If The Universe Is Expanding, What's It Expanding Into?
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Put another way: "Expands into something else" is what "expands" means.
And words sure don't represent reality.
The expanding universe theory is an attempt to visualize what is going on, not to actually describe what is really going on.
Language is a simplification of reality. They create linear narratives (expanding from something smaller to something bigger)
But just as time probably isn't what we think it is, these narratives aren't anything but descriptors of experienced phenomena.
Just as when Dawkins talks about the selfish gene. He isn't saying that they are really selfish (they don't have agency) he say they behave as if they are selfish.
There is absolutely nothing that dictates that we need to be able to express in language what is going on, case in point quantum physics where we have non locality.
The expanding universe theory makes sense because the metaphors used to describe it take our perspective into consideration.
But in reality there isn't necessarily any expansion taking place. In physical reality yes, but that is not all of reality.
Or, put in a better way - the universe humans can perceive is expanding, without expanding into anything else we can perceive.
(Note, if all matter is n-dimensional, then expansion of the n-dimension space into dimension n+1 wouldn't be expansion of the Universe into anything. It would be the expansion of an n-dimension subset into an n+1 dimension universe.)
I've always considered this metaphor really good; it's just it is badly explained. The go to guy for decent pop physics explanations is, in my mind, Dr Brian Cox [1] (he does some really excellent TV shows here in the UK). His explanation with the balloon notes that while the surface area increases (expansion of the universe) the volume of the balloon rubber does not. You have to ignore the air inside and outside of the balloon as having any importance.
Then it should become clear how the universe can "expand" without actually expanding into anything.
And that's the bit of mental acrobatics that makes the metaphor break down. Which is why the advice above ("Don't believe the metaphors") is relevant.
When you blow up a balloon, the volume of rubber remains the same, but the area gets larger-- because it expands into open space.
When the universe expands, the volume of matter remains the same, but the area.....um, what? Expands into open space? That can't be right.
So, if the universe is getting larger, like a balloon, where is it going?
Oh, so it's not really like a balloon, then.
Well, actually you're considering the air there - so it doesn't work. Ignore the air and think about how the balloon skin moves through the medium.
In fact the balloon can be considered as not expanding into open air; simply more air is inside it than before - equally a little less air is outside it than there was before (ignoring pressure differences for the moment). Imagine if the skin was perfectly flat and was stretched. The air "moves" around it but is irrelevant to the expansion (and thinning) of the skin. The example only demonstrates how an object can have a larger area without changing volume.
Same principle - but in the balloon it is easier to demonstrate relative movement.
Im only basing this on my own experience but anyone I've ever given the proper explanation too has gone "oh yeh, I see" (or something the that effect).
In a way, it's like all of the light waves are shrinking locally, and only the ones traveling the distance between galaxies are staying the same. (Which is not what's really happening either.)
In any model of the universe as a brane of n dimension, its expansion can be modeled as expansion in the n+1 dimension. Perhaps "universe" is a paradoxical word that cannot have a meaning? It's hard to imagine a world of infinite dimension.
Do you know of cosmological models proposing an infinite number of spatial dimensions?
EDIT: Note that the "expansion of the universe" is really only "expansion we observe through telescopes." I've never heard anything about astronomical observations concerning dimensions S, 6, or others. 3D space can easily expand without the need for anything outside the universe, in a universe of more than 3 dimensions.
Waiting for the light bulb...
But just because the dimensions we see are expanding, doesn't mean they are expanding into other dimensions we can't see. Here I ask you for some empiricism - what's your evidence? To modify the balloon example, just imagine a flat rubber sheet that is stretched to be a little bigger. No extra dimensions necessary.
Your question contains invalid assumptions. There is no analogy that gives both a satisfying and accurate answer. You want to know about physics? Understand the math. The closest you can get without differential equations is Feynman's QED: The Strange Theory of Light and Matter
If there were ever a reason not to like cosmology, it's statements like that.
