Laser Light & HUP: Understanding How Position & Momentum Affect Each Other

In summary, the Heisenberg uncertainty principle states that measuring the position of a system accurately will result in a loss of information about its momentum. In the case of a laser beam passing through a small aperture, only the direction of the momentum vector is lost, while the magnitude remains precise. This is because energy is conserved and passing through the aperture does not fully confine or measure the system.
  • #1
LarryS
Gold Member
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A beam of light from a laser projected on a screen with a very small hole in it will spread after it traverses the pin hole because of the HUP: Measuring position so accurately causes loss of information about the momentum. I believe that in the case of laser light, only information about the momentum vector’s direction is lost. The magnitude of the photon’s momentum vector is still precise because the photon’s energy is still precise.

If that is correct, then why is it correct? In general, there is nothing in the HUP that allows the momentum magnitude to remain precise.
 
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  • #2
The Heisenberg uncertainty principle would apply if you really confined a system in the x and y directions. Passing it through an aperture is not really the same thing as confining it.
 
  • #3
Bill_K said:
The Heisenberg uncertainty principle would apply if you really confined a system in the x and y directions. Passing it through an aperture is not really the same thing as confining it.

Maybe "measuring" is a better word than "confining".

It is my understanding that the HUP applies to each dimension, x, y and z, separately.

If passing it through a very small aperature is not confining or measuring it in the, say y dimension, then what accounts for the spreading of the beam after the aperature?
 
  • #4
referframe said:
A beam of light from a laser projected on a screen with a very small hole in it will spread after it traverses the pin hole because of the HUP: Measuring position so accurately causes loss of information about the momentum. I believe that in the case of laser light, only information about the momentum vector’s direction is lost. The magnitude of the photon’s momentum vector is still precise because the photon’s energy is still precise.

If that is correct, then why is it correct? In general, there is nothing in the HUP that allows the momentum magnitude to remain precise.

I think that's correct. Energy is conserved. Momentum changes direction, both in the direction of the slit and perpendicular to the slit, but the magnitude of the momentum is the same. I'm not completely sure though.
 

Related to Laser Light & HUP: Understanding How Position & Momentum Affect Each Other

1. What is laser light?

Laser light is a type of light that is produced by a device called a laser. It is a coherent, monochromatic (single color), and highly focused beam of light that can travel long distances without dispersing.

2. How does laser light work?

Laser light is created when atoms in a laser device are excited and emit photons (particles of light) in a particular direction. These photons then bounce between two mirrors, amplifying each other and producing a coherent beam of light.

3. What is HUP?

HUP stands for Heisenberg's Uncertainty Principle, which states that it is impossible to know both the exact position and momentum of a subatomic particle at the same time. This principle is a fundamental concept in quantum mechanics and affects our understanding of the behavior of particles.

4. How does HUP relate to laser light?

In laser light, the photons have a very specific position and momentum, as they are highly focused and coherent. This means that the uncertainty in their position and momentum is very small, in accordance with HUP.

5. Why is it important to understand how position and momentum affect each other in laser light?

Understanding how position and momentum affect each other in laser light is important because it helps us to accurately predict and control the behavior of light particles. This knowledge is also crucial for many technologies that use laser light, such as medical imaging, communication systems, and scientific research instruments.

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