Why might light from distant galaxies not reach us?

In summary, Dave believes that the red shift is caused by the energy being absorbed by the intervening gas and dust.
  • #1
Endervhar
142
0
When I first met the idea that light from an object that was moving away from us at superluminal speed would never reach us, it seemed quite reasonable. However, I seem to have developed a problem arising from the fact that light must always be observed as traveling at c, irrespective of any relative movement between emitter and receiver. It goes something like this.

A distant object is moving away, relative to Earth, at superluminal speed. At a given point (say 15 billion l y away, which we will call “point A”) it emits a photon. This photon should reach Earth 15 billion years later.

Assume the emitting object is receding (relative to Earth) at 1.5c. One year later it is at point B, which is 16.5 l y from Earth. There it emits another photon, which should reach Earth after 16.5 years; ie, 1.5 years after the arrival of the photon emitted at point A.

Whatever the speed of recession of the emitting object, any light emitted must, from the viewpoint of Earth, be approaching Earth at c, so it must eventually arrive.

I accept that there must be something wrong with this reasoning, but at the moment I can’t see what it is.
 
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  • #2
There's a theory referred to as 'tired photons' I believe. As the space between us and the distant galaxy gets stretched more and more the photons that reach us have less and less energy (red shifted). Near the limit, the visible light has turned into microwaves, then LF radio waves and, finally, DC. Even with only a vestigial frequency <1Hz, the photons are still getting here at c but their energy is getting less and less 'till they can't be detected.
 
  • #3
Thanks sophiecentaur. That helps. I have been doing some more thinking, and I suspect that I have failed to take account of the fact that distant galaxies are moving away, not because they are moving through space, but because the intervening space is increasing. Having said that, we would still observe EM radiation as approaching Earth at c. This must be where your point about the increasing red shift enters the picture.
 
  • #4
Which raises the question "Where does all that wasted energy go?"
Yawn - where did all mine go too?
 
  • #5
Yawn - where did all mine go too?

Probably into answering naive questions.
Now you are making me think, again. Dangerous at my age, but I am working on the assumption that it could keep dementia at bay. :smile:
 
  • #6
Whatever the speed of recession of the emitting object, any light emitted must, from the viewpoint of Earth, be approaching Earth at c, so it must eventually arrive.

I accept that there must be something wrong with this reasoning, but at the moment I can’t see what it is.

There is nothing wrong with your reasoning. A photon always moves at c so it will eventually arrive here. Like sophie said it will be redshifted, possibly so far that it moves down into infrared, microwave, or radio wave range.

As to the why, I like to think of it like this:
Imagine you are moving away from me at 0.5c. I flash a green laser beam at you. In my frame the light is emitted with energy that makes its wavelength/frequency equal green light, obviously. However, when the light reaches you it MUST still travel at c and has to "borrow" energy from itself to make up for the fact that you were moving away from its source. This makes it redshift down to let's say orange light instead of green.

For the opposite, moving towards me, the light has to "borrow" energy from its velocity in my frame and add it to the frequency to make itself move at c in your frame, hence Blueshifting it.

Thats merely my personal way of thinking about the concept without getting confused and has no real scientific view, so don't flame me posting it!
 
  • #7
Thanks Drakkith. As a non-scientist, I find explanations such as yours very helpful.
 
  • #8
sophiecentaur said:
Which raises the question "Where does all that wasted energy go?"
1] Much energy is actually absorbed by intervening gas and dust.
2] A fixed amount of energy saturating an increasing volume will be diluted as the volume expands.
 
  • #9
Endervhar said:
Probably into answering naive questions.
Now you are making me think, again. Dangerous at my age, but I am working on the assumption that it could keep dementia at bay. :smile:

thats the best reason I have heard yet ;)

haha



Dave
 
  • #10
Hi Davec... Sorry for intervening but i don't understand your post:
If dust and whatever absorb the energy of photon that is coming to us ---isn't it the only cause of red shift? Because to loose energy that mean to diminish the frequency. This implies that moviment of galaxis is false? This meaning that CMB is not "after glow" and some other wonders. Sorry if i am wrong about your poast.
 
