Just a hypothesis.

[ferunandesu]

If space was emptied of matter, filled with light, and there was only yourself with a powerful telescope, then you could see the back of your head in every direction.

[TelecomClone]

No you couldn't, because you'd be looking back in time. You would see whatever was where you are millions or billions of years ago (so nothing, if space is otherwise emptied).

[ferunandesu]

So you would see yourself if you waited long enough... In every direction.

I mean. What else would you have to do...

[TelecomClone]

So you would see yourself if you waited long enough...
Yes.



In every direction.Yes, but not the back of your head in every direction. You would only see the back of your head when looking in the direction that you were looking all of those years ago. For instance: if you spun in a circle for five thousand years, and then stared off into one direction for x billion years after that, you would see yourself spinning in a circle for five thousand years x billion years ago, and then you'd see the back of your head.



I mean. What else would you have to do...Heh.

[TelecomClone]

The really weird question is whether or not you'd be able to feel your future self staring at the back of your own head from x billion years in the future, just before you bent over the telescope in second 1 of minute 1 of hour 1 of day 1 of year 1.

[SCARYdoor]

Since when is the universe a never ending loop? Hasn't it been proven to be currently expanding, or contracting, or something? Wouldn't you just see empty space that is lit up? But since there's nothing to light up, I think it would actually just look black, because it's empty.

(Just trying to work out how you think you would see the back of your head. Maybe I'm missing something)

[ferunandesu]

Expansion or contraction would have no effect on its ball of yarn status.

[wildcat]

No you couldn't, because you'd be looking back in time. You would see whatever was where you are millions or billions of years ago (so nothing, if space is otherwise emptied).
There is no objective, universal time. You are looking into their past only.
You cannot look back into something that is not there.

[ferunandesu]

You are looking into their past only.


their?

[headfonez]

Since when is the universe a never ending loop? Hasn't it been proven to be currently expanding, or contracting, or something? Wouldn't you just see empty space that is lit up? But since there's nothing to light up, I think it would actually just look black, because it's empty.

(Just trying to work out how you think you would see the back of your head. Maybe I'm missing something)
I agree with this.

[TelecomClone]

There is no objective, universal time. You are looking into their past only.
You cannot look back into something that is not there.It takes light one second to travel something like 299,792,458 meters across a vacuum. In order to see something through a telescope by means of light, you've to see light that was reflected, refracted, or emitted by that something. In order to see that light, however, it has to be physically collected by your telescope - and in order to be collected by your telescope, it has to cross the expanse of space between that something and your telescope. Crossing vast expanses of space, like a distance 1.5 times the unique diameter of the universe, as in this example, takes a vast amount of time from your stationary frame of reference. That is why you see a time-delayed image and, therefore, that is why if there was nothing where you were standing all of those billions of years ago, nothing would have reflected light and you would first see nothing through your telescope. That is why you would see yourself suddenly appear as you did x billion years ago, then see yourself bending over the telescope, and then see the back of your own head.




Since when is the universe a never ending loop? Hasn't it been proven to be currently expanding, or contracting, or something? Wouldn't you just see empty space that is lit up? But since there's nothing to light up, I think it would actually just look black, because it's empty.

(Just trying to work out how you think you would see the back of your head. Maybe I'm missing something
The universe has no center. The idea is that if the universe is expanding slower than the speed of light, then light emitted, reflected, or refracted by an object in the universe would be able to lap the universe.

[ferunandesu]

"if you traveled outward and outward in a straight line, indefinitely and pugnaciously, you would never arrive at an outer boundary. Instead, you would come back to where you began... we are not adrift in some large, ever-expanding bubble. Rather, space curves, in a way that allows it to be boundless but finite... "

- A Short History of Nearly Everything

just think of it as a ball of yarn for now... a ball of yarn with no outer edges or center... uuh... nevermind, don't think of it that way...

[ptGatsby]

In order to see something through a telescope by means of light, you've to see light that was reflected, refracted, or emitted by that something. In order to see that light, however, it has to be physically collected by your telescope - and in order to be collected by your telescope, it has to cross the expanse of space between that something and your telescope. Crossing vast expanses of space, like a distance 1.5 times the unique diameter of the universe, as in this example, takes a vast amount of time from your stationary frame of reference. That is why you see a time-delayed image and, therefore, that is why if there was nothing where you were standing all of those billions of years ago, nothing would have reflected light and you would first see nothing through your telescope. That is why you would see yourself suddenly appear as you did x billion years ago, then see yourself bending over the telescope, and then see the back of your own head.


