# The Naked Scientists Forum

### Author Topic: How does redshift work?  (Read 8161 times)

#### yor_on

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##### How does redshift work?
« on: 11/12/2008 16:45:01 »
You have a spacecraft that moves relative our universe very near to the speed of light.

Let us say that you set up two light-bulbs, one in the nose and one at the ships 'tail/end'.
You yourself are placed inside the middle of the ship observing them both (free line of sight)
My question(s) is:)

light-bulb = o
you = x
-------ship-------
---> 'o'    'x'     'o' --->
------------------

A. Will the frequency's from those light-bulbs differ?
B. how
C. Why ( <--- And that is the most important question of them all :)

That is, will they demonstrate a red or blue shift?
Or will they seem 'just as normal' frequency wise to what you are used to while 'at rest', like at home.
(When observed by you 'at rest' within that frame?)

And no. It's not any physics test.
I have my own view about it and want to get more views and reasons to why :)
So ponder on it and tell me what you think ...And why:)

« Last Edit: 18/12/2008 04:06:47 by chris »

#### DoctorBeaver

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##### Re: How does redshift work?
« Reply #1 on: 11/12/2008 16:57:38 »
As they are travelling with you they will appear normal to you.

#### yor_on

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##### Re: How does redshift work?
« Reply #2 on: 11/12/2008 17:01:21 »
Why:)
-
Ok, you see it as one frame, is that it?
« Last Edit: 11/12/2008 17:03:37 by yor_on »

#### yor_on

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##### Re: How does redshift work?
« Reply #3 on: 11/12/2008 17:18:06 »

As you are having a velocity ---->
The light from the bulb at the tail coming towards you will have a longer way to travel than the light from the front.

The fronts light will have a shorter 'way' as you (x) are traveling towards it.
Graphically something like this:)

---------ship------------------>
'o'-------> 'x' <----'o'

Would that be acceptable?

#### lyner

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##### Re: How does redshift work?
« Reply #4 on: 11/12/2008 17:53:00 »
All I can say is that, if your idea were correct, then we could TELL how fast we were traveling through the Universe and we have no way of knowing - in fact it is not a meaningful concept.
Michelson and Morley tried a similar experiment a long time ago and found that they could detect no motion.
You will agree that doing the experiment on a slow train, you wouldn't expect to measure its speed? Whatever speed you are doing (relative to someone else) your spacecraft would be just the same as the slow train. (Just with a posher loo).

#### yor_on

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##### Re: How does redshift work?
« Reply #5 on: 11/12/2008 18:47:34 »
Ok, SC I think you are right:)
If that was correct then we indeed would have a definition.
The thing here is that I was on a site who had this definition according to special relativity.

Light emitted by a light source is a type of clock depending on its frequency.. For our pilot the light coming from the back is red-shifted. The frequency of light emitted from the nose of the craft rises as the light travels to the back, so the light will be blue shifted.

One could say that the light from the tail appear red in the nose, because the light lost energy as it overcame the spacecraft's artificial gravitational field. Also one could say that the light emitted at the nose appears blue at the tail because the light gained energy as it fell deeper into the artificial gravitational potential well. It is a mathematically valid view and cannot be distinguished from the time-dilation interpretation of the light's Doppler shift.

The reason why, according to this site, is that in special relativity, when one finds that a spacecraft maintains its structural integrity, so that the distance from tail to nose is constant for passengers in the spacecraft, it will experience different rates of acceleration throughout its structure. The nose of the spacecraft accelerates at a lower rate than the tail of the spacecraft.

The amount of acceleration depends solely on your position along the accelerations velocity. The acceleration increases as you move back. As time dilation is defined by the rate of acceleration, the pilot finds that a clock placed in the nose of the spacecraft moves faster than a clock placed in the tail.

As I understood it that is:)
And yeah, it was a surprise for me.
So, would you say that this is correct?

If it is then the definition of being 'at rest' seems very 'limited' as this spaceship would be defined by an 'infinite' number of reference frames as I see it.

