"No valid plans for the future can be made by those who have no capacity for living now." -Alan Watts
"They do not see what lies ahead, when Sun has faded and Moon is dead
." -J.R.R. Tolkien
One of the most amazing facts about the Universe is that, despite only having spent a few hundred years studying the fundamental constituents and forces of what makes us up, humanity has been able to accurately figure out just what all this actually is.
The laws of nature are almost completely understood in a few, very important senses. We know that our Universe is about 13.8 billion years old, despite having human experiences and observations that range from only a few fractions of a second to a handful of years. Our investigations of the laws of nature today allow us to look back into the distant history of the Universe, and understand what it was like 13.8 billion years ago, and how that gave rise to our Universe today.
This is a lot more impressive if we think logarithmically, which is something we're more used to doing for distance. In the Universe's distant past, when it was just 380,000 years old, it was too hot to form neutral atoms; that's what we see as the leftover glow from the Big Bang: the cosmic microwave background! That was when the Universe was just 0.0028% of its current age, or 1/36300th the age it is now.
We can extrapolate back even farther, to the time when the Universe formed the first atomic nuclei, back when we were just 200 seconds-or-so old, or some 4 × 10-16 times our current age. Earlier than that, it was so hot that we spontaneously were creating matter/antimatter pairs, back when the Universe was around 10-18 times its current age, and back when all the particles we've created in accelerators -- including the Higgs -- were common in the Universe, at the highest energies we presently (and robustly) understand the fundamental laws of physics, the Universe was just a few tens of picoseconds old, or around 10-28 its current age.
But that only explains how we got here. What about the other side of the coin: where we're headed? As the famous physicist Niels Bohr once quipped:
Prediction is very difficult, especially about the future.
But just as our laws of physics allow us to extrapolate back into the distant past, they also allow us to extrapolate into the far future! We can start with the night sky.
Over the next three-to-five billion years, the Andromeda Galaxy (and quite possibly the smaller Triangulum Galaxy) will merge with our own Milky Way, causing a spectacular change to our galaxy's structure and to the night sky in general.
While gravitation will cause the local group to eventually merge with us, dark energy will cause all other galaxies and clusters -- the ones that aren't bound to us today -- to eventually redshift away from us, leaving our observable Universe on timescales of billions to hundreds of billions of years. But none of this will, in all likelihood, affect our Solar System, which is what I'd like to focus on today.
The Sun will continue to get hotter as it ages, boiling our oceans in approximately 1-2 billion years and ending life-on-Earth as we know it. Eventually, about 5-7 billion years down the line, we'll run out of nuclear fuel in the Sun's core, become a Red Giant, and engulfing Mercury and Venus in the process. Due to the particulars of stellar evolution, the Earth/Moon system will likely be pushed outwards, and will be spared the fiery fate of our inner neighbors.
After the Sun expels its outer layers to form a planetary nebula, the core of our star will contract to become a white dwarf, the eventual fate of nearly all stars in our Universe. But the planets will still be here, orbiting our cold, dim stellar remnant.
The Earth, for its part, will slow down in its rotation, while the Moon migrates farther away. After about 50 billion years, the Moon's orbital period will be more like 47 days (as compared to the present 27.3 days), and our 24-hour-day will have slowed to match: it will take 47 of today's days to make just one day on the 50-billion-year-in-the-future Earth's day. At this point, the Moon and Earth will be tidally locked, so that the Earth and Moon always appear in the exact same position in one another's skies.
Eventually, white dwarf stars will go black, as they cool and radiate their energy away. This will take a very long time: maybe 1016 years by my estimates (although your mileage will vary), or about a million times the present age of the Universe. The atoms will still be there, they'll be just a few degrees above absolute zero. At this point, the entire night sky will be dark, as all the stars in our local group will have burned out. At this point, space will be really, really black!
