Late night thought...

Late night thought...

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krarkol

Original Poster:

109 posts

109 months

Thursday 28th July 2016
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So I'm up at this time, rediscovering my childhood love of our lovely universe and how fascinating it is

Then I had a thought. If I was on a planet within the Andromeda galaxy, observing Earth, I'd be observing events from around 2.5 million years ago. This is common knowledge.

But, the Milky Way and the Andromeda galaxy are closing in on each other, so the distance the light has to travel is shorter.

Does this mean, in theory, whatever I saw happening on Earth, my view would actually be in fast forward to some degree?

RobDickinson

31,343 posts

253 months

Thursday 28th July 2016
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krarkol said:
Does this mean, in theory, whatever I saw happening on Earth, my view would actually be in fast forward to some degree?
Yes. but..

How fast do you think you could travel?

The exact same thing happens right here.

Look at something 100 meters away, run towards it, its now taking less time for photons bouncing off it to get to your eyes. (interesting side effect of all this is you only ever see history..)

Andromeda is 2.5mil light years away, so is seeing light form us of that amount of time ago because it took 2.5million years to travel.

Consider if you had a space ship capable of light speed it would take you 2.5million years to get here from there. So you would arrive now having taken 2.5million years seeing the whole history of earth, over a 'normal' amount of time **, so no fast forward. For that you would have to travel faster than the speed of light...

  • ignoring time dilation issues with going the speed of light - you would probably only experience a couple of days whilst 2.5 million years passes

Dr Jekyll

23,820 posts

260 months

Thursday 28th July 2016
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That's why when something making a noise comes towards you it sounds higher pitched than when going away from you. You are hearing in fast forward then slow forward.

Simpo Two

85,147 posts

264 months

Thursday 28th July 2016
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I think doppler and relativity are getting confused.

I thought the same of colours. If you took a red snooker ball, and put it some distance away, and then accelerated it towards you very fast, it should appear blue...

Eric Mc

121,773 posts

264 months

Thursday 28th July 2016
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What happens to the light at the ultra violet (and normally invisible) end of the spectrum? Would that not shift into the lower frequency bands and become visible and therefore the colours overall, to us, would remain unchanged.

I always thought that the spectral shift was not actually seen as a change in colour but more a moving of the spectral absorbtion lines into areas of the spectrum where they wouldn't normally be seen.

For example, the absorbtion line for sodium normally appears in the yellow band. In a "blue shifted" spectrum, the absorbtion line will have moved out of its normal position and towards the blue end of the spectrum.

Simpo Two

85,147 posts

264 months

Thursday 28th July 2016
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It's just the electromagnetic spectrum - starting with the longest wavelength first, roughly: radio - IR - red - blue - UV - microwaves - gamma radiation. Only a very small part in the middle is 'visible light'.

Eric Mc

121,773 posts

264 months

Thursday 28th July 2016
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That's exactly what I mean. Therefore, the invisible elements of the spectrum move into the visible bands. Which
invisible elements (i.e. ultra violet or infra red) move into the visible section depends on whether the object is approaching or receding from the viewer.

Therefore, the COLOUR of the object does not actually change (i.e. it doesn't take on a blue or a red tint). The colour of the light stays effectively the same. The "red shift" effect is not a change of colour - it's a change of position of the Fraunhofer Lines.

Nom de ploom

4,890 posts

173 months

Thursday 28th July 2016
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I never understood the train platform 60% of light speed analogy too.

i've read it loads of times and i still struggle with the concept.

I guess if you were wanting to travel to Andromeda at the speed of light and get there in time to turn round and look at earth as it was wouldn't you have to travel faster than light and add on the speed in which andromeda is moving away from Earth?

the galaxies are moving away from each other but the speed of light is constant so therefore the time it takes light to get from earth to andromeda is longer even though the distance light covers in a year is the same????

or have I had a sugar rush???

krarkol

Original Poster:

109 posts

109 months

Thursday 28th July 2016
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My thoughts where based on me already being on a planet in the Andromeda galaxy, observing Earth.

The Milky Way and Andromeda are currently on a collision course with each other, therefore, both will be travelling very very fast I'd assume, but not quite speed of light fast.

I don't think the "running towards something" analogy works, as that is very slow, so there'd be a tiny difference in time passed but you'd barely notice it.

I was wondering if the speed of the galaxies closing speed would mean that you'd be viewing whatever happened on Earth 2.5 million years ago, but say 10% quicker than it actually happened.

To me it makes sense, as light going between 2 fixed points would have a constant rate, but if you are moving towards the source at great speed, wouldn't you be receiving the light earlier, and thus seeing it in fast forward?

Think of a boat going up a river. The faster it goes, the more water will pass it by, so the river will look like it's passing by faster.

Simpo Two

85,147 posts

264 months

Thursday 28th July 2016
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Eric Mc said:
That's exactly what I mean. Therefore, the invisible elements of the spectrum move into the visible bands. Which invisible elements (i.e. ultra violet or infra red) move into the visible section depends on whether the object is approaching or receding from the viewer.

Therefore, the COLOUR of the object does not actually change (i.e. it doesn't take on a blue or a red tint). The colour of the light stays effectively the same. The "red shift" effect is not a change of colour - it's a change of position of the Fraunhofer Lines.
Radiation outside the visible spectrum doesn't have a colour (as we know it - colour is after all only a function of our eyes and brain). But if wavelength changes with speed for EMS as it does for sound, then if you start with an object in the middle of the visible spectrum, say yellow, then it *should* appear more orange if going away and more green if approaching.

