# I Energy levels in atoms & speed of interaction

1. Mar 25, 2016

### DanMP

Hello,

I wonder how the speed of interaction affects the energy levels in atoms. We know that electrons in atom are attracted to the protons in the nucleus through the electromagnetic force. Photon, the force carrier for the electromagnetic force, moves in empty space with the speed c (the speed of light).

My question is: if the speed of interaction would have been different than c, let's say Fc (F>0), how this would affect the atoms and energy levels in atoms? Can someone re-calculate one energy level using Fc instead of c, and then compare the result with the normal energy level (the one with c as the speed of interaction)?

2. Mar 25, 2016

### Staff: Mentor

There is no meaningful way to change the speed of light. How would such a change look like? We define the length of a meter as the distance light travels in 1/299..... second. This definition can always be applied. You can ask "what would happen if everything is suddenly half its size" - which somehow would look like a doubled speed of light. The result would be a massive explosion of everything solid and liquid, to relieve the pressure coming from the compression.

The speed of light is more like a conversion factor between units. It is not a fundamental parameter you could change in a meaningful way.

3. Mar 25, 2016

### DrDu

The speed of light has but little importance on the electronic structure of atoms. In fact, most calculations assume that $c=\infty$, as the speed of the electrons is usually much smaller than the speed of light. There are some notable effects of the finiteness of the speed of light for very heavy elements like gold, mercury or uranium. E.g. the color of gold is due to relativistic effects. If not, gold would appear white like silver. Confer
http://www.insp.upmc.fr/webornano/ressources/2009/pdf_dijon/02_Pyykko.pdf

4. Mar 28, 2016

### DanMP

Don't see it like a change. Consider an alternative universe where the speed of force carriers is different than in our universe. Let's say that F is close to zero and that we can see a gold atom in this alternative universe. How this atom would behave compared with a gold atom in our universe? Can you calculate the differences in size, energy levels, speed of the electrons, etc.?

This is strange. The speed of light (and force carriers speed) is not infinite.

There is no c in energy levels calculation?

5. Mar 28, 2016

### ZapperZ

Staff Emeritus
Try solving the hydrogen atom yourself. Do you get any c in there?

What you are being told is that since the electrons in these light atoms move so slow when compared to c, the interactions can accurately be described accurately as instantaneous. The finite speed of light has no effect.

Zz.

6. Mar 28, 2016

### Staff: Mentor

That does not make sense. If you have two universes, you cannot even calculate that F because it has no physical meaning. It would be a simple conversion factor between units, like the factor between miles and kilometers. Does the universe change if you increase the length of a mile relative to a kilometer? Can you measure the conversion factor between miles and kilometers without using references on Earth? No.

7. Mar 28, 2016

### DanMP

c is present there (https://en.wikipedia.org/wiki/Hydrogen_atom#Energy_levels) but not only in one place (there are constants depending on c), so it's impossible for me to find exactly how c influences the energy levels. I thought that in this forum there are physicists able to do that.

Yes, in this universe, but in an alternative universe, where the speed of force carriers would be close to zero, things could be different ...

8. Mar 28, 2016

### ZapperZ

Staff Emeritus
I mean as in the SPEED of light, not c as a constant, conversion factor (the "c" in E=mc2 does not imply something is moving with speed c, for example). And yes, physicists on this forum are able to do this because this is a standard example and exercise in into QM classes.

But here's the thing. If the peed of light holds the same significance in that other universe, then you can't tell too much differences within context with this, if we want to play with such speculation. This is because c, whatever its value, will still be the fundamental constant in which a lot of other constants depend on.

Maybe you should try to learn how it works in this universe first before attempting to make up other universes?

Zz.

9. Mar 28, 2016

### DanMP

We don't calculate F. F is given.

Anyway, you can see it like this: we have, side by side, 2 gold atoms, one normal and the other with a different speed for force carriers (it is a thought experiment). If one atom emits a photon (electronic transition), would the other absorb it? Are the energy levels the same?

OR, we make a computer simulation for one atom, including the finite speed of force carriers, c. Then, on the same screen, we project another simulation, for the same atom but with Fc instead of c. For F=1, the atoms should look/behave identical. What would we see if we change F?