The galaxy, meanwhile, will become a violent place if we wait long enough. Stars are very, very small entities compared to the distances between them; there's less than a 0.1% chance that a Sun-like star will collide with another star during its lifetime. But between us, Andromeda, and the rest of the local group, there are some one trillion stars and stellar remnants flying around. In this chaotic system, a typical star system may go a very, very long time without colliding with anything else, but we've got all kinds of time.
After an approximate time of 1021 years, or some 100 billion times the present age of the Universe, the now-black dwarf at the center of our Solar System will randomly collide with another black dwarf, producing a Type Ia Supernova explosion, and effectively destroying what's left of our Solar System.
At least, that might happen. That will be the eventual fate of many stars in our local group, but not all! Because there's another competing process that -- by my calculations -- is possibly even more likely to happen to us: gravitational ejection from the local group due to a process called violent relaxation! When there are multiple bodies in a gravitationally chaotic orbit, sometimes one will get ejected, leaving the rest even more tightly bound. This is what happens in globular clusters over time, and explains both why they're so compact and also why there are so many blue stragglers -- or older stars which have merged together -- in the core of these ancient relics!
So if we're one of the ejected star systems, what then? Will the remaining planets just continue to orbit the dead star at the center of our Solar System forever?
If only it weren't for that pesky gravitational radiation! Our orbits -- even gravitational orbits in General Relativity -- will very, very slowly decay over time. It might take an exceptionally long time, some 10150 years, but eventually, the Earth (and all the planets, after enough time) will have their orbits decay, and will spiral into the central mass of our Solar System. It would take even longer -- maybe 10200 years or even more -- for the last few stars that are left in what was once our local group to spiral in to the central mass in the aftermath of the Milky Way-Andromeda merger.
But that's never going to happen, since there's a black hole there, which will have already evaporated thanks to Hawking radiation! Hawking radiation will take out even the most supermassive black holes in the Universe after only some 10100 years, and a solar-mass black hole in a meager 1067 years.
And that's the far future of our Solar System, based on the best physics that we know today!
Thank you for your excellent blog!
However, your confidence in our current human scientific knowledge seems too optimistic...
What do you thing about this news?
In the arXive:
Web (I couldn't find the original paper):
Don't you think that such variables as the age of the Universe, matter distribution ("content" would probably be better term), and thus our futures' predictions could be severely reconsidered in the near future (in light of new data)?
Hmm... that may be the best physics, but it is not the best biology. I think we'll eat the sun before it cooks us.
Could it really ever happen that black holes' HAWKING radiation predominates the infall of matter and other kind of energy?
Will BHs ever be alone in the dark?
Where can I find the related calculations and assumptions?
Thanks in advance.
"Could it really ever happen that black holes’ HAWKING radiation predominates the infall of matter and other kind of energy?"
Expansion would have to have progressed to the extent of removing galaxies from the universe, so that the starlight of a "milky way" isn't an energy source of a black hole.
Wonders of the Universe has a figure for how long it would take for black holes to evaporate, though. I guess having a go in a forum on the BBC site would do.
The biggest assumptions would be the rate of acceleration. The more concrete assumption would be "at what level of expansion at the galactic level would balance radiation out as higher than incoming?".
Working out the "sky temperature" would be solving the rate of radiation from a black hole of mass M and putting that into Stephan-Boltzmann to turn it into temperature. Which may already be one potted equation for showing BH radiation.
the assumptions are pretty straightforward.
1. Eventually all stars will burn up. The old, the current, the new ones yet to form.
2. black holes will still be there at chance coliding, eating one another or flowing through oblivion, sucking what's left.
3. when no barrionic matter is left, there is still cmb. As long as it's above the temperature of Hawking radition, the BH will absorb that.
4. At one point the CMB will be cooler than HR. Then BH's will just emit ever so slowly till they are no more.
5. Even that radiation and huge energy released once they pop, will be absorbed by other BH's.. Untill the last one remains
If I remember correctly, there was an episode of either Horizon or similar about the time frames of end of universe, which goes step by step... the death of stars is nowhere near the end...
