Heisenberg's Uncertainty Principle Explained

Heisenberg's uncertainty principle tells us that it is impossible to simultaneously measure the position and momentum of a particle with infinite precision. In our everyday lives we virtually never come up against this limit, hence why it seems peculiar. In this experiment a laser is shone through a narrow slit onto a screen. As the slit is made narrower, the spot on the screen also becomes narrower. But at a certain point, the spot starts becoming wider. This is because the photons of light have been so localised at the slit that their horizontal momentum must become less well defined in order to satisfy Heisenberg's uncertainty principle.

I based this video on one by Prof. Walter Lewin of MIT: http://bit.ly/100Wk2K

Henry (MinutePhysics) has previously made a video about Heisenberg's Uncertainty Principle where he treats it as less spooky and more a consequence of waves: http://bit.ly/TV3xO5

Sixty Symbols has a great video on Planck's constant: http://bit.ly/11upebY

Thanks to the University of Sydney for hosting this experiment, especially to Tom and Ralph for their assistance getting it working.

Music: Kevin McLeod (Incompetech.com) Mirage and Danse Macabre

• Category

  Education

[hsh8383]

Heisenberg's making Crystal Meth now :b

[qwertyuiopaaaaaaa7]

My problem with this is, the math is based on our observations, not the other way around. So the photons don't move to the left or right to maintain a value greater than h/4(pi) they do it because they do it, and we observe this and see that it relates to h/4(pi). So this doesn't actually tell me why anything is happening, it's just an equation that says "yeah, this is what we see happen..."

[Tom Erlandsen]

Small correction:

ΔxΔp≥h/2π

σxσp≥h/4π

The latter uses standard deviations of position and momentum hence the use of sigma. ΔxΔp≥h/4π is often quoted by sources, and is technically incorrect. The distinction is important :)

[Jack G]

A lot of times we see theoretical physicists writing these equations long before they can be actually be observed in the real world. Stephen Hawking's work is a great example, as is much of Michio Kaku's. In this example, the laser light observation is behaving as Heisenberg predicted through his studies, NOT the other way around (Lasers were invented 30 years afterwards). The Ungenauigkeit Principle predates this experiment by decades.

[kitchenfromhell]

It is not a wide spot when you are not looking at it.

[intrusore724689]

The video is wonderful but it's a description of what we see (easy) not an explaination of why it is, in other term a description of what we don't see (hard)

[Hannes Visagie]

How awesome is this channel

[gregory morgan]

Yeah now let's make a video explaining how mass and energy are linked because Einstein came up with E=mc². They weren't until then and things could get pretty messy at times.

[Invisibleman321]

Thank you. This simple explanation was very helpful. I've been trying to get my mind around this for a while

[SomeCartoonChick]

Oh so NOW I get that Schrodinger and Heisenberg joke!

[James Blackwell]

But is that not why anything happens in the first place? Correct me if I am mistaken but EVERYTHING works the way it does because it works the way it does and is forced in this path.

When you get down to the very foundations, it's almost illogical to ask ''why''

[James Phelps]

i can see in his eyes that he loves what he does and teaching other people