0811: "Starlight"

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Mr. Burke
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Re: 0811: "Starlight"

Postby Mr. Burke » Wed Oct 27, 2010 6:17 am UTC

Ego wrote:Yeah, mining physics knowledge from wikipedia is lame.
No, merely a contradiction.

In other news, “what if inanimate objects were animate” jokes are so 1850's.

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atimholt
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Re: 0811: "Starlight"

Postby atimholt » Wed Oct 27, 2010 6:21 am UTC

Feynman's QED actually explains the reflection thing really well. It all has to do with the wave-like nature of light, and the reflecting surface not absorbing too much of it.
More or less, light can travel in any path, and only travels in a straight line because the integral of all possible paths cancel each-other out due to interference, except in a straight line. But near a flat, non-absorbent surface, the path integrals do funny things and the photon takes a sharp turn without interacting with the surface in a necessarily intuitive manner. The book has illustrations that really clarifies things.
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Re: 0811: "Starlight"

Postby black_beret_guy » Wed Oct 27, 2010 6:41 am UTC

BlazeOrangeDeer wrote:
Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

I thought thermodynamics were a macro effect of quantum events, just as newtonian mechanics are basically the total of quantum events in a large system. So yeah, they basically "go wonky" at the quantum scale and the photon retains the energy.


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Re: 0811: "Starlight"

Postby NotAllThere » Wed Oct 27, 2010 6:47 am UTC

What happens to an individual photon seems to me to be fairly irrelevant. What matters is the information.
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Re: 0811: "Starlight"

Postby Mokurai » Wed Oct 27, 2010 6:52 am UTC

The photon doesn't die in your eye when it gets absorbed. It activates a visual receptor molecule, which activates a nerve fiber. A large number of photons activate a large number of nerves creating signals which are processed in the optic nerve and brain, resulting in the visual sensation of the dots in the sky. The energy from the photon continues to be passed around forever, mixed with energy from many other sources, occasionally resulting in other photons being emitted.

Photons almost never travel at C. They travel at a rate determined by the index of refraction of the medium. Since a vacuum is never completely empty, its index of refraction is never exactly 1. This gives the photon a little bit of proper time when travelling from a star billions of light years away (up to about 1e22 kilometers measured along the world line of a photon from the earliest generation of stars).

Richard Feynman explained in his little book QED that photons reflected by a mirror act as though they are absorbed by many atoms and reradiated along a path of constructive interference over all possible paths.

The handedness of polarization of reflected photons is reversed, since the mirror image of a helix is a helix of opposite handedness.

All of the above is very accurately correct, but fundamentally wrong. The wave equation of quantum mechanics
has been adapted for Special Relativity, but cannot be made to work in General Relativity. This is not a problem for photons, or whatever, which go along doing whatever it is they do with no need to have us understand what that is. There is, however, a problem for Young-Earth Creationists, who want to believe that we see stars by the light that they emit, and that they are really as far away as they seem. Creationism doesn't have time for this to happen, which means that God created the light close enough to us to get here on time as if it came from distant stars, or some other such humbug.

Cosmological redshift, gravitational lensing, and synesthesia are left as exercises for the interested reader.

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Re: 0811: "Starlight"

Postby SocialSceneRepairman » Wed Oct 27, 2010 6:58 am UTC

atimholt wrote:More or less, light can travel in any path, and only travels in a straight line because the integral of all possible paths cancel each-other out due to interference, except in a straight line. But near a flat, non-absorbent surface, the path integrals do funny things and the photon takes a sharp turn without interacting with the surface in a necessarily intuitive manner. The book has illustrations that really clarifies things.



...but light doesn't travel in a straight line. It radiates out in all directions. Otherwise, you wouldn't be able to see anything not directly in front of you.

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Re: 0811: "Starlight"

Postby bjornart » Wed Oct 27, 2010 7:02 am UTC

SocialSceneRepairman wrote:
atimholt wrote:More or less, light can travel in any path, and only travels in a straight line because the integral of all possible paths cancel each-other out due to interference, except in a straight line. But near a flat, non-absorbent surface, the path integrals do funny things and the photon takes a sharp turn without interacting with the surface in a necessarily intuitive manner. The book has illustrations that really clarifies things.