@Wow & Sinisa Lazarek:
Thanks a lot fo this very nice and simple explanation...
Is it correct that all this depends on accelerated expansion, or would this all be like that without acceleration, too?
Does anybody know, what happens to all the particles in a BH while it is vapouring?
Especially when its size is already small and thus its vapourization rate is high.
How do all the particles turn into radiation?
@6. That's a good question, don't know for sure, but think that they took current rate of expansion, if it even matters at all.
The time scale is so amazingly long that these 16bn years form BB to now seem like a second.
@7 Don't think anyone can answer that. I don't even think that what is at the center of BH can be called matter or particles.
Hawking radiation is not radiating "original" matter that fell into a black hole. It's a virtual pair production that drives it, not matter that fell in.
"...virtual pair production..."
OK, I understand. But even this means that there are again particles outside the BH, which could interact. Even those should somewhen return to the BH, or not?
Not sure I follow your last sentence. Return? Interact? Please be more specific.
Pair production leaves one particle in the BH, the other escapes.
This escaping one is not alone - there are a lot of such escapers.
These may interact and at least even fall back into the BH - an endless circle? Or do some escape into the dark without ever coming back?
ONLY those that escape into the dark without ever coming back are considered Hawking radiation.
And only if that one is a positive particle of the virtual pair. If it's a negative one, the BH gained a fraction of the mass.
@Sinisa Lazarek (#13):
Can we actually predict the polarity of a BH related to charge?
Igor: That news doesn't actually change anything in the picture presented here. Maybe one tick on the "history of the universe" diagram moves a little, though also maybe not enough to be visible on that scale.
It's not the charge of BH but of virtual particle pair.
The theory behind Hawk. radiation is due to space around event horizon behaving like a quantum foam (due to strong forces present) producing virtual pairs at a higher rate than just regular vacuum. A virtual pair will always be "+, -" or a particle and it's anti-particle.
Several scenarios can arise:
* both + and - get sucked in... nothing happens
* both + and - don't get sucked, but they cancel a moment later... nothing happens
* + gets sucked, - leaves.. but we can't really detect anti particles
* negative particle falls in, positive (or real particle) leaves, and we detect it. From our perspective, a BH emited a particle. (Hawking radiation)
And since this whole pair production is due to gravitational forces creating vacuum fluctuations in the first place.. it takes some energy from BH.
From what I understand as an amateur, what actually happens is much more complicated. But it can be explained it terms of pair production. As for predictions, since this is a QM effect, we can only calculate probabilities of this happening.
In my opinion, in a universe where the largest time ever measured is 13.8 x 10^9 years (the apparent age of the universe); predictions of 10^200 years are quantitative pseudoscience.
Describing the weather is well done by science; predicting the weather is notoriously a joke. Yet, certified astronomers regularly predict 10 or 100 or even 10^190 orders of magnitude longer than the apparent age of the universe. Is predicting the future states of the universe really so much more reliable than predicting the weather. I recommend to trust no prediction greater than 10 billion years; unless it's about the interstellar stock market.
"The laws of nature are almost completely understood..."
And after many quotes like this from a scientists; scientists wonder why the public often thinks scientists are arrogant and why governments don't want to spend the big bucks for fundamental research.
"I recommend to trust no prediction greater than 10 billion years"
in all fairness, no prediction for the Universe over 100 years is in any way relevant or true to any of current readers.
These 10^100 years things are done as much for amusement as for anything else. Don't see why call it arrogance. It's fun with math and current understanding of things. Nothing more.
"Don’t see why call it arrogance."
Because OKThen thinks that assuming for the sake of argument our understanding of the universe is correct and extrapolating, without ten thousand redundant caveats about how we could be wrong, is "arrogance". Like, the lack of those caveats means Ethan et. al. are simply too arrogant to even IMAGINE that they could be wrong.