And that first definition DB gave me I also have used.
For defining 'times flow' as seen from inside a moving frame that is :)
Then again, it's all the same, isn't it.
But when questioning it I saw this other possibility too ::))
So, what do you think?

« Last Edit: 11/12/2008 19:13:14 by yor_on »

#### lightarrow

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##### Re: How does redshift work?
« Reply #6 on: 11/12/2008 20:19:02 »
You have a spacecraft that moves relative our universe very near to the speed of light.

Let us say that you set up two light-bulbs, one in the nose and one at the ships 'tail/end'.
You yourself are placed inside the middle of the ship observing them both (free line of sight)
My question(s) is:)

light-bulb = o
you = x
-------ship-------
---> 'o'    'x'     'o' --->
------------------

A. Will the frequency's from those light-bulbs differ?
No, unless the spaceship is accelerating; in this last case light from left is red-shifted, from right is blue-shifted.

#### yor_on

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##### Re: How does redshift work?
« Reply #7 on: 11/12/2008 22:08:26 »
So you see it as connected to acceleration only.
What would then differ that from its final motion?
What energy transfer would acceleration give (and deduct) the light that a final very high uniform velocity wouldn't?

-------

Would it be that under acceleration its (the spaceships) frame never would be at rest?
But when 'cruising' :) the frame always keep its uniformity.

Reasonable?

------------

Awh, it sounds good, but it doesn't explain why the energy would differ, does it:)
« Last Edit: 11/12/2008 22:23:24 by yor_on »

#### yor_on

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##### Re: How does redshift work?
« Reply #8 on: 11/12/2008 23:23:53 »
I'm missing something here.
But I can't see what differ acceleration from uniform velocity.
They both contain the same thing, momentum.

But when accelerating you are creating a 'gravity well' at the back of the ship.
So the 'gravity' will be higher the nearer the end you move, isn't that right.
And if added gravity was the reason, then any gravity would do, right?
Nope:)

But gravity is the suspect here:)
As gravity disappears when 'coasting'.
Not momentum as that always will be there.

And seen like that.
Knowing that light obeys gravity.
Knowing that there will be an inequality comparing the end to the front of our spaceship it makes sense.
so gravity relates to acceleration.

But it still doesn't explain why one G accelerating as compared to one G at Earth for ten years will give such a difference when comparing the frames

Or is it wrong to look at the differences (red/blueshift) as energy differences?

------

Awhhh, I should never have started this thread :)
« Last Edit: 11/12/2008 23:49:09 by yor_on »

#### yor_on

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##### Re: How does redshift work?
« Reply #9 on: 12/12/2008 00:13:37 »
Time and gravity goes together.
When you have a uniform velocity time/light won't change as observed from inside.
But if we rotated the ship along its longitudinal axis fast enough that an 'artificial' gravity was created then time would change, and measuring a light bulb at its wall as compared to a light-bulb at the spaceships 'axis' would then show that same effect, right?

If you don't get it, don't worry.
I don't either:)

#### LeeE

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##### Re: How does redshift work?
« Reply #10 on: 12/12/2008 01:33:28 »
Quote
Time and gravity goes together

Hmm... only in the sense that time and space go together, and where gravity is regarded as a change in space-time.

While the idea of everything existing in it's own time-frame is widely known from the traveling clocks experiment, if you go along with the idea that space and time cannot be separated and can only properly be treated as a single spacetime concept, then it would seem that everything also exists within it's own space-frame.  More correctly, everything exists in it's own spacetime-frame and movement, which is what you're talking about here, is the product of space and time.

#### yor_on

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##### Re: How does redshift work?
« Reply #11 on: 12/12/2008 11:49:31 »
So tell me, if I'm coasting at that final velocity (infinitesimally near 'c') will the 'other' frame 'age', or is it just locked to acceleration.

But when light travels it is said to have no acceleration, also it has no 'gravity' per se, only a momentum?
I think I'm right in presuming that it is its mass-lessness that allows it 'c'
But what makes it timeless internally?

