Sun Goes Down. Up Comes A Mystery

http://www.npr.org/blogs/krulwich/2012/10/10/162630285/sun-goes-down-up-comes-a-mystery

by iProject • 14 years ago

166 78 14 years ago

cstross• 14 years ago

Genuine question: at what age is Olber's Paradox explored in high school physics these days? (It was part of my A-level physics curriculum in the UK, age 17, circa 1981 ... I'm finding it rather odd that it was unfamiliar to this journalist!)

lutusp10• 14 years ago

> at what age is Olber's Paradox explored in high school physics these days?

To the best of my knowledge, and in general, it isn't.

> I'm finding it rather odd that it was unfamiliar to this journalist!

It wasn't. It was unfamiliar to his daughter. He posted it because he's aware that the public is generally unaware of it, not that he is.

tfinch• 14 years ago

I was taught about it in my A level physics class circa 2002... Not sure if it was part of the curriculum though.

javert• 14 years ago

Taking physics was not a requirement in my high school, in North Carolina, so I didn't take it.

Most people, even the best students (I was one of those), did not.

Disgusting, isn't it?

salgernon• 14 years ago

It wasn't taught in any of the physics classes I took in the US circa 1990. As a matter of fact, I hadn't heard about it (by name) until watching a Jim Al Khalili documentary from the BBC. I suspect that proper treatment of the subject would have offended those in the US that still believe in the firmament of heaven.

(I'm only partly joking. My high school biology class had a chapter on evolution. The teacher said "I have to teach this but I won't be testing you on it. Read the chapter and let yourselves out when the bell rings." He then left the room.)

_delirium2• 14 years ago

The physics courses I took (in both high-school and college) didn't generally deal with astronomy. Not sure how common that is. Instead they covered mostly "foundational" topics: general & special relativity, electricity & magnetism, mechanics, atomic structure, etc.

jonnathanson• 14 years ago

I think your experience is pretty typical. I had the same. I took what was technically considered a college-level physics course in high school, and we never really delved into astronomy. Only in college itself did the subject start to come up, and that's because I selected those classes.

I suspect this is because the US high school education system has a fairly standardized, one-size-fits-all curriculum. There are some allowances and exceptions, of course. But, for the most part, everyone is going to be covering roughly the same material. And astronomy isn't deemed as necessary, for the beginner, as some other rudiments of physics. College, on the other hand, offers more opportunity for individual choice in one's curriculum.

ohashi• 14 years ago

I took Astronomy and we didn't deal with this.

sean-duffy• 14 years ago

I'm currently taking A-level physics and Olber's Paradox isn't part of the syllabus at all, unfortunately.

shardling3• 14 years ago

Why unfortunately? They can hardly cover everything! And they certainly can't mention everything covered in the syllabus.

Typically it would be discussed in the context of cosmology; if they don't cover cosmology at all that's unfortunate, but not mentioning this very specific footnote to it would be understandable. And even if they do plan on covering it, I'd be a little surprised if it was listed on the syllabus.

Lewton• 14 years ago

I learned of it in 5th grade, watching a space documentary in science class.

Someone3• 14 years ago

I think that that "UK" makes quite a difference. In my experience, British education focuses more on historical and philosophical (aka 'impractical' :-)) knowledge than that of most other countries.

That experience is heavily colored by watching University Challenge and Mastermind, but I do not think that makes a difference when comparing the top levels (which, I guess, we are; those not in the top levels at A level physics will not remember hearing about Olbert's paradox)

lutusp10• 14 years ago

Another discussion of Olbers' Paradox (that predates Dark Energy):

http://www.arachnoid.com/sky/

munyukim• 14 years ago

I hope science is wrong this time, because if it's true a whole generation will never enjoy the beauty of stars at night

danbruc• 14 years ago

That conclusion is wrong - the expansion of the universe will not tear apart galaxies because the stars are bound by gravity.

pi18n2• 14 years ago

According to the redshift and mass measurements of distant galaxies, it seems like the universe is expanding faster than gravity would keep it together.

amalcon• 14 years ago

That generation will not get to live on Earth at all, because by then the sun will have expanded and swallowed it. Even ignoring that humans as we know them will not exist at that point, this is surely a bigger problem than the lack of stars.

ww520• 14 years ago

The future generation will not see most of the stars in the universe due to accelerated expansion in the universe. The distant galaxies will move faster and faster away from us until their light has red-shifted to be dimmer than the CMB. We will be alone with our Milky Way, may be with some others nearby in the super cluster.

darklajid• 14 years ago

This is one of these 'unreadable on my tablet' articles, where sharing buttons cover the content (bad) and something disables pinch/zoom (inexcusable) for reasons I don't get.

