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A universe smaller than the observable

A Universe Smaller than the Observable. Created by Sal Khan.

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  • leaf green style avatar for user James Kang
    what exactly is 4-D? people keep telling me things about it being hard to imagine it. Sal mentioned something about 4-D shapes so i am trying to understand what he is trying to talk about.
    (22 votes)
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    • duskpin sapling style avatar for user elainechen311
      A good way to think of 4D is to think of shadows.

      The shadow of a 1D object, a line, would be a 0D silhouette, or a dot.
      The shadow of a 2D object, a square, would be a 1D silhouette, or a line.
      The shadow of a 3D object, a cube, would be a 2D silhouette, or a square.

      Similarly, the shadow of a 4D object will be a 3D silhouette. Isn't that mind boggling?
      (16 votes)
  • blobby green style avatar for user Ben Barnett
    How does this theory explain the proof of the expansion of the universe?

    (5 votes)
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  • blobby green style avatar for user Dean Barzilay
    How come that when the light passes through our space in the universe we cant see it anymore untill its next pass... isnt light a continues thing or the light we see is a result of one time event like a supernova and such?
    (8 votes)
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    • leaf grey style avatar for user Geoff Kincade
      Any object which is emitting light does so on a continuous basis. Photons are being created and streaming out in a straight line in all directions. Sunshine is photons created about eight minutes ago on the sun. A very small amount hit the earth. The vast majority continue into interstellar space. When a photon hits something, like the earth, it is absorbed and no longer exists. Just as you can never step into the same stream twice, the light you see the second time around is other photons from the same stream.
      (14 votes)
  • leaf red style avatar for user Nathaniel Boyce
    I mostly understand the idea of the universe being smaller than the visble universe. However its difficult for me to comprehend the idea of an object being four dimensional. Is it possible someone could better explain this to me?
    (5 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      This is the best way to describe it. A four dimensional object is incomprehensible. Think of it this way.

      You've heard of the directions forward and backward (first dimension). You've heard of the directions left and right (second dimension). And you've heard of up and down (third dimension). The forth dimension brings two new directions that are incomprehensible to us. Some say that time is the forth dimension.
      (3 votes)
  • leafers ultimate style avatar for user David Spenger
    so a scientific theory is an explanation on how things work right. My question is how do i tell a proven theory like evolution or gravity vs. something like cosmic inflation or the theory of a smaller universe? Are they too supported by facts, or are they just possibilities with small bits of evidence to support them?
    (8 votes)
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    • leafers seed style avatar for user ivysiri
      Theories aren't proven. Accepted models are supported by repeatable experiments and observable evidence. Gravity and evolution aren't proven beyond a shadow of doubt, we just accept them as the most accurate explanation. We end up re-learning a lot of physics, including the concept of gravity once we move beyond classical mechanics/physics.
      (3 votes)
  • leaf orange style avatar for user Alejandro Carvallo
    What does this mean for time travel? If we can see ourselves in the past we could see earth grow with time and it would be like seeing our planet's past wouldn't it?
    (3 votes)
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    • old spice man green style avatar for user Nathan Davis
      For simplicity sake, if you are 10 light years away from earth and had an extremely powerful telescope, yes you can see earth as it existed 10 years ago.

      With regard to time travel, well you are traveling through time right now. If you aren't moving relative to earth you are traveling through time at the speed of light. You can travel slower through time relative to earth just by speeding up. The faster you are moving relative to earth, the slower you move through time again relative to earth. However, to travel back in time would require you to move faster than light which isn't possible.
      (5 votes)
  • leaf orange style avatar for user Jon Donym
    Has anybody tips on how to really visualize a 4d space?
    Or is that something our brains cant really do?
    (2 votes)
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    • male robot hal style avatar for user Andrew M
      You are not going to be able to visualize 4d.
      The best you can do is just use an analogy by comparing 3d to 2d. If you have a 3d sphere, the 2d version is a circle. So a creature living in 2d who encountered a sphere would see a circle, and as the sphere moved in the 3rd dimension, the circle would get bigger or smaller depending on which part of the sphere was in the 2d plane. The sphere could move far enough above the plane that the circle would just become a dot and then disappear entirely into the 3rd dimension that the 2d creature cannot fathom.