The thing about gravity (invariant mass) is that the higher it is the more it will bend spacetime and 'shift time', as seen from our observer when comparing frames.
But when there is no gravity involved as with the photon, and no acceleration, what makes it seen as timeless?

That's probably why I assumed that 'velocity/motion' was bound to time too.
But as SC pointed out, that's not so:) and if I just had thought it through, I would have remembered:)

I used to believe that it was connected to both acceleration and motion.
It's only connected to acceleration and gravity.
Mostly because I saw it as something connected to momentum (photons), but it's not, is it.

Strange how easily you can make yourself believe you understand something.
But as they say, there's no one easier fool to fool, than yourself:)

---

Yes LeeE that's what I was starting to wonder about.
Everything falls back to our definitions.

As you say, spacetime is a whole, but either there are keys to be found for opening it with what we know.
I have problems accepting gravitons, and I also have problems accepting that we need two definitions for encompassing spacetime.

But I have no quarrels with Einsteins ideas, well, other than understanding them that is.
And in that I'm not alone:)

-----------

Another problem I'm having with the photon is how that 'timelessness' is suposed to 'work'.
Is there a significant 'real' difference between something 'timeless' (aka our photon) and our almost 'timeless' comparison, (our accelerating spaceship / matter)?

Considering that if our spaceship had an acceleration good enough :)
It would 'see' our universe die.
The photon doesn't.

Instead it interact with it, in all frames.
And if there is, what does that make what I see as a 'common nominator'.
Namely momentum?
« Last Edit: 12/12/2008 14:17:25 by yor_on »

#### lightarrow

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##### Re: How does redshift work?
« Reply #12 on: 12/12/2008 15:48:49 »
So you see it as connected to acceleration only.
What would then differ that from its final motion?
What energy transfer would acceleration give (and deduct) the light that a final very high uniform velocity wouldn't?

-------

Would it be that under acceleration its (the spaceships) frame never would be at rest?
But when 'cruising' :) the frame always keep its uniformity.

Reasonable?

------------

Awh, it sounds good, but it doesn't explain why the energy would differ, does it:)
yor_on: there is absolutely no difference if you only consider balls, bullets, any object thrown at a given frequency, instead of light waves: if the starship is moving of uniform motion, the frequency you measure at the centre doesn't vary; if the starship accelerates, the frequency you measure increases for those bullets coming from right and decrease for those coming from left.
In this case it should be easier to understand how it works.
« Last Edit: 12/12/2008 15:51:33 by lightarrow »

#### yor_on

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##### Re: How does redshift work?
« Reply #13 on: 13/12/2008 02:26:39 »
Thanks Lightarrow.
Looks like I'm missing something to simple to see:)

To me red/blueshift seems to have more to do with creating a 'gravity well' behind the tail of the ship when accelerating?
(For now, that is:)
.. And, if so, it seems to hold true for both matter and light.

But as I'm not really getting your point here, perhaps you could give me an example?

-----
Hey what can I say:)
I'm sloow..

« Last Edit: 13/12/2008 02:30:42 by yor_on »

#### QertysXasder

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##### Re: How does redshift work?
« Reply #14 on: 13/12/2008 09:35:13 »
i like spacecraft :-)

#### Chemistry4me

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##### Re: How does redshift work?
« Reply #15 on: 13/12/2008 09:49:04 »
If you were travelling at the speed of light, say 5 meters in front of a car that was also travelling at the speed of light, then the car switches on its headlights. Would you be able to see the light/will it reach your eyeballs??

#### yor_on

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##### Re: How does redshift work?
« Reply #16 on: 13/12/2008 10:07:54 »
No 'matter' can get up to 'c' but if it could:)
No, you can't and wouldn't.
That is another 'strange' thing about light.

It will only give you 'c' (at most)not caring from where that 'beam' comes.
So if you were in that spaceship sending out a beam as you traveled an observer no matter where from he saw your ship, coming at him or leaving would get that light beam at 'c'.