FF's reading mode saved me, but is there any permanent workaround for problems like this? Addons?

pi18n2• 14 years ago

I think it is completely absurd that tablet OS's would ever allow a website to disable zooming. It's like a usability nightmare. Permanent solutions... maybe email the websites that do it?

CamperBob2• 14 years ago

(Shrug) I just go to other websites and read something else. On the Internet, there really is something interesting to see in every possible direction you look.

wazoox• 14 years ago

Interestingly, E. A. Poe provided one of the first explanation of Olbert's paradox : https://en.wikipedia.org/wiki/Eureka:_A_Prose_Poem

surrealize2• 14 years ago

Am I the only one for whom the paradox isn't intuitive? It's not clear to me why many distant dim stars should add up to a bright sky. It depends on how distant and how dense they are, doesn't it? Even assuming that the universe is infinite and eternal and approximately equally dense everywhere.

The embedded video describes the paradox by saying that the night sky "should be as bright as the sun". Imagine if that were the case. Then move yourself twice as close to the sun, making the sun four times brighter. Then the night sky is 1/4 as bright as the sun.

So clearly, distance and (lack of) density could make the night sky less bright, presumably to below the threshold of human perception. Right? Even aside from their explanations of distant light not having had time to reach us, and/or being red-shifted out of our visible range.

splat3• 14 years ago

The reason is that the flux from any particular star goes as 1 / distance^2, but the number of stars observed in a patch of sky increases like the distance^2. The two cancel each other out.

An easier way of thinking of it is to think about looking along a single direction. No matter which direction you look, this ray will always be pointed towards the surface of a star.

(Incidentally, when thinking about these sorts of problems, it helps to distinguish flux from specific intensity: http://en.wikipedia.org/wiki/Specific_radiative_intensity)

IsTom• 14 years ago

But wouldn't the ratio of these two matter? It could be that density of stars in space is very low and you'd get shade of almost-black?

drcube5• 14 years ago

It's only a paradox if you think the universe is infinite in size and age, and the stars are evenly distributed and have been shining the whole time.

Basically, the fact that the sky is dark tells us that at least one of the above assumptions is wrong.

Someone3• 14 years ago

Not even that. One can postulate that there are things that shield out starlight. Let's call it "ether" or "dark matter".

Alternatively, most of the stars really far away could be covered in a Dyson sphere.

drcube5• 14 years ago

I should correct myself that historically, the paradox didn't hinge on the universe being infinitely old, but on the speed of light being infinitely fast.

jbri2• 14 years ago

How does distance affect brightness? Distant stars are exactly as bright as close stars - however, the "brightness" is just spread over a greater area of space.

In particular, the perceived brightness of a star falls off according to the square of distance - you can work this out for yourself by comparing the surface area of a sphere of radius d, with the surface area of a sphere of radius 2d.

But how many stars are at that distance? If you assume uniform density, then the answer is exactly the opposite of what we found above! The number of stars at a distance of d is proportional to the square of the distance.

This means that the total amount of light perceived from stars at a distance d is exactly the same, irrespective of the value of d.

Then if the universe was also both eternal and infinite, then there would be an infinite amount of light reaching the earth!

surrealize2• 14 years ago

Okay, if the universe were perfectly homogenous, i.e., uniformly dense at all scales, then this argument would be easy to buy.

If you allow density variation at some scales, like the difference in density between the interior of a star and interstellar space, then it changes a bit, right? If you're really close to a star, then obviously that star is brighter than the average brightness of the universe. Right?

If so, then the night sky wouldn't be "as bright as the sun" (like the video says), as observed from earth.

But would the night sky be uniformly bright? Doesn't it depend on the average density of the universe? I.e., if the average density gets low enough, then don't those small-scale density variations start to matter at some point?

specialist• 14 years ago

> How does distance affect brightness?

Red shift. The universe is filled with photons. But as the universe expands, everything moving away from everything else, the human visible light moves into the infrared spectrum.

gurkendoktor2• 14 years ago

It reminded me of the 'paradox' that 1 + 0.5 + 0.25 + 0.125... is finite and not even very large. You could have an infinite universe with infinitely many stars and still end up with a certain percentage of the sky being dark.

Or maybe I'm mathematically wrong, but the above summation has broken any intuition for me that "the sum of infinitely many things must certainly surpass everything" :(

thaumasiotes• 14 years ago

That sum is even easier to process as finite if you represent it in binary:

1 + 0.1 + 0.01 + 0.001 + 0.0001 +...

clearly, after any number of terms have gone by, you're still not at two (though of course, once they all have, you are).

mikeash• 14 years ago

The sun's brightness per area is constant with distance. If you move twice as close to the sun, the brightness per area is exactly the same as it is on Earth, it just covers four times as much of the sky.

