What is the appropriateness of the term potential in general relativity?

[Jonny_trigonometry]

What is the highest known blue shift?

 

[SpaceTiger]

The largest blueshift that I could find that wasn't listed as a probable error:

http://nedwww.ipac.caltech.edu/cgi-...bj_id&objid=6441063&objname=16&img_stamp=YES"

I would still interpret it with caution, however. Other heavily blue-shifted objects are listed here:

http://nedwww.ipac.caltech.edu/cgi-bin/nph-allsky?z_constraint=Less+Than&z_value1=-500&z_value2=0&z_unit=km%2Fs&flux_constraint=Unconstrained&flux_value1=&flux_value2=&flux_unit=Jy&frat_constraint=Unconstrained&ot_include=ANY&nmp_op=ANY&ra_constraint=Unconstrained&ra_1=&ra_2=&dec_constraint=Unconstrained&dec_1=&dec_2=&glon_constraint=Unconstrained&glon_1=&glon_2=&glat_constraint=Unconstrained&glat_1=&glat_2=&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES"

 

[Jonny_trigonometry]

thanks these are pretty large blue shifts. Would a jet from a black hole have a higher blue shift? Is there more than one way to create a blue shift, like what if light is coming in from an area that is more "stretched" than our area?

 

[SpaceTiger]

thanks these are pretty large blue shifts. Would a jet from a black hole have a higher blue shift?

Interesting question. Yes, in theory, a jet could exhibit a large blueshift, even at very large distances, but they're never actually measured that way. When you're looking down a jet from a quasar or active galactic nucleus, the object is called a "blazar". These blazars can have high gamma factors (i.e. velocities very near c), but their spectra are completely featureless. The lack of features in the jet spectrum means that when we measure the redshift of the blazar, the only spectral features that might be available would be from the host galaxy or quasar. Neither of these things is moving towards us with relativistic velocities, however, so the blazar is given a net redshift.

Anyway, the short answer is yes, there are probably jets whose radiation is blueshifted, but it doesn't go into the catalogs as a "blue-shifted" object.

Is there more than one way to create a blue shift, like what if light is coming in from an area that is more "stretched" than our area?

I'm not quite sure what you mean by more "stretched". The only other blueshifting method that I can think of would be a "gravitational blueshift". This could occur if the object were very nearby and in a shallower potential well than we are (like from a satellite to the Earth's surface). This effect would be pretty much irrelevant for astronomical purposes, though.

 

[Jonny_trigonometry]

huh, weird stuff. I didn't know what the spectrum is like for jets. I suppose that there is no matter in the form of an atom when it's coming out of a black hole, quasar, or galactic center because of the violent nature of the environment there, but I thought there would be a point far out enough from the source where matter starts to condense into atoms... I guess if it does, it probably doesn't emit light? Then again if it did, we could see absorption lines from it, so I guess that's not the case. Thats interesting.

I was thinking along the lines of the stretchability of space-time and how it's "stretched" in the middle of two masses, and "compacted" close to each mass. So I can use the term potential well when speaking of gravity? cool! this makes more sense. Is the space in a void between galactic clusters at a higher potential than the space inside a galactic cluster? meaning, when light traverses into a void from a cluster, it goes "uphill" and when it goes into another cluster it goes back "downhill" into that cluster, so the redshift from going up is canceled by the blue shift from going down, assuming the two galactic clusters have the same "depth" of a potential well? (leaving out the redshift from expansion)

 

[SpaceTiger]

huh, weird stuff. I didn't know what the spectrum is like for jets. I suppose that there is no matter in the form of an atom when it's coming out of a black hole, quasar, or galactic center because of the violent nature of the environment there, but I thought there would be a point far out enough from the source where matter starts to condense into atoms...

The jet will certainly shock the intergalactic medium, but any such line emission would be swamped by synchrotron radiation from the relativistic jet.

Then again if it did, we could see absorption lines from it, so I guess that's not the case.

Well, the spectra might show absorption lines, but with this alone, there's no way to be sure if the absorption is local or intervening. Since blazars are observed at cosmological distances, they often show absorption lines from gas along the line of sight.

So I can use the term potential well when speaking of gravity?

The appropriateness of the term "potential" in GR is being debated in this thread:

https://www.physicsforums.com/showthread.php?t=97793"

Locally, there doesn't seem to be much disagreement that we can have a meaningful definition of "potential". At cosmological distances (such as to voids), the redshift from expansion would swamp any other gravitational effects, so it's not really a concern here.