If the sun's gravity is so strong as to keep whole planets circling it in orbit, how come light and heat can escape?
November 14, 2004 3:33 PM   Subscribe

ScientificIlliterateFilter: If the sun's gravity is so strong as to keep whole planets circling it in orbit (even ones million of miles away), how come light and heat (incredibly fragile things compared to planets) can escape as rays (or whatever they are)?
posted by amberglow to Science & Nature (53 answers total)
Heat doesn't travel. Light does. The object struck by the light heats up.

Light doesn't get disrupted that much by the sun because it's a "whatever it is". It is barely touched by the sun because it can be considered mass-less.

The experiments that proved that light is bent by gravity sources only worked because it was light cutting at a tangent to the object - light scoming from the sun is fighting directly against the gravity and is only effected as a infantessimate slowing of the light (if at all).

By the way, I'm drunk and tired and heading for bed, so some or all of this may be complete bollocks. Especially the spelling. :)
posted by twine42 at 3:49 PM on November 14, 2004

Twine42, I'm pretty sure light radiated from the sun can't be slowed down by gravity because the speed of light (in a given medium, in this case vacuum) is constant; light can't travel slower than light, as it were. If light loses energy to gravity, it loses it as frequency; shifting down the spectrum. Someone who knows their fizzicks can probably correct me on this.
posted by George_Spiggott at 4:04 PM on November 14, 2004

Depending on what level you want an answer, your question is actually deep and touches on topics at the edge of physics.

1) Have you heard of "escape velocity"? In order to escape from a planet or a Sun, all any object of matter needs is to go above that speed, regardless of its mass. This is because of a wierd, fundamental property of gravity that the force of gravity cancels the intertia, exactly.
Anyways, light because it is traveling so fast (at the speed of light of course) can escape all but the strongest gravitational fields (i.e. black holes)

2) Others might chime in and tell you light is massless so it cant "feel" the gravitational field. This is wrong. Light has no rest mass but it does have energy E=hf and is subject to gravity fields. Einstein showed this.
However, because Light cant slow down, it must give up its energy some other way, as it leaves a planet or Sun. As you can see from the equation above, f=frequency is lowered and so light escaping from the Sun actually lowers its frequency as it departs.

3) Viewed this way you can see that all that light-energy coming from the Sun is not fragile at all. The Earth is just a pile of rocks. All the organization you see, the trees, the animals, humans, is fed off of the tiny amount of Light that our planet manages to capture from the Sun.
posted by vacapinta at 4:10 PM on November 14, 2004

thanks (i'm reading The Confusion, and there's all this science stuff that's raising questions for me in the trilogy so far, since i'm clueless at most scientific things--elementary school level prob). Escape velocity makes sense, but then i have to ask -- why does light go so fast? Why can't it slow down, or, why isn't it affected by gravity the way other things (like us, or sound, etc) are? : >
posted by amberglow at 4:22 PM on November 14, 2004

Light goes so fast because it doesn't weigh anything. Light travels in "packets" called photons that behave as both particles AND a wave at the same time. (Look up: wave particle duality). Because they don't weigh anything they travel really really fast - as fast as is possible in fact, before they run into the speed limit of the universe. Light isn't the only thing to travel at this speed, it's just the first thing we discovered that does. That's why it's called "light speed".
posted by Mwongozi at 4:28 PM on November 14, 2004

Also: Light is affected by gravity. Just not much. Also look up: gravitational lensing
posted by Mwongozi at 4:29 PM on November 14, 2004

First, sound isn't affected by gravity either. Regarding light. Think of it this way - gravity affects things with mass. Light doesn't have mass (I know, I know, it isn't that simple, but think of it that way...) so it isn't affected by gravity. Light isn't something that moves but rather is a wave (like a sound wave or any other type of wave...) that travels thru a medium. That medium might be affected by gravity as the medium has a mass, but the light itself doesn't actually have any mass to be affected.

