Recoil of a hydrogen atom emitting Lyman alpha photon

In summary: EPR paradox, photon entanglement and other quantum weirdness. The issue of photon momentum that I mentioned is just another problem with the standard model.
  • #1
Geoff Simmons
5
0
I am interested in what the recoil velocity of an initially stationary hydrogen atom in free space would be when it emits a Lyman alpha photon. I tried to do the calc and got about 3 metres per second which seems rather high.
 
Science news on Phys.org
  • #2
Hi Geoff. :welcome:

Perhaps our General Physics would be a suitable forum where you could post your question. If this is a homework exercise, you will need to use our physics homework forum and include your calculations/attempt.

This Introductions forum is just for saying "Hi".

EDIT: I see someone has moved the thread for you
 
  • #3
Geoff Simmons said:
seems rather high.
Other than offending your intuition, what's bothersome about that result?
 
  • #4
Bystander said:
Other than offending your intuition, what's bothersome about that result?

Errr... Thanks for the respectful response. A hydrogen atom in a nebula emits Lyman alpha. It recoils at 3 m/s. The photon travels through space for 5 years say, before being absorbed into an observer;s retina. In the meantime the hydrogen atom has recoiled hundreds of thousands of kilometers. When the photon is eventually absorbed, who is to say that the hydrogen atom recoiled in the right direction? It;s a failing of the standard model.
 
  • #5
Geoff Simmons said:
I am interested in what the recoil velocity of an initially stationary hydrogen atom in free space would be when it emits a Lyman alpha photon. I tried to do the calc and got about 3 metres per second which seems rather high.
That's about the same answer I get. I don't see any issue with it.

And the standard model doesn't even come into this problem, so I'm wondering how you got the idea that that is an issue.
 
  • #6
Thanks v much. The standard model produces the EPR paradox, photon entanglement and other quantum weirdness. The issue of photon momentum that I mentioned is just another problem with the standard model. We build our physics ever higher but should be ready to accept it may be built on ideas that just "seemed to work" at the time.
 
  • #7
You fail to explain (so far) how is this a problem at all.
And this recoil is not really a matter of standard model. Unless you mean by it just "standard physics" or "current theories of physics".

Edit. Do you imagine the atoms in the hydrogen gas at rest before emitting a photon?
If not, what do you think are their average speed for a gas at the temperatures that make emission likely?
 
  • #8
Geoff Simmons said:
The issue of photon momentum that I mentioned is just another problem with the standard model.

Could we ask you to clarify what you mean when you say "the standard model"? The term has a generally accepted meaning (https://en.wikipedia.org/wiki/Standard_Model), but that meaning makes no sense here.

It's also not clear to me what problem you're seeing. The atom emits some electromagnetic radiation; this radiation carries some momentum so the atom recoils in the opposite direction and momentum is conserved.
 
  • #9
Sorry that my explanation was so incomplete. In the photon-as-a-particle (standard) model there is no problem. Momentum and energy are conserved. I was referring to the problem that the photon behaves as a widely dispersed wavefront while the atom recooils in a very precise direction. It's the counterpart of the "wave function collapse" problem, although it happens long before the wave function collapses. Apologies for the lack of clarity. It's easy to set up an experiment, for example by placing the atom at a focal point of an ellipsoidal mirror and observing the light focused at the other focal point, to prove that the photon behaves as if the wavefront disperses in a spherical form.. How then does the atom know what direction to recoil without knowing where the photon will finally be absorbed? It strikes me there is an interesting issue here. It's similar to the EPR paradox.
 
  • #10
Geoff Simmons said:
Sorry that my explanation was so incomplete. In the photon-as-a-particle (standard) model there is no problem. Momentum and energy are conserved. I was referring to the problem that the photon behaves as a widely dispersed wavefront while the atom recooils in a very precise direction. It's the counterpart of the "wave function collapse" problem, although it happens long before the wave function collapses. Apologies for the lack of clarity. It's easy to set up an experiment, for example by placing the atom at a focal point of an ellipsoidal mirror and observing the light focused at the other focal point, to prove that the photon behaves as if the wavefront disperses in a spherical form.. How then does the atom know what direction to recoil without knowing where the photon will finally be absorbed? It strikes me there is an interesting issue here. It's similar to the EPR paradox.
Simple: The atom also doesn't have a definite direction.
 
