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A question for the physicists here

CTRandall

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I know there are a couple of physicists lurking about the Chrons ( @Biskit @Venusian Broon -- I'm sure there are more) and I've got a question for you.

Let me preface it by saying I wasn't dropped as a baby.

Here goes: what is distance? I know the Planck Length is theoretically the smallest possible unit of distance, but I don't think that is universally accepted and, further, I believe a good number of physicists hold out for the possibility that the universe is smooth, with no fundamental, smallest unit of measure. (Please correct me if I'm wrong!) Either way, knowing the smallest unit (or lack thereof) doesn't tell me what distance is. Before you turn away in disgust at my imbecility, let me clarify what I mean.

The real problem I'm having is the expansion of the universe. If no new mass or energy is being created--if all that was is still all that is--it seems the only difference between the past and now is the distance between things. Teeny-tiny universe = intensely hot and undifferentiated. Great big universe = temperature variations and a variety of structures on different scales, e.g. molecules, stars, clusters.

If that is a valid way of looking at our universe, then what is this magical "distance" thing that separates us from the primordial Hell that followed the Big Band? Is the expanding universe continually generating new Planck lengths to stick between Earth and the stars that are moving away from us? (Yes, that sounds absurd to me too.) Or is distance some kind of relative property? For example, if the universe is all there is (please, please can we avoid multiverses for a moment), there is nothing to measure it's size against. It is meaningless to say it is big or small, maybe even to say it is bigger or smaller. Thus, expansion is, in a sense, relative, only observable by us from within the universe. So again I'm faced with the question, what is this "distance" that separates me from you--is it absolute, relative, one half of a distance-temperature continuum, continuously generated...

Or am I just tying myself into a Xeno-esque paradox by focusing on the concept of distance rather than space-time?

Your replies can be as brutal as you like. Go ahead, take out your frustrations with the scientific illiterati on me.
 

Venusian Broon

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Good questions.

I'll get back to you on the universe expanding, as it's late (got playing Observation and really quite enjoying it. Very sort of 2001-ish...) :)!

However about the Planck length first.

I think most conventional theories assume that Space-time is 'smooth' and will have infinitesimal distances. (Well, technically speaking both the Space-time of Einstein and the rather different space that Quantum mechanics uses, but let's leave it at 'Space-time'!)

Now the Planck length is really the shortest observational measurement we could theoretically make. Essentially because we have the Heisenberg's Uncertainty principle, that relates momentum and distance to a constant. So we are required to use extremely high-energy collisions to probe very small distances, and the smaller you want to get, the more energy you have to put in. At smaller than the Planck length, you put so much energy to 'probe it' you would actually generate a black hole. And putting even more energy into it would just generate an even bigger singularity.

So the expanding universe isn't generating lots of individual Plank volumes as it expands, it's just that if we try and observe something on that length (1) we'd need a vast amount of energy (2) we'd be close to generating black holes. The Planck length is just a limit on our observations, which is not the same as saying that it is the 'smallest unit'. Theoretically there is an infinite number of smaller ones....just we could never observe them because if we tried to all we'd do is create black holes.

However, as you've pointed out, other theories do posit a real minimum length of Space-time that is somewhere close to the Planck length. Sheldon Cooper's nemesis, I believe, namely: Loop Quantum Gravity. But then the sort of assumptions you'd have to make and how they behave in an expanding universe....probably do get a bit spicy and you have/can make all sorts of mathematical tricks to try and explain how these theories reconstruct what we see.

Will get back to you after giving the second half of your post a thought in the morning!
 

Venusian Broon

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Okay, expansion of the universe. What I've written is just, more-the-less off the cuff, but hopefully it'll make some sort of sense.

I'll get back to the relative thing at the end. Penrose has an interesting idea on that.

Naively (I didn't study much cosmology, so I'm taking a conventional position) I see the universe expanding smoothly, not in discrete Planck volumes, as I hopefully argued above.

And assuming just one universe, although in many multi-universe models the separate universes themselves are all expanding slower than the inflationary space between them, so there is no way any of them could ever join up or contact, so effectively we can only experience the universe as if it was the only one.

