BOSTES HSC Physics 2015 Question 33

Question: 
This question is asked in the context of astrophysics. Students are expected to explain the impact of the development of space-based telescopes on an understanding of celestial objects.

Marking Guidelines:

Criteria	Marks
Relate applications of space-based telescopes to an understanding of celestial objects.  Assess the impact of the development of space-based telescopes.	6

(Source: https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-physics.pdf)

Possible answer:
Students are expected to make informed judgments on the impact of the development of space-based telescopes on the understanding of celestial objects. Below are examples of applications of space-based telescopes from a perspective of various types of electromagnetic waves.

1. Visible light: Earth’s atmosphere can reduce the intensity of visible light rays and distort the locations of celestial objects. On the contrary, space-based telescopes can avoid atmospheric distortions, and thus capture images with higher intensities and better resolutions. (In other words, the locations of a celestial object would not be distorted by earth’s atmosphere and the images captured are more accurate.)

2. X-rays and gamma-rays: Earth’s atmosphere can block the X-rays and Gamma rays completely such that ground-based optical telescopes are unable to capture them. However, space-based telescopes can detect celestial objects that emit X-rays or gamma rays, and help astronomers to achieve a deeper understanding of stars and galaxy structure.

3. Radio waves: The atmosphere does not pose problems such as blocking or significant absorptions of radio waves. Importantly, linking a space telescope and a ground telescope can improve resolutions of radio sources, and particularly useful to study star formations.

Feynman’s insights or goofs?:
Physics teachers may use Feynman’s lectures to explain some problems of ground-based telescopes and applications of space-based telescopes from the perspective of electromagnetic waves.

1. Visible light: Feynman explains that the earth’s atmosphere can distort the location of the sun because of the varying density of air. In Feynman’s words, “[a] third interesting phenomenon is the fact that when we see the sun setting, it is already below the horizon! It does not look as though it is below the horizon, but it is. The earth’s atmosphere is thin at the top and dense at the bottom. Light travels more slowly in air than it does in a vacuum, and so the light of the sun can get to point S beyond the horizon more quickly if, instead of just going in a straight line, it avoids the dense regions where it goes slowly by getting through them at a steeper tilt. When it appears to go below the horizon, it is actually already well below the horizon (Feynman et al., 1963, section 26–4 Applications of Fermat’s principle).” In a similar sense, light rays from an object in a pool are refracted and the location of the object is distorted. Importantly, Feynman’s explanation is applicable to any celestial objects or stars. (However, gravitational lensing effect can result in multiple images of the original galaxy due to the presence of black holes and dark matter.)

2. X-rays and gamma-rays: Feynman has an insightful explanation of an origin of cosmic radiations: “[i]n the year 1054, the Chinese and Japanese civilizations were among the most advanced in the world; they were conscious of the external universe, and they recorded, most remarkably, an explosive bright star in that year. (It is amazing that none of the European monks, writing all the books of the middle ages, even bothered to write that a star exploded in the sky, but they did not.) Today we may take a picture of that star, and what we see is shown in Fig. 34-7. On the outside is a big mass of red filaments, which is produced by the atoms of the thin gas “ringing” at their natural frequencies; this makes a bright line spectrum with different frequencies in it. The red happens, in this case, to be due to nitrogen. On the other hand, in the central region is a mysterious, fuzzy patch of light in a continuous distribution of frequency, i.e., there are no special frequencies associated with particular atoms. Yet this is not dust “lit up” by nearby stars, which is one way by which one can get a continuous spectrum. We can see stars through it, so it is transparent, but it is emitting light....What keeps the electron energy so high for so long a time? After all, it is 900 years since the explosion — how can they keep going so fast? How they maintain their energy and how this whole thing keeps going is still not thoroughly understood (Feynman et al., 1963, section 34–4 Cosmic synchrotron radiation).”
       In 1967-8, Jocelyn Bell discovered numerous celestial radio sources including the Crab Nebula that emit clock-like pulses of radiation. Currently, astronomers call these remnants of a class of supernova as pulsars or rapidly spinning neutron stars. They deduce that the neutron star has about the same mass as the sun but it is compressed into a very dense sphere that is only a few miles across. Thus, electrons in the powerful magnetic field around the stellar core are spiraling at nearly the speed of light and emitting electromagnetic waves.

3. Radio waves: We use a specialized antenna and radio receiver to detect radio waves. Feynman explains that “radio waves have been detected from places in space beyond the range of the greatest optical telescopes. Even they, the optical telescopes, are simply gatherers of electromagnetic waves. What we call the stars are only inferences, inferences drawn from the only physical reality we have yet gotten from them — from a careful study of the unendingly complex undulations of the electric and magnetic fields reaching us on earth (Feynman et al., 1964, section 20-2 Three-dimensional waves).” That is, much theoretical analysis or inferences are needed to study radio waves.
       In the 1950’s, Brown and Twiss show that it is possible to measure the angular sizes of celestial radio sources from correlations of signal intensities instead of amplitudes by using independent detectors. In short, Feynman (1985) explains “[t]he relative directions of the two arrows can be changed by changing the distance between the sources or the detectors: simply moving the detectors apart or together a little bit can make the probability of the event amplify or completely cancel out, just as in the case of partial reflection by two surfaces (p. 75).” Note that Feynman does not directly mention Hanbury-Brown-Twiss effect which is used to distinguish single source and a double source of radio waves when they are extremely close together. 

References: 
1. Feynman, R. P. (1985). QED: The strange theory of light and matter. Princeton: Princeton University Press. 
2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley. 
3. Feynman, R. P., Leighton, R. B., & Sands, M. (1964). The Feynman lectures on Physics, Vol II: Mainly electromagnetism and matter. Reading, MA: Addison-Wesley.