# B Particle behavior and the Doppler effect

1. Sep 28, 2018

### itoero

How does particle physics explain the doppler effect? (including blue/red shift)

2. Sep 28, 2018

### Staff: Mentor

Doppler is a wave phenomenon and has no particular connection to particle physics.

3. Sep 29, 2018

### itoero

Yes doppler is only about waves but what happens on a particle level?
In dopplerredshift you measure low energy photons on one side and higher energy photons on the other side.
How can photons just change energy?

4. Sep 29, 2018

### Drakkith

Staff Emeritus
The energy of anything is not invariant. Simply changing reference frames can result in a drastic decrease or increase in the energy of an object or wave. For example, a moving car on a highway has a lot of kinetic energy as viewed from a person standing on the side of the road, but has negligible kinetic energy viewed from another car moving alongside the first.

So then your question would become, "How can a car just change energy?" The answer to which is that part of the energy content of an object or wave is frame dependent. For massless particles like photons, all of its energy is frame dependent, whereas for objects with mass part of their energy content is locked up in their mass and thus forms a 'minimum' energy level that the object can never fall under.

5. Sep 29, 2018

### Mister T

In general, you need collections of large numbers of particles to exhibit wave properties.

There is no change in any one photon's energy. Some are observed to have a high energy and some a low energy, but that difference is due to the observer's relative motion.

6. Oct 3, 2018

### itoero

I don't think that' a valid explanation. You view your own speed due the direct environment (concrete, grass, trees….) the observable speed of the other cars is based on the difference with your speed.
I'm not asking how a car changes energy.
The light emitted from a star is white. Yet we observe blue light on one side and red light on the other. How can photons change energy without inelastic scattering.
Why do you think cosmological redshift is due to the doppler effect? There is a big difference between sound waves and electromagnetic waves.

7. Oct 3, 2018

### Drakkith

Staff Emeritus
No, we just choose to use the Earth as the de facto frame of reference when referring to speed and velocity for most of our everyday lives because it is convenient. However the truth is that in our own frame of reference we simply aren't moving and have no kinetic energy. This is a fundamental principle of physics.

Sure you are. You're asking how a car changes energy because you're asking how a photon changes energy and the two phenomena follow many of the same laws of physics.

The idea that photons have one specific energy is only applicable if you choose to measure from a single frame of reference. If you switch to another the photons may not have the same energy. To put it simply, they don't have a specific energy in general. Energy is conserved, but it is not frame-invariant.

8. Oct 3, 2018

### Mister T

You haven't given an example of a photon changing its energy!

Observers can measure the energy of light. Is it hard for you to accept the notion that the energy observed depends on the motion of the observer relative to the source? One observes a beam of light to be blue while another observes it to be red. The light never changed from blue to red, the only thing that changed is the observer's motion relative to the source.

9. Oct 11, 2018

### Staff: Mentor

What makes you think they changed energy? In the frame where the star’s light is white it remains white and has the same energy on all sides. The red and blue shift is due to the Doppler effect on the detector, not a change in energy of the light.

Energy is frame dependent, so if you want to claim that the energy changed then you need to identify what reference frame you think that happened.

Last edited: Oct 11, 2018
10. Oct 12, 2018

### itoero

Which laws? I don't get why you think that's a valid comparison. A photon always travels at c, regardless the energy it has. A car does not.
[
How do you know which frame of reference to take?
We don't know why motion does that. The doppler is a description of a phenomenon it doesn't explain the phenomenon. Do you deny this?
Why isn't the star taken as frame of reference?

11. Oct 12, 2018

### ZapperZ

Staff Emeritus
I still do not see that you have established the starting point of your entire thread here, i.e. the connection between "particle physics" and "doppler effect". Why do you think particle physics can or should "explain" (whatever that word means) the doppler effect?

Until you can do that, this is similar to asking how particle physics can explain funny.

Zz.

12. Oct 12, 2018

### Mister T

You're free to choose any one you like. There is no wrong or right choice.

Sure we do. The energy of what you observe is dependent on your motion because of the way we define energy.

The Doppler effect is something we observe. It is also something that we can explain.

13. Oct 12, 2018

### Mister T

When a massive object moves at a speed that's nearly $c$ relative to you, this is what you observe. Very very small changes in the object's speed are associated with very very large changes in the object's energy. It is possible to get so close the speed $c$ that when the energy is increased by orders of magnitude there is a negligible increase in speed.

There are people doing this every day at locations all around the world. It's an undisputed fact.

Just what is it you're trying to understand here?

14. Oct 12, 2018

### Drakkith

Staff Emeritus
To take for what?

It can be. There's nothing wrong with that. But since the star is moving with respect to us here on Earth, you will get different measurements for the energy of the starlight if you measure in both frames.

15. Oct 12, 2018

### Staff: Mentor

The Principle of Relativity:
https://en.m.wikipedia.org/wiki/Principle_of_relativity

I'll be succinct: do you understand and accept that the speed and therefore the kinetic energy of a car are frame of reference dependent?

Your responses imply that you don't know what a frame of reference is or is used for.

16. Oct 12, 2018

### Staff: Mentor

It certainly can be, you just have to be clear. In the star’s frame of reference the energy does not change.

I deny it. Not only do we know exactly why motion does that, the derivation is very basic and well known freshman-level physics.

Last edited: Oct 12, 2018
17. Oct 26, 2018

### itoero

Then why motion does that?

18. Oct 26, 2018

### itoero

Yes but what does that matter? The speed of a car does not change for different observers.

19. Oct 26, 2018

### ZapperZ

Staff Emeritus
Yes it does. Different reference frame will measure different speed! That is what "frame dependent" means!

Zz.

20. Oct 26, 2018

### Staff: Mentor

Yep. This is why I said we need to start this walkthrough with these foundational physics concepts before even getting into Doppler shift itself.

So.

What is a reference frame? It's a set of coordinates against which you make measurements. The most common we use every day are coordinate systems centered on Earth (rotating with it) and centered on ourselves.

So, when I say I am driving my car at 10m/s, what reference frame am I measuring it from? What is the car's speed as measured against the other reference frame?

[note: I'm an engineer, not a physicist so my descriptions tend to be less than fully....formal. Some of that is on purpose but not all of it.]

Last edited: Oct 26, 2018