Grand design spiral galaxy
A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm
and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy's radius. Approximately ten percent of spiral galaxies are classified as grand design type spirals, including M81, M51 and M74.
The origin of Grand Design structure
Density wave theory is the preferred explanation for the well-defined structure of grand design spirals.
According to this theory, the spiral arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxy’s disk. Stars are clumped in these dense regions due to gravitational attraction towards the dense material, though their location in the spiral arm may not be permanent. When they come close to the spiral arm, they are pulled towards the dense material by the force of gravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes material to clump in the dense regions.
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The Grand Design is a popular-science book written by physicists Stephen Hawking and Leonard Mlodinow and published by Bantam Books in 2010.
It argues that invoking God is not necessary to explain the origins of the universe, and that the Big Bang is a consequence of the laws of physics alone.
The authors of the book point out that a Unified Field Theory may not exist.
Albert Einstein and other physicists had proposed such a Theory based on an early model of the universe containing three-dimensions and time.
Since then, the model of the universe has changed significantly.
It is now believed that the universe has 10, or even 11 (M-Theory) dimensions.
Published in the United States on September 7, 2010, the book became the number one bestseller on Amazon.com just a few days after publication.
The book was published in the United Kingdom on September 9, 2010, and became the number two bestseller on Amazon.co.uk on the same day.
Synopsis
The book examines the history of scientific knowledge about the universe.
It starts with the Ionian Greeks, who claimed that nature works by laws, and not by the will of the gods. It later presents the work of Fr. Copernicus, who advocated the concept that the Earth is not located in the center of the universe.
The authors then describe, in a language easy to understand by non-specialists, the theory of quantum mechanics using, as an example, the probable movement of an electron around a room.
In the culmination of the book, the authors explain how the theory of quantum mechanics and the theory of relativity together help us understand how universes could have formed out of nothing.
Hawking writes:
Because there is a law such as gravity, the universe can and will create itself from nothing. Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist. It is not necessary to invoke God to light the blue touch paper and set the universe going.
The authors explain, in a manner consistent with M-Theory, that as the Earth is only one of several planets in our solar system, and as our Milky Way galaxy is only one of many galaxies, the same may apply to our universe itself: that is, our universe may be one of a huge number of universes.
The book concludes with the statement that only some universes of the multiple universes (or multiverse) support life forms. We, of course, are located in one of those universes.
The laws of nature that are required for life forms to exist appear in some universes by pure chance, Hawking and Mlodinow explain (see Anthropic Principle).
Reactions
Evolutionary biologist, Dr. Richard Dawkins welcomed Hawking's position and said that "Darwinism kicked God out of biology but physics remained more uncertain. Hawking is now administering the coup de grace."
The Bishop of Swindon, Dr. Lee Rayfield (himself a scientist) has said "Science can never prove the non-existence of God, just as it can never prove the existence of God." Anglican priest, Cambridge theologian psychologist, and
Historian of Science Rev. Dr. Fraser N. Watts, said "a creator God provides a reasonable and credible explanation of why there is a universe, and ... it is somewhat more likely that there is a God than that there is not. That view is not undermined by what Hawking has said."
Dwight Garner in the New York Times was critical of the book, saying; "The real news about 'The Grand Design' is how disappointingly tinny and inelegant it is. The spare and earnest voice that Mr. Hawking employed with such appeal in 'A Brief History of Time' has been replaced here by one that is alternately condescending, as if he were Mr. Rogers explaining rain clouds to toddlers, and impenetrable."
Writing in the Los Angeles Times, Michael Moorcock praised the authors; "their arguments do indeed bring us closer to seeing our world, universe and multiverse in terms that a previous generation might easily have dismissed as supernatural. This succinct, easily digested book could perhaps do with fewer dry, academic groaners, but Hawking and Mlodinow pack in a wealth of ideas and leave us with a clearer understanding of modern physics in all its invigorating complexity".
Craig Callender, in the New Scientist, is not convinced by the theory promoted in the book: "M-theory ... is far from complete. But that doesn't stop the authors from asserting that it explains the mysteries of existence ... In the absence of theory, though, this is nothing more than a hunch doomed – until we start watching universes come into being – to remain untested. The lesson isn't that we face a dilemma between God and the multiverse, but that we shouldn't go off the rails at the first sign of coincidences.
Dr. Marcelo Gleiser, in his article 'Hawking And God: An Intimate Relationship', explains: "contemplating a final theory is inconsistent with the very essence of physics, an empirical science based on the gradual collection of data. Because we don’t have instruments capable of measuring all of Nature, we cannot ever be certain that we have a final theory. There’ll always be room for surprises, as the history of physics has shown again and again. In fact, I find it quite pretentious to imagine that we humans can achieve such a thing. ... Maybe Hawking should leave God alone."
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