What is it like out there in the tremendous void between stars in our Milky Way galaxy? According to a recent article , there are about one million atoms (mostly hydrogen) per cubic meter, so it is not completely desolate. But get into the space between galaxies, and there are only 10 atoms/m3. That is a far better vacuum than anything we can achieve on Earth.
But imagine that you are in a space vehicle (an impossible dream because of the huge amount of energy it would take to get there), staring off into space through a porthole window. What will the “sky” look like? It would be lit up with billions of tiny stars. In other words, that cubic meter of lonely, practically empty space, is crisscrossed by a fantastic number of photonic outputs of the universe—the visible as well as ultraviolet, infrared, and so forth. All of it is electromagnetic radiation.
Recently, cosmologists have been calling our attention to another ingredient in the void between stars in the Milky Way galaxy—Dark Matter. It is not electromagnetic radiation, so we can’t “see” it. It is a mysterious substance—different from ordinary matter. Actually, Dark Matter refers to an effect that fits into the “nothing new under the universe” category, because it was proposed, way back in 1933, by Fritz Zwicky . According to his calculations at that time, which are remarkable in view of the relatively flimsy data upon which they were based, the stars in the galaxies should be flying apart: Given the mass of a typical star and the vast distances to its nearest neighbors, gravitational attraction is insufficient to hold the star in a circular or spiral orbit around the center of the galaxy. Zwicky suggested that “missing matter” was responsible. Eventually, the missing mass came to be called Dark Matter.
In addition to Dark Matter, since 1998, we have Dark Energy. Here is how Linda Rowan and Robert Coontz introduced The Dark Side, Science, 20 June 2003: “Dark stars, the dark age, dark matter, and dark energy are the major components of the dark side of the universe: 96% of the universe consists of mass and energy we can’t see and don’t really understand. Fortunately, the badly outnumbered 4% of luminous matter feels the dark side through gravity and other forces.” In this brief Chapter, Dark Energy is completely avoided; Dark Matter, on the other hand, is something that we can “feel”; it interacts with us via gravitational attraction.
It is no trivial or fly-by-night phenomenon; in fact, cosmologists estimate that the mass of Dark Matter is from five to 10 times that of the luminous material ! This is mind-boggling and ego-crushing; it should completely reverse our perspective and be worthy of headlines in the popular as well as scientific press. In other words, the galaxy consists of a huge blob, cloud, or halo of Dark Matter within which are 100 billion relatively insignificant specks—or stars—of ordinary matter. The ordinary matter flies along stream lines that are determined by gravitational interaction with the Dark Matter in which it is immersed.
The latest word on Dark Matter is that it is a “spherical halo” surrounding our Milky Way galaxy . This is portrayed in the idealized drawing of Fig. 1(a). The diameter of the Milky Way is 120,000 light years ; that is, it requires 120,000 years for light (photons) to travel across the galaxy.
If we assume that the Dark Matter obeys the laws of physics (because we don’t know any better), the spherical halo is not a stable configuration. If it is rotating, it would flatten out, like the disk of the galaxy. If it is stationary, it would fall in toward the center of the galaxy. My conclusion is that we do not have a spherical halo; instead, let us examine the configuration of Fig. 1(b), in which the Dark Matter is uniformly distributed within a sphere 120,000 light years in diameter. In that event, what is the density of the Dark Matter?
It is a simple matter to calculate the density: We are given the specifications listed in Table 1 (some of the values are rounded off to simplify the calculation). The net result is that Dark Matter has a density of 1.3 X 10-20 kilograms/cubic meter (kg/m3). How does this compare with air? It doesn’t – a glance at Table 2 shows that air is 1020 times as heavy! But the Dark Matter does have an appreciable mass – it is equivalent to having eight million hydrogen atoms in that cubic meter (or 8 atoms/cc). It is the equivalent to having an almost-perfect vacuum. Little wonder, then, that it has not yet been detected by laboratory instruments .
