Host: Professor Dingle, you are an expert on the subject of redshifts: what exactly are they?
Dingle: There are two different kinds of redshifts that are relevant to our discussion. The first kind of redshift is called the Doppler Velocity Redshift. It measures the relative velocity between two luminous bodies as they separate from one another in space. It is the only kind of redshift that has been physically confirmed.
The other kind of redshift theoretically results from light losing some of its energy over the great distances that it travels or propagates through infinite space. This so-called “Loss of Energy Redshift,” as compared to the Doppler Velocity Redshift, is the primary subject of our discussion this evening.
Host: Why is that? What do redshifts have to do with whether the universe is expanding?
Dingle: That is a very good question, and the answer is as follows. If the redshifts which Dr. Hubble observed emanating from distant galaxies are determined to be Doppler Velocity Redshifts, then this means that all of the observed galaxies must be systematically moving away from the Earth at ever increasing velocities which are proportional to their distance from Earth and the universe must be expanding.
But if such galactic redshifts are determined to be primarily Loss of Energy Redshifts, then this means that all of the galaxies are not moving away from the Earth and the universe is not expanding. It is just that simple.
Host: That’s really exciting. Let’s discuss the Doppler Velocity Redshift first. What exactly is it?
Dingle: Ok. But in order to properly answer your question we first need to go back into history to the year 1842. In that year an Austrian physicist named Christian Doppler theorized that the number or frequency of circular sound waves emitted by a whistle and moving through the air would change (that is, they would become less or more frequent in number per second) for a distant listener depending upon the linear (or line-of-sight) velocity of the body emitting the sound relative to the listener.
The most familiar example of this Doppler effect of sound is where the circular sound waves emitted by a speeding train’s whistle becomes higher and higher in number for a listener (that is, more and more shrill in pitch) as the train approaches a listener standing in a train station. What happens physically is that the whistle’s circular sound waves moving through the air become bunched together in the direction of the train’s motion, so that each sound wave is received more and more frequently by the listener’s ear in the train station. Please see Figure 9.
Then after the train’s whistle passes through the station its pitch becomes lower and lower (that is, less and less shrill) as the train recedes into the distance, because each sound wave is then received less and less frequently in the station by the listener’s ear. This Doppler Effect does not occur for a distant listener who is located perpendicular to the moving whistle, because for him the sound waves remain more or less evenly spaced apart.
A similar effect occurs in the vacuum of empty space of our Milky Way galaxy when two luminous celestial bodies (such as two stars) emit a spectrum of light waves toward each other as they are either moving linearly toward or away from each other.
Host: Before we go any further, Dr. Hubble please tell us what a spectrum is?
Hubble: Well, let’s see, this can get quite technical. A luminous body in space, such as the Sun, emits many different circular waves of light in all possible directions. These waves of light have many different wavelengths which vary from extremely long radio waves to extremely short gamma waves. All of these different wavelengths considered together are called the electromagnetic “radiation spectrum.” Please see Figure 2.
The very small visible light portion of the electromagnetic radiation spectrum is received by the human eye on Earth as a composite beam of yellow light which contains many different wavelengths of light. The human eye distinguishes each different wavelength of visible light as a different “color,” from the longer wave lengths of red to the shorter wave lengths of blue or violet. When the composite beam of yellow light is passed through a glass prism the individual wavelengths of light are bent or refracted at different angles. The colors are then spread out in an ordered rainbow-like sequence called a “visible light spectrum.” Please see Figure 2A.
Host: Are these the only types of spectra that exist?
Hubble: No. There are two other types of spectra that are very important to our discussion. When an atomic element in gaseous form (such as calcium) is heated to incandescence in the laboratory (in other words, it gives off light), it only emits certain wavelengths of visible light which may be thought of as the “fingerprints” for that particular atomic element. These wavelengths are shown on an optical instrument called a spectrograph as isolated sharply defined lines of color (called “spectral lines”) separated by dark gaps. The colored spectral lines appear in their normal wavelength locations in a rainbow-like spectrum, and all such spectral lines for each element considered together are known as an “emission spectrum.” An emission spectrum observed through a telescope identifies the chemical composition of a distant star.
There is also a fourth type of spectrum which is of critical importance for our discussion this evening. It is called an “absorption spectrum.” When an incandescent or glowing star is surrounded by a cooler gaseous atmosphere, the cooler gaseous atmosphere absorbs from the star just those colors of light which it would emit if it was also incandescent. The result as seen in a spectroscope on Earth is a continuous rainbow-like background of emitted colors interrupted by dark absorption lines which represent the colors absorbed by the stars’ gaseous atmosphere. This pattern of dark absorption lines (called an “absorption spectrum”) identifies the gases absorbed by the cooler gaseous atmosphere. In effect, it is just the reverse of an “emission spectrum.”
The most conspicuous dark absorption lines seen from most stars in our Milky Way galaxy are the H and K dark lines of absorbed calcium located in the violet region of the rainbow-like spectrum.
Host: Thank you, Dr. Hubble. I don’t think that was too technical, and we now know what the four types of spectra are. Professor Dingle, please continue with your description of the Doppler effect of light.
