The Naked Scientists

The Naked Scientists Forum

Author Topic: Question on photons hitting atoms?  (Read 4694 times)

Johann Mahne

  • Guest
Question on photons hitting atoms?
« on: 05/08/2011 21:16:45 »
 An astronaut is conducting experiments on single frozen molecules and atoms in deep space that are far apart.
One a nitrogen molecule, one hydrogen,one a lead atom.

He is beaming streams of light photons at them.He is able to keep the photons trained on the molecules at all times.

 As each photon hits a molecule the astronaut is able to record the status of the molecule.
 
 What basic differences will he see in the behaviours of the molecules?


 

Offline imatfaal

  • Neilep Level Member
  • ******
  • Posts: 2787
  • rouge moderator
    • View Profile
Question on photons hitting atoms?
« Reply #1 on: 05/08/2011 21:54:06 »
Hi Johann - do you have a link for the experiment?  Your description sounds a bit like you have read a (hyped) journalistic write up.  Handling individual molecules, freezing them, hitting them with radiation, and then measuring again - every stage seems like a bit of an exaggeration. 

Just as a quick thought as an initial problem - how does the astronaut record the status of the molecule?  The way we do this is to lob a photon or two are at it - so to measure what has happened after a photon has hit it, is to hit it with a photon...   
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #2 on: 07/08/2011 12:28:32 »
An astronaut is conducting experiments on single frozen molecules and atoms in deep space that are far apart.
One a nitrogen molecule, one hydrogen,one a lead atom.
He is beaming streams of light photons at them.He is able to keep the photons trained on the molecules at all times.
What do you mean?

Quote
As each photon hits a molecule the astronaut is able to record the status of the molecule.
 What basic differences will he see in the behaviours of the molecules?
He will have to send photons one by one with enough time delay from one to the next that the atom/molecule have the time to come back from the excited state to the fundamental level; then he patiently record each photon coming from the atom's de-excitation in a screen/photographic film, after having made them pass through an interference grating.

After many millions of photons have hit the detector, the astronaut will be able to compute the wavelenghts of the lines that appears and from these the atom/molecule energy levels and so he can infer which atom/molecule is (for example).
« Last Edit: 07/08/2011 12:30:29 by lightarrow »
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #3 on: 08/08/2011 15:39:57 »
Another (obvious) difference is the recoil of each atom/molecule: H is the lighter (molar mass = 1g/mol) then N2 (MM = 28) then lead (MM = 207). So, using the same frequency of light, the first one  will recoil faster, then the second, then the third.
 

Johann Mahne

  • Guest
Question on photons hitting atoms?
« Reply #4 on: 10/08/2011 16:01:16 »
Quote
Hi Johann - do you have a link for the experiment?  Your description sounds a bit like you have read a (hyped) journalistic write up.
Imatfaal; Sorry about the weird question.I realize that the question is very hypothetical.I'm just trying to understand,with your help, what happens as molecules asorb photons.

Quote
Another (obvious) difference is the recoil of each atom/molecule: H is the lighter (molar mass = 1g/mol) then N2 (MM = 28) then lead (MM = 207). So, using the same frequency of light, the first one  will recoil faster, then the second, then the third.

 LightArrow;Ok thanks, this is the type of reply i'm looking for.Would it be possible to predict in what direction the molecules take off in or would it be random?I'm asking this because photons have no mass.
  When would they start to move?Would the gas molecules move right away from the first photon even though they are in a frozen state,or would they first spin?Would the lead molecule's electrons jump into new orbits straight away or would it start moving first before that?
 
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #5 on: 12/08/2011 14:14:06 »
Quote
Another (obvious) difference is the recoil of each atom/molecule: H is the lighter (molar mass = 1g/mol) then N2 (MM = 28) then lead (MM = 207). So, using the same frequency of light, the first one  will recoil faster, then the second, then the third.
LightArrow;Ok thanks, this is the type of reply i'm looking for.Would it be possible to predict in what direction the molecules take off in or would it be random? I'm asking this because photons have no mass.
1. The fact photons are massless is irrelevant.
2. You don't even need to talk of photons, the classical treatment is enough: light, that is electromagnetic waves, do have momentum.
Predicting in what direction the molecules take off is easy: in the opposite direction of the light's beam, if this is collimated enough. This effect is exploited in the solar sails:
http://en.wikipedia.org/wiki/Solar_sail
In that page of wikipedia they talk of photons, but as I've written up, there is absolutely no need of photons to describe the effect.

Quote
  When would they start to move? Would the gas molecules move right away from the first photon even though they are in a frozen state,or would they first spin? Would the lead molecule's electrons jump into new orbits straight away or would it start moving first before that? 
Here we are in a more difficult field.
First, you don't exactly know when the photon will arrive, because you don't know exactly when it's started.
Because of momentum conservation law, the molecule would move right away it interacts with the photon, regardless if the photon is absorbed or simply scattered (for example, air molecules don't absorbs photons but interacts by scattering; this provides, by filtering, the blue color of the sky, because higher frequencies are scattered more).

