# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Zer0 on 15/10/2021 19:34:54

Title: Law of Conservation of Energy?
Post by: Zer0 on 15/10/2021 19:34:54
I had a Follow Up Query regarding this Question of The Week.

QotW - 21.09.13 - Where does the extra energy go when light is stretched?

What i grasped from the Above was, that as The Source radiating Light keeps going further apart...it Red Shifts.

And as Space can & does stretch/expand Faster than the Speed of Light, then Red Shift dissolves & disappears completely.

Query :-
But what if that same Space Contracts somehow...then Does the Light/Energy Re-emerge from the Darkness?

Ps - Please keep it Short & Simple, Thanks!
🙏
Title: Re: Law of Conservation of Energy?
Post by: Origin on 15/10/2021 20:28:04
But what if that same Space Contracts somehow...then Does the Light/Energy Re-emerge from the Darkness?
Expanding space will red shift (stretch) photons wavelength.  Contracting space would blueshift (compress) photons wavelength.
In space that is compressing instead of expanding, a photon will be blue shifted, if there is no photon there is no photon to shift
Title: Re: Law of Conservation of Energy?
Post by: Halc on 16/10/2021 14:31:35
@Zer0 My apologies for the sidetrack in this thread. I have moved it all out.
I tried to keep it simple, but anything shorter seemed to border on being wrong.

QotW - 21.09.13 - Where does the extra energy go when light is stretched?

What i grasped from the Above was, that as The Source radiating Light keeps going further apart...it Red Shifts.
Short answer is that energy is not conserved in any coordinate system where space changes over time. The same situation using a coordinate system with static space results in no energy loss or gain.

In inertial coordinates, any light emitted (by say a distant star) stays the frequency it started, and doesn't redshift over time. The redshift as observed by one observer of a receding source is mostly due to Doppler effect, and thus was redshifted in that frame from the moment it was emitted.
In expanding coordinates the same observed thing (distant star) is not moving significantly, but space between it and the observer is expanding, and this 'stretches light' so to speak, causing it to lose energy just like a high speed rock also slows over time. It isn't specific to light at all. Light energy and kinetic energy both go down with expanding space. Potential energy goes up.

Quote
And as Space can & does stretch/expand Faster than the Speed of Light, then Red Shift dissolves & disappears completely.
Redshift doesn't dissolve with recession rates greater than c. We can see plenty of objects receding faster than c.
Secondly, the rate of expansion of space isn't a speed, so it doesn't make sense to compare it to the speed of light.
Speed is in units of km/sec while expansion rates have units of km/sec/mpc. The latter cannot be meaningfully compared with c, which has the km/sec units.

Quote
But what if that same Space Contracts somehow...then Does the Light/Energy Re-emerge from the Darkness?
In expanding space, the light never goes completely away, but just redshifts to arbitrarily long wavelengths. Yes, that light would eventually blueshift back to its original energy (and more) if contraction were to occur. The rocks that had slowed down would eventually gain their speed back, and more.
Title: Re: Law of Conservation of Energy?
Post by: Zer0 on 17/10/2021 16:26:02
Hiya Halcy!
🙋

Nobody should ideally Apologize for doing the Right thing.
👍

Infact, i am soo Sorry to have put you through all this trouble.
🙏
I Realize Now the OP is Absurd & does Not belong in the " Physics & Astronomy " section either.

I Specifically mentioned " Same Space " could somehow Contract.

Observational Evidence states it's Expanding & Accelerating.
So, but obvious, my OP does not conform to Reality.

As far as I'm concerned, this OP is over n out.
👍

Ps - Thank You for the elaborate explanation.
It wasn't expected, but was much Necessary.
Helped in clearing out Alot of misconceptions of mine.
Keep up the Good Work!
🍭