# Uncertainty principle and photon

1. Apr 15, 2008

### spidey

i have always read in almost all sites that we have to shine at least a photon to measure the particle's position and momentum and hence comes the uncertainty principle...why we are using this shining photon technique always...is this the only way of measuring particle's position and momentum...is there any other method other than shining photon method to measure particle's position and momentum so that we can measure position and momentum with great accuracy...am i missing anything?

2. Apr 15, 2008

### ZapperZ

Staff Emeritus
This is not actually correct. For example, in the single slit diffraction, one narrow down the position of a photon passing through the slit using just the slit width. So if the slit has a width of $\Delta(x)$, then the photon that passed through the slit was in that position, with an uncertainty of position being $\Delta(x)$.

You will also notice that if the width is made smaller and smaller, your ability to predict the value of $p_x$ after it passes the slit becomes less and less accurate. The photon can acquire a larger range of momentum values as you make the slit smaller. Thus, the spread in momentum becomes larger as more and more photons passes through the slit. The uncertainty in position ($\Delta(x)$) will corresponds in the spread in this momentum, i.e.$\Delta(p_x)$.

In this case, you'll notice that we did not use any light to shine on the particle that we want to measure (this works for any quantum particle such as photons, electrons, neutrons, protons, etc.). In other words, it has nothing to do with instrumentation accuracy. It is intrinsic.

Zz.

3. Apr 15, 2008

### spidey

thank you for clearing my doubt...

4. Apr 16, 2008

### genneth

I always thought of it as something fundamental about physical laws (about conjugate pairs in mechanics), such that measuring devices *always* causes a back-reaction at least as large as uncertainty principle says. I guess I mean that I think it's both.

5. Apr 16, 2008

### ZapperZ

Staff Emeritus
Maybe it does. However, we should also pay attention to the fact that the uncertainty in a single measurement can be improved with better technique and better technology. I can measure the position that an electron hit a CCD much better than using simply a charge-sensitive plate. That improves the accuracy of a position measurement. Yet, it does nothing to my knowledge of its non-commuting observable within the HUP.

Thus, improving the measurement uncertainty isn't tied to the HUP. Simply having better instruments does not make the HUP go away, or make the non-commuting observable better known.

Zz.