The only thing you would notice would be a redshift (as I see it:) for the light coming at you from the ship leaving your position. And a blueshift to the ships light (more energetic/compressed waves) if it was coming at you.

But when dealing with travels at 'c' and with the postulate that nothing can travel faster than 'c' I guess that there would only be 'nothing':) No light at all coming.
That as you both were traveling at the same velocity (direction and speed)

You could also ask what if light behaved like a light house and could send out light of its own while traveling?
Would the other photons traveling at the same velocity interact with that light?
Do you think that this 'light' could leave the traveling photon?

Why?
« Last Edit: 13/12/2008 10:13:18 by yor_on »

#### yor_on

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##### Re: How does redshift work?
« Reply #17 on: 13/12/2008 13:33:51 »
Yes, it even beats my bike QertysXasder:)

#### yor_on

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##### Re: How does redshift work?
« Reply #18 on: 13/12/2008 14:27:21 »
Quote
Time and gravity goes together

Hmm... only in the sense that time and space go together, and where gravity is regarded as a change in space-time.

While the idea of everything existing in it's own time-frame is widely known from the traveling clocks experiment, if you go along with the idea that space and time cannot be separated and can only properly be treated as a single spacetime concept, then it would seem that everything also exists within it's own space-frame.  More correctly, everything exists in it's own spacetime-frame and movement, which is what you're talking about here, is the product of space and time.

I like the way you see spacetime LeeE, It's the same way as I see gravity (for now:)
As a four dimensional 'net' reacting instantly on any 'force' applied on it locally.
Even though the propagation of it globally obey 'c'.

can you give me an simple explanation on how you see the accelerations effects at one G to become so 'visible' as compared to being on a 'coasting' object at one G (Earth).

One should be able to describe it (acceleration/time) as a curve.
As the spaceship moves constantly building on that one G.
Or am I bicycling in the great yonder again?

And do anyone have an example on it?
Not me bicycling, the curve I meant.

(A stationary object (Earth) as compared to a accelerating.)

It should be easier to see how that one G accelerating 'influences' time then.
As compared to a 'coasting' frame.

« Last Edit: 13/12/2008 14:29:54 by yor_on »

#### LeeE

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##### Re: How does redshift work?
« Reply #19 on: 13/12/2008 14:37:46 »
If I understand you correctly, you could compare the curve for time dilation due to gravity with the curve for time dilation for velocity where the velocity is due to acceleration.  I think.

#### yor_on

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##### Re: How does redshift work?
« Reply #20 on: 13/12/2008 14:55:45 »
Have you seen any such comparisons on the net?
It would make an instructive image.
One could do it in Java perhaps?
Allowing for different objects containing invariant mass, and then graphically show how it would relate to an accelerating object timewise, well sort of:)

#### yor_on

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##### Re: How does redshift work?
« Reply #21 on: 13/12/2008 16:39:41 »
Do you see a deeper significance in using frequency instead of (as I see it now) gravity Lightarrow?
As you use it for both 'matter' and 'light' in your example.
Are you seeing it as an 'wave defined' approach?

Would that make 'gravity' the wrong description for explaining red/blueshift with our accelerating spaceship?

#### lightarrow

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##### Re: How does redshift work?
« Reply #22 on: 13/12/2008 17:55:23 »
Thanks Lightarrow.
Looks like I'm missing something to simple to see:)

To me red/blueshift seems to have more to do with creating a 'gravity well' behind the tail of the ship when accelerating?
Yes, if I understand what you mean, because of equivalence principle: to be inside a starship accelerating in the space far from massive objects towards up, would be equivalent, for what we are considering, to be stationary in a gravitational field down under the tail. So you can actually do the experiment at home! Shooting bullets from up to down means that the bullets will acquire kinetic energy and so speed; so clearly the frequency of arrivals is increased; shooting up, instead, the bullets will decrease speed so arrivals' frequency will decreases.
Light cannot change speed, so it change color    Actually, is quite the same thing if, instead of speed, you consider energy: falling down a gravitational field, light, as any other body, will acquire energy and so its frequency will increase. The opposite going up the field.

