Why is the sky dark at night?

By Phil Plait | October 1, 2012 7:00 am

One thing educators like to say is that there are no stupid questions. I disagree, mildly: unasked questions might fit that category. Still and all, even basic questions are worth asking, and sometimes can lead to profound insight.

For example, one of my favorite questions is also one of the simplest: "Why is the sky dark at night?" The wonderful folks at Minute Physics took this query on, and show you why this question is so very important.

See? Just by asking why the sky is dark, you can figure out that the Universe is neither infinite in space nor in time. It had a beginning! It doesn’t get much more profound than that.

And another thing I like about this video is that it answers this question in a very similar way to how I do. It’s something I’ve been meaning to write about here on the blog, but I suppose I waited too long. Now I don’t have to.

Funny, too. I have a BAFact I was going to post on this very topic on October 7, just a week from now! Why? Well, that’s not a stupid question at all, and I’ll answer it… in a few days.


Related Posts:

- Astronomers find a galactic nursery 12.7 billion light years away
- An ultradeep image that’s *full* of galaxies!
- The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec! So there.
- First light, confirmed?

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (64)

Links to this Post

  1. Science: Why is the sky dark at night? | October 1, 2012
  2. Speedy Light | October 4, 2012
  3. 9. The Universe, in Brief « Tikkunista! | October 26, 2012
  1. adamas hinton

    in mathamitics you can have the sum of an infinite number of things that ends up being finite i.e 1 + 1/2 + 1/4 + 1/8 … = 2 so why can we not have the same thing happen with starlight.the sky at night only suggests to me that there is a finite amount of light that we are observing.i might be missing something.

  2. Rick

    Great explanation….and yet still very complicated. Thank you for posting this.

  3. Brian

    I thought measurements from the CMB indicated that the geometry of the universe on large scales was flat, therefore implying that it’s infinite?

    Or have I been misinformed by popular science TV?

  4. Nigel Depledge

    The BA said:

    See? Just by asking why the sky is dark, you can figure out that the Universe is neither infinite in space nor in time.

    I cannot access the video from work (due to our firewall policy), but I’m not sure I buy this statement.

    Sure, you can work out that the universe is finite in either space or time, but I don’t see how the question demands that the universe be finite in both space and time.

    Besides, I was under the impression that the universe was considered to be infinite in space and finite in time. Is this no longer the case?

  5. Thomas Vaughan

    You write, “Just by asking why the sky is dark, you can figure out that the Universe is neither infinite in space nor in time.”

    However, the dark night sky does not rule out standard hot big bang models with negative curvature. In those models, space is infinite in volume at all times, no matter how early. (Of course, the volume of the *observable* universe is finite at every finite time, even in models with negative curvature.)

    And of course, we could never actually *know* from any observations, present or future, whether the universe is really infinitely old or not (or infinitely voluminous or not). It’s just that the best model that we have right now to fit the data suggests a finite age.

  6. Larry

    If it didn’t get dark at night, the days would all bang up against one another. Duh!

  7. Rebecca Harbison

    adamas hinton @ 1

    Because basically the cross-section of stars falls off the same way as brightness does. So, if we go from Earth to Saturn (let’s round that to 10 times farther away), the Sun is 100 times smaller in the sky in (angular) area (10 times smaller in angular diameter). It’s also 100 times fainter. So the average surface brightness of the Sun is the same at Saturn as it is from Earth… or from Alpha Centauri, the Andromeda galaxy, etc.

    In order to have a dark sky and an infinite line of sight, the brightness needs to fall off faster than the inverse square (as the angular area does).

  8. Wow. By incredible coincidence, I also posted a topic about this in my blog last Saturday (in Spanish, linked in may name).

    And regarding the question of Adamas Hinton (#1): a non uniform distribution of stars could give a dark sky even if there were infinite stars and no horizon. It would require a fractal distribution of galaxies with a fractal dimension smaller than 2. I put a reference to one of the papers in my note. The known dimension of galaxies is just above 2, though.

  9. Todd

    Couldn’t insterstellar dust/objects and whatnot absorb/scatter/shift the light?

    I’m not actually suggesting things are different than what the video suggests. I just wonder if there couldn’t be alternative explanations (though incredibly slim) is all.

  10. kevbo

    What I never get is that, sure, red shift will make visible light invisible to us, but isn’t there ultraviolet light that would be red-shifted into the visible spectrum?

  11. astroboyabi

    A serious question – does seeing the sky thru IR make the sky appear brighter to a snake?

