# What happened to h-bar ?

Homework Helper
What happened to "h-bar"?

Imagine my surprise when I'm looking through the new formula sheets for the new IB Physics curriculum, and I see the uncertainty principle as "equal to or greater than h/4pi."

Over 4pi? When did that happen? I thought I was going crazy. My textbooks all say "over 2pi," and they are publications from 1999 and 2000 (not too old). Only this new text edition (sent as a sample) that was published in 2004 shows the 4pi.

I guess this means we are twice as certain about things than we previously thought.

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dlgoff
Gold Member
Am I missing something? hbar=h/2pi

Homework Helper
Am I missing something? hbar=h/2pi
Yeah, you're missing the first post

HUP has forever been stated as "greater than or equal to h-bar."

Now I find that it's "greater than or equal to h/4pi."

So what happened to h-bar, was it too uncertain?

George Jones
Staff Emeritus
Gold Member
I checked half a dozen of my quantum books, including Messiah written in 1958 and Griffiths written recently, and they all say hbar/2.

It's a matter of definition. Hand-wavy arguments often establish hbar, but if a precise defintion of RMS deviation is used, then it's alway hbar/2. Messiah gives both.

Usually, factors of two don't matter that much; the important things are that there is a lower bound on the the product of the uncertainties, and that this lower bound is quantum in nature because it's on the order of hbar.

According to Griffiths (in his elementary particles book),"When you hear a physicist invoke the uncertainty principle, keep a hand on your wallet."