The Astrophysics Spectator

Issue 2.16, April 27, 2005

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The basic layout of the site is as survey paths, which can be found under the Surveys link at the top of this and most other pages on this site. Each survey begins with a basic overview of the subject. Part of this overview include simulators of astrophysical phenomena that allow the reader to experiment with the phenomena. The later pages in a survey present the subject in greater and more mathematical depth. A path ends with research pages that describe current research projects and results in astrophysics.

The links at the top of each page are Home, which is the current home page of this site, Commentary, which is an index of short essays on topics loosely related to astrophysics, Surveys, which is the index of survey paths, Research, which is the index of research pages and the page leading to recent news items, Background, which is the index page for all background information on astrophysics, including survey pages, simulator pages, tables, bibliographic references, and lists of web resources, Previously, which is an index of previous home pages, and Site Info, which describes the site and its author, and gives contact information.

On the home page is found an addition link. This is the Store link, which leads to reviews of worthwhile books on astronomy and other relates subjects. Links on these pages enable the reader to buy these books from, which helps to financially sustain this web site.

Each Wednesday, a new issue of The Astrophysics Spectator is published that comprises a new home page, a new commentary, whatever news the author notices, and background, research, and simulator pages added to the survey paths. The home page acts as an index to the newly added pages. This site also has an RSS channel, whose link is given at the bottom of the right-hand column of this page.

April 27, 2005

The vision of interstellar travel is such a staple of science fiction that one seldom sees a story about the future without it. But the stories of space travel present a vision that is an extension of travel here on Earth. If any thought to real physics is given, it is to the extreme distances that separate the stars. So we have stories of spacecraft zipping from star to star faster than the speed of light, with only hours or days passing on each voyage, or we have spacecraft moving slowly, but with their passengers in suspended animation, so that they can pass through the aeons that it takes to travel between the stars, or we have the invocation of fantasy physics, such as the worm hole or hyperspace, that gives a shortened path to a destination. But we seldom see real physics in these stories, so we seldom see one of modern physics most arresting conclusions: travelers who accelerate to close to the speed of light slow down the passage of their time relative to the passage of time back on Earth.

Time dilation is part of special relativity. As a traveler accelerates, he experiences a slowdown of time, so that in a round-trip to a distant star less time passes for himself than for those left on Earth. For acceleration over short periods of time, this effect is too small to measure, but if extended over several years, the effect can be dramatic, with the traveler seeing centuries of Earth time pass for each of his own years. For instance, if a traveler accelerates at the average gravitational acceleration at Earth's surface, he can travel to the Andromeda Galaxy and back and experience the passage of only 57 years, but those he left behind would be long gone, as would many generations of their descendants, because on Earth 4.5 million years will have passed. And how would this passing of the generations appear to our traveler? From the radio signals he receives from Earth, he would see time slow to a standstill on his out bound journey, seeing only the passage of a single year, but on his homeward journey, he would see the flow of Earth's history as a flood, with many days of Earth history blasting in with each second that passes on his spacecraft, sweeping away the world that he knew.

But does any of this impact astrophysics? It does, because the effects encountered by our traveler are also encountered within many astronomical phenomena, such as gamma-ray bursts, active galaxies, and cosmic rays. The world experienced by our travelers is the world close to a black hole. Time dilation and event horizons are a part of our astrophysical world.

Two pages are added to the “Special Relativity” survey path that discuss the effects of time dilation on an interstellar traveler. The first page discusses how acceleration and distance produce a large deviation of the traveler's elapsed time from Earth's elapsed time. The second page describes how Earth time appears to move to our traveler. Each page contains a live figure that illustrates the effect discussed in the page.

The commentary this week examines the assertion that the universe is stranger than we can imagine. We theorists have no problem with strange phenomena; it's the complexity that gives us fits.

Jim Brainerd


Complex, Not Peculiar. Many claim that the universe is more peculiar than we can imagine, but it is our theories rather than the universe that are peculiar. It is the universe's complexity rather than its peculiarity that we find difficult to comprehend. (continue)


Travel to the Stars under Special Relativity. A spaceship that is accelerating at a constant rate can travel to the stars in what its passengers regard as a startlingly short time. Accelerating with the average gravitational acceleration at Earth's surface, our travelers can reach nearby galaxies and return to Earth in a human lifetime. The Earth they find, however, will have aged by millions of years. The reader can use the live figure on this page to learn how the rate of acceleration changes these results. (continue)

Earth's Time During Interstellar Travel. As an accelerating spacecraft travels to the stars, its passengers sees the passage of time on Earth slow to almost a stop as Earth falls through the spacecraft's event horizon. On the return trip, the passengers see Earth's days pass in seconds. The page includes a live figure that shows how time on Earth elapses with time on the spacecraft for user-selected distances and rates of acceleration. (continue)


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