Ok, so the sun is radiating gravity and pulling the medium (space) towards it. The light wave is traveling thru the medium away from the sun at the speed of light. From the perspective of the light wave ("I am Jack's light wave...") it is traveling thru the medium at the speed of light. From the perspective of the medium, the light is traveling thru it at the speed of light. HOWEVER from the perspective of the sun the light is traveling away from it at a slightly slower rate than the speed of light BECAUSE the light might be moving at the speed of light, but the medium it is traveling thru is moving backwards towards the sun.

posted by pwb503 at 4:32 PM on November 14, 2004

ahhhh, thanks!...so we haven't found anything else that's as fast as light yet? will we?

and--so does that mean a flashlight shone out from an astronaut in space would go further? as opposed to one down here?
posted by amberglow at 4:38 PM on November 14, 2004

Nothing can go faster than light. 299792458 m/s - it's not just a good idea: It's the law.

Just to confuse you even further, light affected by gravity doesn't slow down, it just bends. It can't slow down, because the speed of light is a constant. Explaining that however, would take a v. long time.
posted by Mwongozi at 4:41 PM on November 14, 2004

The speed of light is the ultimate speed in our Universe. Its nothing I can prove. That just happens to be the speed. Einstein took it as an axiom and nobody has been able to prove him wrong (The case for Relativity has just gotten stronger and stronger)

That said, as Mwongozi said, there are possibly other things that travel at the speed of light besides light, like the gluons or possibly the neutrino (which may actually have mass and thus cant go at the speed of light)
posted by vacapinta at 4:54 PM on November 14, 2004

Light (photons) have no mass. They cannot be affected by gravity (gravity affects mass only), but the spacetime they travel in does, thus they appear to be affected by gravity.

The hubblesite explains it with more detail here.
posted by skallas at 4:54 PM on November 14, 2004

skallas: I was trying to gloss over that to prevent further questioning. :)
posted by Mwongozi at 4:56 PM on November 14, 2004

thanks. (altho that's not a satisfying answer about why the speed of light is the fastest, and only thing that fast)

and of course, one more question: if things that are waves don't have mass, why don't they all travel at the speed of light? sound certainly doesn't go as fast (that whole "you see lightning before you hear the thunder" thing) Would thunder and lightning happen at the same time in a vacuum?
posted by amberglow at 5:01 PM on November 14, 2004

A better way to think of it, amberglow, is that gravity curves space and light photons travel through space.

Take a marble - pretend that marble is a single photon (packet) of light. Now imagine a large sheet of jello. You press down on a spot on the sheet of jello, and it depresses around your finger - send the marble rolling near that spot at a high enough speed that it continues rolling on past - see how the marble's path is slightly affected by the depression in the jello?

Gravity is like your finger, curving space (space in this case is jello), and light is like your marble. A black hole is when you push down so hard (gravity is so intense) that any nearby marbles get caught and stay there. A wormhole is when you take a block of jello (space) and pinch opposite sides together so hard that your fingers are virtually touching.

As to a flashlight in space - that really has nothing to do with the laws of light, and more to do with the fact that light dissipates more quickly in any medium (ie air) than the vaccuum of space (which contains nothing to interfere with the photons). As light shines out into space, though, the area it is spread out over naturally increases (there's an easy formula for this I can't recall offhand) and the number of photons that will hit your retina decreases - so the light gets dimmer (from Pluto the sun appears as a very bright star). The photons from that flashlight in space will go on forever, but at a certain point they're so spread out that it makes no real difference. Down here on Earth there's just too many pesky air molecules for the photons to bump into for light to travel that kind of distance.
posted by Ryvar at 5:01 PM on November 14, 2004

The "things that are waves don't have mass" assumption is wrong. Actually, even "normal" particles like electrons have wave-particle duality.

There is no sound in a vacuum, amberglow. Sound is just a wave that requires the presence of matter.
posted by vacapinta at 5:04 PM on November 14, 2004

Also, skallas is unfortunately talking about the quasi-Newtonian conception of Gravity when he says "They cannot be affected by gravity (gravity affects mass only), but the spacetime they travel in does, thus they appear to be affected by gravity."