Dingle: At this point, Dr. Hubble and I will still only be discussing the Doppler velocity effect of light as it is observed in the local space of our Milky Way galaxy; not in the much more distant space of the universe.
When a composite yellow beam of light from a star or other luminous body in our local Milky Way space enters the spectroscope attached to a telescope on Earth, it contacts a moveable elongated opening or slit which causes the light beam to refract (or bend) and disperse into a visible spectrum of different wavelengths or colors. This rainbow-like spectrum is then recorded on a recording strip. The photo sensors in the spectroscope also detect any absorption spectrum that is contained in such light and superimpose its dark absorption lines over the background of the rainbow-like visible spectrum of colors. However, the dark absorption lines of the absorption spectrum may not be in their normal locations with respect to the rainbow-like spectrum.
Host: Why wouldn’t they be in their normal locations?
Dingle: I will now tell you why. If two luminous bodies I and II are relatively stationary in local space, a spectrograph located on each luminous body would detect the absorption lines of the light received from the other luminous body at its normal position on the rainbow-like spectrum. Please see Figure 5A and Figure 3A.
On the other hand, if such luminous bodies are relatively approaching each other in local space, each body will be at a slightly closer relative position when each successive light wave is emitted from such luminous body and received by the other. As a result, each body will receive the same total number of absorption lines from the other, but they will be bunched closer together than normal. This will cause the spectrograph to detect each successive absorption line emitted from the other body more frequently per second, than if the bodies were relatively stationary. Please see Figures 5B and 3C.
All of such more frequently received absorption lines will naturally be recorded by the spectrograph more toward the shorter (higher frequency) wavelength or blue light portion of the recording strip than would be normal for such absorption lines. Thus, the entire pattern of dark absorption lines is displaced from its normal position toward the blue end of the rainbow-like spectrum. This abnormal blueshift results because of the way the spectrograph is programmed to record spectral lines and absorption lines.
Similarly, if such luminous bodies are relatively separating from each other in local space, each luminous body will be at a slightly farther relative position when each successive light wave is emitted from such luminous body and received by the other. As a result, each luminous body will receive the same total number of absorption lines from the other, but they will be spaced farther apart than normal. Please see Figures 5C and 3B. This will cause the spectrograph to detect each successive absorption line emitted from the other body less frequently per second, than if the bodies were relatively stationary.
Such less frequently received absorption lines will naturally be recorded by the spectrograph more toward the longer wavelength (lower frequency) or red-light portion of the recording strip than would be normal for such absorption lines. Thus, the entire pattern of dark absorption lines is displaced from its normal position toward the red end of the rainbow-like spectrum. Again, this abnormal redshift of the absorption spectrum results because of the way the spectrograph is programmed to record spectral lines and absorption lines.
Host: Thank you, Professor Dingle. Do you agree, Dr. Hubble?
Hubble: Absolutely. In both cases, whether the shift is to the red end or to the blue end of the normal visible light spectrum of colors, each absorption line is shifted by a constant fraction of its normal position, and this fractional displacement is constant throughout a given absorption spectrum. Thus, the constant fraction of displacement of absorption lines is interpreted to describe and measure the relative velocity of the two luminous bodies in their line of sight.
For example, a shift of one part in a hundred thousand (0.00001) represents a relative velocity of 1.86 miles per second. And a greater shift of one part in a thousand (0.001) represents a relative velocity of 186 miles per second, and so on.
Doppler velocity-shifts of this magnitude, both to the blue end or to the red end of the visible rainbow-like spectrum are well known in the laboratory, and among the local planets and the local stars. They are familiar phenomena in local Milky Way space, and their interpretation of relative velocity is not to be questioned.
Now, let me state more specifically what we have been talking about: In local space the displacement or shift of the superimposed absorption spectrum (received from a local star) toward the red end or the blue end of the normal rainbow-like spectrum indicates the total relative velocity between a co-moving light source and the co-moving observer (i.e., on Earth) in the local space of the Milky Way galaxy.
But, like the Doppler effect of sound, only the relative velocity along the line of sight contributes to the Doppler shift of light. Please see Figure 11A. The motion of an observed luminous body which is oblique or at an angle relative to the motion of the observer on Earth will contribute less magnitude of light shift than if such body was moving linearly relative to the Earth’s motion. See Figures 11B and 11C. And if the motion of such luminous body is transverse or perpendicular relative to the motion of the Earth, it will not contribute any magnitude of light shift observed on Earth. See Figures 11D and 11E. All of these facts will become very important for our later discussions of whether the universe is expanding or not.
All of these Doppler velocity effects of light have been physically or visually confirmed in several ways in our local space. For example, the relative magnitudes of red and blue light shifts describe the visually observed motion of nearby planets, the visually observed rotating sun spots of the Sun, and the visually observed orbital motions of nearby binary (double) star systems in the Milky Way galaxy. See Figures 12 and 13.
Host: I thank both of you for those very clear and precise technical explanations of the Doppler velocity effects of light.