I'd argue that it is meaningful, in fact very meaningful, that the universe is getting bigger. At least for us. Yes distance is effectively the distance between 'stuff' and as time has gone on the average distance between baryons has always gone up. This might seem odd at first, because this also implies that the space between the atoms that make yourself up is continually expanding. It's just that stuff that makes us up has the strong nuclear force keeping the nucleus together, EM charges that interact and help keep atoms and molecules together and a general gravitational force that keeps large masses of stuff together. All these things should counteract this expansion for large classes of macro objects, like galaxies, stars, planets and us and therefore we're not being stretched out continuously. (However see later and the Big Rip)

So firstly this expansion explains a lot of the evidence and observations we make of the cosmos. So taking the three 'classic' pieces of evidence. The redshift of galaxies measures the actual expansion, the cosmic microwave background radiation is taken to be the stretched out remains of the EM radiation generated in the big bang and the atomic composition of the early universe at recombination (~77% H ~23%He and a dribble of Li) can be understood by us by assuming that the universe had a certain density of matter and energy when it had expanded enough for atoms to become stable.

There are now loads of other pieces of evidence - for example I was just looking at a video that discussed 'the cosmic dark ages' i.e. there was a period of time after the moment of recombination we had to wait for the first stars and galaxies before they started and could generate light. However you can observe quasars that were born close to this era and the spectra of these objects that we now currently see, and can show that their light has been absorbed by neutral hydrogen gas clouds that must have present and by the expansion of the universe.

There are more lovely pieces of evidence.

Anyway, that's what's happened. What about the future?

Well, we now believe the expansion of the universe is accelerating. Why it does this, we don't know. It's just what we've observed. But assuming that this continues for the lifetime of the universe:

It means that the size of the observable universe will be shrinking. Very roughly, any distant object's light that we can see today, will continue to be red-shifted by the expansion, till at some point the wavelength of all the light produced by the object will be longer than the observable universe and hence will 'drop out' of the observations of beings trying to look for them. Taking this to a logical extreme, at some point we'll only be able to see the local group of galaxies. Not even the gravitational forces of the other galaxy groups will be able to counteract the expansion.

This has interesting philosophical implications. At this moment in time we can still see evidence of the big bang. Microwave background radiation, the first stars, quasars and the first galaxies. But at some point in the future, given the expanding and accelerating universe, all this information will be lost. What would intelligent life at that time, who observed the sky make of this and how would this impact their theories on how the universe came into being? What sort of theories would be 'true' at that point if some of the evidence, that we're aware of, will be lost?

Of course it can get worse. If the acceleration is ever increasing exponentially then we'll get the 'The Big Rip'. In this case the observable size of the universe gets to be extremely small. And just as at some point, light gets so red-shifted that it can't exist in the observable universe, the fundamental forces between matter starts to get affected. So if the observable size of the universe is smaller than the milky way, then the forces that give the milky way it's structure (or whatever the milky way is by then...) can't operate and the milky way would disintegrate, the structure would disappear. When the observational size of the universe gets to be much smaller than the size of the solar system, then all solar systems (again, whatever they are at that time) would break up as gravity can no longer communicate properly with all the various original components and so it goes on, till the size becomes smaller than the smallest pieces of matter and eventually Space-time itself gets 'ripped' apart. (Whatever that means, I'm a tad unclear on the final step!)

So such an expansion does have big implications for us in the universe, if we are thinking about the future. Admittedly we don't know if something like the big rip actually will happen, and by current measurements, even if it were found to be true, the point that it would occur could be so far into the future that perhaps something else would happen instead.

---------------------------------

With regards to the relative part. Essentially the issue here is that we're taking a viewpoint from outside the universe. Which, of course is a bit paradoxical, really.

Anyway if the Big Rip is not going to happen because we're not expanding fast enough, then the expansion will likely lead to the heat death of the universe, populated by stellar remnants such as black dwarfs and black holes. Although eventually the black holes, as the cosmic background radiation gets cooler than the hawking radiation of the black holes, they will just start to evaporate away.

