaps too far for us to imagine, but it really is very close for astronomical objects. The light emitted from this nebula travelled for five thousand years before reaching our telescopes. This means that the image in Fig. 6 is that of the nebula five thousand years ago! What does this nebula look like today? Nobody on Earth would know; we will have to wait till five thousand years later to find out about that. Similarly, when we use large telescopes to watch distant galaxies, we are observing their images long time ago. For example, the small spots of red light indicated by the white arrows in Fig. 7 are believed to be among the first-generation galaxies in the universe. They are over ten billion light years(6) away from us, and it took over ten billion years for their light to reach us!

Here we should introduce the concept of the light cone. If one needs to send some signals from one point to another point separated in space, the fastest way is to use a light beam. Nothing travels faster than light, as far as we know, but even light has a finite speed, of about three hundred thousand kilometres. Therefore, the transmission of signals will take some time. In other words, there will always be some time delay in the communication between any two points in the universe; the farther they are apart, the longer it takes. Without communication, objects at these two points cannot affect each other. This is true no matter how far or how close the two points are apart. No matter how powerful our telescopes are, we can "see" only a small corner of the vast universe (Fig. 8). If you look around and see your classmates, what you see are not their images at this moment. If you see your classmate sitting next to you smiling at you, that happened a fraction of a billionth of a second ago. It is impossible for you to know what your classmates are doing at this very moment; you can only find out what happened in the past. All you can see are history!

(Fig. 7 Images of distant galaxies taken with Hubble Space Telescope. The reddish spots of light indicated by the arrows are emitted from galaxies that are estimated to be over twenty billion light years away. This photo is downloaded from Hubble Space Telescope website http://hubblesite.org/newscenter/ .
Photo courtesy: NASA STSCI)

(Fig. 8 The light cone: from each point in space one can draw a boundary for the space-time region where light (of speed c) emitted from the point can reach. Points outside of this light cone, coloured in blue, cannot communicate with the point at the origin.)

Knowing that the farther one looks, the further back in time one sees, astronomers study the evolution of galaxies by comparing their structure and shapes at various distance. This is like taking snapshots of galaxies at different ages and watching how they age. In Fig. 9(7), one sees images of some galaxies when the universe was about two, five, nine, and fourteen billion years old, the last one being the current age. Young galaxies tend to be small and irregular in shapes, and it seems that they grow bigger and into more definite elliptical (upper panels) or spiral (lower panels) shapes. There are probably many collisions and mergers among neighbouring galaxies during this process , such as the one shown in Fig. 10(8).

(Fig. 9 Images of galaxies at various distance, showing how their structure and shapes evolve. From right to left are images of galaxies at two, five, nine, and fourteen billion years old. http://hubblesite.org/newscenter/ Photo courtesy: NASA STSCI.)

(Fig. 10 Spiral galaxy NGC3314 was formed by a pair of merging galaxies. This photo was downloaded from NASA Hubble Space telescope website http://hubblesite.org/newscenter/.
Photo courtesy: NASA STSCI.)

Well, the answer is the hot primordial fireball(9) of the early universe . Amazingly, everyone has a telescope at home to catch the signals from this object, and it is your television! As the universe expanded, the light waves emitted about fourteen billion years ago by the primordial fireball were stretched longer and longer to become microwaves today, forming the cosmic microwave background radiation, which fills the entire universe almost completely uniformly. The antennae of our television picked up these microwave signals, which show up as a small fraction of the noise, or "snowflakes", on the television screen (Fig. 11). Astrophysicists built specialized microwave telescopes to measure these cosmic radiations and deduced information about the physical conditions of the early universe from these signals. You probably have not realized that your TV antenna is actually an ultimate telescope and time machine!

Astronomers are building larger and better telescopes, so that they can observe farther objects and learn more about the early universe. These will help in answering one of the ultimate questions that human beings ask: what was it like in the beginning, and how did the universe evolve? These research involve hard work of many generations of scientists. Perhaps you will also become one of them?

(Fig. 11: "Snowflakes" on a TV screen. A fraction of these noises comes from cosmic microwave background radiation, which was emitted from the primordial fireball in the early universe.)