When I was young, I was taught in the university class as the usual textbook says; if we measure the position of a quantum mechanical particle, then its wave function shrinks to a Dirac' delta function of positional variable.

But in the recent book of John Gribbin, it is written that; "the collapse of wave function is a device introduced by Niels Bohr without evidence whatsoever." And the book further introduces other theory that can avoid the necessity of introducing the collapse of wave function. It is "transaction interaction"proposed by John Cramer.

Which is true at all? Is there surelly the collapse of the wave function or not?

Please someone tell me the truth.

Old poor stray sheep

Dear Old, Poor, Stray Sheep (aka Mitsuru Yamada),

Many physicists have long been concerned with this question of quantum wave function collapse, beginning, of course, with Niels Bohr, himself. It turns out that a landmark paper (Ph.D. thesis actually) was written on the subject by Hugh Everett III at Princeton in 1957. Everett assumed that the wave function did not collapse upon observation and proceeded from there.

What happened next is quite the story, but one which I am in no way qualified to tell. Like many people who are ahead of their time, Everett's personal story is a tragic one.

The wave function does not collapse as per the "Copenhagen interpretation," but this leads to a bit quantum weirdness that will certainly boggle most minds. Hint: multiverses are involved without number. Yet, it turns out that the current study of cosmology especially results from the increasingly measured fine structure of the cosmic microwave background as well as a growing school of thought (still controversial) is consistent with a multiple universe view.

If you are up for a bit of mind boggling, as it were, may I recommend to you a new book by MIT professor and physicist, Max Tegmark, "Our Mathematical Universe: My Quest for the Ultimate Nature of Reality," Alfred A. Knopf, New York, 2014?

In this book, you will be taken through arguments as to whether the Schrodinger wave function collapses or not. Furthermore, you can use the book as a spring board into the literature on the subject in the "Suggestions for Further Reading" section. Tegmark also includes numerous references in the text to on-line sources for supporting papers.

Prof. Tegmark broaches and then gets into the subject of quantum wave collapse in some depth (but no math whatsoever) in Chapter 8, "The Level III Multiverse," and thence for the remainder of the book. However, to really follow the arguments and as a bit of a refresher, I advise reading the first seven chapters as well.

This book is very well written and is anything but dry. Max Tegmark writes from a candid and personal level so that the reader is drawn into his world and world view (or universe view) in a charming way. He succeeded in sharing his fascination with physics and as the subtitle suggests, "The Ultimate Nature of Reality," with me. So I say that this is an exciting and interesting read. I highly recommend the book.

I rather suspect that the word "mathematical" in the title will be off-putting to many, who otherwise would enjoy this book. Too bad, for there is no math in the book. Just the concepts behind the mathematics and the interpretations that the math suggests.

By the way, if after digesting the prose of the book and maybe dipping into the technical literature referenced therein, if you still have questions or concerns, Max Tegmark is easy enough to contact directly. You can find his email address from the MIT faculty list on-line.

Just don't ask me!

Good luck and welcome to the Multiverse!