It's quite tedious. Especially when they get long-winded about it. This was a short one.
"In my opinion, in a universe where the largest time ever measured is 13.8 x 10^9 years (the apparent age of the universe); predictions of 10^200 years are quantitative pseudoscience."
Yet you don't quantify it. While Ethan on the other hand quantified under which conditions we can trust the predictions, in the same way we can trust the rotation of Earth to bring further days until something changes.
"“The laws of nature are almost completely understood…”
And after many quotes like this from a scientists; scientists wonder why the public often thinks scientists are arrogant and why governments don’t want to spend the big bucks for fundamental research."
No, they wonder why some of "the public" doesn't care for deciding whether scientists are really arrogant or not, an important factor in deciding on arrogance. In fact, it is ironically arrogant of these people not to do their homework before being opinionated. And as already noted quite tedious over time, as it is much easier done than making the long-winded diatribes usually seen.
Here is an excellent description on why the laws of nature are completely understood in the largest observed sector, that of everyday life. The Laws Underlying The Physics of Everyday Life Are Completely Understood:
"A hundred years ago it would have been easy to ask a basic question to which physics couldn’t provide a satisfying answer. “What keeps this table from collapsing?” “Why are there different elements?” “What kind of signal travels from the brain to your muscles?” But now we understand all that stuff. (Again, not the detailed way in which everything plays out, but the underlying principles.) Fifty years ago we more or less had it figured out, depending on how picky you want to be about the nuclear forces. But there’s no question that the human goal of figuring out the basic rules by which the easily observable world works was one that was achieved once and for all in the twentieth century.
You might question the “once and for all” part of that formulation, but it’s solid."
Irony: not just for meteors...
"Is predicting the future states of the universe really so much more reliable than predicting the weather." (sic)
That would be the most mind-bogglingly obvious 'yes' of all time - unless you expect gravity to suddenly become turbulent with age. All those eclipse predictions? Knowing when comets are coming back? Get those guys doing the weather forecasts!
"I recommend to trust no prediction greater than 10 billion years."
In your not-remotely-arrogant opinion, of course.
Meanwhile, in more important news: great post Ethan!
A few years back I had read that the earths fate with the red giant phase of the sun wasn't known. Supposedly tidal forces with a giant star and closely orbiting planet can cause the planets orbit to decay. So it is a race between solar mass loss, how big the solar-red-giant gets,and tidal drag. Is it the best current guess that the planet survives?
As far as I remember, it's not 100% certain earth would get absorbed. But even if it didn't, I wouldn't call it "survival" :D Earth would be a small charred piece of coal in a spitting distance of a Red Giant star... no fun at all :))
Of course, being a Scientist, it hurts to be called arrogant. But at the same time, I can understand why you think so. As scientists, its our ob to be right about things. If you are a scientist, and you are constantly wrong, you dont really have a career. So scientists spend most of their time being right about things. And thats ok, thats our job. But there is an other point to that. We are also humen. Being humen means, we carry our work attitude with us, wherever we go. I find my self often in situations where I have to shut my mouth and not call people on some stupid statement they did, just because I know they are wrong about something.
So yes, I think we scientists seem to be arrogant at times. And denying that seems even more arrogant. But thats a bargain we have to live with.
In any case we have about 1 billion years before the increase of heat from the Sun renders Earth a dead planet.
Between now and then, we'll either spread out into the further reaches of our solar system and to other stars, or we'll go extinct. By "we" I don't just mean humans, but Earth-originated life in general, a certain amount of which we will have to take with us in some form in order to reconstitute human-viable ecosystems elsewhere.
This is Darwinian selection on the cosmic scale, with Earth as a single ecological niche, and other planets orbiting other stars as other potential ecological niches. Our success at this depends on maintaining sustainability on Earth and continued progress in science & technology: in every era of human society, including our own, in the present. One billion years, give or take a few;-) isn't a particularly long timeline given the magnitude of the task.