  • #11
Thanks. However the uncertainty principle doesn't solve this. In the case that the atom is as far away as Alpha Centauri, then it will have recoiled hundreds of thousands of kilometres before the photon reaches Earth. I think that a committed QM expert would be reduced to saying that the atom and the photon are "entangled". When the photon is finally absorbed, the position of the atom is suddenly communicated instantaneously. Not simple.
 
  • #12
Geoff Simmons said:
I think that a committed QM expert would be reduced to saying that the atom and the photon are "entangled". When the photon is finally absorbed, the position of the atom is suddenly communicated instantaneously. Not simple.
For better or for worse, you've restated the EPR problem - which I'll grant is not simple.

It's worth noting that observers in motion relative to one another may describe the interaction differently. One will say that the electron measurement comes first, "causing" the wave function of the photon to collapse; thanks to the relativity of simultaneity the other may just as correctly say that the photon measurement comes first causing the wave function of the electron to collapse first. Thus, whatever is happening doesn't remotely resemble anything that we'd naturally call "communication" - there is no sensible definition of the sender, the receiver, or the content of the communication.
 
  • #13
Geoff Simmons said:
In the photon-as-a-particle (standard) model there is no problem.

It's a bit of a digression here, but the photon-as-a-particle model is a red herring. No matter how we think about the emitted radiation, we end up in the EPR place: We know the total momentum of the system is zero so the momentum vectors of the two particles will be equal in magnitude and opposite in direction... But as far as the formalism of quantum mechanics is concerned, the direction of the two vectors is not determined until we've measure one them, even at an arbitrarily large distance from the other.

And PLEASE stop calling it the "standard model", if only because you're going to mess up other people's search queries.
 
  • #14
Nugatory said:
but the photon-as-a-particle model is a red herring

So is the specific question asked: this question doesn't depend on the magnitude of the recoil.
 
  • #15
Geoff Simmons said:
Thanks. However the uncertainty principle doesn't solve this. In the case that the atom is as far away as Alpha Centauri, then it will have recoiled hundreds of thousands of kilometres before the photon reaches Earth. I think that a committed QM expert would be reduced to saying that the atom and the photon are "entangled". When the photon is finally absorbed, the position of the atom is suddenly communicated instantaneously. Not simple.
I was actually invoking entanglement, not the HUP. :wink: Sorry to be unclear.
 

What is the Recoil of a Hydrogen Atom?

The recoil of a hydrogen atom refers to the change in momentum and velocity of the atom after it emits a Lyman alpha photon. This recoil occurs due to the conservation of momentum, as the photon carries momentum away from the atom.

What is a Lyman Alpha Photon?

A Lyman alpha photon is a high-energy photon with a wavelength of 121.6 nanometers. It is emitted when an electron in a hydrogen atom transitions from the n=2 energy level to the n=1 energy level.

How is the Recoil of a Hydrogen Atom Calculated?

The recoil of a hydrogen atom can be calculated using the equation p=mv, where p is the momentum, m is the mass of the atom, and v is the velocity. The change in momentum is equal to the momentum of the emitted photon, which can be calculated using the equation E=hc/λ, where E is the energy of the photon, h is Planck's constant, c is the speed of light, and λ is the wavelength of the photon.

What Factors Affect the Recoil of a Hydrogen Atom?

The recoil of a hydrogen atom depends on various factors such as the energy of the emitted photon, the mass of the atom, and the direction of the emitted photon. A higher energy photon will result in a greater recoil, while a lighter atom will experience a larger recoil. Additionally, the direction of the emitted photon can affect the direction of the atom's recoil.

Why is the Recoil of a Hydrogen Atom Important?

The recoil of a hydrogen atom plays a significant role in various physical processes, such as the formation of stars and the absorption of light by interstellar gas. It also provides valuable insight into the behavior of atoms and the conservation of momentum in quantum systems.

Similar threads

Replies
4
Views
771
  • Quantum Physics
2
Replies
38
Views
3K
  • Astronomy and Astrophysics
Replies
11
Views
3K
  • Quantum Physics
Replies
11
Views
2K
Replies
9
Views
1K
  • High Energy, Nuclear, Particle Physics
Replies
6
Views
1K
Replies
4
Views
852
  • Introductory Physics Homework Help
Replies
2
Views
2K
  • Quantum Interpretations and Foundations
Replies
3
Views
1K
Back
Top