Why is it dark at night?

[Brett]

Geek44 & I must have had a Vulcan mind meld in the "All-American nose
art thread & we started discussing astronomy. I thought I better move
our discussion here in case there is people interested that didn't check
out the other thread & to keep from totally hijacking the other thead ...


Anyway, here is relevant quotes:

Originally Posted by Geek44

I've often wondered the same thing about the darkness of the universe at night. I can add a question and two points here.

Question; Why can't we see the massive black hole at the centre of the galaxy? It must have the mass of a million suns. As matter falls into it, it flares with all kinds of wavelengths as it crosses the event horizon which must be huge.

Point 1; Photons from stars further than the sun are scattering in all directions...maybe they hit our eyes in small numbers compared to the concentrations we get from the sun which is closer, or even an electric light.

Point 2; The universe is only 'dark' to one's photon sensitive eyes. If we could see x-ray or radio waves, it would be very 'bright'.

I'm a bit of an armchair physicist/astronomer. Currently trying to understand 'string theory'. Now...if you'll excuse me, I'm needed in an alternative dimension.

(Bright non visible - Point 2) Good Point. I had never
thought of that or read it. It just goes to show
why I love the subject so much, the more you
learn, the more you realize there is much more
to learn.


I’m not sure of your math background, so forgive
me if I repeat something obvious to you below.

(photons hit eye - Point 1) I’m not sure I understand this.

One thing I thought of later after hearing the night
sky paradox was – Wait a minute … infinite sums
don’t necessary add up to infinity (for instance
PI = 3 + 0.1 + 0.04 + 0.001 + … = 3.1415 ... & adds up to
less than 3.2).

I asked about this on an astronomy forum
& the reply I got was that you can do a calculus
integration (a generalization of addition for
infinite sums) and if there is infinite time + stars
the integration will add up to a bright night sky.

I have a college Math degree but haven’t used anything but HS algebra for
years, so I got lost on their answer, but its on my list …

(see center galaxy - question) From what I understand
there is dark clouds between us & the center.
I forgot to add that infinite time + stars &
vastness is hard to reconcile with a dark night
sky. If the universe was infinitely old, the
dark clouds would have time to be heated
to visibility.

(string) Have you read “The Elegant Universe”
by Brian Greene? It’s the best book on string theory
I have read (well, it’s the only, but I thought
it was very good). One thing I mean to explore
is the theory says each point in 3-D space is actually
rolled up 10-D points (much as a 3-D pipe looks
2-D if seen from far enough away).

The picture in the book (& try showing 10-D in 2-D!),
reminds me of fractal dimension objects. For instance,
a coastline has a bigger D than a 1D line, but less than
a 2D plane. So it might have a dimension of 1.4. Similarly
mountains has a bigger D than a 2D plane, but less than
ordinary 3D space. So it might have a dimension of 2.36.

What I want to ask is: Could the universe have a fractal
dimension - for instance 6.3 or 5.8?

[Commander Waffle]

[Geek44]

Okay Brett, seems you're educated so I'll try to keep up as far as I can. I'm an armchair type and I really only have mid-high school mathematics. I've read about Chaos, Einstein's theories and even pretended to understand some Hawking, but that's as far as I go.
To clarify the photons hitting the eye idea, which is my guess...let's assume photons are behaving like particles and not waves for my purposes...I'll bring up the 'Double Slit' experiment later.

Imagine your eye is 93 million miles from a star...our sun makes a great example (;(). The sun is putting out countless numbers of photons every second and in order to perceive the sun, our eyes react with a certain number of them...a pretty high order of magnitude. Some are filtered out but we 'see' the sun. Now imagine your eye is 93 BILLION miles from the sun. Seems to me that the number of photons that actually reach the retina is diminished given that the photons radiate from the sun in all directions. Another illustration would be that you get wetter standing closer to the sprinkler.

Galaxy centre question...dark clouds.
Are you referring to dark clouds of ordinary matter or 'dark' matter? Either way, I understand. Although it would seem like the pea sized Mercury blocking out the sun. I suppose if I was closer to Mercury, that could happen.

