# The Naked Scientists Forum

### Author Topic: How much can a lightwave be stretched?  (Read 12387 times)

#### syhprum

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##### How much can a lightwave be stretched?
« Reply #25 on: 27/05/2008 20:28:23 »
Frequency meters for low frequencies work in a somewhat different manner instead of counting the number of zero crossings in a given time period they use the zero crossings to stop and start a local signal generater and measure the number of cycles in this period.
this leads to a much quicker reading but demands that the low frequency signal being measured is very free of noise.
« Last Edit: 30/05/2008 15:27:10 by syhprum »

#### Soul Surfer

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##### How much can a lightwave be stretched?
« Reply #26 on: 28/05/2008 08:58:33 »
You can also measure a single frequency "instantaneously" by using a phase sensitive detector and a short delay line  (less than one cycle at the frequency you wish to measure)  you split the signal into two paths send one directly to the phase sensitive detector and one via the delay line to the phase sensitive detector.  The phase difference is a measure of the frequency.

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #27 on: 28/05/2008 09:10:40 »
You can also measure a single frequency "instantaneously" by using a phase sensitive detector and a short delay line  (less than one cycle at the frequency you wish to measure)  you split the signal into two paths send one directly to the phase sensitive detector and one via the delay line to the phase sensitive detector.  The phase difference is a measure of the frequency.

But if the frequency is once cycle per billion years, you've still got a long time to wait.

#### lyner

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##### How much can a lightwave be stretched?
« Reply #28 on: 28/05/2008 23:55:03 »
S-S and S
What you are both implying is that you know when you have seen just one cycle of a wave what the frequency is. That will only work if you have an 'infinite' signal to noise ratio and if the frequency can be assumed to be constant. You have no idea when a zero crossing  occurs if your signal is buried deep within noise and interference. You have to look at millions of cycles of the signal to establish that. Bandwidth is what counts when discriminating signals.
You have to throw away the 'textbook' pictures of sinewaves and squarewaves when you are considering looking at signals from the fringes of visibility.

The microwave radiation remaining from the Big Bang comes from 'everywhere' so, even when we detect that, we are not not particularly looking just at the limits of visibility.  I am still not sure whether the spectrum of this radiation is due to 'cooling', red shift, or both.

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #29 on: 29/05/2008 12:43:40 »

The microwave radiation remaining from the Big Bang comes from 'everywhere' so, even when we detect that, we are not not particularly looking just at the limits of visibility.  I am still not sure whether the spectrum of this radiation is due to 'cooling', red shift, or both.

Wouldn't the fact that it comes from everywhere imply that it is the result of cooling rather than red-shift?

#### lyner

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##### How much can a lightwave be stretched?
« Reply #30 on: 29/05/2008 14:53:22 »
Yes.
But that may be a bit too obvious.
Radiation from a long way away would have a more recent origin so it would be at a higher frequency (not cooled down so much) but also very red-shifted by  the expansion process. There is a bigger volume at great distances than there is close to us so you might expect more photons to arrive and dominate the received radiation.
It might just turn out that the red shifting mechanism produces just the same effect as the cooling.
This is a good 'un, isn't it?

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #31 on: 29/05/2008 16:59:00 »
There is a bigger volume at great distances than there is close to us so you might expect more photons to arrive and dominate the received radiation.

Bigger volume could also mean that concentration of photons is diluted.

Quote
It might just turn out that the red shifting mechanism produces just the same effect as the cooling.

Wouldn't that be a bit of a coincidence?

Quote
This is a good 'un, isn't it?

I seem to have the knack of asking questions that get people wondering.

#### lyner

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##### How much can a lightwave be stretched?
« Reply #32 on: 29/05/2008 17:31:39 »
Quote
Bigger volume could also mean that concentration of photons is diluted.
uh?
I'm assuming that there are the same number of sources per mcubed, everywhere. You can't do other than that. Yes - it's further away, so each source would spread out (inverse sq) but there are more and more as you go out.
It's the same argument that turns up in the Olber Paradox (resolved by expanding universe etc). Google it.
If there were an infinite Universe no expansion, the sky would be infinitely bright because, between any two stars, you could see another one, further away. Between that star and one of the originals there would be another -and another and another.
As far as the possible coincidence is concerned, the age of the Universe is related to the Hubble constant H0 (which tells you the speed of recession in terms of the distance). In fact it is a simple relationship: age = 1/H0.
So, from that, the amount it has cooled down relates to how long ago the BB was and the red shift relates to H0, which relates to the age. So each hangs on the other. Not such a loony idea, Dr B.

