Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DoctorBeaver on 23/05/2008 20:53:49
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I was thinking about blue-shifted light at an event horizon when BING! - my mind did a flip & I thought of this litle poser.
Can a lightwave be so red-shifted that it becomes a flat line? If so, what would be the effect?
Would the lightsource have to be moving away at c? Or at infinite velocity? Would a photon have to have zero energy for its wave to be flat? Can a photon with zero energy exist?
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A photon with zero energy cannot exist for several reasons.
Firstly if it has zero energy it cannot have any effect on anything and cannot be detected.
Secondly it cannot fit into our universe which has a finite size because it has an infinite wavelength. like the casimir effect where photons longer than a particular wavelength canot exist in a cavity causig a negative pressure in that cavity against the vacuum energy.
This leads on to an interesting thought that I have had for some time on the assumption that gravitons or gavitomagnetic waves (which are in some ways similar to photons are mostly very low frequency compared with photons ie frequuiencies are measured in cycles per year(earth's orbit down to a few cycles per second for a binary neutron star assuming that the quantum enegy is plancks constant times the frequency the quanta are very low energy and there are lots of them.
The motions of galaxies in clusters represent cycles per billion years or more and there may be a lower limit to the frequency of gravitions in the universe because they just cant fit in. Can this be the source of some form of modified dynamics for very low frequency gravitiational effects like MOND?
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A photon with zero energy cannot exist for several reasons.
Firstly if it has zero energy it cannot have any effect on anything and cannot be detected.
Secondly it cannot fit into our universe which has a finite size because it has an infinite wavelength. like the casimir effect where photons longer than a particular wavelength canot exist in a cavity causig a negative pressure in that cavity against the vacuum energy.
I had a feeling the answer was something like that. But I hadn't considered the finite size of the universe as being a reason zero energy photons cannot exist.
This leads on to an interesting thought that I have had for some time on the assumption that gravitons or gavitomagnetic waves (which are in some ways similar to photons are mostly very low frequency compared with photons ie frequuiencies are measured in cycles per year(earth's orbit down to a few cycles per second for a binary neutron star assuming that the quantum enegy is plancks constant times the frequency the quanta are very low energy and there are lots of them.
The motions of galaxies in clusters represent cycles per billion years or more and there may be a lower limit to the frequency of gravitions in the universe because they just cant fit in. Can this be the source of some form of modified dynamics for very low frequency gravitiational effects like MOND?
I'm not sure how you arrive at this. Are you suggesting that the frequency of the graviton/gravitomagnetic wave determines orbital velocity, or vice versa?
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I was thinking about blue-shifted light at an event horizon when BING! - my mind did a flip & I thought of this litle poser.
Can a lightwave be so red-shifted that it becomes a flat line? If so, what would be the effect?
Would the lightsource have to be moving away at c? Or at infinite velocity? Would a photon have to have zero energy for its wave to be flat? Can a photon with zero energy exist?
This is what happens to light emitted by galaxies that are presumed to exist beyond the limits of the observable universe. Because they are receeding at >= c the frequency of the light drops to zero, equivilent to infinite wavelength, and zero energy. The receeding object needs to be travelling at c or greater but it doesn't have to be receeding at infinite velocity.
It's interesting to note that although we can't see anything beyond the observable universe, as centered upon us, we can see regions within the observable universe where energy originating from beyond the observable universe must be present, so although this energy will be present and interacting within those regions, we shouldn't be able to detect it's presence or it's interactions. Hmm... might just be a timing issue...
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It's interesting to note that although we can't see anything beyond the observable universe, as centered upon us, we can see regions within the observable universe where energy originating from beyond the observable universe must be present, so although this energy will be present and interacting within those regions, we shouldn't be able to detect it's presence or it's interactions. Hmm... might just be a timing issue...
Definitely timing. The information from those interactions hasn't had time to reach us yet.
I find it "interesting" to think that although nothing can travel faster than c, there are probably parts of the universe that, due to expansion, are receding from us >c. And how fast does the recession get, from our frame of reference, when you reach the outer limits of the universe?
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Can a lightwave be so red-shifted that it becomes a flat line?
More precisely, it would become a constant electric and magnetic field.
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Can a lightwave be so red-shifted that it becomes a flat line?
More precisely, it would become a constant electric and magnetic field.
Alberto - can you explain that, please?
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Hmm... I'm not sure if it is just a timing issue. Let's say it's sometime in the future where sub-lightspeed interstellar travel is possible and you've hiked off to another system a few lightyears away. Now, because the center of the observable universe is located at the observer, you can see light from a source located a few lightyears outside the boundary of the observable universe when seen from Earth.
