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Observable universe thought experiment


losfomot
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We are only able to see so much of the universe. Can we alter this?...

 

Say we built a rocket telescope and fired it directly toward a specific area of space... say the hubble deep field area. We get this rocket going about .99c and then let it coast for a day or a week... however long it takes to get a good exposure. Then we turn the rocket around and have a look at the photo... what would we see?

 

Would we see farther than the observable universe we experience on Earth? (since distance has contracted in the direction of travel)

 

Would red-shifting be (to some degree) reversed by our velocity toward the light we are collecting?

Edited by losfomot
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AFAIK, the light from distant parts of the Universe doesn't care if the spaceship is coasting close to lightspeed or is in rest relative Earth. The distance that has been traveled by the light will still be the same since the photons themselves already propagates at c. But the light should be blue shifted to some degree due to the high speed relative our view from Earth.

 

Also since the ship will be further out in space in one direction it will be able to see further out than we can observe at the same time, here on Earth in that direction. However when the spaceship returns to Earth with the photo, the image in it has already reached us, since those photons would bypass and outrun the ship. Ergo, it's easier to stay here and wait.

 

As time goes by we will be able to see further and further into the Universe until the accelerated expansion takes the upper hand and distant stars will start to faint and disappear from our sky. In a very distant future the Local Virgo Supercluster might be everything there is possible to observe from our location inside the Milky Way.

 

"Galaxies outside the Local Supercluster are no longer detectable

Assuming that dark energy continues to make the universe expand at an accelerating rate, 2×1012 (2 trillion) years from now, all galaxies outside the Local Supercluster will be red-shifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way."

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

Edited by Spyman
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We are only able to see so much of the universe. Can we alter this?...

 

Say we built a rocket telescope and fired it directly toward a specific area of space... say the hubble deep field area. We get this rocket going about .99c and then let it coast for a day or a week... however long it takes to get a good exposure. Then we turn the rocket around and have a look at the photo... what would we see?

 

Would we see farther than the observable universe we experience on Earth? (since distance has contracted in the direction of travel)

 

No, we wouldn't see any further. Any picture that the rocket telescope could bring us would be a picture that we ourselves produced years before. Because the rocket travels slower than the speed of light, the light that it captures is travelling faster than the rocket, and that light would get to earth sooner than the rocket can. Let's make up some numbers to make the point clear:

 

Imagine that 13 billion years ago there is a supernova explosion in some distant galaxy. The rocket is 1000 light years closer to that galaxy than the earth is, so it sees the explosion 1000 years before we do. Then that rocket travels at 0.99c toward earth, and so it takes 1010 years to get to earth. By the time the rocket gets back, our astronomers would have seen that explosion 10 years prior.

 

Would red-shifting be (to some degree) reversed by our velocity toward the light we are collecting?

 

In principle, I guess the image would be less red-shifted.

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Thanks for the answers. I should have seen that myself.

 

So our (co-moving) observable universe would actually shrink, in the direction of travel, from 46 billion ly to about 6.5 billion light years (because of length contraction). That is, the 'wall' would be in the same place, but that place would be much closer.

 

Even if we can't see more of the universe in this fashion, it still seems to me that we would be able to get a better (clearer) picture, since we have reduced the distance 7 fold?

 

For examples:

 

That new furthest galaxy is roughly 30 billion light years away... but to our rocket telescope, traveling toward it at .99c, it is only ~4.3 billion light years away. (of course it was only 13.1 bly away when the light we see was emitted, so is it that distance that is reduced?.. probably... here's a less confusing example:)

 

That new 'goldilocks' planet is 20 light years away... but to our rocket telescope, traveling toward it a .99c, it is less than 3 light years away.

 

Being roughly 7 times closer should make for a better picture? Or is this advantage somehow nullified?

 

In principle, I guess the image would be less red-shifted.

 

Why such a hesitant answer? It seems to me that, even if our observable universe did not get bigger, the farthest stuff we can see would become much less red-shifted, and therefore easier for us to see.?

 

Edit - perhaps this thread would fit better in the relativity subforum, rather than cosmology?

Edited by losfomot
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Being roughly 7 times closer should make for a better picture? Or is this advantage somehow nullified?

Shutter time in the camera on the spaceship would be subjected to time dilation relative the camera on Earth.

 

From the frame of the camera in the spaceship the distance might seem 7 times closer but clocks on Earth would tick 7 times slower. Thus the camera on Earth has its shutter open and recieves photons for a 7 times longer period, while the camera on the spaceship collects photons from a 7 times longer distance.

 

I seems that their pictures would be equally good, except maybe for the effect of less redshifting.

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Even if we can't see more of the universe in this fashion, it still seems to me that we would be able to get a better (clearer) picture, since we have reduced the distance 7 fold?