So here's my typical layperson's question of the night, for any working scientists here:
Assume three inhabited star systems, A, B, and C, in an expanding universe at the stage where even stars in one's own galaxy are slipping out of one's own light cone.
Assume that B is local to both A and C, but A and C are not mutually local. Astronomers in system B can see stars A and C. Astronomers in both systems A and C can see star B but can't see each other.
It seems to me that B should be able to serve as a relay for communication between A and C until either A or C slips out of B's local universe. Is that correct, or where's the mistake?
"“I recommend to trust no prediction greater than 10 billion years.”
In your not-remotely-arrogant opinion, of course."
We know, for example, that Pluto will be in the same orbit in (IIRC) 100 million years.
We DON'T know which side of the sun it is going to be on, however.
Chaos takes longer to screw things up in the much simpler solar system than the much more tightly knit and energetic weather systems.
"Is it correct that all this depends on accelerated expansion, or would this all be like that without acceleration, too?"
It might happen without acceleration expansion, the problem here being that the acceleration from hubble over the scale of a galaxy (where the BH got its meals) is very very small and gravitational attraction much higher, so it may be that galaxies will remain cohesive (if alone) until every star dies out.
At that point, incoming radiation from the stars is nil and CMB is practically nil now too.
"3. when no barrionic matter is left, there is still cmb. As long as it’s above the temperature of Hawking radition, the BH will absorb that."
The BH will absorb more than it radiates.
It ALWAYS absorbs the CMB. Your statement is a shortcut but one that leads AGW deniers to claim that the greenhouse effect cannot happen because it would disobey the second law of thermodynamics.
The radiation is absorbed from a colder body. The warmer one just radiates more than it gets and cools (but cools slower).
It's the same with the BH.
Re. Semmel @ #24:
And rationalist laypeople also have a stake in science being right. But the thing we all have to keep in mind is that underneath the competition for discoveries and their material rewards, is ultimately a mindset of humility that recognizes the fact that our best theories today may be overturned (or circumscribed, in the manner that Newtonian physics is circumscribed: it's still "right" within a limited domain) by some clever undergrad tomorrow. We might turn out to be wrong after all (which also means that nature might turn out to be more interesting than we expected).
The statement that "the laws of nature are almost completely understood", isn't arrogant: stated in the right way, it conveys a sense of awe along with understanding, and an invitation to join in sharing that knowledge. It also inherently points toward a progressive worldview: that over time there is progress in knowledge, and thus progress in its potential applications. The corollary of this is the necessity of progress in society at-large, via changes in the law, economics, and culture. As a result of that progress, we no longer tolerate witch-burnings or slavery, we seek to accord equality under law to all persons regardless of race, gender, etc.
An analogy that may be useful:
If you're a police officer, you have a professional obligation to be friendly and respectful to members of the general public. This because the uniform and the badge you wear are symbols of the very real power you hold, to question and arrest people, and have them charged with crimes, and use force if needed.
If you're a working scientist, you have the equivalent of a badge for being able to define a specific field of knowledge, and thus a specific set of aspects of reality as we understand it. That's very real power. And that power should be wielded with an attitude of friendship and respect toward members of the general public.
"The BH will absorb more than it radiates."
correct. Meant that, but in not so many words. :)
The thing that is amazing IMO is this... Even when there is no more matter or BH's or anything we know nowdays... there will still be space, time and vacuum energy. If time persists long enough, QM says anything can indeed happen. You, me, another BB... etc.. and so on and on. Not 10^100 years.. How about 10^10^100 years....
The questions that I ponder is not what's gonna happen to our planet or galaxy in 10^xx years.. but what will happen with spacetime eventually. Can expansion actually tear spacetime appart into oblivion? If not... time persists.. forever.... And anything that could happen in terms of quantum fluctuations.. will happen. What I wonder about is, with our current understanding of spacetime.. could we try to infer an educated guess?
