How come the sky isn't constantly light, considering the billions upon billions of light sources in not only our galaxy but the entire universe. Even allowing for the time light takes to travel, we should be seeing light arriving all the time from every direction from millions and billions of years ago. Yet instead we only see the light from nearby stars. (Even if you take away the effect of our atmosphere as an absorber of light and go up into space, this is still a puzzler to me.)
The lightwaves stretch over time or distance(?), becoming narrower, so by the time they reach us they are weaker.
Quote: Leevil @ September 6 2009, 3:07 AM BSTThe lightwaves stretch over time or distance(?), becoming narrower, so by the time they reach us they are weaker.
Good. But I don't buy it. (I presume it's for sale?)
Quote: Tim Walker @ September 6 2009, 3:08 AM BSTGood. But I don't buy it. (I presume it's for sale?)
It's for rent boi!
Today I saw the strings
Connected to my wings
They made me live and dance
Nothing left was for chance
Quote: Rob0 @ September 6 2009, 2:27 AM BSTYup, one big mind-f**k.
Didn't know the moon doesn't rotate - is that so?
Anyway, a fun time-lapse of it moving slightly over a month:
http://antwrp.gsfc.nasa.gov/apod/ap010218.html
It does rotate, but only once per month. The effect is that as it goes around the earth always the same side is towards the earth.
And it's not a coincidence, it is synchronised. One theory is that the centre of mass is not at the centre of the sphere, so the 'heaviest' side is always closest to the Earth. It presumable did spin before, but over millions of years the gravity drag (which causes our tides) slowed it down until nowadays it always presents the same surface to the earth.
The 'dark side of the moon' is not dark of course we just used to call it that because we had never seen it till rockets went around the other side; just like the near side it has a day of 28 days; when we have no-moon or new moon it means the other side 'the dark side' is in full sunlight. You can deduce that from the photos that Rob0 found.
Quote: Tim Walker @ September 6 2009, 3:01 AM BSTHow come the sky isn't constantly light, considering the billions upon billions of light sources in not only our galaxy but the entire universe. Even allowing for the time light takes to travel, we should be seeing light arriving all the time from every direction from millions and billions of years ago. Yet instead we only see the light from nearby stars. (Even if you take away the effect of our atmosphere as an absorber of light and go up into space, this is still a puzzler to me.)
Some kind of light must be getting here, even from the black bits, because we managed to pick it up on the Hubble Ultra Deep Field
http://en.wikipedia.org/wiki/Hubble_Ultra_Deep_Field
You might also like:
http://en.wikipedia.org/wiki/Olbers'_paradox
Quote: Leevil @ September 6 2009, 3:07 AM BSTThe lightwaves stretch over time or distance(?), becoming narrower, so by the time they reach us they are weaker.
Doppler effect. Redshift. The galaxies are moving away from us, fast, causing the light to shift down to microwaves, rather than visible light.
I think.
Quote: Kevin Murphy @ September 6 2009, 12:40 PM BSTSome kind of light must be getting here, even from the black bits, because we managed to pick it up on the Hubble Ultra Deep Field
http://en.wikipedia.org/wiki/Hubble_Ultra_Deep_Field
You might also like:
http://en.wikipedia.org/wiki/Olbers'_paradox
Doppler effect. Redshift. The galaxies are moving away from us, fast, causing the light to shift down to microwaves, rather than visible light.
I think.
crush crush crush... munch munch munch
Cheers, chaps. I suppose that makes sense, yes. 
Here's another one... (apologies if I may have mentioned this before)
It's regarding the speed of light. Now, according to the two models of the theory of relativity, nothing can travel faster than the speed of light.
Now, say that our Sun suddenly vanished out of existence, just like that. (This is obviously a hypothetical!) OK, so due to the limitations on the speed of light in a vacuum, it would take us on Earth something like 8-9 minutes to notice that the Sun had disappeared due to the time taken for the light to travel to us.
Right, but according to quantum theory and gravitational theory, as soon as the Sun vanished we would instantenously notice the gravitational effects of the Sun disappearing. Now, that would mean than gravitational "information" travels faster than the speed of light. How can this be?
Yours,
Puzzled of Bristol.
Quote: Tim Walker @ September 6 2009, 6:57 PM BSTCheers, chaps. I suppose that makes sense, yes.
Here's another one... (apologies if I may have mentioned this before)
It's regarding the speed of light. Now, according to the two models of the theory of relativity, nothing can travel faster than the speed of light.
Now, say that our Sun suddenly vanished out of existence, just like that. (This is obviously a hypothetical!) OK, so due to the limitations on the speed of light in a vacuum, it would take us on Earth something like 8-9 minutes to notice that the Sun had disappeared due to the time taken for the light to travel to us.
Right, but according to quantum theory and gravitational theory, as soon as the Sun vanished we would instantenously notice the gravitational effects of the Sun disappearing. Now, that would mean than gravitational "information" travels faster than the speed of light. How can this be?
Yours,
Puzzled of Bristol.
My brain hurts. I'm sticking it on the ever-increasing list of things to ask the quantum physicist at the next opportunity.
Sovereignty travels faster than light!
As soon as a king or queen dies, the sovereignty passes instantly to the heir.
Even if s/he was on Mars s/he would instantly be King (or Queen).
Quote: Tim Walker @ September 6 2009, 6:57 PM BSTCheers, chaps. I suppose that makes sense, yes.
Here's another one... (apologies if I may have mentioned this before)
It's regarding the speed of light. Now, according to the two models of the theory of relativity, nothing can travel faster than the speed of light.
Now, say that our Sun suddenly vanished out of existence, just like that. (This is obviously a hypothetical!) OK, so due to the limitations on the speed of light in a vacuum, it would take us on Earth something like 8-9 minutes to notice that the Sun had disappeared due to the time taken for the light to travel to us.
Right, but according to quantum theory and gravitational theory, as soon as the Sun vanished we would instantenously notice the gravitational effects of the Sun disappearing. Now, that would mean than gravitational "information" travels faster than the speed of light. How can this be?
Yours,
Puzzled of Bristol.
Because the light disappearing would be the last stage of the stars burn out. That energy has to go somewhere, I doubt it would just turn off like a light bulb, so a hypothetical question like that, would not relate to answering it on a quantum and gravitational level.
What affects would we notice? A loud bang? Loss or gain of gravity? The Sun, I'm totally guessing, would most likely show signs of imminent extinction by becoming increasingly unstable, these are the first affects we would probably notice.
But I'm probably talking absolute bollocks.
I was only using the sudden disappearance of the Sun as a hypothetical about the paradox between the speed of light and the "speed of gravity", that's the bit that I'm unsure about. I know it might be silly to say forget about the energy involved, but in that case... oh, I don't know... 
Thanks anyway. 
Isn't light gravity? Or light is energy is gravity?
Quote: Tim Walker @ September 7 2009, 12:01 AM BSTI was only using the sudden disappearance of the Sun as a hypothetical about the paradox between the speed of light and the "speed of gravity", that's the bit that I'm unsure about. I know it might be silly to say forget about the energy involved, but in that case... oh, I don't know...
Thanks anyway.
I too have pondered the 'speed of gravity', particularly with regards to black holes and their immense gravitational pulls that can suck in light. Does matter entering a black hole get any where near the speed of light as gravity sucks it in? And how does something become infinitely dense? (I mean aside from watching Coming of Age?)
