A brief question on quantum thermodinamics.

  • Context: Graduate 
  • Thread starter Thread starter Alex Cros
  • Start date Start date
  • Tags Tags
    Quantum Thermodinamics
Click For Summary

Discussion Overview

The discussion revolves around the energy dynamics of an electron absorbing and re-emitting a photon, particularly in the context of quantum thermodynamics. Participants explore whether energy is lost during this process and how classical thermodynamic principles apply to quantum scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how an electron can re-emit a photon with the same energy as the absorbed photon without energy loss, suggesting a conflict with classical thermodynamics.
  • Another participant proposes that classical thermodynamics applies to large numbers of particles, while quantum mechanics deals with individual atoms, where statistical laws may not hold.
  • It is suggested that the re-emitted photon may not necessarily have the same energy, as multiple electrons could be excited, or energy could be transferred to vibrations in a solid.
  • Several participants assert that no energy is lost during the transition between energy states, as the energy given up by the electron equals the energy required to excite it, although interactions with other atoms could lead to energy transfer without photon emission.
  • A participant inquires about experiments observing interference after photon absorption and emission in free or loosely coupled atoms, seeking clarification on the term "free or loosely coupled." Another participant defines it as an atom decoupled from a complex system.
  • There is a discussion about the concept of "lost" energy, with participants noting that energy is conserved but may change forms, becoming unusable or distributed among many particles in complex systems.

Areas of Agreement / Disagreement

Participants express differing views on whether energy is lost during the photon absorption and emission process, with some asserting that energy is conserved while others suggest it can be transformed into other forms. The discussion remains unresolved regarding the specifics of energy loss and the implications of quantum versus classical thermodynamics.

Contextual Notes

Participants highlight the complexity of energy transformations and the statistical nature of thermodynamic laws, indicating that the discussion is limited by the assumptions and definitions of energy loss and transformation in quantum systems.

Alex Cros
Messages
28
Reaction score
1
Hi, I am new to this forum so I don't really know if this question already exists..

My question is: When an electron absorbs a foton and climbs to the next energy gap and then returns again, as the energy is quantum energy, how is possible that the foton reemited by the electron has the same energy as the original foton, without any energy lost as the classical thermodinamics would suggest? Is there any energy lost during this transformation?

// I am 17 years old, be gentle :D

Thanks!
 
Physics news on Phys.org
I think there are two answers to this.

1: classical thermodynamics depends on many (as is ##10^{23}##) particles interacting. Thermodynamic laws are statistical laws that apply in those situations, so in the realm of QM where you are dealing with a single atom, the statistical rules don't apply.

2: The reemitted photon does not need to have the same energy. In an atom with lots of energy levels that are close together the incoming photon could excite multiple electrons and then two lower energy photons could be reemitted. In atoms that are part of a solid, the energy could get transferred into vibrations in the solid and a lower energy photon could be reemitted.
 
No energy is lost because the energy given up by the electron as it falls to a lower energy state must be equal to the amount of energy required to excite it into that state to begin with. Now, before the electron can drop into a lower energy state and emit a photon, it is possible that something else interacts with it and takes the energy instead. For example, the atom with the excited electron can collide with another atom and the electron can give up this energy to the other atom without emitting a photon.
 
Drakkith said:
No energy is lost because the energy given up by the electron as it falls to a lower energy state must be equal to the amount of energy required to excite it into that state to begin with. Now, before the electron can drop into a lower energy state and emit a photon, it is possible that something else interacts with it and takes the energy instead. For example, the atom with the excited electron can collide with another atom and the electron can give up this energy to the other atom without emitting a photon.

I've been wondering for a while, is there any experiment which has observed interference after a free, or losely coupled, atom has absorbed and emitted a photon?

Or are we left with only more complex effects to confirm this this type of mechanism?
 
Last edited:
What is a "free or loosely coupled" atom in this context?
 
Drakkith said:
What is a "free or loosely coupled" atom in this context?

An atom which is sufficiently decoupled from a complex system.
 
Drakkith said:
No energy is lost because the energy given up by the electron as it falls to a lower energy state must be equal to the amount of energy required to excite it into that state to begin with. Now, before the electron can drop into a lower energy state and emit a photon, it is possible that something else interacts with it and takes the energy instead. For example, the atom with the excited electron can collide with another atom and the electron can give up this energy to the other atom without emitting a photon.

And would that energy transformation be done without any energy lost in the process of transformation?

Thanks for all the answers so far!
 
Alex Cros said:
And would that energy transformation be done without any energy lost in the process of transformation?

Thanks for all the answers so far!

What you are reading are already examples of energy being "lost". The expression "lost" refer to energy changing form into something unusable or different from the original (e.g. kinetic energy becomes thermal energy). No energy is ever lost strictly speaking, because energy is conserved.
 
Alex Cros said:
And would that energy transformation be done without any energy lost in the process of transformation?

"Lost" energy is energy that has been transferred into a system that is too complicated to keep track of the energy. As ddd123 points out, it is not really lost. When you talk about energy being "lost" in something like an inelastic collision, the energy is just being distributed to the (more than ##10^{23}##) atoms involved in the collision. Once numbers like that are involved, it is preferable to deal with statistical measures of energy like temperature. That is when the thermodynamic law apply.

Every transfer of energy transfers all of the energy without losing any energy.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 36 ·
2
Replies
36
Views
5K
  • · Replies 29 ·
Replies
29
Views
3K
  • · Replies 38 ·
2
Replies
38
Views
7K
  • · Replies 36 ·
2
Replies
36
Views
9K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 46 ·
2
Replies
46
Views
9K
  • Sticky
  • · Replies 0 ·
Replies
0
Views
9K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 16 ·
Replies
16
Views
4K