  • #11
sophiecentaur said:
There's a theory referred to as 'tired photons' I believe. As the space between us and the distant galaxy gets stretched more and more the photons that reach us have less and less energy (red shifted). Near the limit, the visible light has turned into microwaves, then LF radio waves and, finally, DC. Even with only a vestigial frequency <1Hz, the photons are still getting here at c but their energy is getting less and less 'till they can't be detected.

So does this mean that light will always reach us, and that light always moves at the same speed, but it just gets down graded in frequency from the "visible spectrum" to one undetectable by known sensors?

And to pony off that question, since it has been one I have always wondered about, can a photon with no frequency still be a photon?
 
  • #12
mquirce said:
Hi Davec... Sorry for intervening but i don't understand your post:
If dust and whatever absorb the energy of photon that is coming to us ---isn't it the only cause of red shift? Because to loose energy that mean to diminish the frequency. This implies that moviment of galaxis is false? This meaning that CMB is not "after glow" and some other wonders. Sorry if i am wrong about your poast.

I was not addressing red shift at all; I was addressing the original question of "Why might light from distant galaxies not reach us?" coupled with "Where does the energy go?"
 
  • #13
mAMBOkING said:
So does this mean that light will always reach us,
Not all light will reach us. Most of the universe is forever beyond our ability to see.

As the universe ages, more and more of the universe will slip outside our observable bubble. It is speculated that billions of years from now, observers looking up (from the tropical paradise of their homeworld Pluto) will see only our Milky Way Galaxy and its close neighbours floating in an otherwise empty universe, and will have no way of understanding how the universe came to be. Cosmology, like archeaology, is a science doomed to attrition.

mAMBOkING said:
and that light always moves at the same speed, but it just gets down graded in frequency from the "visible spectrum" to one undetectable by known sensors?

And to pony off that question, since it has been one I have always wondered about, can a photon with no frequency still be a photon?
'Very low' is not the same as 'zero'. To have a zero frequency would require an infinitely long wavelength, which won't happen in a finite universe.
 
  • #14
Endervhar said:
When I first met the idea that light from an object that was moving away from us at superluminal speed would never reach us, it seemed quite reasonable. However, I seem to have developed a problem arising from the fact that light must always be observed as traveling at c, irrespective of any relative movement between emitter and receiver. It goes something like this.

A distant object is moving away, relative to Earth, at superluminal speed. At a given point (say 15 billion l y away, which we will call “point A”) it emits a photon. This photon should reach Earth 15 billion years later.

Assume the emitting object is receding (relative to Earth) at 1.5c. One year later it is at point B, which is 16.5 l y from Earth. There it emits another photon, which should reach Earth after 16.5 years; ie, 1.5 years after the arrival of the photon emitted at point A.

Whatever the speed of recession of the emitting object, any light emitted must, from the viewpoint of Earth, be approaching Earth at c, so it must eventually arrive.

I accept that there must be something wrong with this reasoning, but at the moment I can’t see what it is.

there are a few things i noticed right away that you might attribute to your uncertainty in your reasoning:

1) firstly, superluminal motion is a phenomenon caused by the observation of matter traveling almost directly TOWARD the observer at relativistic speeds, not AWAY from the observer. due to the nature of the propagation of light, matter moving more or less directly toward us at relativistic speeds will appear to be moving toward us faster than it truly is (sometimes to the point where it appears to exceed the speed of light - this is the phenomenon of superluminal motion). likewise, matter moving more or less away from us at relativistic speeds will appear to be moving away from us slower than it truly is, again, due to the nature of the propagation of light.

2) nothing in our universe actually travels at superluminal speeds with respect to its observer. if the speed of light, c, is observed to be the same by any and every observer regardless of the position and motion of the source, and if nothing can travel faster than c, then it stands to reason that superluminal motion is an optical illusion, which it is.

3) if there are objects so far away from us in our universe that they would seem to be moving away from us at superluminal speeds (these objects would have lie outside our visible universe of ~13.7 billion light years in all directions...in other words, they would have to be more than 13.7 billion light years distant), it is not b/c they are actually moving away from us faster than the speed of light - it is b/c the expansion of the universe is contributing greatly to the illusion of superluminal motion as these objects recede from us at relativistic (but not superluminal) speeds.


