# Particle behavior and the Doppler effect

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

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

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

Drakkith
Staff Emeritus
How can photons just change energy?

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.

Svenns, nasu and russ_watters
Mister T
Gold Member
In general, you need collections of large numbers of particles to exhibit wave properties.

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?

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.

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.
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.
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.
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.

Drakkith
Staff Emeritus
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.

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.

I'm not asking how a car changes energy.

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 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.

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.

Dale and russ_watters
Mister T
Gold Member
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.

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.

Dale
Dale
Mentor
2021 Award
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.
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.

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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.
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.
[
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.
How do you know which frame of reference to take?
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?
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?
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.
Why isn't the star taken as frame of reference?

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

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.

Mister T
Gold Member
How do you know which frame of reference to take?

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

We don't know why motion does that.

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

The doppler is a description of a phenomenon it doesn't explain the phenomenon.

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

Mister T
Gold Member
A photon always travels at c, regardless the energy it has. A car does not.

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?

Drakkith
Staff Emeritus
How do you know which frame of reference to take?

To take for what?

Why isn't the star taken as frame of reference?
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.

russ_watters
Mentor
Which laws?
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.

Dale
Mentor
2021 Award
Why isn't the star taken as frame of reference?
It certainly can be, you just have to be clear. In the star’s frame of reference the energy does not change.

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

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I deny it. Not only do we know exactly why motion does that,
Then why motion does that?

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

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

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

Zz.

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

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

Zz.
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.]

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sophiecentaur
Dale
Mentor
2021 Award
Then why motion does that?
Start with “In truth, relativity has just a single formula for the Doppler shift ...” here:

http://mathpages.com/rr/s2-04/2-04.htm

The geometry and algebra are very straightforward

The speed of a car does not change for different observers.
Obviously it does. It is e.g. 100 kph for observers on the side of the road and 0 kph for observers in the car.

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[
Start with “In truth, relativity has just a single formula for the Doppler shift ...” here:

http://mathpages.com/rr/s2-04/2-04.htm

The geometry and algebra are very straightforward
Yes but that doesn't explain why motion does that. The 'fact'' that motion can change wavebehavior doesn't explain why it happens.

Logic dictates gravitation is a main cause. Gravitational redshift shows such an effect of gravity. Gravity can lengthen and shorten waves.https://en.wikipedia.org/wiki/Gravitational_redshift
Obviously it does. It is e.g. 100 kph for observers on the side of the road and 0 kph for observers in the car.
Not true. The speed of a car is set, regardless of the observers. You need measuring devices to know the speed of a car. When you simply observe speed then the 100 kph is an interpretation without scientific value.
This is about the observer effect.https://en.wikipedia.org/wiki/Observer_effect_(physics)#Particle_physics
When you observe a phenomen, you change it. This is in a sense an uncertainty.
You should try to reduce the observer effect as much as you can...by being in correct reference frame and using correct measuring devices//technology. If you can't then you're not dealing with science.
You can explain certain phenomena with 'reference frame' but it doesn't have predictive value.

Dale
Mentor
2021 Award
L
Yes but that doesn't explain why motion does that.
Yes, it does.

A derivation is an explanation why. That is the purpose of derivations. A derivation starts from some basic accepted principles, in this case the geometry of signals sent between observers in straight line motion. Then it shows why those initial principles lead logically to the conclusion.

So yes, the linked derivation does show why motion does that.

You should try to reduce the observer effect as much as you can...by being in correct reference frame and using correct measuring devices//technology.
There is no uniquely correct reference frame. That is the whole point of the principle of relativity. The roads frame and the vehicles frame are both valid, and the speeds in the respective frames are also both valid.

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Mister T
Gold Member
Yes but that doesn't explain why motion does that. The 'fact'' that motion can change wavebehavior doesn't explain why it happens.

When you jump onto a slow-moving train one of the things that can make it easier is if you first run along with the train, matching its motion.

Can you explain why motion does that?

You need measuring devices to know the speed of a car.

You mean like one of those radar guns that you point at something and it tells you how fast it's moving? If you stand still as that train approaches you the reading on the radar gun is different than when you don't stand still.

Can you explain why motion does that?

Can you change the speed of that train just by running alongside it? How can two people, one standing still and the other running along with the train, each measure different speeds for the same train. How is it possible for the same train to have two different speeds at the same time?

sophiecentaur
Gold Member
The doppler is a description of a phenomenon it doesn't explain the phenomenon.
This reads like a statement along the lines 'Scientists are not as smart as they think they are'. The doppler effect is extremely well known and there is a very simple argument about how the spacing between the peaks and troughs of a wave in a medium and how a moving source or observer will produce or detect those peaks and troughs.
If you want to include phonons in an explanation of the effect then that's up to you but that requires you to understand the true nature of phonons. As with photons, those quanta of energy are only 'there' when there's an opportunity for a wave to interact with an object.
There is no real answer to the "Why" question in Science, when it's asked in any context. We can only make models that work to varying degrees of accuracy. The model used for the Doppler effect works very well - even when relativistic effects are introduced. I reckon that's pretty good by any standard.

Drakkith
Staff Emeritus