      Try reading the book Flatland if you are interested in doing more imagining like this. You still won't be able to picture 4d but you will be able to have some understanding of it. For example, now you can see that a 4d "hypersphere" in our 3d universe would look to us like a sphere that could change size or even disappear, as its intersection with our dimension changed according to movements in that mysterious 4th dimension.
      (6 votes)
  • hopper cool style avatar for user The Unique One v2.0
    Some people say that the 4th dimension is time, whereas others say that the 4th dimension is something in a tesseract. Which one is true? Please explain.
    (2 votes)
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    • male robot hal style avatar for user Charles LaCour
      OK, lets get a few things straitened out.

      A tesseract also known as a hyper-cube is just a 4 dimensional cube, no more no less. It has no more significance than a cube, square, line or point.

      There is no 4th dimension just like there is no 1st, 2nd or 3rd dimension. In the world we experience on a day to day basis you need 4 coordinates to specify an event therefore it is 4 dimensional.

      The current idea of our universe is that it is a 3 dimensional surface embedded and evolving in a 4 dimensional space. To help visualise this you can think of the traditional analogy of a balloon expanding where the 2 dimensional surface of the balloon is the universe and 3 dimensional volume it is expanding in is space-time. This is a very crude analogy since it implies that there is something outside the univers for the expansion but that doesn't have to be true.
      (5 votes)
  • blobby green style avatar for user rgpusey
    this is just a theory right? or is this like... proven?
    (3 votes)
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    • leaf green style avatar for user omri.alkabetz
      It's a hypotheses, no one has any reason to think that it's true as for today.
      But it's a problem to call it "just a theory", because a theory in science is very different than a hypotheses: A scientific theory is "a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment".
      It's not a theory yet, because it has no evidence.
      (Sorry for my OCD about these definitions, but I'm sick of people who say that "Evolution is just a theory").
      (4 votes)
  • aqualine seedling style avatar for user Niv
    If you started at one point and came back around the universe. would an example be going straight across the earth from America and instead of leaving earth, you would come right back around to America? If so would it work in all directions? up, down left right-no matter how you go you will still pop back to where you were?
    (4 votes)
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Video transcript