DaveC426913 said:
'Very low' is not the same as 'zero'. To have a zero frequency would require an infinitely long wavelength, which won't happen in a finite universe.

i agree with your conclusion that a photon cannot have an infinitely long wavelength (and thus zero energy). but while the visible universe (the par of the universe that we can actually observe) is most definitely finite due to the finite speed of light, we cannot yet say that the whole universe is finite...or infinite for that matter. perhaps a better reason would simply be the fact that a photon is a quantum of light, and as such, is a manifestation of energy. if it has a specific energy, then it must have a specific wavelength, even if that energy is almost zero and that wavelength is almost (but not quite) infinitely long.
 
  • #15
94JZA80 said:
1) firstly, superluminal motion is a phenomenon caused by the observation of matter traveling almost directly TOWARD the observer at relativistic speeds, not AWAY from the observer.
3) if there are objects so far away from us in our universe that they would seem to be moving away from us at superluminal speeds

In (1) you say the phenomenon of superluminal motion is restricted to objects moving toward the observer, but in (3) you acknowledge that there may be objects that seem to be moving away at superluminal speeds.

Is it me, or is there a contradiction here?
 
  • #16
3) if there are objects so far away from us in our universe that they would seem to be moving away from us at superluminal speeds (these objects would have lie outside our visible universe of ~13.7 billion light years in all directions...in other words, they would have to be more than 13.7 billion light years distant), it is not b/c they are actually moving away from us faster than the speed of light - it is b/c the expansion of the universe is contributing greatly to the illusion of superluminal motion as these objects recede from us at relativistic (but not superluminal) speeds.

The expansion of the universe IS causing them to move away from us faster than the speed of light. It is not merely an optical illusion, but a consequence of an expanding universe.
 
  • #17
almost (but not quite) infinitely long.

How can something be "almost" infinite? Surely, if it is finite, however large it becomes, it is still infinitely far away from being infinite.
 
  • #18
Endervhar said:
How can something be "almost" infinite? Surely, if it is finite, however large it becomes, it is still infinitely far away from being infinite.

True, but I think we are starting to split hairs here with this.
 
  • #19
True, but I think we are starting to split hairs here with this.

Sorry, hair splitting was not my intention, I just have this "thing" about infinity, and the ways in which the term is used. My son says I am an "infinity Nazi". :devil:
 
  • #20
Endervhar said:
Sorry, hair splitting was not my intention, I just have this "thing" about infinity, and the ways in which the term is used. My son says I am an "infinity Nazi". :devil:

Lol...
 
  • #21
Endervhar said:
How can something be "almost" infinite? Surely, if it is finite, however large it becomes, it is still infinitely far away from being infinite.
The point here is that it needs to be effectively infinite - long enough that the edges don't affect the area of interest.

Beyond that, it doesn't matter whether it's finite or infinite.
 
  • #22
The point here is that it needs to be effectively infinite

This just adds to the various ways in which the term "infinity" is used, which adds to the confusion of thought.
 
  • #23
Endervhar said:
This just adds to the various ways in which the term "infinity" is used, which adds to the confusion of thought.

I disagree. Almost infinite is a perfecly valid concept even if it may not be mathematically rigorous. This is the land of words, not the land of numbers.
 
  • #24
Dave, is it not difficult to maintain scientific veracity without mathematical rigour?
 
  • #25
Wow, look at all these split hairs on the floor...:rofl:
 
  • #26
Endervhar said:
Dave, is it not difficult to maintain scientific veracity without mathematical rigour?

It's not difficult at all. This is the real world; physics is not paralyzed just because mathematical definitions don't fit well.

Remember the context here:

For a photon to have zero frequency, it would need to have a wavelength infinitely long. But a photon whose wavelength is longer than the diameter of the universe is effectively infinite. To-wit: being 100 billion ly long and being infinitely long makes no change in it the property of its frequency; it's still undefined.

So: a wavelength greater than the diameter of the universe is "as close to being infinity as is meaningful".
 
  • #27
DaveC426913 said:
This is the real world; physics is not paralyzed just because mathematical definitions don't fit well.