  12. Random Rambler

    Not denying the video, but couldn’t we be a sort of bubble in an infinite universe, because of the Hubble limit? And since this is hypothetical, I’ll suppose non-infinite with respect to past time, but infinite in space.

  13. Bramblyspam

    There are any number of times that Phil has posted about objects that would be much more easily visible if there weren’t a big dust/gas cloud in the way. In principle, you could easily have a dark universe just because there’s non-shining matter in front of distant galaxies.

  14. Great explanation but hated the typical, wrong images of evolution at about 1:30. It’s a branching tree not a steady progression as depicted.

  15. Tim Ford

    Hi Phil,

    As others have mentioned, the sky being dark definitely doesn’t tell us the universe is finite in space. It’s not true that when we look between the stars at darkness we’re seeing the universe before there were stars (not to mention the fact that that statement doesn’t make much sense).

    Instead, when we see darkness we’re seeing areas where there aren’t any *observable* stars. Stars that may be in that direction are so far away that they’re being pushed away (by virtue of the fact that the universe is expanding) faster than the speed of light. I guess you can think of this as being red-shifted to 0, not sure. And don’t let “faster than the speed of light” throw you off. Nothing can move faster than the speed of light, but expansion means the distance between objects can increase faster than the speed of light.

    Science has no evidence to suggest that the universe is finite in size, or that there is a finite amount of mass in the universe. We don’t know that the universe was “very small” at the big bang — just that it was very compact. If you tack “observable” onto the beginning of most these invalid statements, they then become right.

    Thanks for all you do to spread knowledge! I hope you’ll help eradicate this misconception that science believes the universe to be spatially finite.

  16. ceramicfundamentalist

    @ Todd #9 and bramblyspam #13

    “Couldn’t insterstellar dust/objects and whatnot absorb/scatter/shift the light?”

    interstellar dust could absorb light for a finite amount of time, but would brighten and start radiating infrared or visible light over very long time frames, because of that very energy it absorbed. in an infinitely old universe intervening dust would become as bright as background stars.

  17. Chris A.

    Here’s a fun bit of trivia: The first published solution to Olbers’ Paradox (as the question “why is the night sky dark in an infinite universe?” is known) came from, of all people, Edgar Allan Poe!

  18. Eddie Janssen

    I think this is called Olbers’ paradox?
    But Chris A. beat me to it and I do not know how to cancel my comment.

  19. Vívian

    I love minutephysics!

  20. Craig Hartel

    I watched the video twice. I think my mind is blown because I fail to grasp some of the fundamentals of astrophysics.

    One of the things I find the biggest challenge to understand is how we can see light that is from billions of years away – it seems paradoxical to me that we can see that with an aided eye on earth but the light hasn’t really reached earth yet but it has because we found a spot with a telescope.

    So we see something from earth which hasn’t reached earth yet and probably won’t given that the universe is continuing to expand and by virtue of the expansion all of the light going away from us red-shifts to the point where it becomes infrared light.

    Am I just over thinking this? I mean I get that we can use binoculars to see things far from us that our naked eye couldn’t see – or rather our brains can’t resolve because there is so much light coming in that you can’t pick out details that far away. Is this the same with a telescope – is it fair to say that a telescope is simply a big set of binoculars, or better, a big monocular?

    Please, can someone ‘splain this to a science lover that is not even a BSc?
    Thank you!

  21. jcr

    Tim Ford #15 appears to be misremembering his special relativity. Distant stars cannot be moving away from us at faster than the speed of light. To someone else moving at a different velocity, the distance between two stars may be increasing at greater than c, as long as all velocities are less than c with respect to the observer. In the case of our reference frame, where Earth is at rest, all other massive objects must have velocity less than c, so the change in the distance between Earth and any other sun can’t be greater than c.

  22. Roach

    I’m curious exactly what the implications are for the night sky in the future.

    If we fast forward a few billion years, ignoring any change undergone by our sun, what does the rest of the universe look like? If we live in a decelerating universe, wouldn’t that imply that the redshift of distant objects is decreasing, thus bringing more light into the visible spectrum?

    On the other hand, if we live in an expanding universe, redshifts would be increasing. At the same time, small amounts of visible light should be coming from yet more distant space than we can perceive now, so it might be a question of which is the greater delta.

  23. mike burkhart

    The sky isn’t as dark as it use to be, thanks to street lights. If you live in the city like I do the light polution is realy bad and only bright stars and planets appere in the sky. Thats why I have to use a ultra block filter on my telescope.

