Callisto and Arcas: Loving mother and dutiful son.
Many mortals languishing under the dominion of the Oympians suffered cruelly and unjustly at their hands. Of this lamentable fact we are all well aware. None suffered more than Callisto, a woman who was not only remarkably beautiful and uncommonly kind, but was also devoted to her only son, Arcas. Both mother and son were skilled hunters who passed many happy days in the earnest pursuit of quarry through forest, fields and mountains. One day, the unfortunate Callisto drew the attention of the lustful Zeus, notorious for his philandering ways and his wanton use and abuse of women. Keenly aware of Zeus' propensities, Hera, his tormented and perpetually jealous wife, quickly noticed how Callisto had unknowingly beguiled her husband. She promptly descended onto the land and confronted Callisto as the latter was hunting in a deep forest while her son pursued game elsewhere. The astonished Callisto at once beheld a furious goddess who materialized from thin air while a sky-wide band of storm clouds darkened the forest. (Hera conjured the clouds to conceal herself and Callisto from Zeus.) Hera identified herself at once to the now terrified Callisto and without any explanation transformed her into a large bear. Although she altered Callisto's body, Hera cruelly allowed her to retain the mind and soul of a woman, so she would remain fully conscious of her bestial transfiguration. Looking down at her claws, fur and the other ursine features, Callisto tried to scream, but could only produce a fierce growl. She then watched helplessly as Hera, smirking maliciously, vanished. Horrified by her alteration, Callisto fled madly away. She found that not only could she run swiftly, but her immense bulk allowed her to move almost effortlessly against the same tree branches that proved so obstructive to her as a woman. Soon, she was hidden within the deepest forest hollows; so remote, in fact, that the darkness prevailed even though the storm clouds had melted away. Callisto was miserable: more miserable than she could have ever imagined possible during her short, but otherwise happy life. Throughout each day she foraged constantly for food, feeling profoundly ashamed at her voracious appetite and beastly means of food gathering. Only she alone knew what it was to suffer the incongruous aches of a bear's incessant hunger and a woman's broken heart. Not just broken, but shattered, for she dearly missed her son Arcas. Every moment engendered a grief more bitter and intense than the one preceding it. She often looked through the lattice work of trees within the forest and knew that contained therein was Arcas, no doubt looking desperately for his mother. The pain of their separation was often so sharp that she cried out, only to hear the same bear's roar that she had heard so often when she had pursued them as a huntress.
Within a week of her transformation, Callisto was awakened from her uneasy sleep by the sound of footfalls on branches. She at once spied Arcas walking through the trees nearby, His sweet, but despairing, voice often called out "Mother! Mother! Are you well?! Where are you?! Mother!" Forgetting her current condition, Callisto leapt up and called out, "Arcas! Arcas! I am here! I am here!" To her delight, Arcas looked over toward her and she bounded toward him, ecstatic at what she believed would be a joyous reunion. She didn't realize until it was too late that Arcas had only heard a fierce growl and at the moment was looking at a large bear running toward him. The young hunter deftly removed an arrow from his quiver and placed it in his bow. To her terror, Callisto remembered her appearance and realized that she was well within range of Arcas' arrow. She was about to be killed by her own son. Zeus, who had been curiously absent until that moment, looked down on the scene and in one stroke transformed Arcas into a bear. The arrow, bow and quiver fell to the ground. He then grasped both bears by their tails and cast them into the night sky where they became Ursa Major and Ursa Minor, the Great and Little Bear, respectively. In his new form, Arcas recognized his mother at once and they started playfully pursuing each other through the circumpolar sky. Hera, incensed at their reunion, persuaded Posiedon, god of the ocean, to deny them access to the sea so that they would be forever condemned to roam the northern skies. It would have infuriated Hera to have known that they regarded their perpetual placement in the northern skies as the best of all possible fates: for they would never again be apart, their star fields would remain forever verdant, and although they would never be human again, neither of them would ever die. Astronomically, one can still find the Big and Little Dippers in the northern sky. The "Big Dipper" is an asterism within the constellation Ursa Major. The "Little Dipper" comprises almost all of Ursa Minor. Being circumpolar constellations, they never set, or at least won't set in our lifetimes. One can use the Big Dipper's outer bowl stars, Dubhe and Merak, to find Polaris, the end star in the Little Dipper's handle. Throughout the course of one day, the Big Dipper will appear to describe wide circles around Polaris. In fact, the entire sky seems to revolve around this remarkably unremarkable star merely because it is aligned almost directly with Earth's north celestial pole.
THE DAILY ASTRONOMER
Thursday, November, 5, 2020
Thursday, November, 5, 2020
Remote Planetarium 118: Questions
The farther afield we travel, the more exotic the Universe becomes. Today we're devoting a class to more subscriber questions. We enjoy receiving questions because they enable us to cover the topics we missed, or expound on those topics to which we paid insufficient attention. This latest question batch includes inquiries about black holes, dark galaxies and parallel Universes, all of which transcend the mundane, or, more correctly, what we often perceive as mundane.
"Are there galaxies made completely of dark matter? If there are, how could we ever find them?" -D.H.
Galaxies consisting entirely of dark matter might exist. We know that ninety percent of our home galaxy consists of dark matter. It stands to reason that a galaxy comprised of nothing but dark matter could form in the Universe. In August 2016, a team of astronomers determined that an ultra diffuse galaxy dubbed Dragonfly 44 had no discernible stars despite being nearly as massive as the Milky Way. These observations suggest that Dragonfly 44 is composed almost entirely of dark matter.