I have tried to test it with snooker balls but keep injuring myself and breaking ornaments.

Nom de ploom

4,890 posts

173 months

Thursday 28th July 2016
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good analogy.

and I agree but as the galaxies got closer and closer light itself wouldn't be "squashed" as at still arrives at the same speed you just see it sooner than you would if the two planets were in the same position relative to each other?

if you could see things faster than they happened because your were heading towards it rather than going away from it wouldn't it appear slower not faster? What about time?

Eric Mc

121,773 posts

264 months

Thursday 28th July 2016
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Simpo Two said:
Radiation outside the visible spectrum doesn't have a colour (as we know it - colour is after all only a function of our eyes and brain). But if wavelength changes with speed for EMS as it does for sound, then if you start with an object in the middle of the visible spectrum, say yellow, then it *should* appear more orange if going away and more green if approaching.

I have tried to test it with snooker balls but keep injuring myself and breaking ornaments.
What I am suggesting is that the "colourless" invisible wavelengths are shifted INTO the visible spectrum because of the motion of the object. Once this invisible radiation has shifted into the visible spectrum it will, of course, become visible and will have a colour. For instance, Infra Red (invsible) radiation which shifts into the lower end of the visible spectrum become visible red - which last time I looked, was a colour.


krarkol

Original Poster:

109 posts

109 months

Thursday 28th July 2016
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It's more confusing the more I think about it because I suppose it depends on how we process the light?

For example,
You are on a conveyor belt and along the way there a photos from a film hanging up. Each photo displays just 1 frame. So if you speed up the belt, when you get to passing around 24 photos a second, you should be seeing the film in real time, as intended. But if you go even faster, you'll see it sped up. E.g. Fast forwarding a video tape

But...
Usually if you increase the frames per second, time actually slows down as you are collecting more information within a shorter space of time. E.g. slow motion cameras

zollburgers

1,278 posts

182 months

Thursday 28th July 2016
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krarkol said:
For example,
You are on a conveyor belt....
Probably better to stay clear of that example.

Eric Mc

121,773 posts

264 months

Thursday 28th July 2016
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ash73 said:
The overall EM signature would be the same but shifted; the perceived colour would therefore change.
Why?

I can only see stuff sliding in one end of the visible spectrum at the lower energy end and the top end (high energy) sliding out - leaving the overall visible spectrum unchanged.

XM5ER

5,087 posts

247 months

Thursday 28th July 2016
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I believe that the spectrum would be blue shifted (as the wave lengths are compressed by the velocity of the emitters relative to the receivers) but also that the movement that is detectable would appear ever so slightly faster (because time taken between frames to reach your eye, to use the analogy above, is less reduced due to the shorter distance of travel each time).

Einion Yrth

19,575 posts

243 months

Thursday 28th July 2016
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Eric Mc said:
Why?

I can only see stuff sliding in one end of the visible spectrum at the lower energy end and the top end (high energy) sliding out - leaving the overall visible spectrum unchanged.
If infra red is shifted to become red, then red is shifted to become orange, thus that red thing you were looking at appears to change colour to be a bit more orangey.

Eric Mc

121,773 posts

264 months

Thursday 28th July 2016
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And the orange becomes yellow - and so on up (or down) the visible spectrum, depending on whether the object is approaching or receding - so overall the colour balance doesn't change

The colours that have shifted up or down the visible spectrum have been replaced by the colour above or below them - and those at the furthest end of the visible spectrum that may drop out are replaced by the near visible electromagnetic frequencies which move into the visible range and can be seen - therefore restoring the full spectrum.

The only circumstances where the colour MIGHT change would be where the object is moving so fast that ALL the frequencies from the lowest to the highest have been "used up" as they travelled through the visible spectrum and there were no more frequencies left to replace them at either end of the spectrum.. For that to happen, the object would have to be moving toward or away from the viewer at a substantial portion of the speed of light.

Einion Yrth

19,575 posts

243 months

Thursday 28th July 2016
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My point was that any specific thing that you had had your eye on would have changed colour, not that the entire scene would have become bluer.

Simpo Two

85,147 posts

264 months

Thursday 28th July 2016
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Eric Mc said:
What I am suggesting is that the "colourless" invisible wavelengths are shifted INTO the visible spectrum because of the motion of the object. Once this invisible radiation has shifted into the visible spectrum it will, of course, become visible and will have a colour. For instance, Infra Red (invsible) radiation which shifts into the lower end of the visible spectrum become visible red - which last time I looked, was a colour.
Agreed. You just happen to picking an example of a change that involves moving from 'invisible' into visible light.

Eric Mc said:
And the orange becomes yellow - and so on up (or down) the visible spectrum, depending on whether the object is approaching or receding - so overall the colour balance doesn't change

The colours that have shifted up or down the visible spectrum have been replaced by the colour above or below them - and those at the furthest end of the visible spectrum that may drop out are replaced by the near visible electromagnetic frequencies which move into the visible range and can be seen - therefore restoring the full spectrum.
If I may switch from EMS to audio to make it simpler, you're saying that when the ambulance goes past you with its horns blaring they don't drop in tone...?