...but light doesn't travel in a straight line. It radiates out in all directions. Otherwise, you wouldn't be able to see anything not directly in front of you.


You're joking, right?

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StClair
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Re: 0811: "Starlight"

Postby StClair » Wed Oct 27, 2010 7:04 am UTC

The alt/title text made me "awww."

But the starlight's purpose is not "so we can see pretty dots." The photon travels until it hits something - that is its purpose. It would do the same if we, or our planet or our star, did not exist. It is not here for us; we are here to observe it. The universe, knowing itself.

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Re: 0811: "Starlight"

Postby paotrip » Wed Oct 27, 2010 7:23 am UTC

that was a mirror? I thought it was just a frame. I guess the the white dots in the 5th frame were a giveaway. Call me slow.

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Re: 0811: "Starlight"

Postby Essah » Wed Oct 27, 2010 7:36 am UTC

that was deep man...damn

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Re: 0811: "Starlight"

Postby existentialpanda » Wed Oct 27, 2010 7:37 am UTC

LucasBrown wrote:Ruined by the title-text. I like the comic and I like the title-text, but the combination of the two is awful.


Gee, I never thought not knowing anything about relativity would be useful! [/sarcasm]

I thought the comic by itself was quite good, once I figured out that that was a mirror and not a painting of stars that he was holding up.

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Re: 0811: "Starlight"

Postby littlelj » Wed Oct 27, 2010 8:15 am UTC

Meem1029 wrote:Do you have a link or title for that, because that would be a very interesting article to read?

(O.T., but does it seem awkward to anyone else when you ask a question with a side-note at the end as part of the sentence because the punctuation just seems bizarre for that? And I just realized that I did the same thing in my question right there.)


The way I learned it - although that's British English punctuation which differs from American - a question mark can occur in the middle or at the end of the sentence. So you might like to try:

Do you have a link or title for that? because that would be a very interesting article to read.

Does it seem awkward to anyone else when you ask a question with a side-note at the end as part of the sentence? because the punctuation just seems bizarre for that.


Also,

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I heart this comic. Whimsy. Awesome.
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lank
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Re: 0811: "Starlight"

Postby lank » Wed Oct 27, 2010 8:25 am UTC

although it's a cute popular misunderstanding of relativity, the pedant in me must nitpick:

1) in relativity there is no "from the light's point of view" there is no lorentz transformation which can put you in an inertial frame with speed v = c.

2) because there is no such transformation, your eye and the star it was emitted from can never be considered the same place and time (thank god!) unless they are in the same place at the same time. you might be able to pick an inertial frame which contracts the distance to become arbitrarily small (by picking positive epsilon << 1 and v = c-epsilon), but they are still two distinct points in spacetime.

remember the spacetime difference between two events P1 and P2

s2 = (ct)2 - x2

is conserved between inertial reference frames. s2 > 0 => timelike difference (which can be traversed by a massive particle in a frame with x' = 0 and t' = s and t' > 0 in all reference frames), s2 < 0 => spacelike difference (in which s is imaginary and t' can be positive or negative) and s2 = 0 => lightlike or null difference (for which frames with x' = 0 or t' = 0 do not exist).

thus if s2 = 0 (as it must if P1 is a photon's emission and P2 is its absorption), the only way t' = 0 makes sense is to put x' = 0 and thus P1 and P2 coincide. and even if it happens to be warm where you are today, it's just not that warm.

that said, i too appreciate the whimsy!

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Re: 0811: "Starlight"

Postby Singulaire » Wed Oct 27, 2010 8:40 am UTC

great, like I needed another reason to get off my lazy ass and read Q.E.D.
I would still love to have some well-founded information on what happens to the laws of thermodynamics in the sub-atomic level.

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Re: 0811: "Starlight"

Postby xhable » Wed Oct 27, 2010 9:13 am UTC

I'm pretty certain Feynman did a lecture on the photon bouncing from a mirror, I believe I torrented it from isohunt, you might be able to find it if you're interested (it was in a set of 10).

