High School How does the Doppler shift explain changes in photon energy without interaction?

[itoero]

According to doppler shift you can measure low energy photons at one side of a star and higher energy photons at the other side of a star. How is this possible? I know the classical explanation but what happens on an atomic level?

 

[phyzguy]

The atoms on one side of the star are moving toward you and the atoms on the other side of the star are moving away from you. Although they emit the same energy photons in their rest frame, you measure lower energy photons from the atoms moving away from you, and higher energy photons from the atoms moving toward you.

 

[jbriggs444]

According to doppler shift you can measure low energy photons at one side of a star and higher energy photons at the other side of a star. How is this possible? I know the classical explanation but what happens on an atomic level?

Absolutely nothing at all. The photon does not have a wavelength or an energy in any absolute sense. It only has a wavelength and an energy in the context of a frame of reference. Change the frame of reference and you change the wavelength and energy.

 

[itoero]

Ok, but why is the red and blueshift not subject to the same frame of reference?

 

[jbriggs444]

Ok, but why is the red and blueshift not subject to the same frame of reference?

They are moving in different directions. Pick a frame fixed to the star and both may have the same energy and wavelength. Pick a different frame and they won't.

 

[itoero]

They are moving in different directions. Pick a frame fixed to the star and both may have the same energy and wavelength. Pick a different frame and they won't.

I don't get why it's not a fixed frame.
Rayleigh scattering explains the blue and red light we get from the sun (a star)...it deals with a fixed frame.
https://en.wikipedia.org/wiki/Rayleigh_scattering#Cause_of_the_blue_color_of_the_sky

The movement of the Earth compared to a star (the sun)causes the red and blue light and we use a fixed frame to study it.
The movement of the Earth compared to a star causes the red and blue light but we don't use a fixed frame to study it.
It seems to me that in order to really explain the doppler red and blue shift we need to study the atomic behavior (scattering).
Like is said in the Ewald–Oseen extinction theorem https://en.wikipedia.org/wiki/Ewald–Oseen_extinction_theorem
"An important part of optical physics theory is starting with microscopic physics—the behavior of atoms and electrons—and using it to derive the familiar, macroscopic, laws of optics."

 

[jbriggs444]

I don't get why it's not a fixed frame.
Rayleigh scattering explains the blue and red light we get from the sun (a star)...it deals with a fixed frame.

You asked about Doppler.

Please nail down a scenario that we can discuss intelligently.

 

[itoero]

You asked about Doppler.

Please nail down a scenario that we can discuss intelligently.

I asked why people don't take a fixed frame in dopplershift but they do, to study the blue and red light we get on earth...
Is that such a silly question? Should I create a new thread for it?

 

[phyzguy]

I asked why people don't take a fixed frame in dopplershift but they do, to study the blue and red light we get on earth...
Is that such a silly question? Should I create a new thread for it?

If the star is rotating, then the gas on one side of the star is moving at a different velocity from the gas on the other side of the star, so they are in different reference frames.

 

[jbriggs444]

I asked why people don't take a fixed frame in dopplershift but they do, to study the blue and red light we get on earth...
Is that such a silly question? Should I create a new thread for it?

Can you nail down a scenario, please. What are you talking about?!

At a guess, @phyzguy nailed it. You are looking at a star and seeing one edge red-shifted and the other edge blue-shifted. You ask what happened at the atomic level on the star. The answer is as in post #3: Absolutely Nothing.

The difficulty is that the wavelength of the light is at its normal standard for an emitting (or absorbing) atom as judged from the rest frame of the atom at the star's surface. It is not at its normal standard when you measure that wave length from your rest frame on the Earth.

If you marvel at the unexpected measured wave length, it is because you have failed to account for the change in reference frame.

In special relativity, it is inertial frames extend globally and all one is concerned with is the relative velocity of the frames. In general relativity, inertial frames are only local, relative velocities are not always unambiguous and gravitational effects enter in.

 

[itoero]

The difficulty is that the wavelength of the light is at its normal standard for an emitting (or absorbing) atom as judged from the rest frame of the atom at the star's surface. It is not at its normal standard when you measure that wave length from your rest frame on the Earth.

Yes and how can the energy of a photon change without any interaction? When a photon interacts with an electron, you get inelastic scattering...the corona of a star contains free electrons. They scatter inelastic with photons.

Velocity can cause a shift in concentration of electrons, which explains doppler red and blue shift.

 

[jbriggs444]

Yes and how can the energy of a photon change without any interaction?

The energy of a photon is not a relativistic invariant. It does not makes sense to say that it "changes" when you use two different frames to measure it and get two different measured values.

Velocity can cause a shift in concentration of electrons, which explains doppler red and blue shift.

What are you talking about?

 

[itoero]

If you marvel at the unexpected measured wave length, it is because you have failed to account for the change in reference frame.

I knew about that explanation before I created this topic.

 

[jbriggs444]

I knew about that explanation before I created this topic.

Then why are you asking for a different explanation?

 

[Dale]

Yes and how can the energy of a photon change without any interaction?

It doesn’t. It is different in different frames, but in each frame its energy is constant and is equal to the energy lost by the emitter.

 

[Drakkith]

Yes and how can the energy of a photon change without any interaction?

The scenario is similar when talking about kinetic energy. The kinetic energy of an object will be different for different frames, despite no interaction taking place. For example, the frame of a cosmic ray (high speed proton) moving past Earth would see the Earth as possessing an immense amount of kinetic energy. But for you and I here standing still on the surface, that kinetic energy is zero. For the Apollo astronauts on the way back from the moon, the Earth's kinetic energy was non-zero but still far less than that seen by the cosmic ray's frame.

 

[sophiecentaur]

Yes and how can the energy of a photon change without any interaction?

You can't because the Energy depends on a reference frame.
It's just the same with a 'speeding' bullet. Its Kinetic Energy may be high in the frame of the gun. It can do a lot of damage when it hits someone standing nearby but it will do very little damage to someone traveling away from the gun in an aeroplane at a velocity that's just a bit less than the bullet (in the gun's reference frame). Photons are not 'mechanical' like a bullet but the same thing applies. Relativity tells us that they must be traveling at the same speed in any reference frame so something else has to change and that's the frequency.