Edit: the human perception of time is an interesting subject. Physics (GR) holds that there is a 4D space-time. What we experience as separate constructs is really just too sides of the same coin. Although thermodynamics seems to define a "direction" to time, there's no real reason why you couldn't move in both time directions. For example, the production of an anti-particle in a particle accelerator (e.g a positron) is really just a particle (electron) travelling back in time to the production point. You can get around cause and effect issues if you assume the universe is in fact a 4D static environment, where time travel (forward or backward) has been predetermined by the static layout created during the big bang. This does of course have implications for free will... However, the multi-verse interpretation of QM gets you out of this.
Basically, I think most physicists would say these two scenarios are the same thing, just expressed (or observed) in two different ways.
Edit: actually this highlights an important distinction between the quantum and macro-worlds. Things can only happen in one direction in the macro-world thanks to entropy (everything -> disorder). Whereas in the quantum world there's no such restriction, and things can happen in either direction.
My brains segfaults imagining that there was a time where time didn't exist...
So if space didn't exist (in any meaningful way that we would understand today) before the big bang, time couldn't exist either.
http://feeds.tvo.org/~r/tvobigideasVideo/~5/QOpqIcl2UDk/BI_F...
It's long, but it won't merit a tl;dw unless you have no curiousity, imagination, sense of humour or innate geekiness.
The way my puny layman mind can figure out ... if 2 objects move away from one another at slightly more than half the speed of light, then one would see the other as apparently moving away at greater than the speed of light.
Also, iirc, the expanding balloon metaphor was more to give an idea how all points are moving away from one another, and each sees itself as the center. It was not meant to talk of dimensions, or moving into ... (btw, thanks a lot for the link. great read)
IIRC, mass grows towards infinity as it approaches the speed of light. So that's not possible if the objects have mass.
If it is the latter; it's perfectly possible. Ultimately it is just mathematics - as pointed out on Wikipedia apparent "FTL" travel is allowed by most models.
With massless particles is of course possible. My physics is a bit rusty so I can't really argue on this matter :)
From what i know, a great deal of energy is required to accelerate it, as the speed increases, but its still a finite amount.
Objects with no mass on the other hand, all move at c.
First, there's no such thing as objective space. There is no objective Frame of Reference. You can pick a frame of reference, and it should probably be an inertial (non-accelerating) frame of reference, but it's just as valid as any other frame of reference. Inertial frames of references move with respect to each other at constant, less-than-C velocity.
Ok, so you picked a frame of reference, and in that frame of reference you have two objects A and B moving away from each other, each with a velocity of 0.5c, for a total relative velocity of c. What does this look like to either of the objects? Well, to answer that question, we have to jump into their frames of reference, which are moving at 0.5c with respect to ours.
In this new frame of reference, one of the objects A is stationary. Object C that was stationary in the previous frame of reference is now moving at 0.5c. And the other object B, that in the previous frame of reference was moving away at c? In this frame of reference, it is moving away at less than c. Why? A combination of time dilation and space compression, which you probably know happens in special relativity.
In the original frame of reference, object A was moving at 0.5c because it took 1 year to travel from one point to another than are half a light year apart. In the new frame of reference, the object A is standing still and the two points are moving, and they're more than half a light-year apart, and it takes less than 1 year for the new one to get here. Similarly, time & distance separation between points that the other object B is passing have altered in the new frame of reference. There's some math which isn't that complicated but is a bit onerous, but the punchline is that it appears to be traveling less than the speed of light in the new frame of reference.
Why does space and time contort like this? So that a light particle traveling will appear to be moving at the speed of light in any inertial reference frame, regardless of the relative velocities of those reference frames.
Forget the philosophical/physics questions, what do those words even mean?
Is this the conventional view. I always thought the universe was expanding as a result of the big bang momentum. This expansion would keep going unless there was a critical mass to pull it back. (that was the way things were when i was a kid). Or am i misreading.
Carl Sagan would have provided an interesting anecdote or analogy to explain the answer instead of ridiculing whoever ask this.
Sadly, my writing skills are way lesser than those of Sagan.