Rather, as Ryvar, points out, Gravity is the property of matter which causes it to alter the spacetime around it. Other objects then respond to the shape of spacetime. The degree to which spacetime is altered depends on an object's mass-energy.

So, does Light have mass-energy? Yes. Can it alter spacetime? Theory says yes. Does it respect the shape of spacetime? Yes. So, by all definitions, Light does have mass.

My guess is that this is why Mwongozi (and me) didnt want to bring this up.
posted by vacapinta at 5:09 PM on November 14, 2004

As to things that are faster, there are theoretical particles dubbed 'tachyons' that may in fact travel faster than light.

Electromagnetic waves certainly don't have mass - but a sound wave is really just a cascade of vibrations transferred through a medium (usually the atmosphere). When you clap, some air molecules knock into other air molecules knock into your ear drum which transfers those vibrations into a neural signal which travels to your brain.

In water, which is denser than air, sound travels faster (and further) than in air, because the increased density ensures a more thorough transfer of vibrations between molecules. This is why sonar is so popular for submarines and the like.
posted by Ryvar at 5:11 PM on November 14, 2004

but it's just so little mass that it doesn't act like other things with mass?
posted by amberglow at 5:13 PM on November 14, 2004

It should be noted, that as vacapinta points out - electromagnetic energy (including visible light) may have some kind of mass, but it's so small that for the purposes of helping you get a leg up on physics, it's best that we pretend it doesn't.
posted by Ryvar at 5:14 PM on November 14, 2004


This is reminding me of that whole thing about our old tv shows now a certain number of miles (lightyears?) away, and still traveling. So it's just the light of those and not the sounds (or vibrations from the sounds)?
posted by amberglow at 5:18 PM on November 14, 2004

Here's how I learned it:

Stars emit energy in the form of leptons, which include electrons, neutrinos and gamma particles.
This is accomplished through a process called stellar mucleosynthesis; the matter expelled from atomic reaction isn't nearly as dense nor identically "charged" as the material residing within the stellar core.

Concentrated clusters of matter, such as vapor and dust, are more succeptable to the pull of the sun's gravitational well. (This is partially due to their atomic makeup, as well as the process of accretion.) Some of the leptons colliding with the neighboring material are reflected/deflected, as in the case of "moonlight". Subatomic particles, like neutinos and quarks, can pass through the surrounding material; concentrations of particularly dense matter, such as planets, can slow down the rate in which the neutrinos travel...but only to a (very) limited extent.

It's only in the case of extreme solar density that leptons have any greater "difficulty" in escaping; neutron stars have considerable dimmness, yet still expell/emit gamma particles, as do black holes and/or gravistars.
posted by Smart Dalek at 5:21 PM on November 14, 2004

>skallas is unfortunately talking about the quasi-Newtonian conception of Gravity

I'm taking the simple approach to a simple question. My explanation is perfectly reasonable, unless the question was a some kind of mandate to go off on QM and advanced physics thus leaving everyone in the dark. Heh, pun intended!
posted by skallas at 5:23 PM on November 14, 2004

I appreciate it--skallas, and all of you--for the simple and the not-so-simple answers. : >
posted by amberglow at 5:26 PM on November 14, 2004

amberglow: TV is transmitted via electromagnetic waves in the radio portion of the frequency (IIRC), both data for pictures and data for sound (which is then sent to speakers which convert that signal into vibrations through the air which hit your ears) - anyone who can figure out what to do with both parts of the signal will be able to enjoy both pictures and sound.
posted by Ryvar at 5:27 PM on November 14, 2004

>So it's just the light of those and not the sounds (or vibrations from the sounds)?

Nope. Television broadcasts are radio broadcasts. Radio waves are photons too. Microwave energy, satellites broadcasts, gamma radiation (all radiation), etc are al photons.

When your TV gets the radio waves it decodes it into light and sound. Like how your computer gets digital information and turns it into video with sound, yet we all know its just binary "one's and zero's."