I don't understand...

Light is emitted from every direction, but from no source - ok, we'll take that for granted. Light travels everywhere, eventually hitting your head, the only source of matter. Light bounces off your head, creating the 'image' of your head, bouncing it off into empty space.

Later, you take a telescope and look into space. You see... nothing. Your image is already projected outwards from your head. You'd have to instantly transport yourself ahead of the light being bounced off your head in order to see your head.

All this assuming infinite space and all that.

[ferunandesu]

Later, you take a telescope and look into space. You see... nothing.

You see the light that bounced off your head after it traveled the length of warped space.

The light from the back of your head would shine on the tip of your telescope. The light from your telescope would shine on the back of your head.

[ptGatsby]

You see the light that bounced off your head after it traveled the length of warped space.


What warped space? I thought space only warped in the presence of matter. Far as I know, space expanded at c the moment of creation...

[ferunandesu]

What warped space? I thought space only warped in the presence of matter.

No. Space itself is one large warp.

[ptGatsby]

No. Space itself is one large warp.


I guess that depends on if you believe in that particular theory of space expansion.

From Nasa;


Matter plays a central role in cosmology. It turns out that the average density of matter uniquely determines the geometry of the universe (up to the limitations noted above). If the density of matter is less than the so-called critical density, the universe is open and infinite. If the density is greater than the critical density the universe is closed and finite. If the density just equals the critical density, the universe is flat, but still presumably infinite. The value of the critical density is very small: it corresponds to roughly 6 hydrogen atoms per cubic meter, an astonishingly good vacuum by terrestrial standards! One of the key scientific questions in cosmology today is: what is the average density of matter in our universe? While the answer is not yet known for certain, it appears to be tantalizingly close to the critical density.


If you are the only matter, the universe should be infinite, far as I can tell.

[Hustler]

The answer to the question is maybe. If the universe had a total mass of 1 ferandesu, it would have serious ramifications for the warpature of spacetime. Furthermore, it is theoretically possible for spacetime to expand at a rate greater than the speed of light and, as such, the light bounced off the back of your head would never have a chance to reach your telescope. Also, depending on the nature of the energy which fills this hypothetical universe of yours, it is possible a photon has a mean free path less than infinity and, as such, has an upper-bound associated with the age of existence for your universe (or proton half-life), so a photon, even in a slowly-expanding (less than the speed of light) universe would effectively not have time to reach from the back of your head back to your telescope. Note that this could still occur in a universe with an infinite mean free path for a photon if the expansion rate of the universe were close enough to the speed of light to make this process take long enough. I guess the only way to know for sure is for you to conduct the experiment.

[ferunandesu]

I guess the only way to know for sure is for you to conduct the experiment.

I'll do that.

[Google Monster]

lol

[wildcat]

their?
You are looking into their past. Time is a useful tool. Ontologically, it is not there. Time is not the fourth dimension. It is already included in the three dimensions.

[Google Monster]

"if you traveled outward and outward in a straight line, indefinitely and pugnaciously, you would never arrive at an outer boundary. Instead, you would come back to where you began... we are not adrift in some large, ever-expanding bubble. Rather, space curves, in a way that allows it to be boundless but finite... "

- A Short History of Nearly Everything

just think of it as a ball of yarn for now... a ball of yarn with no outer edges or center... uuh... nevermind, don't think of it that way...

I see where you think you would see the back of your own head. But there is a flaw in your thought experiment because you mentioned that you rid of all matter and filled the space with only light. The lack of matter would result in a lack of curved space therefore the light will not travel back to where it started or reflected.

[Johnny]

You are looking into their past. Time is a useful tool. Ontologically, it is not there. Time is not the fourth dimension. It is already included in the three dimensions.

no the other 3 are tools also

and it is only correct to say the first 3 are included in the fourth

[TelecomClone]

I see where you think you would see the back of your own head. But there is a flaw in your thought experiment because you mentioned that you rid of all matter and filled the space with only light. The lack of matter would result in a lack of curved space therefore the light will not travel back to where it started or reflected.And the lack of pressure would result in an instantaneous and massive anoxia, and the low temperature would freeze him solid such that he couldn't bend over a telescope, and the lack of a source of light would preclude the existence of light, and the lack of a catalyst would preclude his existence in the first place, and an insufficiently massive universe never would have expanded at all, and and and...

I think it is implied that the universe is the same size and shape as it is now - that along with his being able to float in space and look through a telescope in the first place.

