# The Naked Scientists Forum

### Author Topic: What does it mean if something is "redshifted", and what is a redshift based on?  (Read 6844 times)

#### thebrain13

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##### What does it mean if something is "redshifted", and what is a redshift based on?
« on: 29/12/2006 06:21:45 »
Would you view a redshift if a planet was moving around you without getting any closer or farther away?

Would it increase or decrease based on the velocity of the planet moving around you?
« Last Edit: 01/01/2007 12:43:21 by chris »

#### eric l

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #1 on: 29/12/2006 14:58:53 »
Are you thinking of a planetthat reflects light, or of a star that emits light ?
In the case of the planet, the light that reaches it would be subject to blue shift as it moves closer to the source of the light, and to red shift as it moves farther away.  I reckon that this would also be true for the reflected light that reaches the observer.  So each revolution around the observer would show blue shift, no shift, red shift, no shift, and this over and over again.

#### thebrain13

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #2 on: 29/12/2006 15:55:02 »
the planet is never moving towards or away from you, its just orbiting around you. would the redshift increase if the planets velocity increased?

#### eric l

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #3 on: 29/12/2006 16:15:30 »
As the planet revolves around you, in one revolution it will move both closer to and farther away from the source of the light.  If the speed increases, both blue shift and red shift will increase.

#### Heliotrope

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #4 on: 29/12/2006 19:33:27 »
Let us assume you're on planet 1 that is rather larger and planet 2 which is rather small is orbiting around you.
The Sun is somewhere far away and both our planets are orbiting around it together.
A bit like the Earth and Moon system going around the Sun.

You get a friend on planet 2 to turn on their flashlight.
Then using a very large telescope that you just happened to have in your back pocket you have a look at planet 2 and check out the light coming down from your friend's torch.
You measure the redshift of the torch light.
You find there is no red shift.
The distance from the torch on planet 2 to your position on planet 1 is not changing. Hence no red shift.

Now you point your telescope at a place on planet 2 that your friend with the torch is nowhere near.
All that you get down your telescope is reflected light from the Sun.
This time you measure a red shift.
The red shift is caused by the planet's motion around you.
It is because it's distance from the Sun is changing whereas yours is constant.
It's distance from the Sun is changing so there is a relative velocity between them that can be measured by looking at the doppler shift in the reflected light.

#### thebrain13

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #5 on: 31/12/2006 22:44:22 »
If an object is moving around you without getting any closer or farther away, time dilation still occurs, correct? And if an objects time is relatively slower than yours, you would see it redshifted, right? Time is based on relative motion, but relative motion is not based on your position relative to the other object. For example the object that is moving around you is not getting any closer or farther away from you but it still has relative motion to your point of view, correct? Is this Einsteins interpretation?

And heliotrope, I don't believe planet two would view a redshift from the suns light bouncing off to you. Or at least not based off relative velocity that is, but it would be gravitationally redshifted. The relative frame of reference from the sun to planet 2 is equivalent. Whatever happened to the light in between is irrelevant. For example if the hubble telescope were viewing a distant redshifted supernova, it is most likely largely redshifted. However if the hubble telescope moved forward to match the velocity of the supernova, right before it viewed the redshift, no effect would occur, even though the light was redshifted the whole time. Only the beginning and ending of the light matters.

#### eric l

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #6 on: 01/01/2007 21:44:57 »
Hello thebrain (and a Happy Newyear to you), I think you are mixing up speed of light (and time dilation) with speed of the planet relative to the source of the light.
Redshift is an example of the Doppler-effect.  This Doppler-effect is what you hear as the tonality of the siren of an ambulance or police car changes as it moves nearer first, and farther away after comming past.  This change in tonality is related to the speed of the vehicle (or to be more precise to the ration between the speed of the vehicle and the speed of sound).
As the planet turns around you, an observer will see the light of a star redshifted as the planet moves away from the star.  The light reflected by the planet to you is the same as seen by this observer.
Anyway, even if the planet remains at the seem distance from you (and the same relative speed), the total distance for the light will be constantly changing as it travels from the source to the planet and via the planet to you.

#### thebrain13

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #7 on: 03/01/2007 04:27:40 »
So, bottom line what you guys are trying to tell me is that if an object is moving around another, without getting any closer or farther away, that it would not be redshifted?

Even if the object was traveling at 99 percent the speed of light, it still wouldnt be redshifted. Is that what your saying?

#### ukmicky

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #8 on: 03/01/2007 04:52:14 »
yes, as long as its distance is not increasing relative to the observer then their would be no redshift

#### syhprum

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #9 on: 05/01/2007 15:19:05 »
If the object was orbiting you at .99c it could only be doing so because it was subject to a very intense gravitational field which would make its orbital path complex and then there would surely be some red or blue shifts.

http://www.fourmilab.ch/gravitation/orbits/
« Last Edit: 05/01/2007 15:22:47 by syhprum »

#### thebrain13

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #10 on: 07/01/2007 00:37:25 »
True, there would have to be some affect from a gravity/acceleration induced redshift. However if the object was propelled by rockets and it was traveling sufficiently far enough away from the planet the acceleration induced redshift would become negligible.

#### ukmicky

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #11 on: 07/01/2007 03:12:11 »

the top line blue shift, the frequency of the wavelength has increased, the wavelength is being squash like a spring, the waves are closer than they should be for the frequency of light being emitted by the source because the distance between the object and the person viewing it is decreasing at a speed which is close to the speed of light.

The bottom line redshift, the frequency of the wavelength has decrease the wavelength is being stretched ,the waves are further apart than they should be for the frequency of light which is being emitting by their source because the distance between the object and the person viewing it is increasing at a speed which is close to the speed of light.
« Last Edit: 07/01/2007 03:16:58 by ukmicky »

#### Soul Surfer

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##### Re: What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #12 on: 08/01/2007 12:28:17 »
The doppler effect (red and blue shifts in spectrum lines) depends only on the relative radial velocity of the observer and the observed that is the velocity component along the line of sight.  The velocity of the object across the line of sight has no effect even if it is very fast  (However remember a vary fast moving object will change its relative position quickly unless it it a very long way away.

Strong gravitational fields also create a red shift so as an object aproaches the event horizon any radiation from it that is viewed from a long way away (outside the gravitiatinal field)appears reddened.

#### thebrain13

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##### What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #13 on: 11/01/2007 17:54:55 »
I dont know but that seems weird to me. The object traveling around the observer is time dilated, therefore it emits light slower, from the stationary objects viewpoint. For example the gravitational redshift is just a measure of the two objects time differences, same thing with the velocity redshift, both objects view each others frame of reference as slower when they are traveling away from each other, which causes a redshift, when both objects travel towards one another both view each others time as faster hence causing a blueshift. I would understand if you bounced light off the moving object (with no radial velocity, thanks for teaching me that word btw) and didnt see any change, but if it was emitting the light it would create it in a slower frame of reference.

And how can a time dilated object create non time dilated light?
« Last Edit: 11/01/2007 18:00:15 by thebrain13 »

#### The Naked Scientists Forum

##### What does it mean if something is "redshifted", and what is a redshift based on?
« Reply #13 on: 11/01/2007 17:54:55 »