>I appreciate it--skallas, and all of you

NP. I think most sciencey people have an inner Carl Sagan and really want to explain this stuff to others.
posted by skallas at 5:29 PM on November 14, 2004

ok...so are those waves losing frequency as they go along too--getting weaker? like light?
posted by amberglow at 5:31 PM on November 14, 2004

Ignoring redshift issues not germane to this conversation, they're not losing 'frequency' but they are getting weaker like light.
posted by Ryvar at 5:34 PM on November 14, 2004

Amberglow: "sound vibrations" in space, such as the tones captured by satellites passing Jupiter are really energy waves creating feedback. Even with layers of dust present in space, the overall effect it's closer in nature to static than it is to acoustic vibrations, which rely upon a medium such as gas or water.

With all due respect toward astronomers and physicists alike, an audio engineer would tell you that "noise" is noise, regardless. It's just not necessairily the same noise you're seeking. Were there enough air or water in the cosmos for producing a "traditional" soundshow, the captured eminations would be very different from the spectral profiles caught by Voyager and company.
posted by Smart Dalek at 5:34 PM on November 14, 2004

I wonder if whoever's out there is pissed at us for continually sending static to them, and noise-polluting the whole area?

Does Earth make a noise tho too? If someone sent a Voyager this way, would ours be like Jupiter's? or is the makeup of Earth too different, and the sounds would be too?
posted by amberglow at 5:41 PM on November 14, 2004

>...so are those waves losing frequency as they go along too--getting weaker? like light?

They're getting weaker (or better yet, harder to detect), but only because as these transmissions travel they get wider. Like how a flashlight creates a cone of light. You really have to capture that entire cone to recreate the signal. So the aliens many lightyears away that would want to watch Ally McBeal would have to build a dish the size of solar systems to watch it! Also their hardware would have to be especially good at picking up the signal (sensitivity).

This page tackles the issue, its a bit techy though.

He has a pretty good example:
Table 2 and Figure 4 further demonstrate that if the Arecibo radio telescope could be transported to Alpha Centauri, scaled up in size and then pointed back towards Earth, then a telescope with a diameter of over 33,000 kilometers would be required to detect UHF television pictures leaked from Earth. This is equivalent to a dish diameter just over two and a half times the diameter of the entire Earth.
Alpha Centuri is our neighbor. Its only 4.something light years away.

Someone better informed than me could probably say something about fidelity too. As these waves intersect with other waves, some alteration/damage has to be done, so even if ET built a dish the size of ten solar systems, the signal would probably be too messed up to decipher. I thinkt he whole "ET could be listening" is a big misconception as it assumes A LOT. There's this "broadcasts are immutable" assumption which I dont think holds water. Not to mention eventually, they should be impossible to decipher from background radiation regardless of how high tech you are.
posted by skallas at 5:48 PM on November 14, 2004

Note: the above applies to leaked signals, UHF, etc only. NASA uses narrow microwaves (and other types of radio) to talk to far away satellites, rovers, etc. The difference is that both parties know where they are at so communication isn't this big gamble. You just point your antenna to the spot on mars where the rover is sitting and it shoots back info to Houston (or wherever) on Earth.

When it comes to leaked TV/Radio etc its a very big gamble.
posted by skallas at 5:56 PM on November 14, 2004

Single Female Lawyer, skallas ; > (and thanks--that's kinda disappointing tho--but probably just as well for us. But that means we'll never hear/see them either, right?)
posted by amberglow at 5:58 PM on November 14, 2004

Theres this joke in astronomy circles that aliens have been trying to contact us forever but have just been too obvious about it.

That is, they'll arrive and complain "We exploded entire stars (supernovas) and even left huge shining beacons inside (pulsars) to get your attention, so surely you guys knew we were coming, right?"
posted by vacapinta at 6:04 PM on November 14, 2004

Escape velocity is really all you need to understand to answer amberglow's original question. At a given distance from a given mass, there is some speed above which things will leave, below which things will fall, and at which things can orbit. The closer you start, the faster you have to move to get away.

