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I have had this question in my mind for a long time, I thought you guys might enlighten me easily.

I am confused about some space photographs and claims like "this galaxy is 13 billions light years away from us.": how we can take the photograph of something that far, if it is 13 billion light years away wouldn't it take 26 billion light years to take those pictures?

today this post led me to ask this question, at last: a space picture

There certainly is something I don't know about photography or light years; if you could tell me the logic behind this, I would appreciate it.

I am not a physicist or any science guy, so please tolerate my ignorance.

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    $\begingroup$ You need a really powerful flash unit. $\endgroup$ – Hot Licks Dec 22 '14 at 18:12
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    $\begingroup$ @HotLicks there is some neat stuff if you do get that flash working on that scale. There's some rather interesting science around it. $\endgroup$ – user20936 Dec 22 '14 at 23:52
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    $\begingroup$ Actually, it should be pointed out that you don't get a picture of something 26 billion light years away. Rather, you get a picture of something that WAS 26 billion light years away, 26 billion years ago. Since then it's all been replaced with a megamall and a football stadium. $\endgroup$ – Hot Licks Dec 23 '14 at 2:22
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    $\begingroup$ For a simple comparison, the Sun is 8 light-MINUTES away from Earth. Would you expect that it takes 8 (or 16) MINUTES to take a picture of the Sun just because it's that far away? The same physics applies. $\endgroup$ – user2338816 Dec 23 '14 at 3:32
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The error is probably in this statement

if it is 13 billion light years away wouldn't it take 26 billion light years to take those pictures?

I think you are imagining that cameras send out light to the objects, and when this light comes back records the light as an image. Not really. Cameras merely record the light they see from that area. So if that area is 13 billion light years away (not sure how credible source is) then all that means is that the light you are capturing today is the light that galaxy emitted 13 billion years ago.

Imagine for instance Anna and Bob are playing catch with a ball. Anna throws the ball to Bob. Bob receives the ball, and says the ball came at 3:00pm sharp. But the ball was in the air for 1 minute (anna is a slow thrower). That means Anna threw the ball at 2:59, even if Bob recorded it at 3:00. In this scenario, Bob is acting much like a camera acts, by receiving information (in this case a ball, in a camera's case it would be light from galaxies) The reason that Hubble took photos for 4 months (this might be wrong, I'm no good with photography) is that the longer it receives the information, the more 'background' light that we don't want to capture can be removed.

Hoped this makes sense.

P.S. may have misunderstood the question. You say

if it is 13 billion light years away wouldn't it take 26 billion light years to take those pictures?

as if light years are a measure of time. A light year is a measure of distance, the distance light travels in a year in a vacuum.

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    $\begingroup$ Dear Joshua, maybe Tolga wants to take a picture of someone this far, but she wants him to smile, so she first says "cheese", waits when the subject starts to smile, and press the trigger. Incidentally, the end of the visible Universe is currenly about 40+ billion light years from us, not 14, because those regions continued to expand between the moment when light was emitted and now. $\endgroup$ – Luboš Motl Dec 22 '14 at 7:31
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    $\begingroup$ I don't know if this has anything to do with my question =) but I just wanted to make it clear that Tolga is a "he" here :) $\endgroup$ – Tolga Evcimen Dec 22 '14 at 7:36
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    $\begingroup$ didn't make much sense :( $\endgroup$ – Tolga Evcimen Dec 22 '14 at 8:00
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    $\begingroup$ @TylerH No. There is no perspective of light. $\endgroup$ – Jim Dec 22 '14 at 17:01
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    $\begingroup$ @TylerH Now I wish we were talking about dark matter so I could say "pfft, wimp!" $\endgroup$ – Jim Dec 22 '14 at 20:11
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[…] if it is 13 billion light years away wouldn't it take 26 billion light years to take those pictures?

Only if you were using a flash. With a flash, you'd trigger the flash here, 13 billion years later the flash light would have traveled all the way there and illuminate the distant galaxy, and yet another 13 billion years later it would have traveled back again and you could open the shutter and take a picture from the reflected light from your flash.

But you don't take photographs of deep space objects using a flash. You use the light these objects (or some bright objects nearby) emit by themselves. So you simply open the shutter now, and record the light emitted by the galaxy 13 billion years ago. Which means you're depicting the object as it was 13 billion years ago, but since it was kind enough to emit light all that time ago all by itself, without you asking for it or providing illumination, that does not prevent you from taking your picture.

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    $\begingroup$ That'd be a pretty powerful flash! $\endgroup$ – aslum Dec 22 '14 at 17:39
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    $\begingroup$ @aslum - or a very sensitive detector... $\endgroup$ – Bob Jarvis Dec 22 '14 at 18:10
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    $\begingroup$ This would be correct if the universe weren't expanding. But trying to send a photon round-trip near the edge of the observable universe, well...it'll take a lot longer than 26 billion years. $\endgroup$ – Tim S. Dec 23 '14 at 21:20
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    $\begingroup$ Yes, I'm simplifying a lot. I'm disregarding expansion, I'm disregarding relativistic aspects, I'm disregarding the infeasibility of such a flash, and so on. (If anyone wants to calculate the rate of returned photons we could expect from a standard flash unit, that might be interesting.) All of this to keep the explanation simple and intuitive. I'd consider these white lies. $\endgroup$ – MvG Dec 23 '14 at 21:50
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I got my answer from my colleagues :) And I guess you were trying to say the same.