Essentially if I recall correctly the photon is both bounced and absorbed, once from the surface of the mirror, and an infinite amount of times through the mirror simultaneously while been absorbed into molecules in the glass, once at the bottom of the mirror and then all the way back up the same occuring, till you get to the surface of the mirror again... where it both reflects back into the mirror and passes through.. what results is the probability that a certain amount of energy is emitted back. I believe he stated "this can be roughly summarised to 'it bounces with less energy'".

Mirrors were one of the early lectures, he goes on to imply that light is everywhere all the time taking many random paths, what you see is an average.

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Re: 0811: "Starlight"

Postby teelo » Wed Oct 27, 2010 10:39 am UTC


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Re: 0811: "Starlight"

Postby Steroid » Wed Oct 27, 2010 10:48 am UTC

What you don't see is that there's some schmuck on a planet orbiting the star who also has a mirror, so rather than sending the light home, it just has to make a triple journey.

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Re: 0811: "Starlight"

Postby snowyowl » Wed Oct 27, 2010 11:18 am UTC

big boss wrote: If one looks at the mirror as a high potential barrier (a Dirac delta function possibly?) in the path of the wavefunction of the photons. When the wavefunction of the photons hits this barrier they have a nonzero probability of reflecting and going in the opposite direction (this is not to say that they were absorbed by the mirror, they were just reflected).

I think this makes sense.
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Re: 0811: "Starlight"

Postby Switch31 » Wed Oct 27, 2010 11:28 am UTC

snowyowl wrote:
big boss wrote: If one looks at the mirror as a high potential barrier (a Dirac delta function possibly?) in the path of the wavefunction of the photons. When the wavefunction of the photons hits this barrier they have a nonzero probability of reflecting and going in the opposite direction (this is not to say that they were absorbed by the mirror, they were just reflected).

I think this makes sense.


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Re: 0811: "Starlight"

Postby Seamus » Wed Oct 27, 2010 11:59 am UTC

Wow. That's the astronomical equivalent of holding up a boombox playing "In Your Eyes" by Peter Gabriel.

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Re: 0811: "Starlight"

Postby DBPZ » Wed Oct 27, 2010 1:10 pm UTC

Than you XKCD.
I couldn't get rid of the fear of infinity of the universe for years, until today. The Relativity saves the day again!

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Re: 0811: "Starlight"

Postby Hale » Wed Oct 27, 2010 1:39 pm UTC

I think this is my first true GOOMHR moment, as I was just talking about this yesterday. Kinda scary. Even scarier when you consider how far the light from galaxies that we never even see with our naked eyes have traveled.
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Re: 0811: "Starlight"

Postby plasticup » Wed Oct 27, 2010 1:52 pm UTC

My immediate though was "ah, but to the photon no time has passed!" but then Randall beat me to it.

ps. retroreflector is the only way to make sure it gets back home

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Re: 0811: "Starlight"

Postby nahkaimurrao » Wed Oct 27, 2010 2:21 pm UTC

alt:"Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again."


Wow! I have studied physics at least since junior high (more than 12 years ago) and taken a few graduate courses in physics but have never heard this explanation of "from the light's point of view" but I swear it has been bugging me for the last few years because I had a hunch that this is how it works but never could get anyone else to see it and never had anyone explain it this way, until now!

I had this revelation a few years ago, when you take this idea to its logical conclusion, that time does not pass for light and that there is no distance between two points in lights point of view then in one sense all things connected by light are concurrent, that the emitter and receiver of a photon are connected by zero time and zero distance from lights point of view.

This leads me to believe that ALL matter and energy in the universe is a single point (possibly a single photon) somehow expanded to exist in multiple locations.

I have also heard Feynman's idea that anti-electrons might just be electrons moving backward in time and that its possible that there is only one electron in the universe that exists in 4 dimensional space as a giant knot but appears to be in multiple locations in 3D space.

And this fits with the single photon theory since light and electron-positron pairs are interchangeable.