 

I don't think it would. The picture quality depends on the size of your telescope. The wave nature of light causes a distortion pattern called an Airy disk, which is the maximum resolution a telescope can have, and it depends on the size of the telescope. Moving the telescope very fast would not change this. It would collect photons faster, but by the time the rocket telescope gets back to earth, all those photons would have arrived here anyway, so we would have gotten the same picture by just leaving the telescope camera on the whole time.

 

Why such a hesitant answer? It seems to me that, even if our observable universe did not get bigger, the farthest stuff we can see would become much less red-shifted, and therefore easier for us to see.?

 

Why does red-shifting make things harder to see? It just moves the light to a different part of the spectrum...

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Shutter time in the camera on the spaceship would be subjected to time dilation relative the camera on Earth.

 

From the frame of the camera in the spaceship the distance might seem 7 times closer but clocks on Earth would tick 7 times slower. Thus the camera on Earth has its shutter open and recieves photons for a 7 times longer period, while the camera on the spaceship collects photons from a 7 times longer distance.

 

I seems that their pictures would be equally good, except maybe for the effect of less redshifting.

 

I thought the purpose of longer exposures was to capture more light from very dim objects... I didn't know this had an effect on the actual resolution of the image.

 

compare.jpg?t=1288293292

 

These two images (photos of a photo) were taken with the same camera, but the 2nd one was from 7 times farther away. Are you saying that, if I had a long enough exposure, I could get the same quality image as the first photo?

 

Why does red-shifting make things harder to see? It just moves the light to a different part of the spectrum...

 

I was under the impression that the further you go to that side of the visible spectrum, the worse the resolution got (with the same size instrument)... I don't know why I have that idea.

Edited by losfomot
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Are you saying that, if I had a long enough exposure, I could get the same quality image as the first photo?

No, I am not an expert on cameras and I am not claiming that longer exposures will improve the quality of an image. I would hope that the people behind the two images presented was skilled and used the proper shutter time.

 

Your comparison with the images is not fair to the question asked in the OP, since the first image is not taken from the same location with such higher speed that distance is compressed 7 times, instead it is taken from a location that actually is 7 times closer, and I would guess with close to the same speed.

 

My point was not about camera exposures, what I am trying to say is that both cameras will capture identical photons if everything else is equal and thus reveal the same image.

 

Let's say that the spaceship is coming from behind Earth and exactly when it passes, both cameras simultaneously takes the picture. The spaceship going very fast covers a large distance while the shutter is open and thus recieves more photons than it would if it would have been standing still. The camera on the Earth on the other hand instead has it shutter open for an equal longer duration letting the same amount of photons entering, which then also will come from the same distance the spaceship is covering. Both cameras thus captures almost exactly the same amount of nearly identical photons from a matching location during an interval they both individually percieve to be of an indistinguishable shutter time. So why would their images be different?

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Your comparison with the images is not fair to the question asked in the OP, since the first image is not taken from the same location with such higher speed that distance is compressed 7 times, instead it is taken from a location that actually is 7 times closer, and I would guess with close to the same speed.

 

And this is kind of the gist of my question... I thought special relativity said that, from the rocket telescope frame, that distant galaxy (or whatever we are looking at) really is 7 times closer. Not 'seems', not 'compressed 7 times'... the distance is actually 1 billion ly instead of the 7 billion ly in Earth's frame (for example).

 

My point was not about camera exposures, what I am trying to say is that both cameras will capture identical photons if everything else is equal and thus reveal the same image.

 

Let's say that the spaceship is coming from behind Earth and exactly when it passes, both cameras simultaneously takes the picture. The spaceship going very fast covers a large distance while the shutter is open and thus recieves more photons than it would if it would have been standing still. The camera on the Earth on the other hand instead has it shutter open for an equal longer duration letting the same amount of photons entering, which then also will come from the same distance the spaceship is covering. Both cameras thus captures almost exactly the same amount of nearly identical photons from a matching location during an interval they both individually percieve to be of an indistinguishable shutter time. So why would their images be different?

 

I see what you are saying and it does make sense. I just thought this might be a way to take advantage of the effects of SR (one day).

 

After watching

, I don't see that the rocket-telescope would get any kind of useful image.
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I thought the purpose of longer exposures was to capture more light from very dim objects... I didn't know this had an effect on the actual resolution of the image.

 

I am pretty sure that this is the case. I cannot imagine any way that a longer exposure improves resolution, it just gathers more light (allowing you to see fainter objects). To improve the resolution you need a larger aperture (assuming the telescope is in space, or you have adaptive optics).

 

I was under the impression that the further you go to that side of the visible spectrum, the worse the resolution got (with the same size instrument)... I don't know why I have that idea.

 

Technically, this is correct, but instead of a rocket telescope you can just make your telescope wider and get the same result. Basically, the resolution limit of a telescope is equal to wavelength divided by telescope width. In principle, I guess that a rocket telescope travelling close to the speed of light would get an advantage with the wavelength, but it is much easier to spend your money making the telescope larger instead.

Edited by DanielC
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