Once you've got there and unpacked, you radio a message back to us about a supernova that has just been observed, just within the limit of your observable universe, but just outside ours - "nice bang fellas - pity you'll never see it. I'll send you a picture". Your radio message gets to us but the light from the supernova doesn't because it's too red-shifted for us to see?
In this case, we're seeing at least one result of the interaction between the supernova, which should be unknown to us, and the remote region of space - you sending us the message.
If you want to do away with the human element, we could just substitute a remote probe to relay the information. The radiation from the supernova interacts with a sensor on the probe and is detected, triggering the appropriate reponse routines in the probe's C&C subsytems, resulting in the data being sent to us.
Either way though, we've ended up knowing about something outside our observable universe. Time doesn't seem to be a factor.
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Lee E
Your scenario is interesting but I think the apparent paradox can be resolved in terms of Energy.
One effect of the Universe's expansion is to reduce the energy of the photons as they are traveling through the expanding space.
The photons from the distant event have, effectively, lost nearly all of their energy (the frequency has got to near DC) by the time they reach the remote observation post. Once the frequency has reached zero, no information about the original event can be carried because there is zero bandwidth in which to transmit it. The remote observer gets photons with enough energy to detect the event and then produces some new photons for communicating with 'base' by introducing more energy and regenerating the information.
The expression 'observable universe' should really only be used for 'direct' observation. There isn't a fundamental reason why information shouldn't get to us from beyond this limit as long as some energy can be supplied on the way.
btw, I think the apparent duration of the supernova would be stretched as well as the wavelength of the em waves received at the remote observer. It would take longer for the observer to actually register the occurrence than if it took place nearby.
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LeeE - let me make sure I've got this right. The light from the supernova has enough energy to reach the observer, but not enough to reach Earth. Correct?
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That's 'one way of looking at it'. The fact that the wavelength has been stretched can be interpreted as if the photons have less energy - this tends to zero as you get far enough away and the frequency approaches zero.
The mechanism of Relativistic Doppler effect means that the effect is asymptotic so there is no actual distance which corresponds to a 'limit'; it just gets harder and harder to detect the energy / information.
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Like an inverse square law where the value can approach, but never reach, zero?
So with a mega-powerful telescope we would still be able to detect the supernova?
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It depends on what you mean by 'mega-powerful'. A better word would be 'sensitive' and, possibly, 'directive' too.
As the wavelength stretches you need a receiver tuned to a lower and lower frequency (it's not light anymore!) and, for the same resolving power / gain you need a bigger and bigger aperture. With a reflector the size of the solar system, presumably, you could 'look' a bit further and then, with one the size of a galaxy, presumably, with one the size of a Galaxy, you could look further still. There is also the problem that, as well as the photon energy getting less and less, the original signal will spread out and the inverse square law would also apply (or, at least, some equivalent version of it which would be affected by the actual geometry of Space on that sort of scale).
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Sophie - that's an interesting comment re adding energy along the way, at the remote observer's location.
As I understand it, the light isn't red-shifted by the expansion of the universe, which increases the distance the light has to travel, but by the speed of recession from us of the emitting body at the time of emission. If the light was red-shifted due to the wavelength being streched by expansion during it's journey it would mean that everything else in the universe would also be stretched - everything would be getting bigger and the universe would not appear to be expanding at all.
Everything beyond the observable universe is receeding from us at >= c, so any light emitted from anything beyond that limit starts with zero frequency at the time of emission and has zero energy, from our point of view. It can't ever reach us directly because, from our point of view, it never existed.
Even with the most sensitive instruments, we'd still just see zero frequency.
The result seems to be that we should be able to see things in remote regions of space that appear to have no directly observable cause.
But then thinking about it again, if we imagine that this supernova was taking place now, it would take nearly 13 billion years for the light to get to the remote location, and by the end of that time we could be receeding from the remote probe at >= c, so perhaps we wouldn't be aware of it and perhaps it _is_ a timing issue after all:)
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If everything expanded by 0.001%, the space between two very separated objects would expand by a huge distance but the objects would not expand significantly. So that particular objection doesn't apply to the expanding Universe idea. Remember, it's the speed of recession which increases with distance - not the distance increasing with distance.
The concept of a distance increasing at >c is not meaningful.
It would be regarded as nonsense to talk of two objects moving apart at 1.1c in the Lab so how can it make sense for two well separated objects?