So its possible that the universe is a single entity, a single electron or a single photon that is wrapped up in a 4D knot in such complexity to give us the universe we live in, but that the separation and distinction of one particle to another is illusory as we are all the same particle.

I also came up with a theory while studying the effects of circular waves on a long cylindrical molecule that when there is a circular wave traveling around the cylinder at right angles to a linear wave traveling down the cylinder it acts just like a wave traveling down a 1 dimension line with a certain amount of mass. So this could mean that all mass is the effect of waves which are spinning at right angles to our normal 3 space and these could be the curled up dimensions that are so small we can't see them but when a photon is caught in one of these dimensions it appears to stop moving in the large spatial dimensions, but really it is still moving at light speed but just curled up in the small dimensions and if it moves in the large dimensions it appears to have mass which is caused by its curled up momentum in the small dimension which is perpendicular to the large spatial dimensions. This is how light can appear to turn into a particle and vice versa, really the light is still light but just got caught in the small dimension and now appears to be a particle with mass instead of a massless photon.

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Re: 0811: "Starlight"

Postby Cloud Walker » Wed Oct 27, 2010 2:32 pm UTC

I really liked this one. I interpreted it as beret guy wanting the stars to see how beautiful they are. But having the photons go back home I like as well.
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Re: 0811: "Starlight"

Postby ritvax » Wed Oct 27, 2010 2:44 pm UTC

Cloud Walker wrote:I really liked this one. I interpreted it as beret guy wanting the stars to see how beautiful they are. But having the photons go back home I like as well.


At first I thought Beret Guy was getting an empty picture frame to make the event of viewing stars even more special and in an way, give some additional meaning to the act of viewing stars, so that their trillion-mile journey was not wasted. Then I realized he was holding up a mirror to help the starlight get back "home."

What always blows my mind is the idea that many of the stars we see in the sky no longer exist... Then I try to imagine the great expanses of space and my head hurts.

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Re: 0811: "Starlight"

Postby Sprocket » Wed Oct 27, 2010 3:02 pm UTC

Not to mention the light would do that if we weren't here to see it. Yey romantic Randall! We haven't gotten any of that in a long time!
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Re: 0811: "Starlight"

Postby davidhbrown » Wed Oct 27, 2010 3:06 pm UTC

One of the best discussion threads yet; thanks physics folks :)

I also saw an empty picture frame, thinking that beret=artist and so likely to have such at home. But looking carefully at panel 5 you can see a doubled image of the arm and body in addition to the white dots paotrip mentioned. Definitely a mirror. Makes sense; otherwise it's just an "ooh, aah; love the beauty of the universe" thing which doesn't quite seem xkcd-ish.

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Re: 0811: "Starlight"

Postby TheSavageNorwegian » Wed Oct 27, 2010 3:50 pm UTC

Faranya wrote:I doubt that bouncing is the solution to this. I would hypothesize that the photon is absorbed by the material, and then the material of the mirror emits another photon of approximately the same, but slightly less energy.

Excuse me, but doesn't a photon have a specific quanta of energy? What would happen would be a number of photons would be absorbed, and a slightly smaller number of photons would be emitted. Am I right?

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Re: 0811: "Starlight"

Postby LaserGhost » Wed Oct 27, 2010 5:05 pm UTC

TheSavageNorwegian wrote:Excuse me, but doesn't a photon have a specific quanta of energy? What would happen would be a number of photons would be absorbed, and a slightly smaller number of photons would be emitted. Am I right?

The answer to this is pretty much the same as the one coder0xff gave for the electron quantization. Bound photons (like, stuck in a cavity or whatever) have to exist at certain quanta of energy. Free photons (like, travelling through space) can be at whatever energy level. So the mirror absorbs a photon, then spits one out which is slightly redder.

Here's (one explanation of) why the emitted photon has to have lower energy than the absorbed one. Actually, this isn't quite true; observed energy levels of the photons will depend on how fast the observer is moving (that's the Doppler effect). So let's say we're in a reference frame in which the mirror is stationary. For concreteness let's say the photons are coming in from right to left, and the mirror is set to reflect them back to the right. Photons coming in have momentum going to the left, and reflected photons have momentum going right. So conservation of momentum says that when a photon hits the mirror, the mirror will start going left. Now, usually, the mirror is anchored so it stops almost immediately after the photon hits, but there's still that instant where it's moving. But moving mirrors have greater kinetic energy than stationary ones, so that extra energy has to come from somewhere, and the only place it could come from is the photon, which has to leave with less energy than it did coming in.