At Earth's disance from our Sun, that orbital velocity is (roughly) 2 x pi x 93000000 miles / 365.25 days, and around we go - if we were moving this slowly nearer the Sun we'd fall right in. But the speed of light is high enough that even at the Sun's surface, the Sun's gravity can't hold it back.

Now, if all the Sun's mass were close enough to its center - which would require it to be crushed into a smaller, denser star - then escape velocity at its surface would be greater than the speed of light, and it would become a black hole. (Which won't happen to our Sun but that's another story.)
posted by nicwolff at 6:20 PM on November 14, 2004

ok then! : >

so...back to aliens: are they there or not? (now that i have all you smarties in one place)
posted by amberglow at 6:36 PM on November 14, 2004

It would be nice, amberglow, as it would shut up some (not all) of the fundies - but I have a feeling if there is other intelligent life out there, the odds of it being recognizable AND able to communicate with us in a meaningful fashion are very very remote. Chances are we'll find it, it'll turn out to be a fascinating new energy source and we'll commit wholesale genocide for profit until they get their shit together and wipe us all out.
posted by Ryvar at 6:47 PM on November 14, 2004

that would suck. I think there's definitely someones/somethings out there, but i think we're not at all ready to meet them (and may never be)--without doing something like what you said.
posted by amberglow at 7:01 PM on November 14, 2004

>so...back to aliens: are they there or not?

You can play with the Drake Equation and come up with some number. You really have to consider the lifespan of a species or civilization (which is L in the equation). On top of it all the universe is a big and old place, so you can ask yourself "Did 1 million advanced alien civilizations nearby to us die out by the time we were still trying to get out of Africa?" And the more depressing "Will humans civilization die out before we can make contact?"

Let's not ignore the relative slugginess of the speed of light (talking to aliens 50,000 light years away is doable, but will humans be alive to hear the reply in 100k years?). The univrese is 156 BILLION lightyears wide and 14 BILLION years old.

We can get a bit more sci-fi too like so:

You can do away with lightspeed if you assume some ancient civilization exists and has sent out exploration ships millions of years ago and are searching out the universe. So they could be right outside the solar system. Then you have some very interesting philosphical questions:

1. Why make contact? Ever? To an ethical alien the best thing (to them) to do would to leave us alone like a wildlife refuge.

2. Is communication even possible/meaningful? If we discovered a planet full of dogs, would we try to teach them calculus? Or germ theory? Of course not.

I think it takes a special kind of hubris (perhaps not the best word) to think that human civilization would be something advanced aliens would care much about. Or perhaps they do but see humans as a dead end race. In then end, it seems this all comes down to alien psychology, which we know nothing of.
posted by skallas at 8:57 PM on November 14, 2004

Does Earth make a noise tho too? If someone sent a Voyager this way, would ours be like Jupiter's? or is the makeup of Earth too different, and the sounds would be too?

Crank up your speakers, or jam on your headphones. There's a fair amount of concert footage produced from Earth's own magnetosphere.
posted by Smart Dalek at 3:51 AM on November 15, 2004

that's so cool--thanks Smart! (but is that what you hear in space?)

skallas, why wouldn't they be curious about us? We've always been curious about what's out there--throughout recorded history, no?
posted by amberglow at 4:16 AM on November 15, 2004

altho that's not a satisfying answer about why the speed of light is the fastest, and only thing that fast

i didn't see a direct answer to the "philosphical" part of this.

if you think of something very basic, like numbers, then there are some concepts that seem to eb fundamental. in the case of numbers, zero is the obvious choice. if you start with zero, you can make other numbers by saying, for example, "i'll say a zero all by itself is just one zero; if i put another next to it i have two".

maybe that seems stupid - it's actually the best way i can find to describe the basics (as many people see them) of mathematics (set theory).

anyway, my point is that some things are just axiomatic. you have to start somewhere. for maths, zero is a pretty good place to start.

obviously there's a degree of arbitrariness in this, because - assuming you believe analytic statements are possible (i think it's fair to say that since quine we are all quietly pretending this is the case, unless we're crackpot european philosophers, in which case we don't know about science anyway) - you can start with something different but equivalent. so you might start with 1 and define zero as what you have when you take 1 away from 1.