If every sight line ended on a star, you'd expect the entire sky's brightness per area to be that of a star.

iscrewyou• 14 years ago

Click the picture for The Milky Way from Mars. It's worth it.

splat3• 14 years ago

It's not usually noted, but Olber's paradox relies on a false assumption. The argument goes that the night sky is dark and therefore the universe must be finite in extent or duration or both. But the night sky isn't really dark. Olber was just looking at the wrong wavelength. If you look in microwave wavelengths, of course, you see the CMB. The fact that the CMB is out in microwave wavelengths rather than being somewhere where Olber could see it is strong evidence for an expanding universe.

shardling3• 14 years ago

Ehh, the CMB doesn't come from stars, so bringing it up in this context is a bit of a red herring.

Dylan16807• 14 years ago

Being right for the wrong reason can be even worse than being wrong. CMB is very relevant to the question of the color of the sky, it just happens to be pre-star material.

splat3• 14 years ago

True, but the physics that produced the CMB during recombination is the same as the physics that produces light in stellar atmospheres.

Retric• 14 years ago

There is actually a much better explanation than this. When you look at the nights sky you see a snapshot of the universe. The further back you look the more stuff there is, but while there more objects there proportionally dimmer.

Now, in an unaging and infinite universe with random stars everywhere you would get what amounts to unlimited light. But, our universe is finite and does age, so you get a finite amount of light from that fixed volume. As to how much light you end up seeing it's a function of:

A) Being close to our Galaxy the Milky Way with a large clump of stars inside it.

B) The average of all the galaxy's in the observable universe which relates to the average amount of matter in the universe which is not that high.

Thus, a fairly dim sky outside of the disk that is the milky way.

tkahn6• 14 years ago

Um is it just me or does the conclusion of the article not match the ultimate conclusion of the video?

In the last 15 seconds of the video they say that it's actually because once you get far enough away from earth, the stars are moving so fast away that they redshift to infrared.

waynecochran• 14 years ago

I always thought the sky was dark for the same reason we don't see the bright center of the Milky Way at night: dust.

Space is mostly vacuum, but there is enough dust over the vast volumes to occlude light.

T-hawk• 14 years ago

Dust does not resolve the paradox. Dust occludes by absorbing the light, which means that the dust would eventually heat up enough to glow itself. Dust can't reduce the amount of radiation and energy in the universe, just shift it in time or wavelength or direction.

kfury• 14 years ago

My biggest problem with this explanation is that even if the star-filled universe were infinite that doesn't necessarily mean there would be a star at every point you look at in the sky.

Depending on the density of the stars in space, it's quite probable that as you zoom in farther and farther you see more and more distant stars, you also see space between those stars. It becomes a limit problem where the 'star density' of the sky (vectors from your viewpoint which will eventually hit a star) gets closer and closer to a specific percentage the further you extend your sphere of view (or magnification level) but will never exceed it, and won't ever go to 1.

shardling3• 14 years ago

Assuming that there's some small but non-zero chance of a randomly selected line intersecting with a star in a finite volume of space, it's pretty clear that for an infinite volume, the probability of hitting at least one star will approach 1. (So long as the density of stars is approximately constant, or at least has some lower bound.)

And since stars are not point like objects, a line does indeed have a non-infinitesimal chance of intersecting a star.

Although all of this is a bit different from arguing about the net brightness of light coming from distant stars.

dredmorbius2• 14 years ago

The theory also assumes that objects in the Universe can only emit light. This isn't true: most matter in the Universe is dark (dust, planets, dark matter), and absorbs light.

We know that this is highly true at intragalactic scales -- the core of the Milky Way is completely obscured from Earth due to the dust between us and it. There's good reason to believe it's also true at intergalactic scales.

Other aspects of the paradox contribute, but I strongly suspect that various dark matter effects are stronger.

drcube5• 14 years ago

I think you're wrong here. Given an infinite amount of time and an infinite number of stars distributed evenly in an infinite universe, even the dust and planets will start to radiate[1].

The paradox hinges on the assumption that the universe is infinite in size with an even distribution of stars which have been radiating for eternity. So, it's only a paradox if you assume those things. Remember, this question was first asked in the 1600s.

[1] https://en.wikipedia.org/wiki/Black_body_radiation

dredmorbius2• 14 years ago

The dust and planets will start to radiate ... at the blackbody rate.