Every video until now, we've been working from the assumption that the observable universe is smaller than the entire universe. And if you go by the cosmic inflation theory-- and it was founded by Alan Guth. And I have almost personal connection to Alan Guth. When I was at MIT, I always used to go to this Chinese food truck. And I always used to show up at the food truck like two seconds before Alan Guth. Like he was always one or two people behind me in line. But anyway, he was the founder of the cosmic inflation theory, which is basically this idea that in the very early moments or the very early period after the Big Bang, we went through this major inflation in the expansion of space. But anyway, based on the theory of cosmic inflation, then the observable universe is on the order of-- or maybe another way to say it, the entire universe is on the order of-- 10 to the 23 times the size of the observable universe. So that would mean that this is just a tiny, tiny fraction. I mean, this is an unimaginable large number. In fact, it is unimaginable. So already everything we've talked about, this itself is a huge-- this is an incomprehensible amount of space. But this is an incomprehensible multiple of this incomprehensible amount of space. And that's just based on that theory. But it is possible-- we cannot rule out even the idea-- that the actual universe is smaller than the observable universe. And that one is in some ways even more mind blowing than the idea that the universe is this big, the fact that what we're observing is actually larger than the actual universe. And so you might say, well, Sal, that's impossible. But just think about it a little bit. This is the observable universe. And the way we've depicted it, it's based on how long the light has taken to reach us. We've already covered before that this point in space is now 46 billion light years away, not 13.7, the way it looks right over here. It just took 13.7 billion years to reach us. If there's any photon that would take longer than 13.7 billion years to reach us, it hasn't reached just yet. Because it could have only started 13.7 billion years ago. So they're on their way. And they started at some point outside of our observable universe. So our observable universe will grow over time. But with that said, let's imagine that the actual universe is a subset of this observable universe. Let's say it's roughly half the diameter. So let's say it looks like this. Maybe I'll make it a little bit of an oval. Maybe the actual universe-- and this is just to be a little bit provocative. And it's not impossible. Let's say that this is the actual universe. And the way I drew it, it makes it look like Earth is the center. That we're the center of it. But remember, this is very likely to be the surface-- or it is curved. It has a slight curvature. But it could very well be the surface of a four-dimensional object. And maybe the simplest one to visualize is four-dimensional sphere. So if you really wanted to visualize this right, this whole volume-- and remember this whole picture, it keeps looking two-dimensional. But it has depth. It is a volume of space, an incredibly vast volume of space. And so what I've done here is this is an ellipsoid right here. It's elliptical volume of space that I've bubbled out right over here. But if this was really the entire universe, and if the entire universe really were the surface of a four-dimensional sphere, then the reality is that this entire space could be represented like this. It could be represented as the surface. If this was a four-dimensional sphere-- obviously, I can only draw three-dimensional spheres. But let me show you that it's not just a circle, that it actually has some depth to it. And I can even shade it right over here. And so you can imagine that this point over here is actually the same thing as that point over there. That they have wrapped around and that they're connected right at the back over here-- let me go behind-- that they're connected right over there. And that this point and this point are actually the same point, that they've wrapped around. Maybe they've wrapped around. Actually, the way I've drawn it right here, they would actually all wrap around right back there at that point, if I'm visualizing properly. But if you go in any one direction, you would come back on the other side of the surface. Let's say that Earth is right here. The way we depicted it, Earth is the center. But we see that when you look I like this, there is no center to the surface of a sphere, even a four-dimensional sphere. So in this sense, if you go in any one direction, you'll come back out the other side. So if you start from Earth and you go in that direction, once you get there, you're really here again. And then you would come back to Earth. And so if this were the case, if the actual volume of the true universe was smaller than what it looks like, the observable universe, then what's all this stuff on the outside? And to think about it, think about what would happen if 13.7 billion years ago, when we were in that primitive state, where that background radiation, those photons are being-- those electromagnetic waves are being released, let's say they get released. And those photons on their first pass-- and I think you know where this is going-- on their first pass, they would get to us in about-- this looks like a distance of about, I don't know, this looks like about 6 billion years. Then they would pass us up. And then they would get back to this point again in another 6 billion years. And then they would come back here. And so that very first past photon are going to be right over here. And from our point of view, we're not going to see them for a couple of billion years. And so when we do see them, we're going to perceive them-- we're going to say, wow, it took 15, 16 billion years for that photon to get to me. That must be from something out here. But the reality is it's a photon from something within a smaller physical universe, within a smaller actual universe, that's just taken several passes by us. And we're just seeing a pass after 14 billion years. We just think it's from something further out. Now the other thing is you say, well, if this was the case, if we could just go in one direction of the universe and then come out of the other side, and if all of that was within the observable universe, wouldn't we be able to tell? Wouldn't we be able to look in two directions and see the same thing from a different perspective? And the answer there is to think about what happens-- or actually, wouldn't we even be able to see ourselves? Because we emit some light, and it would take maybe-- I don't know how far that is. Let's say that's 6 or 7 billion light years to get right over here, which would be right over there. And then it would take another 6 or 7 billion light years to get over there. So maybe that background radiation we're seeing is actually background radiation emitted from that exact point in space that we are right now, or from a very similar point in space, to where we are right now. Or part of the background radiation is from a similar point in space that we are right now. So how come we can't just see ourselves? Well, I kind of just answered the question. That second pass, if you're observing the same point in space, if you're observing light from the same point in space on a previous pass, that light was emitted a long, long, long time ago, maybe 13 billion years ago. And so it would be unrecognizable. This region of space, the region of space that we are in right now, if we saw the same region of space 13 billion years ago, we just wouldn't recognize it. Now, there are some people attempting to see if there are some patterns, see if you can model how the universe would change and if you see patterns. And maybe the actual universe is a subset of the observable. We just haven't seen it yet. But it's completely a possibility. Hopefully, I didn't confuse you. I actually find this kind of an interesting idea. That this light that has taken 13-- let's say the light that's taken us 8 billion years to reach us. We think it's from something, based on this scale, 8 billion light years out. It's actually further because the universe is expanding. So it would have actually traversed more space than that. But we think it's from something like that. But it could've been something further in, if the actual universe is smaller. And it's just on its second pass. It's actually coming back again. And that's why it took 8 billion years to reach us. And we don't even recognize it because it looks very different than that region of space right now. Or that region of space after 4 billion years looks completely different than it did when it first released. Anyway, hopefully I didn't confuse you too much. But I think this is a fascinating, fascinating topic.