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> We can only see a finite volume—a finite number of galaxies. That’s essentially because there’s a horizon, a shell around us, delineating the greatest distance from which light can reach us. But that shell has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean.
No, that's not true. You can rise above the surface of the ocean. Barring some major unforeseen revolution in our understanding of physics, you cannot transmit information faster than light. The oceans horizon is grounded in a technological limit, but the cosmological horizon is grounded in a fundamental physical limit.
According to current cosmological thought, the laws of physics at that shell are very close to if not identical to the laws of physics here. And yet, that shell is retreating from us faster than the speed of light. The shell is ever expanding, but we shall never see what that current shell will look like when it is 10 billion years.
How can that be, you ask? It is quite simple. The speed of light is a limit on how fast light can travel through the universe. But the universe is expanding. Light trying to get to us is like a bug crawling over an expanding balloon. If the balloon is expanding faster than the bug is crawling, it can crawl forever but never reach the point it is trying to crawl to.
So there is a fundamental epistemological limit - we can't actually know that our current models are correct. But within existing theory, the location of "the farthest we can see" is not particularly meaningful physically.
> within existing theory, the location of "the farthest we can see" is not particularly meaningful physically
Except that you can always put a finite upper bound on that distance. That is what matters with regards to the topic under discussion.
Which means that it is exactly as meaningful as the event horizon of a black hole. The last point where the external observer (ie us) can see what happens. But in a different coordinate system, not particularly special at all.
Yes, that's right. Different observers have different light cones. Yours is a little different from mine. But that is missing the point, which is that every observer has a light cone beyond which they cannot see -- not even by collaborating with other observers with different light cones.
It's also worth noting that, in a sense, the shell is actually shrinking. Every moment, more of our observable universe disappears "beyond the veil", so-to-speak, as the universe on our side of the edge transitions from near-light-speed to beyond light-speed outside the shell. (I know that's a sloppy way to use the speed of light, but it works for me.)
Small nitpick, that shell---the Hubble sphere---is shrinking due the accelerating expansion of space. Constant expansion cooresponds to a constant Hubble radius, and decelerating expansion with an expanding radius.
Good point, thank you.
it only has physical significance from our perspective.
Ah, but if someone on a distant planet were to record their perspective, e.g., data about their visible universe, and transmit it to us, it would be consistent with our perspective by the time it reaches us. For instance, stars that they can see but we can't, would become visible to us as their message reached us.
They couldn't transmit such a message; the part beyond "the wall" is moving away faster than light: the data would never reach us. From our perspective, their earlier era is "frozen" in time: red-shifted to zero. We cannot receive any "new" signal from them. Therefore, the seeming "faster than light" violation is not really the case. Relativity "fixes" it by mucking around with time.
From a practical perspective, the red-shift horizon is a pretty solid "wall". The Orange Dude would love it. There could be a million Darth Vaders or run-away planet-eating nanobots on the other side, and they can never touch us. Maybe that's a good thing. The Egyption Empire lasted an extra long time largely because it was so hard to invade. It might even be the Anthropic Principle "protecting" us from really dangerous things.
> the part beyond "the wall" is moving away faster than light: the data would never reach us
To nitpick, the wall itself is moving away at about one light-year per year. The photons that are not moving from our perspective because they are receding away from us at the speed of light will soon find themselves in a space where they are again moving towards us.
"we routinely observe galaxies that have, and always have had, superluminal recession velocities" -- Davis & Lineweaver.
Any information that gets to us via some alien friends will have to travel even further than photons from those stars. By the time we got the alien signal, we would have seen photons from those stars already.
Well, yeah, but our perspective is the only one that can possibly matter for us. There may be unicorns and leprechauns out beyond our light cone for all we know, but what possible reason would there be to care?
Is the only reason to investigate the universe to look for ways to exploit it?
The argument is it's beyond our observable universe. It's not possible to be investigated as we can receive no information.
What happens on the far side of the moon doesn't matter for us, either, but we still looked.
> What happens on the far side of the moon doesn't matter for us, either,
How do you know? Maybe the far side of the moon is chock-full of exploitable resources. You can't know until you look.
> but we still looked.
Sure, because we could. We cannot look beyond our light cone.
i don't understand what you are "no, not true"-ing. it's just analogy. you pointed out a flaw if one happens to take the analogy too far, but i don't see the point when the original analogy was clear enough what they are getting at.
I am disputing the claim that "that shell has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean".
>Barring some major unforeseen revolution in our understanding of physics, you cannot transmit information faster than light.
There's been a significant amount of research done in the recent years into quantum entanglement. Although we're not there yet, it wouldn't surprise me if we eventually manage to pass information via entanglement.