But the matter, Dark or otherwise, is far from being ended, for Dark Matter is reminiscent of the aether. Once upon a time (1864), James Clerk Maxwell and his contemporaries “invented” the aether, which is approximately analogous to our atmosphere. It filled all of empty space; that is, space that is devoid of mass such as neutrons, protons, and electrons. It was invented because a medium is needed in which an electromagnetic field (EMF) wave can propagate, just as sound cannot travel in a vacuum. Eventually, especially in the United States, the spelling was changed to “ether,” which causes minor irritation if one is looking up aether or ether in an index. Major irritation was caused in the 1920s, however, because the “big shots” of physics abandoned the aether. They couldn’t measure its motion with respect to the earth, so they declared that it doesn’t exist; that electromagnetic fields somehow propagate in a perfect vacuum.
There is no way an EMF can propagate in a vacuum. But in 1887, Albert A. Michelson and Edward W. Morley showed that the aether, if it exists, is traveling with the earth. On the other hand, star aberration data  received with telescopes indicate that it is not being dragged along by the earth: Picture a star at the zenith (overhead): When its light exits from the Milky Way’s aether on the way to being captured by the earth’s aether, the light should be bent by an angle of 0.0001 radians (20.5”), which represents the earth’s rotation around the sun (3 X 104 m/s) relative to the velocity of light (3 X 108 m/s). After capture, the star’s light should travel vertically, relative to the earth beneath. This does not happen at all; instead, a telescope has to be tilted at an angle of 20.5” to compensate for the earth’s rotation. The 20.5” is the “aberration of starlight.” If a telescope is pointed directly upward for a year, its star images will appear as tiny circles, 41” in angular diameter.
How can we explain the fact that the earth’s aether is dragged along by the earth, while aberration data indicate that starlight does not encounter the earth’s aether? Here it is my turn to say “Somehow, by relativistic effects which are described below, and by the curvature of space.”
Let’s consider another aspect of the earth dragging its aether along. Since the earth travels around the sun at a speed 0.01% as fast as the velocity of light, there must be a transition zone, in front of the earth, where the aether is compressed as one goes from the earth’s aether to that of the sun. Similarly, behind the earth, there must be a transition zone of expansion. This is illustrated in Fig. 2(a). The speed of light has to match that of the aether in the transition zones.
Alas, Fig. 2(a) is incorrect. If we send out a spaceship to measure the velocity of light in the “compression” or “expansion” zones, it will always measure 300 million m/s. Over 100 years ago, it was known that the velocity of light is always 300 million m/s, independent of whether the source is moving towards us or away from us, provided the measurement is made at an inertial (not accelerating) platform. (There is a radial acceleration of the earth around the sun, and of an object around the center of the earth, but these effects are relatively small.) The correct scenario is depicted in Fig. 2(b), where the uniform spacing signifies constant measured velocity of light. In 1905, Albert Einstein explained this with his theory of special relativity: space and time are not the same to all observers; and space can curve.To illustrate with two examples :1) A meter stick has a length L when it is stationary here on earth. But when it moves with velocity v in a direction parallel to its length, towards or away from us, we on earth measure that it has contracted to a length of
L* = L[1 – (v2/c2)]1/2,
where c is the velocity of light. If v = 0, the ruler remains 1 meter long; if v = c, it disappears with L* = 0; if v = 0.866c (260 million m/s), the length we measure is 1/2 meter long. A person traveling with the ruler is not aware of any length changes. All of this has of course been verified, not with meter sticks but with atomic particles.
2) A clock runs accurately when it is stationary here on earth. But when it moves with velocity v in any direction with respect to us, we on earth measure that it runs slow in accordance with
t* = t[1 – (v2/c2)]1/2.
If v = 0.866c, we measure one hour on a local clock, but only a half hour on the traveling clock; and so forth. A person traveling with the clock can use it as an accurate morning-waker-upper. Although this has spawned any number of stories about a traveler returning to earth much younger than his/her children, only atomic particles have been made to go at relativistic speeds for an appreciable amount of time.