If the photon is absorbed, the atom/molecule will later re-emit another photon, this time in a *random* direction, and will recoil in the opposite direction of the photon emitted, so in this case the atom/molecule recoils two times: the first as soon as it absorbs the photon from the light' beam, the second when it will re-emit the other photon.
« Last Edit: 12/08/2011 14:18:55 by lightarrow »
 

Johann Mahne

  • Guest
Question on photons hitting atoms?
« Reply #6 on: 15/08/2011 16:53:45 »
Quote
that is electromagnetic waves, do have momentum
Why do they have momentum if they have no mass?
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Question on photons hitting atoms?
« Reply #7 on: 15/08/2011 17:23:34 »
Basically it is the only thing left that they can have!  A beam of photons has an energy and a direction but no mass if you wish to deflect it it must cost you something and this is the momentum change of the photons.

I think that this is probably the best place to introduce and try to explain an important principle that affects all of physics but most people ignore it and get these hangups in their thinking.

We tend to think of the universe in terms of space and time containing moving particles with mass and energy in the form of photons and gravitational waves.  Now this is really wrong because it ignores a vital fact.  It is impossible to make any sort of observation without affecting the system being observed.

The original question talks about beaming photons on an atom and "observing" the result.  This is not possible!   The beam of photons IS the observation all that we can do is observe the photons after they have interacted with the atom and infer what it has done from the nature and direction of the observed photons coming out of the interaction.  Let me stress this again an Observation CANNOT be made without an interaction.

It follows from this that our universe really consists of energy (some of which is localised in the form of particles) and momentum (things moving about including massless photons) and that our familiar time and space time and space is produced from this by inference and has absolutely no independent existence!

Once you realise and accept this a lot of the paradoxes of quantum theory become more understandable.  The somewhat frightening reality is that that our universe can be (and probably is)  very large and very small at one and the same time!

For a more detailed explanation of this vital principle see   New Scientist  6th August issue  Page 34  "Beyond space-time".
« Last Edit: 15/08/2011 17:31:21 by Soul Surfer »
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #8 on: 16/08/2011 12:14:36 »
Quote
that is electromagnetic waves, do have momentum
Why do they have momentum if they have no mass?
And why shouldn't they have? Momentum is not m*v. M*v is valid *only* if 2 special conditions are satisfied:
1. The object has mass
2. Its speed is very little, compared to c.

In general, for all speeds and mass or massless objects:

p = sqrt( E2/c2 - m2c2)

p = momentum
E = total energy
m = mass

For a photon, m = 0 and E = hν
find its momentum.

About a classical electromagnetic wave, it has momentum because when a charge decelerates (for example) it loses momentum and, at the same time, it appears a pulse of em radiation ahead of the charge. Since we want momentum to be conserved, this em pulse must carry momentum. It comes out this momentum is proportional to E*H where E is the em electric field, H the em magnetic field.

You can also make some classical computations on an em wave hitting a charge: the electric fiels of the wave moves the charge up and down, the magnetic field acts on this moving charge with a Lorentz force, pushing it ahead.
« Last Edit: 16/08/2011 12:16:40 by lightarrow »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Question on photons hitting atoms?
« Reply #9 on: 16/08/2011 14:37:55 »
Quote
that is electromagnetic waves, do have momentum
Why do they have momentum if they have no mass?
And why shouldn't they have? Momentum is not m*v. M*v is valid *only* if 2 special conditions are satisfied:
1. The object has mass
2. Its speed is very little, compared to c.

Yeah.  There are a lot of things in the universe that are very small, massless, or are moving very fast.  I think the major conceptual breakthrough that started a lot of modern physics was when scientists stopped trying to apply laws that work for every day objects (massive and slow-moving) to these extreme cases and started asking if it were possible that our everyday laws are just a special case of more general laws that also work for extreme cases.
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #10 on: 16/08/2011 18:21:49 »
Yeah.  There are a lot of things in the universe that are very small, massless, or are moving very fast.  I think the major conceptual breakthrough that started a lot of modern physics was when scientists stopped trying to apply laws that work for every day objects (massive and slow-moving) to these extreme cases and started asking if it were possible that our everyday laws are just a special case of more general laws that also work for extreme cases.
And in this sense, quantum mechanics is the astonishing emblematic example  :)
 

Johann Mahne

  • Guest
Question on photons hitting atoms?
« Reply #11 on: 17/08/2011 18:58:03 »
who discovered this new momentum equation?
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Question on photons hitting atoms?
« Reply #12 on: 18/08/2011 01:00:28 »
who discovered this new momentum equation?
Don't know, maybe Einstein, but I'm not sure.
 

Johann Mahne

  • Guest
Question on photons hitting atoms?
« Reply #13 on: 18/08/2011 09:02:33 »
 If the astronaut in question, beams the photons at the molecules and also varies the wave lengths what will he observe?
 Will the molecules all simply accelerate faster at the higher frequencies ad nauseum, or will any of them have electrons jump to higher orbits at certain frequencies?

  According to another forum member, Nitrogen does not like having it's electrons go to higher orbits so maybe it will simply keep speeding up.But what about the other molecules?
 
 According to everyone in this forum, heat and kinetice energy is the same thing.
 If this is the case, then how could the astronaut in question detect the temperature of a speeding nitrogen molecule as it is not radiating any photons?To the astronaut, the temperature of the molecule (which was at 2.7 K) might not change from before he started beaming the photons at it.
 

The Naked Scientists Forum

Question on photons hitting atoms?
« Reply #13 on: 18/08/2011 09:02:33 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums