Difference between special relativity and redshift?

[BadgerBadger92]

TL;DR Summary

What is the difference between time dilation in special relativity vs. time stretching in redshift?

For quite a while I thought that time dilation was the effect of bodies moving close to the speed of light dilates the passage of time.

I also have read about red shift, which seems to be the same thing, though I’m sure they are totally different.

I learned in special relativity (the time dilation aspect at least) that the speed of light is the same in all frames of reference. For a very fast body, light seems the same as it would for a body outside of it watching it. The light “gap” I thought would change how they experience time.

Then there is redshift, where when light stretches and causes time to “stretch.”

What am I missing here? I’m no expert but I have been doing a lot of self studying on this, so please don’t bash me for the question.

I have been doing some hard core math studies lately.

Please try to explain as clearly as possible.

 

[anuttarasammyak]

You,in an IFR1, see red light. Say another IFR2 move toward source of light. IFR2 people see blue light.
$$\lambda_1 \nu_1= \lambda_2 \nu_2=c$$
$\lambda$ : wave length
$\nu$: frequency
Wave length and frequency vary according to IFR but their product is same, light speed.

[EDIT] I corrected blue and red.

 

[BadgerBadger92]

You,in an IFR1, see blue light. Say another IFR2 move toward source of light. IFR2 people see red light.
$$\lambda_1 \nu_1= \lambda_2 \nu_2=c$$
$\lambda$ : wave length
$\nu$: frequency
Wave length and frequency vary according to IFR but their product is same, light speed.

I understand this part, but I also heard that redshift includes a warping of time. Is this true or was what I was reading unreliable?

 

[anuttarasammyak]

"warping of a time" is not a scientific word. It seems not reliable to me.

 

[BadgerBadger92]

"warping of a time" is not a scientific word. It seems not reliable to me.

Warping of time may be a bad representation of what I’m asking.

I just read that redshift is the stretching or condensing of light in space. I read that the red shift “shifts time.” Or how it’s experienced.

 

[BadgerBadger92]

"warping of a time" is not a scientific word. It seems not reliable to me.

I learned this from this video. I guess they could have mentioned stretched time as in a reference to relativity.

What do you think about it? I heard Neil DeGrasse Tyson was a good source, but I may be wrong.

 

[anuttarasammyak]

>Difference between special relativity and redshift?​

I now see redshit you say is about light from galaxies in expanding universe. A short answer is it is beyond SR. An interpretation in framework of SR ( e.g., moving away galaxies ) may cause confusion, I am afraid. First SR then GR and proceed to the cosmologiacal topics as in the video you refer is the right way to learn.

 

[BadgerBadger92]

>Difference between special relativity and redshift?​

I now see redshit you say is abot light from galaxies in expanding universe. A short answer is it is beyond SR. An interpretation in framework in SR ( e.g., moving away galaxies ) may cause confusion, I am afraid. First SR then GR and proceed to the cosmologiacal topics as in the xideo you refer is the right way to learn.

Thank you!

 

[Ibix]

Any wave can be Doppler shifted due to a moving source or receiver. You will have heard the effect on the engine note when a car passes you on the street. It is caused by the source being a different distance from the receiver at the end of one wave cycle than at the beginning, so the wave is either stretched or compressed compared to the same emission process from a stationary source.

In special relativity, you get Doppler shift of light. You can decompose the frequency change into two components - one due to the distance change exactly as above, and one due to the slower ticking of the emitters clocks in your frame. The first component can either increase or decrease the received frequency depending on the direction of motion, but the second one always lowers the frequency. It is important to realise that there is absolutely nothing that could remotely be described as "warping" going on. There is nothing more complex here than the Lorentz transforms.

I can't see the video you are referencing, but @anuttarasammyak's response seems to imply that it is referring to cosmological redshift, where light from distant galaxies is redshifted compared to that from nearby ones. This can be explained a number of ways, but not within the framework of SR because it is a process in curved spacetime.

The easiest way is to adopt comoving coordinates, where the space between galaxies expands over time. The space between successive crests of a travelling light wave also expands, so the light becomes redder as it travels. This explanation is correct, but it involves adopting one particular point of view. Don't get too tightly wedded to it.

 

[BadgerBadger92]

Any wave can be Doppler shifted due to a moving source or receiver. You will have heard the effect on the engine note when a car passes you on the street. It is caused by the source being a different distance from the receiver at the end of one wave cycle than at the beginning, so the wave is either stretched or compressed compared to the same emission process from a stationary source.

In special relativity, you get Doppler shift of light. You can decompose the frequency change into two components - one due to the distance change exactly as above, and one due to the slower ticking of the emitters clocks in your frame. The first component can either increase or decrease the received frequency depending on the direction of motion, but the second one always lowers the frequency. It is important to realise that there is absolutely nothing that could remotely be described as "warping" going on. There is nothing more complex here than the Lorentz transforms.

I can't see the video you are referencing, but @anuttarasammyak's response seems to imply that it is referring to cosmological redshift, where light from distant galaxies is redshifted compared to that from nearby ones. This can be explained a number of ways, but not within the framework of SR because it is a process in curved spacetime.

The easiest way is to adopt comoving coordinates, where the space between galaxies expands over time. The space between successive crests of a travelling light wave also expands, so the light becomes redder as it travels. This explanation is correct, but it involves adopting one particular point of view. Don't get too tightly wedded to it.

This is a wonderful answer. Thank you.

 

[vanhees71]

For more on general waves in media in the simplest case of normal dispersion in special relativity, see

https://itp.uni-frankfurt.de/~hees/pf-faq/rela-waves.pdf

 

[BadgerBadger92]

For more on general waves in media in the simplest case of normal dispersion in special relativity, see

https://itp.uni-frankfurt.de/~hees/pf-faq/rela-waves.pdf

I think this is a little too advanced for me at this point, but I’ll keep up my math studies.