10. Mar 28, 2016

### ZapperZ

Staff Emeritus
This is a fallacy that most people have, that in physics, you can change one thing without affecting another. It is why one cannot learn physics in bits and pieces.

It makes scenarios such as this meaningless, because the rules aren't clear and some can change on a random whim.

Zz.

11. Mar 28, 2016

### DanMP

We actually do learn (and use) physics in bits and pieces. Sometimes we consider light as a wave (refraction), sometimes as particles (Compton effect). And we don't use relativity and QM at the same time ...

What is not clear?

12. Mar 28, 2016

### Staff: Mentor

But F has no meaning. It's like proposing a parallel universe where a mile is exactly two kilometers long. Who cares?
"What would the laws of physics predict if we violate the laws of physics?" does not have a useful answer.

There is one thing you can change - and DrDu was probably thinking about that: the fine-structure constant. It is a dimensionless constant, so it is independent of your unit system. In our universe it is about 1/137. If you change this, you change the energy levels - the binding energies are (to a good approximation) proportional to this constant.
Note how many different ways there are to express it in terms of some physical constants that have units. The speed of light for example appears in the numerator in some ways and in the denominator in others. You cannot just say "increase the speed of light" without context, it doesn't mean anything.

13. Mar 28, 2016

### DanMP

So, it's impossible/meaningless (or very hard) to calculate how the speed of force carriers affects the atom. This may be caused by the fact that "speed" is something defined and measured using atoms?

14. Mar 28, 2016

### Staff: Mentor

It is meaningless, if the force carrier is massless (and therefore the speed of light in relativity and the speed of electromagnetic interaction are the same). You just do not have an indepedent reference to compare with.
A photon with mass would change energy levels.

15. Mar 29, 2016

### DrDu

Most calculations of hydrogen like atoms only consider the instantaneous Coulomb potential. The effect of the finite speed of the EM field is called retardation. It is fully taken into account when solving the Dirac equation for the hydrogen atom. Retardation effects can also be described using the Dirac-Breit hamiltonian. Have a look e.g. at Landau Lifshitz, vol. 3.

16. Apr 1, 2016

### DanMP

As far as I understand, those equations take into consideration the relativistic effects in order to obtain more accurate results in QM calculations for the atom. It is interesting but not very helpful. Those equations use/contain time (t), but time (the second) is defined based on energy levels:
(https://en.wikipedia.org/wiki/Second)

If we want to understand how force carriers speed really affects energy levels in atom, we should use something else, something that deals with force carriers traveling between electrons and protons/nucleus ... maybe a computer model where the speed of force carriers is considered as distances on the display (pixels) per computer second, and not as is considered in real life, where the time is given by the atom(s) in the same model we investigate ... In this manner we can see on the same display 2 atoms with different (computer related/defined) speeds for force carriers.

17. Apr 1, 2016

### ZapperZ

Staff Emeritus
Sorry, but you got this all wrong.

We define "1 second" as the frequency of transition in Cs atom, but TIME isn't defined that way! "Time" is a dimension. "1 second" is a UNIT of measurement of that time. There's nothing that says that another civilization can't define another unit of measurement for time.

So the fact that the unit "second" is defined using atomic transition has nothing to do with us using time-dependent dynamics.

Zz.

18. Apr 1, 2016

### DanMP

Sorry for mixing notions. What I meant was that when we calculate/express the energy levels we use the second (to express time intervals or speeds), and the second depends on energy levels in Cs atom.

19. Apr 1, 2016

### ZapperZ

Staff Emeritus
You still don't get it.

The "second" is a UNIT OF TIME measurement. I can express that unit of time as "unit of oscillation of pendulum" if I so wish without using the "second". Another alien civilization will probably define a unit of time as the rate of change of the zoodoniun compound under the exertion of Ethon. It STILL will not change the period of time!

You really should not be suggesting how physics should be done without actually learning physics itself first. Do you really think what you are doing here is rational?

Zz.

20. Apr 1, 2016

### DrDu

So here is your original question:
In a relativistic quantum chemistry package you can do exactly this. You could change c to another value and see how this influences the energy levels in atoms and molecules. c is nothing else but the propagation speed of the electromagnetic field (the carrier of force).