Why, then, was the aether discredited and abandoned in the 1920s? Probably because physicists had their hands full with special relativity and quantum mechanics; solving the earth’s aether versus star aberration problem was an annoying distraction. But by declaring that the EMF can propagate without a carrier medium, that the aether did not exist, physicists ended up with some weird quantum-mechanical effects that the aether could explain. These ideas are further pursued in what follows:
Let’s take a closer look at an EMF. Just as ordinary matter is quantized into electrons, protons, neutrons, etc., the basic building block of an EMF is a photon. An EMF consists of photons. But what is a photon? Nobody knows. We know enough to represent a photon as the wave packet of Fig. 3. The frequency of the wiggles corresponds to the “color” of the photon. If it looks green to a human, the frequency is approximately 5.2 X 1014 Hz; that is, 520 trillion vibrations per second. How can anything wiggle that fast? Electrons can, and that is how the photons are usually generated. Actually, “green” is a slow vibrator; a typical gamma ray wiggles at 3 X 1021 Hz – around six million times as fast as “green.”
Let’s get back to that porthole window in outer space. Figure 3 shows two ways to record a particular photon. In (a), we have an imaginary sensor that hops up and down, at a particular place in the space vehicle, when it detects a photon. How far apart, in time, are the hops? For “green,” the reciprocal of 5.2 X 1014corresponds to a period of 1.9 X 10-15 second = 1.9 femtosecond.In Fig. 3(b), we take a photograph of the photon by opening the shutter as it zips by at a particular place in the space vehicle. For a “green” photon, what is the wavelength? It is 5.8 X 10-7 meter = 580 nanometers. (One billion nanometers = one meter.)All of these calculations about a photon are very impressive, especially since we don’t know what it really is. It seems to me that there are two models:
In Model 1, a photon is analogous to a minuscule projectile. It carries a certain amount of energy (for a “green” photon, 3.4 X 10-19 joule). This blob of energy leaves its source and flies through space, sometimes for billions of years, at the speed of light, until it encounters a material object. Then it deposits 3.4 X 10-19joule of energy (usually in the form of heat). What happens if two of these photon “projectiles” hit each other? Nothing much. They act as if they have zero diameter, so they pass “through” each other unscathed. But projectiles with zero diameter don’t make much sense. Also, how does it turn out that the speed of a projectile, regardless of its launching platform or frequency, is 300 million m/s?
In Model 2, a photon is analogous to a wave on the surface of a lake: The ripple is transmitted from one molecule of water to the next. All of us have experienced the energy carried by a water wave. But how is the ripple of a photon transmitted from one “molecule” of vacuum to the next? The answer, it seems to me, is that the “vacuum” is actually filled with that mysterious “substance” called the aether. Furthermore, if a photon is a ripple in the aether, we expect its velocity to be independent of the source or frequency, just as the speed of a sound wave is independent of the loudspeaker or the audio frequency.
In addition to the fact that the speed of light is 300 million m/s, regardless of frequency, there are other arguments favoring an all-pervading aether. They are based on the electromagnetic nature of a photon. As illustrated in the “end view” of Fig. 4, the wiggles of Fig. 3 (also shown as the E,H waveform of Fig. 4), represent an electric field (E) at right angles to a magnetic field (H). The end view depicts E as vertical and H as horizontal, but any orientation (technically called “polarization”) is possible. Of course, we are all familiar with E and H fields: lightning is an example of a powerful discharge due to the electric field between clouds and the earth; the imaginative shapes that hold promissory notes against the sides of a refrigerator are examples of a magnetic field. These are the same fields that, when rapidly oscillating, propagate as photons, with E and H lines that are at right angles to each other (and, also, to the direction of propagation).
