# Doppler and enegry conservation

• malawi_glenn
In summary, the conversation discusses the relativistic doppler shift and whether or not energy is lost when a photon is redshifted. It is explained that the energy of the photon remains constant, but may be measured differently by the emitter and receiver due to their relative motion. It is also noted that energy is not an invariant of motion and may be measured differently from different inertial frames.

#### malawi_glenn

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In relativistic doppler shift, the energy of the photon is increasing och decreasing, due to realative motion of observer vs emitter. Let say that the photon is redshiftet, does any energy dissapear? :uhh:

The energy isn't lost. While the frequency decreases, the wavelength increases. Each individual photon retains its initial energy; it just takes a 'longer path' to get here. That's a pretty lousy response. Best wait for ST or other to clear it up.

Is this what you mean?

When the photon is emitted, its wavelenght is less than the wavelenght as the observer see it. The energy must have dissapeared, science the wave length is increasing. There must be a relativistic answer to this. Of course the frequency is increasing if wavelenght is decreasing.. But that is not the anszer to my question i think. You must compare the initial wave length and the final wavelenght.

So i guess that the answer lies in the relativisic area. According to the emitter the photon energy is constant, and according to the observer the photon energy is constant, science we can't follow a photons path. If I would make a guess. But energy is an invariant of motion.. ? That is why Iam asking.

malawi_glenn said:
In relativistic doppler shift, the energy of the photon is increasing och decreasing, due to realative motion of observer vs emitter. Let say that the photon is redshiftet, does any energy dissapear? :uhh:

There will be difference in energy measured by emitter and receiver and that is fine. But the "change in energy" is not observed respect to emitter(and receiver). Difference in energy measured b/t two reference frame is not the same thing as the change in energy observed by one frame.

malawi_glenn said:
But energy is an invariant of motion.. ? That is why I am asking

What makes you think Energy is invariant of motion? For one, kinetic energy is obviously a relative quantity respect to different reference frame.

Energy is an invariant of force-free motion, but that energy will be measured differently from different inertial frames.

## What is the Doppler effect?

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This effect is commonly observed in sound waves, such as the change in pitch of a siren as an ambulance passes by.

## How does the Doppler effect relate to energy conservation?

The Doppler effect does not directly relate to energy conservation. However, it is a result of the conservation of energy, as the energy of a wave remains constant even though its frequency or wavelength may change due to the relative motion of the observer.

## How does the Doppler effect impact different types of waves?

The Doppler effect can impact all types of waves, including sound, light, and water waves. The amount of change in frequency or wavelength depends on the speed of the observer and the wave speed, as well as the angle of observation.

## What is the significance of the Doppler effect in astronomy?

In astronomy, the Doppler effect is used to determine the speeds and distances of celestial objects. By observing the shift in frequency of light from stars and galaxies, scientists can calculate their velocities and distances, which helps in understanding the structure and movement of the universe.

## Can the Doppler effect be observed in everyday life?

Yes, the Doppler effect can be observed in everyday life. Examples include the change in pitch of a car horn as it passes by, the change in frequency of a siren on an emergency vehicle, and the change in pitch of a train whistle as the train approaches and then passes by.