# Why might light from distant galaxies not reach us?

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 travelling 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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sophiecentaur
Gold Member
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.

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.

sophiecentaur
Gold Member
Which raises the question "Where does all that wasted energy go???"
Yawn - where did all mine go too?

Yawn - where did all mine go too?
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.

Drakkith
Staff Emeritus
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 lets 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!

Thanks Drakkith. As a non-scientist, I find explanations such as yours very helpful.

DaveC426913
Gold Member
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.

davenn
Gold Member
2019 Award
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.
thats the best reason I have heard yet ;)

haha

Dave

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.

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?

DaveC426913
Gold Member
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?"

DaveC426913
Gold Member
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.

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.

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 travelling 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.

'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.

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?

Drakkith
Staff Emeritus
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.

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.

Drakkith
Staff Emeritus
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.

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".

Drakkith
Staff Emeritus
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".
Lol...

DaveC426913
Gold Member
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.

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.

DaveC426913
Gold Member
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.

Dave, is it not difficult to maintain scientific veracity without mathematical rigour?

Drakkith
Staff Emeritus