But what are E and H fields? We don’t have the foggiest notion as to why the universe was built this way. An E field is some kind of stress. How can a stress exist in a vacuum? But stress applied to an aether can be visualized. Similarly, how can a magnetic field exist in a vacuum? Recent evidence is that the magnetic fields in certain “magnetar neutron stars” , due to fantastically high currents, can be as high as 1011 teslas! (On earth, 101 teslas is a very strong magnetic field.) Again, one can imagine a field of 1011 teslas as being sustained by something – the aether – rather than nothing, such as a perfect vacuum.Why should the speed of light be 300 million m/s? Because the aether has definite characteristics, analogous to the density and elasticity of air where the transmission of sound is concerned. We can learn much from the transmission of sound [but be careful – in sound, the molecules are pushed and pulled in the direction of propagation (longitudinal vibration) while, in the aether, the “molecules” are pushed and pulled at right angles to the direction of propagation (transverse vibration)]. The analogies between sound and an electromagnetic field are given in Table 2. The transmission of sound is resisted by the inertia (density) of its molecules, but assisted by its elasticity (modulus of elasticity). A simple equation yields the velocity of sound;exactly the same equation gives the velocity of light, with permeability analogous to density in sound, and the reciprocal of permittivity analogous to elasticity in sound. For audio transmission, we can exactly measure the density and elasticity of most materials (note that air is surprisingly heavy). Similarly, one can measure the permeability and permittivity of most materials; the handbooks list these for vacuum, air, mica, water, and so forth.If you have read this far, perhaps it has occurred to you that the velocity of light can change if, for some strange reason, the permeability and/or permittivity of the aether changes. Please, banish such dangerous, subversive thoughts. They have already started to elicit shock waves amongst physicists and cosmologists , , , , .
More recently, however, the vacuum has been dignified with additional properties: subatomic particles randomly appear and disappear. It is therefore confusing to call empty space a “vacuum” while so much is going on. But our physicists and cosmologists came to the rescue, in 1998, with a fashionable new word – quintessence . (Actually, it is only a new application, because it appears in old dictionaries.) Now, empty space is permeated with quintessence (Dark Energy). So, since the aether was rejected, and the “vacuum” is misleading, let’s all jump on the “quintessence” bandwagon. Well, not so fast: The latest descriptions of DarkMatter reveal that it is a “dark horse”; it could be the aether, after all!”
Does the Dark Matter make its presence felt locally, in any way? Yes, in answer to the following question: How does the energy, in the form of photon radiation from the sun, cross 1.5 X 1011 meters of vacuum to nourish the earth? As mentioned above, Maxwell’s proposal was that photons propagate through the aether in a manner somewhat analogous to the way sound is transmitted through air. In other words, Dark Matter is not all that mysterious since we know that electromagnetic waves travel through space at a speed of 300 million m/s.Do we have any evidence relating to the gravitational interaction between the aether and ordinary matter? Again, the answer is “Yes,” but it is highly controversial, and would never stand up in court as an example of reliable evidence. Nevertheless, here is the unpalatable conjecture:
By analogy: The earth retains its atmosphere because of gravitational attraction between the gases and the solid earth; in fact, barometric pressure measures the weight of air above the instrument. Similarly, the earth retains its aether, because of gravitational attraction between the aether and solid earth, so that the aether moves with the earth. This is associated with two unlikely implications:
1) Every large object, such as the sun, planets, moon, and so forth, carries its own aether because of object-aether gravitational attraction.
2) If the aether compresses or expands, it probably changes the speed of light. The velocity of sound is given by the square root of elasticity/density; if air is compressed, the elasticity increases faster than the density, so velocity increases. For an EMF, velocity is given by the square root of 1/(µ
ε) (see Table 2). If aether is compressed, does the velocity of light increase or decrease? It can go either way, but nobody knows because it is impossible to compress or expand a bottle of aether. Joao Magueijo thinks that the velocity of light was much higher in the early days of the Universe  when (according to my interpretation) the aether was more compressed than it is now because, obviously, the Universe was smaller.What are some of the other peculiar conjectures about the aether? In an atom, an electron can make millions of circular or elliptical revolutions around the nucleus, repeating the same path over-and-over again. This is known as a “standing wave.” Here the aether may act to stabilize the electron against lateral displacement out of its orbit (see ).