I absolutely agree! I still thing there is considerable scope for confusion arising from the variety of ways in which "infinite" is used. Thanks to Cantor, we have an infinity of mathematical infinities, then, as you point out, there are effective infinities, and that's before you get to the infinity (oops!) of cases in which "boundless" would be a better term. I am not suggesting that we should hog-tie ourselves with sterile terminology or mathematical pedantry, only that we should make an effort to ensure that there is a reasonable degree of unity in our usage.
 
  • #28
Davec... Now the layman is totaly out of track. Can be the frequency les than 1 (One) ?!.
May be i need to shut up! Sorry for intrusion.
 
  • #29
mquirce, Surely there is no such thing as "intrusion" with a legitimate question. Don't apologise when you have as much right in a thread as anyone else.
 
  • #30
Drakkith said:
The expansion of the universe IS causing them to move away from us faster than the speed of light. It is not merely an optical illusion, but a consequence of an expanding universe.

That's essentially right. I wouldn't go so far as to say that the distant galaxies MOVE away from us faster than c. There are problems with defining distance and motion at cosmic scale so to avoid confusion you have to say what DEFINITION of distance. In this connection (Hubble Law expansion) the commonly used definition is what cosmologists call "proper distance" which basically means imagine you freeze the expansion process at a given moment (to give yourself time to measure) and then use any ordinary means like radar or a giant tape measure. Proper distance is the distance at a specified moment.

The distances to most of the galaxies which we observe today are increasing faster than c, and were already doing that back when they emitted the light we are now getting from them. The "charley" link in my signature explains how the light gets here despite this.

It is a bit confusing to think of that as MOTION because they aren't going anywhere. The distances to them are simply increasing. Ordinary idea of motion is you are traveling towards some destination. But they aren't getting closer to anything. Aside from some petty random local motions what we witness is just pure distance-expansion without any motion towards.

Superluminal expansion of distance is certainly real. In fact it is typical---most of the galaxies we can see today are participating. But this is not the kind of motion relative to local surroundings that would conflict with the Special Rel "speed limit".
Special Rel is a geometry that only applies approximately in local patches over small distances like within one cluster of galaxies. It is the nonexpanding local approximation to General Rel. So its "speed limit" is meaningless at cosmic scales and can be ignored at those scales.

DaveC made a good point, that is based on the discovery that the expansion process is gradually intensifying---the slope of the "size-curve" is getting steeper with time. That leads into a whole other discussion.
DaveC426913 said:
Not all light will reach us. Most of the universe is forever beyond our ability to see.

As the universe ages, more and more of the universe will slip outside our observable bubble. It is speculated that billions of years from now, observers looking up (from the tropical paradise of their homeworld Pluto) will see only our Milky Way Galaxy and its close neighbours floating in an otherwise empty universe, and will have no way of understanding how the universe came to be. Cosmology, like archeaology, is a science doomed to attrition.
...

Much of what 94J says here is misleading. "Superluminal motion" refers to an illusion we get with jets of luminous matter aimed in our direction. An interesting but comparatively rare optical illusion. This is a different topic from the superluminal expansion of distance we typically encounter over cosmic scales (more exactly beyond redshift 1.7)
94JZA80 said:
...
1) firstly, superluminal motion is a phenomenon caused by the observation of matter traveling almost directly TOWARD the observer at relativistic speeds, not AWAY from the observer. due to the nature of the propagation of light, matter moving more or less directly toward us at relativistic speeds will appear to be moving toward us faster than it truly is (sometimes to the point where it appears to exceed the speed of light - this is the phenomenon of superluminal motion). likewise, matter moving more or less away from us at relativistic speeds will appear to be moving away from us slower than it truly is, again, due to the nature of the propagation of light.

2) nothing in our universe actually travels at superluminal speeds with respect to its observer. if the speed of light, c, is observed to be the same by any and every observer regardless of the position and motion of the source, and if nothing can travel faster than c, then it stands to reason that superluminal motion is an optical illusion, which it is.

3) if there are objects so far away from us in our universe that they would seem to be moving away from us at superluminal speeds (these objects would have lie outside our visible universe of ~13.7 billion light years in all directions...in other words, they would have to be more than 13.7 billion light years distant), it is not b/c they are actually moving away from us faster than the speed of light - it is b/c the expansion of the universe is contributing greatly to the illusion of superluminal motion as these objects recede from us at relativistic (but not superluminal) speeds.
...