Taking a photo does not require sending some photons and waiting for them to reflect back from object. Taking a photo is basically getting the photons that are thrown by the object. In my case photons thrown from the objects 13 billion years ago.

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    $\begingroup$ That is excatly what @Joshua Lin is saying. You should flag his answer as correct :) $\endgroup$ – Per Hornshøj-Schierbeck Dec 22 '14 at 12:02
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    $\begingroup$ From Joshua Lin's answer: "I think you are imagining that cameras send out light to the objects, and when this light comes back records the light as an image. Not really. Cameras merely record the light they see from that area.". This seems to be saying the exact same thing. $\endgroup$ – Chris Dec 22 '14 at 13:40
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First, a light year is just a unit of distance, and not a unit of time. It is 9 460 538 400 000 000 metres. It's widely used as a measure of distance in astronomy as the numbers come out more reasonable, and very conveniently, light travels at 1 light year per year.

You can take a photo of an exploding star 13 billion light years away in exactly the same way as you can take a photo of a candle on the other side of the room. The only difference is how long the light takes to reach you. In the case of the candle, the light will take a few nanoseconds to get from one side of the room to the other, in the case of the exploding star, the light will take 13 billion years.

Anything that is glowing is emitting photons (little particles of light). These stream out in all directions. If you point a camera at the glowing thing, then your camera will capture some of those photons. If it's a film camera, then the photons will cause a chemical reaction in the bits of film they hit. If it's a digital camera, the photons will cause electrons to be knocked out of place, and the camera can measure this.

When photographing things a very long way away, you usually need a long exposure. This is because most of the photons emitted by the exploding star will never reach you - they are spreading out in all directions, and only a few will happen to go exactly in your direction. This exposure could be minutes or hours to collect enough photons to measure. However, there is no particular relationship between the length of the exposure and the distance away of the thing you are photographing.

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  • $\begingroup$ NB: most astronomers I know either use centimeters or parsecs, rather than lightyears, for their unit of measure. $\endgroup$ – Kyle Kanos Dec 22 '14 at 14:19
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    $\begingroup$ @KyleKanos most astronomers you know use centimeters for their unit of measure? What are they measuring? $\endgroup$ – Michael McGriff Dec 22 '14 at 14:26
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    $\begingroup$ @MichaelMcGriff: Any length really: orbits (though often that's in AU, but defined as $\sim10^{13}$ cm), radius of SNR (usually in pc, but defined as $\sim10^{18}$ cm), and so on. It's only in popularized media (talks, books, etc) that you'll see things using MKS. $\endgroup$ – Kyle Kanos Dec 22 '14 at 14:30
  • $\begingroup$ @KyleKanos interesting. Never would've occurred to me that they would use centimeters as opposed to meters or kilometers etc. Any idea why they use a unit so small for such large distances? $\endgroup$ – Michael McGriff Dec 22 '14 at 14:45
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    $\begingroup$ @MichaelMcGriff: Historical & peer-pressure reasons mostly ("Everyone else is doing it too, so we might as well do it ourselves!"). The IAU recommends publishing with SI units, but no one really enforces that and not many people do use them, as far as the publications I've read. $\endgroup$ – Kyle Kanos Dec 22 '14 at 14:54
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As I read the other answers, I find them all to make sense and be correct. Unfortunately, our friend still doesn't seem to grasp them, so I'm going to try to say the same thing (I feel it's correct) from a different approach.

If you take a picture of someone at night time, you might need a flash. This spits out a bunch of light that can get reflected off your subject and captured by your camera. But if you take the same picture of someone in the day time, there's a good chance you won't need the flash: there's already plenty of light floating around for your camera to capture.

Well, that's the same thing going on in space: there's a lot of light floating around. It's going to float around whether or not a camera is there to capture it. It's just going to go outward from the star or quasar or whatever produced it, and it's going to keep going until it runs into something. Maybe it runs into something close to what produced it. (Light from our sun does this when it hits us, we're pretty close.) Or maybe it runs into something super far away, like on the other side of the universe.

The key is that the light is already there. That's why we put the camera there. To capture the light that's already just kinda floating there.

Your 26 billion year scenario could make some sense: if we wanted to shine a "flash" at our target, flooding it with light to see what bounces back to us so we get a clearer picture, we could do that, and it would indeed take 26 billion years. (Probably longer, since it's probably accelerating away from us.) Well, in theory we could. The flash we made would probably be powerful to incinerate the earth, but that's a small price to pay to give some quasar its own selfie, right? We just have to hope we get it developed in time.

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Basically, what you (and the camera) see today maybe does not exist anymore. In the case of objects millions light years away, they were there millions of years ago and now we see something that does not exist, we see the image of what that "thing" looked originally millions of years ago. We see it because the image (light) only now reached us

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Just snap photo. You will have a photo of the object how it appeared 13 billion years ago, if it is 13 billion light years away. As the light has been traveling for that length of time. A light year is just a unit of measuring vast distances, the distance it takes light to travel at 186,000 miles per second in a year. One single light year is about 5.88 trillion miles. If you want a photo of the object as it looks at this moment, you will need to wait 13 billion years to take the photo plus or minus any change in distances. The object may not still be there.

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when we get the light of it we can have a picture of it. it means if the light has not reached us till now then we can not get the picture.

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protected by Qmechanic Dec 23 '14 at 8:24

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