In a double-slit experiment, depicted in Fig. 5 (which is taken from ), a laser beam is fired at two adjacent, narrow slits. Some of the EMF gets through the upper slit, and some through the lower slit. When the laser beam passes through a narrow slit, it spreads out laterally, so that light passing through each slit spreads over the photographic film at the right. Two of the rays thus formed, of lengths
l1 and l2, are singled out as they come together on the sheet of photographic film. What pattern will the exposed film show? In some locations, the EMF from l1 is in phase with that of l2 when they meet at the film, thus increasing film exposure (constructive interference). At other locations, they have opposite phases, and the EMFs cancel (destructive interference). The net results of constructive and destructive interference are the idealized set of peaks and valleys of Fig. 5(b). But now reduce the laser beam intensity so that only a single, isolated photonat a time is fired at the two slits. The photon (which cannot split in half) goes through one of the slits but, somehow, it induces a wave packet (which has zero energy) in the second, adjacent slit. The sequence is illustrated in Fig. 6, with the photon going through the upper slit. The second wave packet combines with the first slit’s wave, reinforcing where the two wave packet peaks add, and canceling where they subtract. This is a difficult experiment, but it has been sufficiently duplicated so that there is no doubt about the results of Fig. 5(b). The mystery here is — how can the first wave packet give rise to the second slit’s wave? One possibility is that the second wave packet is a shock wave that results because the photon, acting as a projectile, flies through the aether at the speed of light. Furthermore, as shown in Fig. 6(g), the photon veers off to register a constructive interference peak at y = 4; this requires a lateral push-and-pull that can be supplied bygravitational forces between the photon and the aether.
But enough of this subatomic stuff. Let’s end this highly conjectural essay by looking at the big picture — the entire universe. If the aether and Dark Matter are the same, and since electromagnetic fields are transmitted across all of space, then all of space is filled with Dark Matter. The Dark Matter is the Universe (with relatively insignificant galaxies, stars, and planets floating around here and there). This picture is satisfying from one point of view: As the cloud of Dark Matter expands (or contracts), the outer reaches of the cloud define space. The answer to the question of how space can be limited is this: Signals that reach the outer boundary of the Dark Matter are reflected; we pick up the signals as the Cosmic Background Radiation (CBR). In the void beyond the Dark Matter cloud, space is meaningless.
Remember, however, that if the density of the aether changes as the Dark Matter blob expands (or contracts), we can be in deep trouble, for that implies that the velocity of light has not been constant at 300 million m/s. We may have to discard many of the so-called natural constants and, with that, the history of the universe for the past 14 billion years, including the Big Bang concept.
Using numerical values given in Table 1:
Assuming that 2/3 of the baryons are neutrons or protons, and ignoring the mass of electrons, the mass of the Universe is
1080 baryons X 1.67 X10-27 kg X 2/3 = 1053 kg.
The mass of a typical galaxy (Milky Way) is
1053 kg/100 X 109 galaxies = 1042 kg.
The mass of Dark Matter in the Milky Way is
1042 kg X 10 = 1043 kg.
The radius of the Milky Way is
60,000 light-years X 9.46 X 1015 m = 5.67 X 1020 m.
The volume of a sphere is 4πr3/3, so the volume of the Dark Matter sphere is
4π(5.67 X 1020)3/3 = 7.65 X 1062 m3.
The density of Dark Matter is
1043 kg/7.65 X 1062 m3 = 1.3 X 10-20 kg/m3.
This is equivalent, in hydrogen atoms, to
1.3 X 10-20 kg/m3/1.67 X 10-27 kg = 8,000,000 atoms/m3.
What is the centrifugal force due to radial acceleration around the sun? The force is given by
F = mv2/r.
For a 1 kg mass, with v = 3 X 104 m/s and r = 1.496 X 1011 m, we get
F = 1 X 9 X 108/1.496 X 1011 = 0.006 newton.
This is negligible compared to the weight of the object, which is 9.8 newtons at sea level. What is the centrifugal force due to radial acceleration around the center of the earth at the equator? Here, v = 466 m/s and r = 6.4 X 106 m, so
F = 1 X 4662/6.4 X 106 = 0.034 newton.
This is 0.35% as much as that due to gravity.
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