String
Currently watching the DVD of the 'Elegant Universe' but I don't have much time lately...I can't watch it and concentrate on large enough chunks with demanding two and a half year old and a three week old both needing attention.

Fractal dimension
Not sure I'm following your expression correctly. When you ask 'Could the universe have a dimension of 6.3?' I'm not sure if you're asking if it could have 6.3 dimensions...which implies 0.3 of a dimension as well as the other 6 which I find difficult to visualise.

How modern is your personal understanding of physics? Mine can be a little 'Carl Sagan' at times, so please bear with me.
Peace.

[Brett]

(photons hit eye) I think we are talking about the
same thing here, in different ways. Your point about
the light from distant sources getting vanishing small
is what I am making in my discussion of infinite sums.

From what I understand, while any one light source
would vanish from long enough away, IF there was
an infinite source of them, each of the tiny points
from them would add up enough in our eyes to
make the night sky look like day.

That is, unless an assumption is wrong – the universe
isn’t infinite or isn’t infinitely old & thus light from
more distant stars hasn’t had time to reach us.

Or another way of looking at it is:

Q: Why is the night dark?
A: You just answered it

Or rather, if the night sky ISN’T dark, then we won’t
be around to wonder about it (i.e. Anthropological Cosmology).
After the big bang, for millions of years there was no dark sky
-- but it was too hot for creatures such as us. And you can
make similar arguments for possible ultimate fates of the
universe.

(dark matter) I was referring to “ordinary” matter in the
clouds between us & the center of the galaxy. As I understand
it, Dark matter does not interact with other matter (or forms
of energy such as light) except through gravitation. So if there
were Dark matter clouds between us & the center of the galaxy,
we could not tell, unless we detected gravitational affects.

Gravity is incredibly weak (it may not seem so when
stepping off a cliff, but remember the whole earth
is pulling there). Every time you lift a beer off the
table demonstrates how much more powerful other
forces are (puny muscles beat gegabazzilion tons
of the earth’s gravity! )

In any case, light could blow through any Dark matter
without any problems, unless it was hugely massive.
(& from what I’ve read, most is thought to reside
in halos around galaxies or between galaxies in
clusters).

(fractal dimensions) “Chaos” was a great book.
I believe that is where I 1st saw the concept of
fractal dimensions.

Let me know if below is more detail than you want
to know. I will add in the next post what I understand
of fractal dimensions if you want.

I’m not sure if you ever covered multi-dimensional
spaces in your math classes (I never did till college),
but I was surprised that it was a fairly “natural” extension
of concepts of HS algebra.

The idea was that, for instance 2D you could give x & y
coordinates for anything on the plane. For 3D you
could give x, y, z coordinates for anything in ordinary
space (x could be Left-Right, y forward & back, z
up & down).

For 4D you could give w, x, y, z coordinates for anything
in the 4D space, 5D v, w, x, y, z coordinates, etc. There
is no way I can visualize higher dimensional spaces
(except maybe if I’ve had enough to drink )
but its possible to algebraically manipulate them.

[kentek]

Originally Posted by Brett

(photons hit eye) I think we are talking about the
same thing here, in different ways. Your point about
the light from distant sources getting vanishing small
is what I am making in my discussion of infinite sums.

From what I understand, while any one light source
would vanish from long enough away, IF there was
an infinite source of them, each of the tiny points
from them would add up enough in our eyes to
make the night sky look like day.

That is, unless an assumption is wrong – the universe
isn’t infinite or isn’t infinitely old & thus light from
more distant stars hasn’t had time to reach us.

Or another way of looking at it is:

Q: Why is the night dark?
A: You just answered it

Or rather, if the night sky ISN’T dark, then we won’t
be around to wonder about it (i.e. Anthropological Cosmology).
After the big bang, for millions of years there was no dark sky
-- but it was too hot for creatures such as us. And you can
make similar arguments for possible ultimate fates of the
universe.

(dark matter) I was referring to “ordinary” matter in the
clouds between us & the center of the galaxy. As I understand
it, Dark matter does not interact with other matter (or forms
of energy such as light) except through gravitation. So if there
were Dark matter clouds between us & the center of the galaxy,
we could not tell, unless we detected gravitational affects.