  24. Tim Ford

    Craig,

    When you see something in a telescope, you’re seeing light that is just then hitting the telescope. If you looked up, it would hit your eye in exactly the same way. The only reason you can’t detect it with your eye is because it’s too small. A telescope makes it bigger. It also has the advantage that it can take a long exposure and collect those photons over time. Your eye only detects the photons hitting it at any given moment (speaking simplistically).

  25. CatMom

    Space is big.
    Space is dark.
    It’s hard to find a place to park.

    (Just a silly Monday quote – I forget who came up with it. Cheers!)

  26. Buzz

    I have the same question as @10 Kevbo. Stars put out UV too–shouldn’t that become visible when it is red-shifted? Is it because the star puts out more intense visible than UV?

  27. Pat

    Tim,
    Careful — a telescope doesn’t make fainter objects visible because it makes them bigger, but because it’s aperture is bigger, collecting far more photons than our puny eyeballs. A 6″ scope can collect about 350 times the light of our eye. (In fact, just making a dim object bigger actually would make it harder to see — the light would be spread out over a larger area.

  28. What’s a stupid question?

  29. Bramblyspam

    @ ceramicfundamentalist #16: Remember though, the universe is not a static affair. There are constantly old stars dying and new ones being born. Likewise with dust clouds. Even in an infinitely old universe, couldn’t you have young (and thus dark) dust clouds between yourself and the distant stars?

  30. 1:09 “And since only a finite amount of time has passed since this so called beginning, that means that some of the stars necessary to fill up the brightness in every direction are so far away that light from them plain hasn’t had time to reach us yet.”

    This is misleading. It suggests that there are stars that are farther away than c * (age of the universe). If what is meant is there are stars we can’t see because their age in years is less than their distance from us in light-years, then the quoted statement would be true but far less meaningful. The implication is that there are so many stars they essentially create a unbroken wallpaper of stars. Even if this were true the magnitude of most stars from an Earth perspective are so dim as to be luminously moot. Additionally, it may be true that the sky on the night-side of Earth *is* illuminated by starshine but only trivially so.

    As the distance to an object doubles its apparent area quarters. That is to say, 4×4 square twenty feet away could be eclipsed by a 1×1 square ten feet away. If there were two stars, physically identical, but one twice as far as the other, the closer star should seem four times brighter. Given the distance to most stars, it shouldn't be any wonder that they are too dim, even as an ensemble, to provide the kind of illumination that our own single star does.

    And as if to contradict myself, if any point in the universe may be considered the center of the universe, then the size of the universe is necessarily infinite despite its finite age and finite rate of expansion.

  31. Tom

    There are a lot of very authoritative-sounding comments on this post that disagree with each other. Coincidentally, Bob Berman’s column in the latest Astronomy magazine talks about how it is seeming more likely that the universe is, in fact, infinite (seemingly in both space and time, if I read it correctly (the Big Bang may have been a “local” phenomenon)).

    Phil, how about an update? Maybe clear things up a bit on this issue (if that is possible).

  32. ceramicfundamentalist

    @ bramblyspam #28

    in the scenario of an infinite universe nothing can be young and new. certainly atoms or molecules may be arranged into a new cloud but those atoms and molecules have been around literally forever. they have been irradiated upon by all the stars that have ever been for all of eternity. imagine the densest possible dust cloud, so dense that it is completely impossible to see any stars from within it. the matter at the edge of the cloud has been irradiated for so long that it begins to glow in infrared or visible light. that lights up the next inner layer of matter, and the next, and so on. this may take a long long time to penetrate the cloud (or indeed every atom in the universe), but compared to infinity a long long time is just peanuts.

    so, in an infinite universe, either a cloud must be made up of particles that were as hot as the rest of the universe to begin with, or it must’ve been around long enough for it to have illuminated even its own deepest darkest corners. i guess the corrallary of this line of thinking is that the observed heterogeneity of the universe is proof that it is not infinite.

  33. MattTheTubaGuy

    I would actually ask the opposite, why is the night sky bright?
    while flying from KL to NZ, I flew over a part of the Indian Ocean. the water was completely black, yet the midnight sky (near the equator) was not completely black, it was a very dark blue, and noticeably brighter than the black ocean.
    Is this an atmospheric effect, or is the night sky really that bright?
    (keep in mind I was flying in a commercial aircraft 10km above the ocean)

  34. Nigel Depledge

    Bramblyspam (13) said:

    In principle, you could easily have a dark universe just because there’s non-shining matter in front of distant galaxies.