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #33 on: 29/05/2008 22:58:37 »
I was thinking that as the expansion of the universe is driving galaxies apart, there would be the same number of sources spread over a greater volume.

#### lyner

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##### How much can a lightwave be stretched?
« Reply #34 on: 29/05/2008 23:16:05 »
Oh yes, I see what you mean. The aveage density could be getting less (in the simpler model) but, compared with here, it would be the same.

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #35 on: 30/05/2008 07:33:22 »
Quote
The aveage density could be getting less (in the simpler model) but, compared with here, it would be the same.

If less is the same, I wonder if I could get away with paying less for my electricity bill

#### lyner

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##### How much can a lightwave be stretched?
« Reply #36 on: 30/05/2008 11:30:28 »
same density as here, locally!
So, comparatively, there would be no difference. Like your electricity bill is growing at the same rate as your neighbours'.
But, yes, the sources will be getting fainter because of inverse sq law (fewer photons arriving per msquared) and red-shifted. There will still be the same number of them, tho..
« Last Edit: 30/05/2008 11:34:05 by sophiecentaur »

#### LeeE

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##### How much can a lightwave be stretched?
« Reply #37 on: 30/05/2008 14:27:11 »
I am still not sure whether the spectrum of this radiation is due to 'cooling', red shift, or both.

I think it's due to redshift - Wikipedia says "The largest observed redshift, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background radiation; the numerical value of its redshift is about z = 1089 (z = 0 corresponds to present time), and it shows the state of the Universe about 13.7 billion years ago, and 379,000 years after the initial moments of the Big Bang"

I'm not sure how cooling could work - there needs to be somewhere cooler for the energy to go, although if the CBR is spreading into 'new' space the effect must be for there to be less energy per unit of volume - would this equate to cooling?

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #38 on: 30/05/2008 18:41:21 »
I am still not sure whether the spectrum of this radiation is due to 'cooling', red shift, or both.

I'm not sure how cooling could work - there needs to be somewhere cooler for the energy to go, although if the CBR is spreading into 'new' space the effect must be for there to be less energy per unit of volume - would this equate to cooling?

I would think so. You'd be spreading the same total temperature over a greater volume. Surely that would have the effect of cooling it.

#### lyner

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##### How much can a lightwave be stretched?
« Reply #39 on: 31/05/2008 00:45:27 »
If you let a gas expand, whilst it does work, you get a drop in temperature. Likewise, if work is done in the expansion of the universe process (which it is because of the increase in gravitational potential) then you would expect a drop in temperature.

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #40 on: 31/05/2008 09:38:22 »
I feel another thread coming on

#### shmengie

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##### How much can a lightwave be stretched?
« Reply #41 on: 01/06/2008 17:42:11 »
Light is being redshifted by blackholes stretching the ether of the universe.  In the beginning, there were very few blackholes so light was shifted less then.  As time goes by, more blackholes developed and more stretching of light.  Don't know if the ether exists or not, but it's easier to word in than "gravity warped space/time".

This theory goes against the accepted theory that the universe is expanding and seems to be accelerating.

Can't help but wonder if the original poster wanted to know if light reverts back to matter after shifting x number of times.  What speed would the matter be going when that happened?  I suspect that's where cosmic rays (not originating from our sun) come from...
« Last Edit: 01/06/2008 17:44:52 by shmengie »

#### DoctorBeaver

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##### How much can a lightwave be stretched?
« Reply #42 on: 01/06/2008 20:14:27 »
Can't help but wonder if the original poster wanted to know if light reverts back to matter after shifting x number of times.

Speaking as the original poster - no, that's not what I wanted to know.

#### The Naked Scientists Forum

##### How much can a lightwave be stretched?
« Reply #42 on: 01/06/2008 20:14:27 »