Just because we are discussing something going on at a great distance we should still be talking in terms of the limits imposed by Relativity. The 'limit of visibility' doesn't have to be a 'hard edge' it just needs to be where the recession speed is near enough to c for the effects of relativity to become significant.
The idea of the Hubble recession applying at all distances, without Relativity coming into it, can't be really sensible.
As far as receiving photons from great distances goes, my point is that you would never be presented with a signal with 'zero frequency'. You would see a signal with a frequency approaching zero frequency from a very distant object. As with all ideas involving 'infinity', it is much more fruitful to discuss the limits as distance (or whatever other variable you are discussing approaches infinity. Anything else can lead you into serious problems and non existent solutions.
I know there is a sort of comfort in the idea of limits but there don't in fact, have to be limits of the kind we associate with familiar objects when we are dealing with the Universe.
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More precisely, it would become a constant electric and magnetic field.
Alberto - can you explain that, please?
Let's consider electric field only, for simplicity, and in a specific instant of time.
If the wavelenght is, let's say, one metre, it means that in one metre the electric field varies from zero at the initial point, to its maximum positive, e.g., 1 (25 cm away the initial point) to zero again (50 cm away) then to -1 (75 cm away) then to zero again at the final point 1 metre away.
If however we limit our analysis to what happens between two points separated by 1 mm only, you understand that the electric field doesn't vary much between them: if they are near E = 0, then the field will vary let's say from 0 to 0.01, if they are near 0.8 the will vary, e.g., from 0.8 to 0.78 ecc. The same if you take a wavelenght of 1 km and you consider two points 1 metre apart, or a wavelenght of 1000 km and two points 1 km apart ecc. If you imagine an almost infinite wavelenght, you understand that you will need billions of light years of distance to find a slight variation in E field.
If we instead stay in the same point and we want to analyse variations in time, we can have that previous variation letting light arrive: it travels at c so you have to wait billions of years to have that slight variation; in other terms, we can compute the variation in time, fixed the point in space, simply by dividing that space separation by light's speed c.
In formulas:
ν = c/λ
ν = frequency; λ = wavelenght.
If λ goes to infinite, frequency goes to zero; but frequency is the number of cycles per second in a specific point. If this number is almost zero, it means the field is almost non varying; when ν = 0 the field is constant.
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Thank you, Alberto. Nicely explained.
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You would see a signal with a frequency approaching zero frequency from a very distant object.
Sophie - taking into account what Alberto has just explained, wouldn't it take an inordinate length of time to measure a frequency that is approaching zero?
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Absolutely. Measuring a change in level of a signal of frequency f would involve observing a fair number of cycles. A cycle takes i/f seconds so, if 0.01Hz is 'slow enough for you' and you needed to look at10 cycles, to decide if it was there or not, you'd need 1000 seconds. To avoid the effects of interference, you'd actually be looking at thousands of cycles, probably.
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There is another point involving red shift observations.
The amount of red shift which has been measured actually only represents recession velocities which are far from c. I am not aware that anyone has identified optical spectral ines being shifted down into the microwave region. This implies that we are looking far from the limit, at the moment.
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The cosmic microwave background radiation is the result of a great flash of light as the universe became transparent and a lot of the atoms deionised being stretched into the microwave recion of the spectrum.
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Oops! - got myself confused there. I was thinking that a red-shift of 1 equalled recession at c, for some reason.
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I've seen something similar explained by measuring drips from a tap (faucet). If you measure for 10 seconds and count 10 drips, you can have an error of 10%. If you measure for 100 seconds & count 100 drips, you can have an error of 1% etc. (I think that's how it works)
So, if the tap drips only once every billion years, you'd be hard pressed to get any kind of accuracy. The same must apply to measuring wavelengths.
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Yes; the noise in the system is the main problem. For a tiny signal such as you'd get from even a massive event, the noise can only be eliminated by observing for an inordinately long time; millions of your water drops! That is effectively using a vanishingly narrow bandwidth for your measurement.
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So what we're saying is that any photon-emitting event that occurs anywhere in the universe will, theoretically, eventually be detectable anywhere that falls within its lightcone given enough time and sensitive enough equipment.
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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.
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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.
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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.
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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.
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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?
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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?
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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.
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?
This is a good 'un, isn't it?
Are you glad I asked the question? [:D]
I seem to have the knack of asking questions that get people wondering. [;D]
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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.
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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.
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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.
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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 [:D]
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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..
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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?
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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.
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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.
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I feel another thread coming on [:P]
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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...
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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.