    But in an infinite universe, such matter would be warmed by the radiation it absorbed until it glowed as any hot black body does, so that argument does not work.

  35. Nigel Depledge

    Chris A (17) said:

    Here’s a fun bit of trivia: The first published solution to Olbers’ Paradox (as the question “why is the night sky dark in an infinite universe?” is known) came from, of all people, Edgar Allan Poe!

    His answer didn’t involve ravens, did it?

  36. Nigel Depledge

    Craig Hartel (20) said:

    One of the things I find the biggest challenge to understand is how we can see light that is from billions of years away – it seems paradoxical to me that we can see that with an aided eye on earth but the light hasn’t really reached earth yet but it has because we found a spot with a telescope.

    So we see something from earth which hasn’t reached earth yet and probably won’t given that the universe is continuing to expand and by virtue of the expansion all of the light going away from us red-shifts to the point where it becomes infrared light.

    Am I just over thinking this? I mean I get that we can use binoculars to see things far from us that our naked eye couldn’t see – or rather our brains can’t resolve because there is so much light coming in that you can’t pick out details that far away. Is this the same with a telescope – is it fair to say that a telescope is simply a big set of binoculars, or better, a big monocular?

    It seems to me that you might have a misconception about how ‘scopes and bins work.

    Although it often feels like the ‘scope brings a terrestrial object closer, this is not what it is doing. By collecting more light and magnifying the image, the ‘scope is simply allowing you to see detail that you would not normally be able to resolve. (Collecting more light is necessary when magnifying an image because when you magnify an image, you “spread out” the light that makes up the image, which makes the image fainter.)

    Very distant galaxies are not visible to the unaided eye not because the light has not yet reached Earth but because the light is too faint for us to see. A telescope allows us to see fainter objects than we can see unaided because it collects more of the light that is arriving at Earth than our eyes can. Following this principle, the bigger the telescope, the fainter the object one can see with it.

    Earth’s atmosphere makes images fuzzy (this is a different story, so I’ll gloss over it), so Hubble is able to get very sharp images – far sharper than a similar-sized ‘scope based on the ground would get. (Hubble’s mirror is only 2.4 m, or about 94.5 inches, in diameter, but Hubble gets sharper images than the 200-inch ‘scope at Mount Palomar.)

    Another way to collect more light is to take a long-exposure photograph. By looking at the same patch of sky for many hours on end, Hubble can collect enough light to see objects that are very, very faint indeed. Hubble uses digital detectors rather than photographic plates, but the principle is the same.

    So, by collecting many many times more light than can the human eye, Hubble can “see” objects that are many times too faint for the unaided eye to see. But the light from those objects is arriving at Earth all the time.

    D’you see?

  37. Nigel Depledge

    JCR (21) said:

    Tim Ford #15 appears to be misremembering his special relativity. Distant stars cannot be moving away from us at faster than the speed of light. To someone else moving at a different velocity, the distance between two stars may be increasing at greater than c, as long as all velocities are less than c with respect to the observer. In the case of our reference frame, where Earth is at rest, all other massive objects must have velocity less than c, so the change in the distance between Earth and any other sun can’t be greater than c.

    But objects can indeed have an apparent velocity greater than c when the space between Earth and the distant object is expanding so as to cause a recession velocity > c. The distant object is moving at less than c relative to local space, but its light will never reach Earth because the universal expansion causes the object to recede at > c. This is why there is a “horizon” to the observable universe, and why the observable universe is finite, and (as far as we can tell) always will be.

  38. Mike

    Twinkle twinkle little star.
    I used to wonder what you are.
    But now I know you’re like the sun.
    Just too far away so you’re no fun.

  39. Bramblyspam

    Scintillate, scintillate, globule vivific
    Fain would I fathom thy nature specific
    Loftily poised in ether capacious
    Strongly resembling a gem carbonaceous

    (taken from some book I read decades ago)

  40. #18 Eddie:
    Correct, but the name is unfair. The paradox was first thought of by de Cheseux; Olbers was simply the first to publicise it.

  41. #40 Bramblyspam:
    OK, after that, I can’t resist this one…

    The Scientist’s Love Song

    Girl:
    First, you must tell me, what makes the stars shine?
    What makes the ivy around the tree twine?
    Next, you must tell me, what makes the sky blue?
    And finally tell me, why should I love you?