[ferunandesu]

I think it is implied that the universe is the same size and shape as it is now - that along with his being able to float in space and look through a telescope in the first place.

Exactly.

Since I don't want this to be a one word response, I might as well tell you what I'm doing at the moment. I'm posted up in the UGA Student Learning Center. I have my final in Anthropology on Thursday. The class is a guaranteed A, so I'm not stressing...

[ben from below]

if there is no matter, what would CAUSE the light emitted from the back of your head to end up approaching you from the other direction?

[wildcat]

no the other 3 are tools also

and it is only correct to say the first 3 are included in the fourth
Yes. The three dimensions are separate dimensions for us only. We measure existence.
In rainbow we see the spectre of the white.
But it is only a spectre.

[Google Monster]

And the lack of pressure would result in an instantaneous and massive anoxia, and the low temperature would freeze him solid such that he couldn't bend over a telescope, and the lack of a source of light would preclude the existence of light, and the lack of a catalyst would preclude his existence in the first place, and an insufficiently massive universe never would have expanded at all, and and and...

I think it is implied that the universe is the same size and shape as it is now - that along with his being able to float in space and look through a telescope in the first place.

Well I didn't mention those because thought experiments tend to ignore those kinds of things. But the thought experiment did mention curved space and getting rid of all the matter so those two ideas in the thought experiment conflict with each other.

[TelecomClone]

But the thought experiment did mention curved space and getting rid of all the matter so those two ideas in the thought experiment conflict with each other.No no, there is no conflict. Getting rid of the matter in this example clearly serves to the end of having an unobstructed line of sight. Look at the variables: a telescope with which to see, light by which to see, space in which to see, and a lack of obstruction. The thought experiment has to do with space as an infinite finite which requires its being the same size and shape as it is normally. It's a thought experiment, not a physics problem.






Since I don't want this to be a one word response, I might as well tell you what I'm doing at the moment. I'm posted up in the UGA Student Learning Center. I have my final in Anthropology on Thursday. The class is a guaranteed A, so I'm not stressing...I can't help stressing before any final. Even if the final were on elementary grammar, or the names of the days of the week, waiting for it to be done with would drive me rabid.

[Google Monster]

No no, there is no conflict. Getting rid of the matter in this example clearly serves to the end of having an unobstructed line of sight. Look at the variables: a telescope with which to see, light by which to see, space in which to see, and a lack of obstruction. The thought experiment has to do with space as an infinite finite which requires its being the same size and shape as it is normally. It's a thought experiment, not a physics problem.

I realized this soon after I posted my reply. However I do strongly believe that space can only be finite or infinite but can not be both. Sure light can travel an infinite distance in a finite universe but I guess if blackholes or a big crunch came along then even light ends up traveling a finite distance. I don't have a point to make, just saying.

[patelmotels]

Is light not considered matter? Am i missing something

[joft]

there's no such thing as matter, it's just all probabilities

[TelecomClone]

Am i missing somethingYes. The point.

[Google Monster]

E=mc^2 says that energy and matter are the same. But I think so far it only works one way. Matter can be converted to energy but they haven't converted energy to matter.

[wildcat]

E=mc^2 says that energy and matter are the same. But I think so far it only works one way. Matter can be converted to energy but they haven't converted energy to matter.
Energy is converted to matter all the time.

[Google Monster]

ok ok, I should have added "naturally" inside there somewhere and minus the "they" eh?

[ferunandesu]

there's no such thing as matter, it's just all probabilities

the probabilities flesh out rather nicely on our scale

[ben from below]

E=mc^2 says that energy and matter are the same. But I think so far it only works one way. Matter can be converted to energy but they haven't converted energy to matter.

Energy is most certainly turned into matter all the time in particle accelerators. Two counter-revolving proton beams collide and create particles with WAY more mass than just two protons.

[Google Monster]

ok ok, I should have added "naturally" inside there somewhere and minus the "they" eh?

I've added this after that.

Which should turn out to sound like this:

E=mc^2 says that energy and matter are the same. But I think so far it naturally works one way. Matter can naturally convert to energy but hasn't naturally converted energy to matter.

[Edmond Zedo]

Yes, wait a minute. Carl Sagan told me the "finite yet unbounded" universe was only possible, and indeed depended on total mass. Then again, that was 20 years ago.

[Edmond Zedo]

ok ok, I should have added "naturally" inside there somewhere and minus the "they" eh?
Big bang, maybe. We don't know.

[Google Monster]

From what I believe, yes the big bang.