Stopped Light and the Heisenberg Uncertainty Principle

Factual Questions
1

It seems that researchers have now been able to stop light. Here’s one of many articles about it:
http://www.nature.com/nsu/010125/010125-3.html

Now, since the momentum of the photons is very precisely known (it’s zero), doesn’t that mean that the position is then extremely unknown? Shouldn’t these photons be smeared across the entire universe?

2

Photons do have momentum.

momentum = p = h/wavelength

3

To expand:

p = mv in Newtonian physics
So in Relativity, p approx = m[sub]0[/sub]v for v << c

For any v < c, p = v * relativistic mass.
relativistic mass = m[sub]0[/sub]/sqtr(1 - v[sup]2[/sup]/c[sup]2[/sup])

p = m[sub]0[/sub]v is false for particles with zero rest mass like photons where v = c.

4

So, the stopped photons still have momentum? How can they have a wavelength if they aren’t moving?

5

Sorry, I read your post too quickly. I was talking about photons in a vacuum. My WAG here is that the velocity is approx zero which would allow some knowledge of the position. I don’t know about the wavelength of slowed (stopped) light.

Sorry.

Note to self: Read the OP carefully before answering.

6

No problem Dr.M.

What I see as a weird/impossible is that even if the momentum is only approx zero, then still it’s “position probability cloud” would be WAY bigger than than the test apparatus. Wouldn’t the photons leak away and the scientists be unable to “restart” the photons?

7

Revtim, you are correct. If a single photon was frozen with exactly p=0, it would be infinitely delocalized and hence effectively gone. Poof!

However, I read the [referenced article](http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v409/n6819/full/409490a0_fs.html&_UserReference=C0 A804EF46B421B3B30715E7900A3B6981D9) (I don’t know if that link will work – you can also get to it by going to Nature and typing in volume 409, page 490) in the link you posted, which describes the experiments in more detail. If I understand it correctly, individual photons are not being slowed and/or stopped. Instead, the Bose condensate coherently couples to the laser pulse and stores the information contained in it so that even if you turn off the laser for a short while, when you turn it back on the information can be retrieved. So, while you’re not necessarily slowing the velocity of the photons down to zero, you are slowing the transmission speed of information to zero. Pretty amazing stuff.

(Take this all with a grain of salt – I need to read about the experiments in more detail.)

8

Thanks Giraffe, I think you nailed it. Looks like another case of the media simplifying a breakthrough to the point of near falsehood, like those light-going-faster-than-light articles a year or so ago.