    Boy:
    It’s nuclear fusion that makes the stars shine.
    The process of tropism makes ivy twine.
    Scattering of light by the Earth’s atmosphere makes the sky blue, you see,
    And a good dose of pheromones would make you love me.

  42. Nigel Depledge

    T Ray (30) said:

    As the distance to an object doubles its apparent area quarters. That is to say, 4×4 square twenty feet away could be eclipsed by a 1×1 square ten feet away.

    Slight maths error: A 4×4 square has sixteen times the area of a 1×1 square, so would appear twice as large in linear dimensions (four times in area) at twice the distance of the 1×1 square.

  43. Ori Vandewalle

    #26 Buzz:

    You’re pretty much right. Stars emit radiation in almost exactly the same pattern as a black body, and black bodies have a very characteristic curve to them. Specifically, the hotter an object, the sharper the curve gets, and it peaks in the visible light wavelengths. UV light intensity increases as well, but not as much as visible light.

    So as long as a star is approximating a black body, it’s always going to be putting out more visible light than UV light, which means that redshifting it will make it dimmer.

  44. Tom K

    “One thing educators like to say is that there are no stupid questions.”

    As Mr. Garrison on South Park said, “There are no stupid questions, only stupid people.”

  45. Alank

    Big Bang evangelism is all this is about. Wasn’t it a priest who came up with this idea of a beginning with all the connotations of a supreme being that Fred Hoyle disparaged as the “Big Bang”?

  46. I agree, no stupid questions (as long as they honestly are real questions). What about answers? I won’t say the answer in the video is stupid, because it isn’t. However, there are certainly less than optimal answers, and this is one. The basic thrust in the video is correct, and it does highlight the two reasons why the sky can be dark. However, it is wrong in stating that the redshift effect is more important; it isn’t, at least not in our universe. Also, on a topic such as this, an answer like this, which is essentially “shut up and calculate”, isn’t very helpful. Many alternative explanations have been raised in the comments. They are the same ones which have been raised before. The video should have been twice as long and countered these objections, or at least a bit longer and pointed viewers to where to find more complete answers.

    Let’s face it: some things are too complicated for sound bites. This is one of them. There is no debate on this issue among professional astronomers now, because it has been solved, but there was in the past, which shows it is not a simple issue.

    At first, I thought I would respond to all the wrong (in the sense of proposing an explanation which doesn’t work) comments here, but it is easier for me, but more importantly much more enlightening (pun intended) for those interested, to read the “Darkness at Night” chapter in Edward Harrison’s textbook Cosmology: The Science of the Universe. Everyone interested in cosmology should read this book. (Harrison also wrote an entire book on this topic, but I think it is out of print now. Still, libraries have some copies.) Thus, I will correct only other comments which are wrong. (Note that all of the alternative explanations proposed here—and many more—are wrong, as shown by Harrison.)

  47. “I thought measurements from the CMB indicated that the geometry of the universe on large scales was flat, therefore implying that it’s infinite?

    Or have I been misinformed by popular science TV?”

    First, CMB and other measurements indicate a nearly flat universe. However, it cannot be ruled out that it is finite, but extremely large. In fact, many theories of inflation as well as other reasons make this look more likely. Alas, it is so big that normal astronomical observations won’t be able to measure a finite radius of curvature.

    Second, even if it is flat in the sense you mention, it can still be finite if the topology is non-trivial.

  48. “However, the dark night sky does not rule out standard hot big bang models with negative curvature. In those models, space is infinite in volume at all times, no matter how early. (Of course, the volume of the *observable* universe is finite at every finite time, even in models with negative curvature.)”

    Most of your comment is correct, including the first sentence quoted above. So is the second. However, the third, in parentheses, is wrong. A necessary and sufficient condition to have a finite observable universe is the existence of a particle horizon. (See the famous paper by Rindler or, again, Harrison’s textbook.) Even in models with negative curvature, it is possible to have no particle horizon, namely then when there is no matter (but, perhaps, a cosmological constant) in the universe. In these models, the speed of expansion was not arbitrarily large in the past, so it is possible, in principle, to see the entire universe in these models.

  49. JB of Brisbane

    “Sky’s blue during the day, and dark at night – can’t explain that!”
    – not Bill O’Reilly, but probably what he would say.

  50. @ Nigel Depledge (43) Good catch. I botched the example.