#### yor_on

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##### Re: How does redshift work?
« Reply #23 on: 13/12/2008 19:21:24 »
Nice explanation:)

And that's how I thought too.
And 'gravity wells' seems to create strange phenomena.
Like creating an infinite space (seen from the inside)if of sufficient 'mass'.

I saw another thread here discussing that.

Soulsurfer wrote "Basically you will get particles orbiting together with very high energies unaware of any grvitational field or gravitational gradient because they are in free fall but the energies of the particles are all very similar so the particles are essentially cold with respect to each other. Because of relatavistic and gravitational distortion space has contracted and become one dimensional and time like while "time" has expanded to become multidimensional and space like.  As far as the particles are concerned this would look very much like a big bang with space expanding very uniformly and evenly without the need for mystic inflation."

I find it very interesting, and somewhere I used to have a very good link to a site describing the physics of spacetime when 'surrounding' very massive objects, like neutron stars or Black Holes.

« Last Edit: 13/12/2008 19:23:14 by yor_on »

#### yor_on

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##### Re: How does redshift work?
« Reply #24 on: 18/12/2008 03:00:19 »
I will come right out here and admit.
Yes, I'm stupid

I don't see how acceleration and uniform motion coexist?

As someone good wrote to me as I asked
"Am I right in understanding that all frames of reference relative each other, no matter if they're accelerating or just 'coasting', will give those 'results'."

" Unfortunately, no. When two frames are coasting relative to each other, you'll get one result. If one frame is accelerating, you'll get a different result. And, if two frames start with no motion relative to each other and one accelerates away, coasts, and then returns to no motion relative to each other, you'll get still another result. When there's no acceleration, the (relatively simple) rules of special relativity apply. With acceleration, the far more complicated rules of general relativity apply. "

Now according to the rules of the general theory of relativity.
You know:)

" gravity is just a curvature of the geometry of spacetime.
An observer in free fall no longer feels his own weight."

" One can produce gravity in gravity free space merely by reversing the process.
Acceleration creates a gravitational field"

This is in fact Einsteins "principle of equivalence."
And the idea that created 'spacetime'.

So by treating an object (you:) placed near/at a much bigger object (Earth) you could treat it as.
Something in a 'free fall' together 'attracting each other'.
As you could see it as an object of invariant mass having one G 'attracting you' as you it.
In fact, viewed that way you are both 'falling' towards each other.
Or if you like :) Accelerating at each other.

Like, something constantly trying to 'push' you uniformly, at one G.
And when you fall you are in fact going back to an 'inert' state, if I get it right.

But Uniform motion (coasting) is not the same as accelerating and therefore does not,
As far as I understand, create the 'age differences' experienced in the accelerating 'twin experiment'.

That sounds okey to me, but then I have this difficulty with the twin experiment.
In it we have 'someone' inside an accelerating frame versus his twin that stays at their original 'coasting' frame (Earth).
The first twin is accelerating at, let us define it as a 'steady' one G, going to some star, then turning back.
Accelerating and decelerating towards Earth and that 'original' frame, but at no 'place' accelerating over that one G.
Will there be a age-difference shown between the twins?
As I understand it there will be.

But how?
If me accelerating at one G is the same as me 'pushed' by Earths invariant mass of one G?
Where does it differ?

Am I mis/stating/understanding the 'equivalence principle ' here?
Explain it as simple as you can, and I will be you eternally grateful:)

-------------------

How does two objects 'pushing/pulling at each other differ from a accelerated object?
In the accelerated object the so called 'gravity well' was placed behind the stern?
In fact it was placed where no invariant matter exist.

Can there be any 'gravity wells' defined to those other two objects?
We say that gravity is a product of spacetime 'adapting' itself to invariant mass, am I right in that?
So in this case?
« Last Edit: 18/12/2008 04:56:53 by yor_on »

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##### Re: How does redshift work?
« Reply #24 on: 18/12/2008 03:00:19 »