On average, that's somewhere in the neighborhood of 3K.

rcthompson• 14 years ago

The fundamental question here is, if you pick a random vector going outward from the earth, what us the probability of that vector intersecting a star? If it is near 100%, then you would expect a bright sky at night, and if you don't see one then you have a paradox.

chime• 14 years ago

Even if it is 100%, it would have to be multiplied by the probability of photons from that star making it past all the interferences like nebulae, Oort cloud, and Earth's atmosphere along that exact vector. That I think is a much lower percentage.

lutusp10• 14 years ago

> Even if it is 100%, it would have to be multiplied by the probability of photons from that star making it past all the interferences like nebulae, Oort cloud, and Earth's atmosphere along that exact vector.

You're missing the point that, given enough time, all those objects would be heated up by stellar radiation to the temperature of the originating star.

Consider the temperature over time of a body that is energetically coupled to a star, however far away. Because the star's temperature is relatively constant (a property of fusion reactions), it is the receiving body's temperature that changes, according to this equation:

q = (e^(-t/k) - 1)*(a - b) + a

Where:

t = time

k = energy transfer factor

q = temperature at time t

a = temperature at time 0

b = temperature of source

The above expresses Newton's Law of Cooling:

https://www.dropbox.com/s/bt63bt59t9q76th/newtons_cooling_la...

All the bodies exposed to a star's energy radiation follow the above law. And given enough time and barring any other effects, all of them reach the star's surface temperature. Which leads us to Olbers' Paradox -- given billions of years and copious energy sources, why didn't this happen?

erikpukinskis• 14 years ago

Paradoxes and assumptions aside, is it literally true that there is a star at the end of every vector you can draw from my eyeball into space? I understand it's true if you assume the universe is infinitely large, but we know that's not true, right?

svachalek• 14 years ago

My understanding is that it would be true but for the fact that the longer those vectors get, the farther back in time they reach, and thus eventually reach into a time when stars did not exist and thus you hit the CMB instead. The stars you would have seen are in space that has moved away from us faster than c.

My understanding is that the universe is believed to be literally infinite in the three spatial dimensions that are familiar to us, and that its mass is also believed to be infinite. It really blows the mind.

Disclaimer: I'm a programmer not a physicist. :-)

EDIT: I did a little more reading on this. Answers are all over the place. But from what I can make out of the most recent sources, it seems that modern models treat the universe "as if" it were infinite although there is no way to know whether it is or not.

maaku• 14 years ago

There are very few patches of the sky where if you look you won't see (very many) stellar objects or galaxies. Astronomers know about them--they're great for calibration of instruments.

That said, Hubble took a many-day exposure of one of these ultra-deep regions, and this is what it saw:

http://hubblesite.org/gallery/tours/tour-hudf/

dotborg2• 14 years ago

  *A few billion years after that, you'll be standing on a hill looking up on a clear night, and the sky will be close to pitch black*

not that long ago we didn't know that we live on a sphere

what if universe is a 4 dimensional sphere?

lutusp10• 14 years ago

> what if universe is a 4 dimensional sphere?

If the sphere is expanding, then the result is the same -- a gradual decline in energy density per unit of volume. So the specific geometry of the universe is unrelated to Olbers' Paradox.

But there is strong evidence that the universe is geometrically flat at large scales, which in turn argues that it's infinite in size.

How does the apparent large-scale flatness of the universe make an argument for an infinite size? To explain, and just as a simplifying example, imagine that the universe is the surface of sphere. Now include the implications of the fact that we observe large-scale flatness.

Picture this -- imagine that the universe is the surface of a sphere, but the sphere's surface is perfectly flat. How large must the sphere's radius be for its surface to be perfectly flat? Think about how a sphere's surface is defined -- it's the unique surface that's equidistant from the center of the sphere, the surface that has a distance of R (R = radius).

To make both properties true -- to accommodate (a) that it is a sphere and (b) that its surface is perfectly flat, all you need to do is make the radius infinite.

In the case of the universe, with more dimensions, to achieve the measured large-scale flatness, all one need do is assume that the universe is infinite in size.

dotborg2• 14 years ago

the idea of universe size increasing speed now comes to my mind,

if the speed is greater or equal to speed of light, how we can be sure that we won't see ourselves in telescope?

I mean, even if the radius is infinite, because speed is increasing, we should see sun light returning to us over universe surface at some point

Nick_C• 14 years ago

> not that long ago we didn't know that we live on a sphere

Actually, the ancient Greeks knew it was a sphere, and everyone knew by the Middle Ages. Surprisingly perhaps, given their other astronomical work, the Chinese were the last hangers on. The notion that we discovered it relatively recently seems to have been due to a propaganda campaign in the 19th Century.

lelf• 14 years ago

http://en.wikipedia.org/wiki/Olbers_paradox

Sun Goes Down. Up Comes A Mystery

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