  51. TB

    Interesting video, but obviously incorrect. It says that all the stars are about as bright as the sun. But if you look up at night, you will see that some stars are much dimmer than others. The reason is that they are farther away, so less light reaches us. So even if you were looking directly at a very distant star and the light was arriving at your eye at that moment, you still would only see darkness. I could go into the details about angles and aperture sizes and such, but if you want to know about such things take an intro to optics course. At the end you should be able to understand the mistake in this video.

  52. @52: I’ll make an exception for you. What you say is completely wrong. First, the remark about brightness refers to absolute brightness. OK, maybe not clear. Second, stars are dimmer when they are farther away, but there are also more of them at a given distance, and the effects cancel exactly (regardless of the curvature of space). This line of reasoning has been used in connection with this question for several hundred years.

    Please read Harrison.

  53. Skydaddy

    “The heavens declare the glory of God;
    the skies proclaim the work of His hands.
    Day after day they pour forth speech;
    night after night they reveal knowledge.
    They have no speech, they use no words;
    no sound is heard from them.
    Yet their voice goes out into all the earth,
    their words to the ends of the world.” :-D

  54. Matt B.

    Infinity of volume and consistent density of stars doesn’t necessarily lead to covering the entire sky. In a Euclidean plane, there are directions you can draw a ray so that it doesn’t touch any points with integer coordinates. In a real-life infinite universe, the stars wouldn’t be spaced so exactly, so I would expect a fractal of light and dark.

    It might be possible to use calculus to prove that an infinite number of stars doesn’t necessarily add up to 100% of the sky, but I’m not up for that task right now.

    Oh, and I should add: It’s /TEM-por-al/, not /tem-POR-al/. (I often hear a similar mistake with “electoral”.)

  55. Cosmonut

    Couple of things glossed over in the video, but really important.

    As stars get farther away, their light gets fainter. So maybe that will take care of it ? Turns out not, due to a neat mathematical fact.

    Consider a sphere 1 AU in radius centred on Earth. It will just about touch the Sun and thus only 1 star on the surface of the sphere.

    Now consider a sphere say 1 million times larger. Every star lying on the surface of that sphere is 1 million times further away, so the light they contribute is 1/(1 million)^2 times that of the Sun.

    But on average, if a sphere of size 1 AU touches just the Sun, a sphere of 1 million AU touches (1 million)^2 stars ! So, the overall light coming from that sphere = Same as from the Sun.

    Now if space goes on till infinity, we get an infinite amount of light hitting us ! That’s the paradox.

  56. ceramicfundamentalist

    Matt B., #57
    “there are directions you can draw a ray so that it doesn’t touch any points with integer coordinates.”

    interesting thought, but the human eye can’t see a point, it has a limit of practical resolution. would any starless area of the sky be large enough to be resolved by a human eye (or for that matter even the most advanced telescope)? if not, then the sky would still look bright in every direction.

  57. TB

    @53. Please pardon my sloppy writing earlier. I understand what you are saying, but I still disagree. Even if you have a detector which integrates all photons from all directions, you still get a finite light density. The reasoning is simple. If you have some star density which is uniform over space and time (no die-outs or blockages) you will get a photon density which is, in effect, the average temperature of the volume. If you double the star density, you will double the photon density because you have effectively doubled the energy density inside your “black body”. So if we could put a detector in the middle of intergalactic space we could get an approximate value for the star density in the universe. Obviously this is not zero, as my previous post may have implied, but neither would it be infinite or the same as being inside of a star. (I will read Harrison as you suggested, but I have taken many physics courses and have been involved in various camera light detection systems for several years, so I think I already know a bit about this).

  58. Nick L

    “you can figure out that the Universe is neither infinite in space nor in time.”

    Unless an infinite universe has virtually all of its diameter at around a plank length.

  59. Nick L

    That should be Planck length. (We don’t live in a lumber universe regardless of the number of blockheads you might know)

  60. @#58: OK, this is a bit better; one can argue that the brightness isn’t infinite but equal to the average surface temperature of a star. OK, but still that is not what we observe. (This is of course an idealized thought experiment; if the photon density in space were the same as that at the surface of a star, then stars could not exist, but that is not relevant to the paradox.) If the stars have always been shining and the universe is infinite, then every point would indeed be as bright as the surface of a star.

    @#55: Yes, this is possible, but would require that we are at a special place in the universe.

  61. Chalkboard

    Ah yes, the intellectual equivalent of “yah huh…nut uhh…yah huh if this way…ok, but nut uhh if that way” Thanks Phil, keep up the good work you do for those of us with brain pans of a lessor volume.

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