I pretty much took this from the laser elevator post a while back.

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Re: 0811: "Starlight"

Postby herbys » Wed Oct 27, 2010 5:07 pm UTC

syko_lozz wrote:surely it bounces? wouldn't absorption and reemission mean that mirrors were made out of stuff that can emit many different wavelengths of light? I dont think they're that special. But then again I'm a biology major, so who am I to comment?


Surfaces that absorb and reemit are white. Surfaces that actually bounce the photons are reflective.

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Re: 0811: "Starlight"

Postby SpaceShipRat » Wed Oct 27, 2010 5:27 pm UTC

Aww, Beret Guy is so touching :)

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Re: 0811: "Starlight"

Postby Komahawk » Wed Oct 27, 2010 5:38 pm UTC

ritvax wrote:
Cloud Walker wrote:I really liked this one. I interpreted it as beret guy wanting the stars to see how beautiful they are. But having the photons go back home I like as well.


At first I thought Beret Guy was getting an empty picture frame to make the event of viewing stars even more special and in an way, give some additional meaning to the act of viewing stars, so that their trillion-mile journey was not wasted. Then I realized he was holding up a mirror to help the starlight get back "home."

What always blows my mind is the idea that many of the stars we see in the sky no longer exist... Then I try to imagine the great expanses of space and my head hurts.

-ritvax

So once the stars finally reach their home again it's too late. Their home is gone. That adds a whole other level of sadness to the comic.

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Re: 0811: "Starlight"

Postby LSN » Wed Oct 27, 2010 5:50 pm UTC

Beret Guy is hands down my favorite XKCD character.

Also, comic gets a +1 for a having an awesome title.

"Far away, this ship has taken me far away. Far away from the memories of the people who care if i live or die."

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Re: 0811: "Starlight"

Postby Rackum » Wed Oct 27, 2010 5:52 pm UTC

ritvax wrote:
Cloud Walker wrote:I really liked this one. I interpreted it as beret guy wanting the stars to see how beautiful they are. But having the photons go back home I like as well.


At first I thought Beret Guy was getting an empty picture frame to make the event of viewing stars even more special and in an way, give some additional meaning to the act of viewing stars, so that their trillion-mile journey was not wasted. Then I realized he was holding up a mirror to help the starlight get back "home."

What always blows my mind is the idea that many of the stars we see in the sky no longer exist... Then I try to imagine the great expanses of space and my head hurts.

-ritvax

In that case, how depressed are those poor photons gonna be when they finally make it back home only to "find" that it's been gone for millions of years ...

Edit: Changed the wording to (somewhat) reduce the anthropomorphization of the photons; still wanted to retain some of it though.

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Re: 0811: "Starlight"

Postby DamnedHeathen » Wed Oct 27, 2010 6:41 pm UTC

I didn't get that that was a mirror at first. I thought it was a frame so that the character (who is wearing a beret and possibly implied to be an artist) could look at the sky as if it were a painting. The other possibility was that it was an HDTV frame, which would make this comic similar to a Calvin and Hobbes when he ponders his insignificance looking at the night sky, and then runs to the comfort of a warm house and television set.

As for wheather the same photon is reflected from a frame, I'd say that you reach a level of abstraction similar to wondering if when typing on a keyboard, you left-arrow over some text that you have typed and insert some other text, are the letters that got scootched over the *same* letters that you typed or merely copies of them? That is to say, is there anything distinguishing those letters from the letters that you would see if you deleted and retyped the text. Does the 'j' on your screen the *same* 'j' after you space it over to the right a bit?

Also, after you have lived long enough, and a lot of the individual molecules and cells in your body have been replaced, are you still the same person?