so we have to assume a certain aesthetic stance too when choosing axioms. generally we follow - damn the name escapes me, but you know - the person who said avoid unnecessary complications. we pick the simplest, in some vague sense (and people think science is a well defined thing ;o)

anyway, so we have things that are axiomatic. like zero in maths.

what i am getting to is: the speed of light is axiomatic in physics. and this is far from intuitive. in fact no-one believed it, really, at first, when einstein suggested it. but when you follow the logical consequences you find that making the speed of light axiomatic implies a pile of freaky shit that happens to be true (like time not being the same for everyone, and space being curved).

which is the second reason why einstein was a genius.

occam. is the name i forgot earlier. anyway, the speed of light just is. in the same way that zero just is.

it's odd, and not very satisfying. on the other hand, presumably you have to start somewhere. at the moment, physics is in a bit of a mess because there are too many things that you have to arbitrarily fix to explain the world. most of them are way more obscure than the speed of light. so if you're going to be worried about that then, really, it's the least of your worries.

but when things settle down, and physics sorts itself out (whether philosophers do or not, i suspect) it looks likely that the speed of light could remain axiomatic. if so, it's a fundamental thing about the universe. so on the plus side, you can at least say that it's something very deep, that you know.
posted by andrew cooke at 4:36 AM on November 15, 2004

but it's just so little mass that it doesn't act like other things with mass?

Einstein showed that mass is energy, and energy is mass.

If you whack your hand against your desk hard, it makes a louder sound and hurts more than if you tap your desk softy: the fast-moving hand has more kinetic energy than the slow one.

That means it has more mass, because energy is mass: it weighs more the faster it's going, though that's not why it makes a louder sound, really -- it weighs only a tiny bit more unless you're some kind of superhuman hand-moving freak. E = MC2 is a translation equation, like "one mile is 1.8 kilometers": miles and kilometers are both measures of the same thing (distance), and similarly, mass and energy are measures of the same thing.

So let's rewrite E = MC2. Divide both sides by C2 and you have E / C2 = M. (In english: the amount of energy, divided by the-speed-of-light-squared, is the amount of mass.)

And the-speed-of-light-squared is a huge-ass number: about 89,875,517,900,000,000 m2 / s2. About 90 quadrillion (that's the one after trillion).

And, of course, when you divide by a huge number, you get a tiny number. The mass equivalent of any familiar amount of energy is immeasurably tiny. But the speed of light is so high that once you've accellerated far enough toward it, you've added enough speed (kinetic energy) to start making a noticable difference in mass -- and it's harder to move something with more mass; it requires more energy for each mile-per-hour you increase the speed, and because you used more energy for that small increase, your mass increased by more than it would have if you'd only used a little bit of energy, and then you need even more energy to increase the speed by another mile per hour.

This is one of the reasons that anything that already weighs something can never get to light speed: it would weigh an infinite amount, it would take an infinite amount of energy to get it there.

Light, if it wasn't moving, wouldn't have any mass. But because it's going at light speed, it does have mass. Sort of.
posted by Tlogmer at 5:01 AM on November 15, 2004

I'm no physicist, but I'm fairly sure all of these claims that light has mass because it has energy are wrong. Even though light has energy, it has no mass, because the equation E=mc2 is a simplification for low-speed situations. In fact, the full equation that works at low and high speeds is

E = mc2 / sqrt(1 - v2/c2)

Notice the sqrt denominator. You'll see how this precludes an object with non-zero mass from moving at the speed of light: if you substitude c (the speed of light) for v (the speed of the object), you get sqrt(1 - c2/c2) = sqrt(1 - 1) = 0 in the denominator -- division-by-zero! However, if the mass is also zero, then you get 0/0... which is also an error, but one that can be fixed.