Dragonfly 44, an ultra diffuse galaxy located about 330 million light years from Earth. Nearly as massive as the Milky Way, this galaxy consists of no discernible stars. It is the closest thing astronomers have yet found to a dark galaxy, a galaxy consisting entirely of dark matter.
Finding dark galaxies is not a simple matter. One technique of detecting such galaxies involves using radio telescopes to detect the famous 21 cm emission from atomic hydrogen. This emission is caused by transitions of neutral hydrogen atoms. This wavelength,-which is not precisely 21 cm, but slightly above- is within the microwave section of the electromagnetic spectrum. These emissions in regions that lack any luminous material might be indicative of a dark galaxy, one consisting of neutral hydrogen reserves but no stare (or very few.) Another method involves searching for hydrogen absorption lines within distant quasars. The theory is that light emitted from these distant objects would pass through a dark galaxy. Material within that galaxy would then absorb the light at the wavelengths consistent with hydrogen. The problem with these techniques is that intergalactic hydrogen clouds could be confused for dark galaxies.
"I'm having difficulty grasping why time would appear to slow down infinitely around a black hole. What would the functions of the time=distance/speed calculation be that would cause this phenomenon, or would it be some other calculation that explains this?" E.C.B.
The difficulty is understandable. Time dilation around a black hole is a tricky matter. Let's begin by repeating the thought experiment involving two observers: one on a platform well away from a black hole and the other on a vessel moving toward the black hole. Both observers will seem to experience time just as we do sitting here on Earth. Neither will "sense" any time dilation, just as a person moving at 99.999% light speed won't know that time is dilating. A day for that person passes like any other day. However, for a stationary observer, much more time elapses during what, for moving observer, is a 24-hour period. The black-hole bound observer's trajectory will be affected by the black hole's extreme space-time curvature. As the observer draws close to the event horizon, the dark region surrounding the black hole, the curvature increases and then becomes infinite. At this point, the vessel's time stops from the perspective of the outside observer.
In this simplified diagram we see three massive bodies and their associated space-time curvatures. The more massive and dense the object the greater the curvature. An astronaut approaching the massive body experiences gravitational time dilation due to this curvature. The extent of that dilation depends on body's mass. It is very important to note that the astronaut still experiences time though he/she were stationary. An outside observer's time flow would differ from their as a consequence of their proximity to that curvature.
Let's imagine that we give the person approaching the black hole a device that issues a beam of light every time he/she presses a button. The platform-bound observer asks the moving observer to press the button every second. Just after the vessel leaves, the light beam flashes every second. However, as the vessel draws closer to the black hole, the platform observer notices that the more than a second elapses between successive light transmissions. [Also, at the same time, the light frequency will decrease due to the gravitational red-shifting.] The time separating these successive light flashes increases as the moving observer moves ever closer to the black hole. Eventually, the transmissions stop altogether and the black-hole bound observer seems suspended from the stationary observer's perspective. However, the moving observer doesn't experience any suspension...time passes "as normal."
I hope this response actually helped.
"Could you explain what a parallel Universe is? Do black holes lead to them? How would we know they exist?" -S.C.
A parallel Universe is a space-time system which has no connection to our Universe at all. The laws governing physical phenomena in the parallel Universe could be completely different from ours, as well. The notion of parallel Universes used to be the sole reserve of science fiction writers. Now, however, the concept is one that many scientists now regard seriously. One issue that brought the parallel Universe notion into vogue pertained to life in our Universe. The cosmos obviously has conditions conducive to life's development because we exist to discuss those conditions. However, if the fundamental forces were slightly different, the Universe wouldn't be capable of producing the conditions necessary for life's development. For instance, if gravity were slightly weaker than matter wouldn't have coalesced to form galaxies, stars or planets. Without stars, the matter wouldn't have undergone the complex thermonuclear fusion reactions necessary to manufacture the heavier elements on which life is based (carbon, oxygen, phosphorous, et cetera.) If gravity were slightly stronger, the cosmos would have imploded in our itself within an instant. Having only ONE universe with all the conditions precisely fixed to allow for life's development makes some scientists very uneasy. It implies the presence of a designing intelligence that ensured all the parameters were properly adjusted. On the other hand, if myriad Universes exist, then life would only develop in a certain subset of them, those with the appropriate conditions.
It is possible that black holes might provide a conduit to other Universes due to something called the Einstein-Rosen Bridge that, in theory, could connect black holes to other Universes or to distant parts of the same Universe. The Einstein-Rosen Bridge is named for its discoverers, Albert Einstein and Israeli physicist Nathan Rosen (1909-1995). By discovery, we mean that its existence is implied by the unfathomably complex field equations within the General Theory of Relativity. The problem is that the tidal forces around a stellar-mass black hole would reduce an astronaut to his/her component subatomic particles long before they recached the black hole's event horizon.
As far as finding evidence of parallel Universes, that one is a complete puzzlement. However, it has been suggested that dark matter could be matter within a parallel Universe juxtaposed with our own. The "membranes" separating the two space-time systems might somehow permit gravity to seep through. Still, the task of locating parallel Universes -if they even exist- remains a highly daunting one.
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