/sci/ - Science & Math

Potential Energy

>>8160295
Learn what frequency means first.

Photons can travel through space without losing energy. Red shifted light looks that way because it's traveling away from you so the light waves 'stretch' out. It's not until that photon hits something that it gets absorbed

>>8160321
>>8160325
Ya. But Plancks relation says E=hf so how is energy conserved if the frequency is dropping?

Because it does not change. Just set yourself in the relativ motion system of the observer and emitter.

At least not from the relativ motion. It does change because the observer and emitter might be in a different gravitational potential.

>>8160295
the change in frequency comes from the change in observer.


if you are the emitter, the photon has a certain energy when emitted, that doesn't change over time.

if you change frames of reference, energies change accordingly, including the one for photons. It doesn't mean the photons lost energy.

This is just like saying "I claim that at sea level, gravitational potential energy is 0", and then changing our mind and saying "well now I want that to be true but at 10km up in the air"

>>8160295
It's a pretty good question OP.

>>8160364 is wrong, he's confusing the optical redshift with the Doppler-Fizeau effect that is only relevant to phonons.
The loss of energy caused by the redshift is real and caused by the expansion of space, not a consequence of a change of referential.

The answer to this is that energy conservation is a local law and not a global one. It doesn't apply when volumes are not constant in time. Which is why energy has no global meaning in general relativity, unlike work.

>>8160325
>>8160359
light speed is constand lads...
The redshift is caused by distance only, it would be exactly the same if the source traveled toward you. It's caused by space expanding, not by things moving away from you.

Law of conservation of energy.

Photos don't "lose" energy.
They're a cosmological constant.
"c".

>>8160379
>>8160393
>>8160415

we got some good trolling here.
keep it up

>>8160436
>>8160379 here, I'm serious.
The red shift isn't the doppler effect.

>>8160482
there's a redshift due to the metric expansion of space

there's also redshift due to the doppler effect

what's your point?

>>8160349
Now learn attenuation.

>>8160373
This guys right, energy isn't conserved between reference frames. In the frame of a thrown baseball it has no kenetic energy, but in the batter reference frame it does.

You are all wrong. The total energy of all universe is zero, seen by ANY observer.

>>8160742
Really because a neutrino that sees everything moving at .99999c would see everything as pretty energetic

>>8160744
Nope. It would see the total energy as zero.

>>8160748
Rest energy in mass? Regardless of reference frame that is energy.

>>8160744
>>8160748
>>8160742
>>8160758
>mixing up what "energy" means

Brainlets.

>>8160758
Photons have no rest mass

>>8160373
So are you saying that the frequency if the photon is reduced is all reference frames?

>>8160712
That isn't his point. Conservation of energy follows from to time-translation invariance via the Noether theorem. There is no time-translation invariance in GR (at least not always.)

>>8160295
What energy?

>>8160769
undergrad

>>8160295
If a person who is moving away from you punches you in the face, it will hurt less than if a person moving toward you punches you in the face.

Where did the energy go?

E^2=m^2+p^2. This equation is true in any inertial reference frame. If you change reference frames, the changes in energy and momentum compensate each other. Obviously if you switch reference frames, the total energy is going to be different...but so what?