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Re: 0811: "Starlight"

Postby Berke » Wed Oct 27, 2010 6:55 pm UTC

Another favorite of mine. So deep and, well... deep.

Love the Alt-text too, I've always tried imaginining myself seeing things from the perspective of a photon.

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littlelj wrote:Also,

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I heart this comic. Whimsy. Awesome.


Where is this comic from? I couldn't locate it... But I could LOVE it.
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Re: 0811: "Starlight"

Postby Diadem » Wed Oct 27, 2010 7:19 pm UTC

fow wrote:
Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again.


Actually, from the light's point of view, everything is in one of two places. Everything behind it in one, everything in front in the other.

So your eye and home are as far apart as they can be. So, so sad.

No, you are incorrect. From the point of view of light there is no 'in front' and 'behind'. So that's wrong. But there is a 'to the side'.

Relativity teaches us that moving things experience length contraction. Distances in the direction of motion contract, become smaller. Imagine a cube with length, width and height 10 meters. If you move through it sufficiently fast its width and height will still be 10 meters, they never change. But its length gets reduced from your point of view. At 50% of the speed of light its length is only 8.7 meters. At 90% of the speed of light its length is only 4.4 meters. At 99.9% of the speed of light its length is only 45 cm. All the while width and height are still 10 meters.

So what happens at the speed of light itself? Well, the length goes to zero. Everything in its direction of travel (forward and backwards!) is compressed to a point. What does that mean. It means that you lose a dimension. Light lives in a 2 dimensional universe. It can look to the sides in every direction, but it can't look forward or backwards. That dimension simply does not exist anymore for light.
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Re: 0811: "Starlight"

Postby lank » Wed Oct 27, 2010 7:36 pm UTC

Diadem wrote:
fow wrote:
Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again.


Actually, from the light's point of view, everything is in one of two places. Everything behind it in one, everything in front in the other.

So your eye and home are as far apart as they can be. So, so sad.

No, you are incorrect. From the point of view of light there is no 'in front' and 'behind'. So that's wrong. But there is a 'to the side'.

Relativity teaches us that moving things experience length contraction. Distances in the direction of motion contract, become smaller. Imagine a cube with length, width and height 10 meters. If you move through it sufficiently fast its width and height will still be 10 meters, they never change. But its length gets reduced from your point of view. At 50% of the speed of light its length is only 8.7 meters. At 90% of the speed of light its length is only 4.4 meters. At 99.9% of the speed of light its length is only 45 cm. All the while width and height are still 10 meters.

So what happens at the speed of light itself? Well, the length goes to zero. Everything in its direction of travel (forward and backwards!) is compressed to a point. What does that mean. It means that you lose a dimension. Light lives in a 2 dimensional universe. It can look to the sides in every direction, but it can't look forward or backwards. That dimension simply does not exist anymore for light.


light lives in a 4D universe (just like the rest of us) - it just lives on the null paths within it, because the metric is (ds)2 = c(dt)2 - (dx)2. there are no frames in this metric where any of the lengths is zero or a dimension "removed" - no matter how fast you're going in any direction relative to one frame, light is always moving at c in that direction in both frames. i.e. you can only approach the speed of light relative to another frame, and no lorentz boost can put you in the same frame as a photon - because that frame does not exist.

when you examine timelike trajectories, they can be parameterised by time. when you examine spacelike trajectories, they can be parameterised by spatial distance. when you examine lightlike trajectories, they are parameterised by an affine parameter with no direct physical meaning. there is a difference in x and in t between events P1 and P2 even on a lightlike trajectory (unless P1 = P2), it's just that the spacetime distance on that curve is null.

everyone seems to have forgotten, in the idea of relativity, that the speed of light is invariant between frames. how could you thus go into a frame with speed c?

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Aikanaro
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Re: 0811: "Starlight"

Postby Aikanaro » Wed Oct 27, 2010 7:57 pm UTC

I hate bittersweetness, and the trope of True Art is Angsty. Therefore, to me, this comic is pure win. :D
Dear xkcd,

On behalf of my religion, I'm sorry so many of us do dumb shit. Please forgive us.

Love, Aikanaro.


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