Nope, not possible:
> At first sight, the concept of parallel universes might seem too arcane to have any practical impact. But it may (in one of its variants) actually offer the prospect of an entirely new kind of computer: the quantum computer, which can transcend the limits of even the fastest digital processor by, in effect, sharing the computational burden among a near infinity of parallel universes.
I'm far from an expert on quantum computing, but this seems inaccurate to me. There may be an explanation for a quantum computer's operation that invokes parallel universes, but I don't believe that they are actually required for the systems to function. Quantum mechanics is sufficient.
I think the allusion is quantum mechanics -> many worlds interpretation -> quantum computers are executing in parallel universes. But quantum computers get their abilities from the quantum states and quantum operators, neither of which require any particular interpretation of quantum mechanics.
If we can form arbitrary size superposition we can put the universe in superposition - many worlds. If we cannot - limit to how big quantum computers we can build.
Similarly, by the copernican principle, wouldn't our "local" quantum computer also then be burdened by the work being sent from a near infinity of parallel universes?
I think the answer to that is no and the reason involves entropy. Of all the possible universes most have very low probability, so when we "create" a bunch of parallel universes for our computer, those universes could already be existing which would interfere with the calculating (not in the sense of slowing it down because of too many things running on it, more like a radio channel with lots of interference from nearby channels), but this won't happen in practice as these universes will have very low corresponding probability.
On the other hand there is self interference, from poor engineering of the computer. I think this is a very significant problem now - but it will usually be described in terms of decoherence.
I would say "yes" to that. But fortunately, with the load being so widely distributed, the load on our "local" quantum computers would effectively be zero (ie. x/inf). Unless, of course, our universe is the oddball and most others are running at full capacity. That's a depressing possibility.
The load could be zero, it could be infinite, or anywhere in between. Infinite universes sending infinite work is inf/inf. It's not possible to know if that's going to tend towards something like 0 or something like positive infinity without having some way to measure. But it's an error to just assume it's zero.
If they are relevant for a quantum computer, then they are not parallel universes. They are part of "our" universe.
> if the universe stretches far enough, everything could happen <
The article says this. Is this an assertion that the number of things that could happen is finite, or is it an assertion that the number of elements in one infinite set is greater than or equal to the number in another infinite set? Which infinity is equal to the number of things that could happen? How many dimensions would the universe need so that the number of things that would happen in it would equal the number of things that could happen in the most inclusive case?
It's also not such an easy assertion to make, distribution of things that could happen matters, and there could be events with 0% probability.
> “The long-term future probably lies with electronic rather than organic ‘life.’”
I guess this is the standard transhumanist position, but it strikes me as both pessimistic and overconfident. Pessimistic because it doesn’t see the life that arose against all odds on this rock as fit for or worthy of continuation on a solar timescale, and overconfident in its implication that we will inevitably create something in our image that will succeed us.
The middle of the article reminded me of Asimov:
The last question was asked for the first time, half in jest, on May 21, 2061, at a time when humanity first stepped into the light. The question came about as a result of a five dollar bet over highballs, and it happened this way...
It always interested me that this could be Asimov's (1956) response to Arthur C Clarke's (1953), 9 Billion Names of God, where the stars went out.
Your comment led me to search for the latter title, if I could read it online. Instead, I found the following project. http://ninebillionnamesofgod.com/index.html
Those are both fantastic stories.
That's a classic, for sure!
What exactly would be "copernican" about it? Also 3 of the 4 "copernican" revolutions outlined have nothing to do with copernics. Even copernicus's "revolution" was just a rehash of ancient greek idea of heliocentrism ( aristarchus of samos ).
What a mess this article is.
Proving our multiverse is one among many will be the fifth Copernican revolution. Proving this sequence of Copernican revolutions is one among many will be the aleph one Copernican revolution.
I think it would be the aleph naught revolution, no? :)
Did you apply the Y Combinator there?
> if the universe stretches far enough, everything could happen
This means, I presume, that in an alternative universe, children-like creatures are being boiled alive forever with no hope of dying (since they are immortal), there exists creatures being tortured alive forever, although they have evolved to be a million times more sensitive to pain, and a God-like being exists... the possibilities are truly endless.
If other universe contain all logical possibilities, then yes. David Lewis talks about this and points out the moral issue is that even if you choose to do good things in this universe, in some universe your alternate is a psychopathic torturer, so decisions to do good don't actually lessen universal suffering, if calculated across all alternate universes. I think his conclusion was that it's still desirable to locally decrease suffering anyway.
But, apart from the moral absurdities, there are logical inconsistencies.
In a certain universe, there will exist a God capable of influencing all other universes, and thus constraining the set of possibilities, no?
The gods are sandboxed, so unless they've discovered an escalation vulnerability in the multiverse substrate, they can execute any permutation of possible behavior but never affect the lower layer. If that happens, it's back to "turtles all the way down" as we then try to figure out what's running the multiverse VM.
And yet in another universe there’s a god that controls the other god.