Key Concepts
- The basic laws of physics work equally well forward or backward in time, yet we perceive time to move in one direction only—toward the future. Why?
- To account for it, we have to delve into the prehistory of the universe, to a time before the big bang. Our universe may be part of a much larger multiverse, which as a whole is time-symmetric. Time may run backward in other universes.
More to Explore
- Sidebar
Time's Arrow FAQs - Sidebar
The History of the Observable Universe - Infographic
Entropy in the Kitchen - Infographic
What Gravity Does to Entropy - Infographic
Restoring Symmetry to Time - PDF
"The Arrow of Time" feature from 1975 issue of Scientific American
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June
2008 Issue- Recommended Reviews: American Nerd, Coming of Age in Second Life
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- News Scan Blogging--It's Good for You
- Buy the Digital Edition
The universe does not look right. That may seem like a strange thing to say, given that cosmologists have very little standard for comparison. How do we know what the universe is supposed to look like? Nevertheless, over the years we have developed a strong intuition for what counts as “natural”—and the universe we see does not qualify.
Make no mistake: cosmologists have put together an incredibly successful picture of what the universe is made of and how it has evolved. Some 14 billion years ago the cosmos was hotter and denser than the interior of a star, and since then it has been cooling off and thinning out as the fabric of space expands. This picture accounts for just about every observation we have made, but a number of unusual features, especially in the early universe, suggest that there is more to the story than we understand.
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