Now Roger Penrose suggests that on top of this, all matter will eventually decay - including whatever dark matter is. Leaving the universe as a sea of photons. And he suggests that such a universe will then become 'conformally invariant'. Which I take to mean that 'distance doesn't matter' :). Someone with proper mathematical training will hopefully explain it technically and clearly! He also states that the universe at the big bang was conformally invariant, so....he implies that when the universe reaches this state, it is ready to be the next 'seed' for another cycle of big bang and expanding universe. However I'm not clear that this conformally invariant space of a heat death'ed universe - which in our eyes would be unbelievably vast (We are talking about trillions of years of expansion into the future, I think) - would somehow become tiny for the next stage of the universe, because photons don't 'notice distance'! Does this mean that the 'new' hadrons and atoms that form in that new cycle be, in absolute terms, unbelievably vast compared to ours?

However, Penrose's theories has problems. We have never observed the decay of any mass that he needs to occur. And how can one actually get any evidence of the earlier cycles?

It's interesting though.

Sorry for the splurge. Like I said, great questions. What I've stated above is not gospel, just my incomplete understanding, and is highly dependent on experimental results, such as the acceleration of the universe and a hundred and one other things. So take it all with a pinch of salt. ;)
 

CTRandall

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Now Roger Penrose suggests that on top of this, all matter will eventually decay - including whatever dark matter is. Leaving the universe as a sea of photons. And he suggests that such a universe will then become 'conformally invariant'. Which I take to mean that 'distance doesn't matter' :).
This is what I was trying to articulate. Glad Roger Penrose is on my side, now I don't feel like such an idiot! (Even if there is no evidence to support his idea.)

One of the interesting points you touched on is the "observable" aspect of all of this. You questioned how an intelligent race in the far future might understand the universe but it also begs the question of what we can and cannot perceive now.

Thanks for your excellenct summary!

Now I'm off to write my sci-fi epic about a race of intelligent crab-creatures who evolved next to sunless, geothermal vents and, as a result, perceive the universe solely in terms of hot and cold.
 

Venusian Broon

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This is what I was trying to articulate. Glad Roger Penrose is on my side, now I don't feel like such an idiot! (Even if there is no evidence to support his idea.)

One of the interesting points you touched on is the "observable" aspect of all of this. You questioned how an intelligent race in the far future might understand the universe but it also begs the question of what we can and cannot perceive now.

Thanks for your excellenct summary!

Now I'm off to write my sci-fi epic about a race of intelligent crab-creatures who evolved next to sunless, geothermal vents and, as a result, perceive the universe solely in terms of hot and cold.
One of the good things about Penrose is that he is trying to find observational evidence.

Off the top of my head I believe he is interpreting structures found in the Cosmic Background Radiation with dark matter and possible matter decay. Although I might have that a tad mixed up.

There are other technical reasons based on our understanding of the laws of the universe that throw a spanner in the works too...but they are technical!

Yeah the observation and when thing is fascinating. What if an intelligence develops when virtually all galaxies are unobservable? How could they work out the universe was expanding? (Even although in this scenario it was) How would they think they came into existence? Perhaps there is other ways of measuring it on the small scale? And yes it says something about our own hypotheses and the real world.
 

RJM Corbet

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Off the top of my head I believe he is interpreting structures found in the Cosmic Background Radiation with dark matter and possible matter decay. Although I might have that a tad mixed up.
I think he is also looking for evidence of his Erebon particles in the Ligo 'waste paper basket'?
 
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RJM Corbet

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Erebons: Planck mass Dark Matter particles
"Erebon theory is a novel explanation of dark matter recently invented by Roger Penrose. Erebons are scalar particles of the order of a Planck mass which can only interact gravitationally. When erebons decay, they release their energy as oscillating classical gravitational waves on the order of the Planck frequency. Erebon theory is actually part of Penrose' Conformal Cyclic Cosmology scheme, which is a competitor to the theory of inflation. Moreover, Penrose claims erebons may have already been detected by accident, quite analogous to the first accidental detection of the CMB. Here is one of the lectures from July 2017 in which Penrose first introduces the idea of erebons:"


The second one is basically the same talk but is a better production. NB: SKIP THE FIRST SIX MINUTES of introduction waffle by the presenter!


They are both an hour long
 
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RJM Corbet

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Cool! Penrose comes across as the original bumbling genius, lol. He won't use a computer presentation; he uses his own felt-tip drawings on plastic and he keeps losing sheets and getting them backwards and upside down. Its quite funny. But the broad idea is reasonably accessible to a layman like myself.
 
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