There is a lot of confusion in this post, no time to touch on all the points. Properly speaking, our currently visible universe extends way far beyond 13.7 billion lightyears.

Most of the galaxies which we observe with telescope, if you could stop expansion to give yourself time to measure, are at this moment much farther than 13.7.
And, as said earlier, the distances to them are typically increasing faster than c.
 
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  • #31
Endervhar said:
I absolutely agree! I still thing there is considerable scope for confusion arising from the variety of ways in which "infinite" is used.
Except that the word infinity is not being misused.

Think of this analogy:
That purse's colour is Tuesdayish red.

The word red is not being misused - it is most definitely an appopriate word, considering the context here is colour. However, the modifier creates a new concept, that of 'Tuesdayish red' which, I'll grant, might cause the raising of some eyebrows.


Regardless, if you are still uncomfortable with the concept of 'almost infinity' then mentally substitute the word 'effectively' to make the perfectly reasonable phrase 'effectively infinity'.

mquirce said:
Davec... Now the layman is totaly out of track. Can be the frequency les than 1 (One) ?!.
Precisely, which is why I said undefined.
 
  • #32
It occurred to me that this thread was in need of some expert attention, and so I am glad to see that marcus stepped in. I don't claim to be an expert, but I am able to comment on a couple of the ideas that were raised in this thread:

1. The "Tired Light" Hypothesis

This was proposed as an alternative explanation for the observed redshifts of distant objects (i.e. instead of cosmological expansion). It was not really meant to be something that went along with expansion as sophiecentaur suggested in post #2. In any case, to answer mquirce's question in post #10, the tired light hypothesis has been thoroughly ruled out. There are a couple of fairly obvious reasons why it can't work, including:

i) Scattering at this level would require that distant objects be blurred, with the blurring getting worse the farther out you looked. We don't see this.

ii) The scattering and/or absorption would have to be frequency dependent. We don't observe this. The amount of redshift is independent of the original wavelength (in the rest frame of the emitter). The entire spectrum of the emitter gets shifted by the same amount

iii) More generally, the tired light hypothesis just doesn't explain a broad range of observations as well as big bang cosmology does (e.g. the CMB, primoridal abundances of light elements, large scale structure, Type Ia supernova data etc. More details here: http://en.wikipedia.org/wiki/Tired_light#Criticisms).

That's not to say that there isn't any absorption or scattering of photons along the line of sight at all. It just isn't the dominant effect and can't adequately explain the observed cosmological redshifts.

2. Event Horizons

It may be true that light from some objects is redshifted to the point that we can no longer see it, but that is not the sole answer to the question of why we may never see the light from some distant galaxies. Depending on what cosmological model you use, there may very well be a distance beyond which light will never reach you, at any time in the future. We call this distance the radius of the event horizon, since you as an observer can never have information about events that take place beyond it. This is not the same thing as the horizon that cosmologists speak about more frequently, which is the particle horizon: the maximum distance that light can have traveled so far (since the beginning of the universe). The particle horizon sets the size of the region of everything that you can see now, but it expands to encompass more and more volume as time goes on. The particle horizon scale is therefore the radius of our currently observable universe (which is how you may also have heard of it referred to).

Getting back to event horizons -- again, whether or not one exists depends on your cosmological model (i.e. the parameters of the universe you think you live in). For some models, there is no finite event horizon radius. It turns out to be infinite, meaning that if you could just wait long enough (i.e. forever) then you would eventually see everything. However, I seem to recall (and I will double check this) that our standard lambda-CDM cosmological model does have a finite event horizon. Due to the accelerating expansion, there is a distance beyond which you will never be able to see. As for how this can be reconciled with the argument the OP made about how we should always see photons traveling towards us at c, I suspect it has something to do with general relativity that he/she wasn't taking into account (sorry I don't know enough to explain further). I should point out that the OP failed to take into account that the universe is expanding, so that the distance you would infer from the light travel time does not correspond to the proper distance to the object (neither at the time when the light was emitted, nor at the time when it was received).
 

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