If you solve the equation for m, you get

m = E * sqrt(1 - v2/c2) / c2

Here, if you set v=c, then you get m=0, which means that only a mass-less object can travel at the speed of light. Consequently, light can travel at the speed of light c because photons have no mass. Not a very tiny mass, not so-close-to-zero-it-might-as-well-be-zero-mass-for-our-purposes. Just plain zero.
posted by Khalad at 7:04 AM on November 15, 2004

khalad - there are two different things "rest mass" and, hmm, "mass". and people aren't being clear about which they're talking about. in your case, you're talking about rest mass, and it is zero for a photon. however, the effective mass (from the point of view of inertia or gravity) is the other one, which gets bigger with speed.

so everyone is right. awww. :o)
posted by andrew cooke at 7:09 AM on November 15, 2004

light can't travel slower than light, as it were.

no, I don't think that's right. Light travels more slowly in water, e.g. - here - maybe you just meant in a vaccuum, but just to make sure amberglow didn't get confused on that. I didn't see anyone respond to his question "Why can't it slow down" - so the answer is, it can.

Fun book on searching for things faster than light is "superluminal loopholes" by nick herbert (obviously, his final conclusions are always that nothing really can travel faster, but he discusses all the things that have been considered possibilities).
posted by mdn at 7:46 AM on November 15, 2004

(thanks, but now i'm back to being confused)

so--when light hits the ground here, or an asteroid on its way to us, or any solid thing, etc, it's stopped, or just slowed down?
posted by amberglow at 10:54 AM on November 15, 2004

It's absorbed and the energy is converted into another form, such as heat, and thusly do the Sun's dull rays warm our pillowed Earth.
posted by Khalad at 11:14 AM on November 15, 2004

Well, assuming memory serves me here, two things happen - some of the photons bounce off the asteroid/Earth (are reflected) and travel through space - a few (billions) might happen to hit the Hubble Telescope and the result is that we 'see' the asteroid. Or several . . . can't even guess the number . . . bounce off the Earth and astronauts floating in the space station can use them to see the Earth.

The rest are absorbed by the asteroid/Earth and are converted into heat.

Take a really bright flashlight. Hold it up against the palm of your hand and turn it on. Notice how the back of your hand lights up? Your hand isn't dense enough nor thick enough to absorb all the photons streaming out of the flashlight (it's a bit more complicated than this - photons do all sorts of crazy shit in semi-transparent mediums like flesh, but this is close enough for today's purposes). Do the same thing with a handsized rock - notice the back side doesn't light up at all. Why? Too many densely-packed rock molecules getting in the way of the photons.

Asteroids are made up of the same stuff, generally, as your rock there.
posted by Ryvar at 11:16 AM on November 15, 2004

mdn:no, I don't think that's right. Light travels more slowly in water, e.g. - here - maybe you just meant in a vaccuum, but just to make sure amberglow didn't get confused on that. I didn't see anyone respond to his question "Why can't it slow down" - so the answer is, it can.

Thats not really true, mdn. Light always goes at C. The reason it appears to go slower in certain mediums is because they are measuring the group velocity not the phase velocity. In essence, it goes "slower" because its bouncing around a lot and so its progress in one direction may be slower than light but Light never actually slows down.

You can play with these ideas with this applet. By moving the slider bars you can get the "waves" to go any speed you want. But the speed of any individual wave is always 1 (e.g. speed of light) its just their superposition which appears to travel slower.
posted by vacapinta at 11:46 AM on November 15, 2004

incidentally, getting back to the original question. the panets pull back too. gravity is about both masses. which is why big "heavy" planets are pulled round the sun, while motes of light can stream away. (which, i know, gets us back to what others have said).
posted by andrew cooke at 1:25 PM on November 15, 2004

i think my mind needs a for instance, for all this. I'm thinking of when you blow on a dandelion, but that's not quite it...
posted by amberglow at 2:40 PM on November 15, 2004

(my last comment above was wrong, unfortunately. ignore it... sorry!)
posted by andrew cooke at 4:07 AM on November 16, 2004

no prob. thanks tho, everyone : >
posted by amberglow at 3:09 PM on November 16, 2004

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