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John Gribbin attributes to Victor J. Stenger the speculation that "there should be a mirror universe expanding in the opposite direction of time on the other side of the Big Bang", in his review of The Comprehensible Cosmos (Books, April 13).

This idea goes back at least to the early 1970s, when J. R. Gott III published it in the Astrophysical Journal as demonstrating the most symmetric solution of Einstein's field equations. I recall this distinctly, having just received a "revise and resubmit" for, in essence, the same concept (though less mathematically expressed, as a basis for postulating a relation between our universe's quasars and "their" black holes) from the then editor of Foundations of Physics , Henry Margenau.

It wouldn't be surprising to find the idea goes back further - perhaps even into the mirror universe itself.

Paul G. Ellis
London Business School