Red shift and gravity bending light

[Modelworks]

I watch a lot of shows like The Universe and it is often mentioned that they use red shift for measurements. I understand the general idea but I don't understand what they are using for a baseline. If they use the light from a distant star and then apply the red shift math, how do they know that the light they are receiving is not the original color of the star and something not related to the red shift ?


Basically what is the standard they are using for comparison ?


Similar question for gravity bending light. If they say that light from a distant star is being bent by gravity, how do they know ? If all we can see is the light from the source wouldn't it be just like bending a fiber optic cable where we see the output only regardless of how many times we bend the cable ?

So what is their point of reference for the original star ?

 

[f95toli]

They use the spectrum of the incoming light. We know -from experiments in the lab- what frequencies there should be lines from e.g. helium and hydrogen so these form a "fingerprint" to which you can compare the light from the stars.

Note that you shouldn't take the use of the words "red" and "blue" too litterally here, that something is redshiftet simply means that the frequency is lower than you reference.
In theory thismeans that the object has changed its colour but real shifts are really tiny so it would be extremely hard to see by the naked eye.
Moreover, the word are used even for parts of the spectrum well outside the visible band; sometimes people refer to "red" and "blue" sidebands even for microwaves.

 

[Born2bwire]

As for light being bent by gravity, it's as simple as measuring the position with and without the presence of gravity if possible. The first set of experiments in this regard were taken during a total eclipse of the sun. The sun's gravity distorts the light of the stars made visible near its edge during the eclipse. The observed positions were compared against the known position in absence of the sun.

 

[bsobel]

http://astronomy.swin.edu.au/c.../Cosmological+Redshift

 

[Modelworks]

Thanks for the replies. I think I better understand the red shift now.
The gravity bending of light I'm still a bit curious about. If light from a given star is only seen by us when it is affected by gravity, how can we make a comparison and still know that the light is being bent by gravity ?

 

[TuxDave]

nvm

 

[silverpig]

Originally posted by: Modelworks
Thanks for the replies. I think I better understand the red shift now.
The gravity bending of light I'm still a bit curious about. If light from a given star is only seen by us when it is affected by gravity, how can we make a comparison and still know that the light is being bent by gravity ?

The answer is that we can see the effect directly using an appropriate target. If there is a massive object between us and a distant galaxy, then the galaxy's light will appear as a ring around some dark point.

Google "gravitational lensing" to see this effect.

Image 1

Image 2

If us, the distant light source, and the lensing object are all in a nice line, the distant object will appear as a ring instead of as a point.

If there is some slight angle to the arrangement, then the distant object will appear as a crescent arc. Oftentimes, the lensing mass is a large collection of masses such as galaxies. In this case the distant object can appear as a series of arcs all over the place.

 

[soccerballtux]

Hm, would like to see this in action. All the pictures using the sun do not count-- the sun has a huge atmosphere; of course it's going to bend light.

 

[Biftheunderstudy]

Here's one
And another

There are quite a few nice hubble images with giant arcs in them and even a few with full Eistein Rings.

There is also another type of gravitational lensing called Weak Lensing, this is a more statistical approach which looks for stretching of galaxies on a statistical basis. They do this because it is a smaller effect.

Then there's a third type called Cosmic Shear which is gravitational distortions from large scale structure in the universe. This is similar to weak lensing, except instead of a single lensing object or collection of objects its the mass density of the universe which causes the lensing.

 

[Modelworks]

So they are using the arcs of light as a sign that gravity is effecting the light ? If the arc did not appear would there be any way for them to determine gravity was having an effect ?

 

[silverpig]

Originally posted by: Modelworks
So they are using the arcs of light as a sign that gravity is effecting the light ? If the arc did not appear would there be any way for them to determine gravity was having an effect ?

Well sometimes yes. There are other things you can look for... stretching etc.

But if you're asking "If you look up at the sky and take a single image of a single star, can you tell if it is being lensed at all?" then the answer is probably no.

One reason is because if you're looking at a single star, then it's within our own galaxy and would have to be lensed by a black hole or something like that.

 

[TuxDave]

Originally posted by: Modelworks
So they are using the arcs of light as a sign that gravity is effecting the light ? If the arc did not appear would there be any way for them to determine gravity was having an effect ?

The other choice is to look for stars in the sky that are acting very similiarly that you'd suspect they were the same star being bent by gravity thus appearing as two. And then you have one of them supernova and X years later you see the other one super nova in a very similiar way, then it's pretty good evidence of gravitational effects on light.