Gravity is incredibly weak (it may not seem so when
stepping off a cliff, but remember the whole earth
is pulling there). Every time you lift a beer off the
table demonstrates how much more powerful other
forces are (puny muscles beat gegabazzilion tons
of the earth’s gravity! )

In any case, light could blow through any Dark matter
without any problems, unless it was hugely massive.
(& from what I’ve read, most is thought to reside
in halos around galaxies or between galaxies in
clusters).

(fractal dimensions) “Chaos” was a great book.
I believe that is where I 1st saw the concept of
fractal dimensions.

Let me know if below is more detail than you want
to know. I will add in the next post what I understand
of fractal dimensions if you want.

I’m not sure if you ever covered multi-dimensional
spaces in your math classes (I never did till college),
but I was surprised that it was a fairly “natural” extension
of concepts of HS algebra.

The idea was that, for instance 2D you could give x & y
coordinates for anything on the plane. For 3D you
could give x, y, z coordinates for anything in ordinary
space (x could be Left-Right, y forward & back, z
up & down).

For 4D you could give w, x, y, z coordinates for anything
in the 4D space, 5D v, w, x, y, z coordinates, etc. There
is no way I can visualize higher dimensional spaces
(except maybe if I’ve had enough to drink )
but its possible to algebraically manipulate them.

The scarcity of light is partly due to the age of the universe (about 14 billion years) and the age of the Earth (about 4.5 billion years). Our 'light horizon' is thus 4.5 billion light years, so a good chunk of the light simply hasn't had time to reach us.

Add to that the Inverse Square Law (light intensity varies as to the square of the distance) and a point light source at twice the distance has only one quarter the intensity. And we have the Doppler Effect caused by stars and galaxies moving away from us since the Big Bang. This red shift moves visible light into the infrared, microwave and radio parts of the spectrum. The universe is far 'brighter' at these wavelengths.

As for the Milky Way's center, it is screened from our viewpoint by a dense globe of massive stars and the dust and hydrogen cloud Sagittarius B2, which has a mass 3 million times that of our sun. The center can only be studied at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths. Our instruments aren't nearly powerful enough to study the black hole itself, Sagittarius A*.

Are we bored to death yet? Sorry, I've always had a thing for astronomy. I'll leave now.

[cyberia]

This thread if fascinating!

[Geek44]

Keep it coming guys...Brett, you make some excellent points, some of which might take my brain a while to assimilate. I hadn't thought of the 'light horizon' concept and the idea that all the light hasn't reached us yet. To be honest though, I'm eating a little bit of dust at this point.
Peace.

[kentek]

Originally Posted by kentek

The scarcity of light is partly due to the age of the universe (about 14 billion years) and the age of the Earth (about 4.5 billion years). Our 'light horizon' is thus 4.5 billion light years, so a good chunk of the light simply hasn't had time to reach us.

Add to that the Inverse Square Law (light intensity varies as to the square of the distance) and a point light source at twice the distance has only one quarter the intensity. And we have the Doppler Effect caused by stars and galaxies moving away from us since the Big Bang. This red shift moves visible light into the infrared, microwave and radio parts of the spectrum. The universe is far 'brighter' at these wavelengths.

As for the Milky Way's center, it is screened from our viewpoint by a dense globe of massive stars and the dust and hydrogen cloud Sagittarius B2, which has a mass 3 million times that of our sun. The center can only be studied at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths. Our instruments aren't nearly powerful enough to study the black hole itself, Sagittarius A*.

Are we bored to death yet? Sorry, I've always had a thing for astronomy. I'll leave now.

Um ... I must have been asleep when I wrote this. The 'light horizon' is how far our sun's light has traveled since it ignited. That has nothing to do with the light we see in the sky. The rest is OK.

[Geek44]

That's cool mate. You lost me after 'scarcity'.




Just kidding.

[MAGNA]

I would have thought 6.3 would equate back to 3.14 if the sun is a PI in the sky.