That's why I said it was a shortcut, but the abuse taken from it to fuel denialists is why it's not a safe shortcut when there are so many actively looking for misunderstanding as long as it leads to comforting reinforcement of their prejudice and fear.
It's just no longer safe to rely on people being honest any more.
It seems to me that B should be able to serve as a relay for communication between A and C until either A or C slips out of B’s local universe. Is that correct, or where’s the mistake?
The key is remembering that A is moving away from B too (not just C), and just because you see A from B doesn't mean that a signal B sends out at a future date will ever reach A.
So, let's say C sends a signal out today. It takes umpty billion years to reach us at B. By that time, A is beyond our reachable horizon; the signal from C will never get to A. We can still see A (and C), because for the next umpty billion years all the signals those stars have been sending out in the past will reach us. But new signals will not. Then, umpty billion years after A and C have passed beyond our reachable horizon, they will blink out of our sky.
Now, just to throw a monkey wrench in, remember that photons from C have been greatly red-shifted by the time they reach us at B. Because the expansion increases with distance, 'reaching A' may be mathematically equivalent to being red-shifted to infinity. I'm not sure about that but I'll throw it out there for people to chew on.
Anyway, I think that first paragraph is the answer. I welcome corrections. The second paragraph is more of a curious thought.
Quite right, 10^100 years is just amusement.
At the end of the 19th century, physicists generally accepted that all the important laws of physics had been discovered. They were wrong. But now, at the beginning of the 21st century “The laws of nature are almost completely understood.”
Amen. Now please go to church and thank "God" there is nothing else fundamental to learn.
"Niels Bohr once quipped:
'Prediction is very difficult, especially about the future.'"
I always thought that was a quote from Yogi Berra.
I found it included here:
Maybe they were both great minds. ;-}
As a biologist, hearing "The laws of nature are almost completely understood" grates a bit. Yes, I understand we're talking about the fundamental physics here -- but a combination of measurement limitations, chaos theory, and simple computational limits means that our ability to expand from those fundamental equations to understanding our complex world (yes, including weather, but also the complexities of biochemistry, ecology, etc.) is severely limited. Which means that formulating new "laws of nature" will continue well into the future.
Biology is an emergent property of those laws of nature.
The law can be completely understood but not the emergent property "laws" seen from their reaction. See "Langton's Ant".
Re. Wow @ 32:
And for a stunning example, it turns out that Glen Beck managed to get hold of Jacob Barnett for at least a visit and possibly a program. Hopefully the Barnett family will realize what they were dealing with there and not get sucked in. (Jacob Barnett is the 14-year-old Ph.D. student who is currently working on PT-symmetric lattices and is thought by many working physicists to be the next Einstein or better.)
And then there are also plenty of more-or-less innocent idiots out there, like yours truly, who sometimes get lumped in with the intellectually dishonest by way of blundering into the wrong subject matter, per our conversation earlier;-)
Speaking of which....
Eric @ #33:
D'oh! Yep, you got me there, and worst of all, I know about those factors, and in my often Newtonian intuitions I didn't consider them. Blush. Bigtime. Sigh. Thanks for that.
The way I'd summarize it in really oversimplified terms is: there comes a point at which an attempt at communication won't be able to catch up with the receding target. We can consider that point (rather than visibility) to be the boundary or limit of communication between A and C. And based on observations of the red shift, we should be able to at least approximately predict when that point will occur.
What I'm looking for here is to try to scope out the ultimate limits on the persistence of intelligent civilization on an interstellar scale (meaning, inhabited planets in various star systems that can communicate with each other in a meaningful or useful way). And my "agenda" (as I've often mentioned) is to promote the idea that interstellar migration represents Darwinian success on the cosmic scale, and that we are in some way morally obligated to not take steps (such as squandering certain resources or crashing Earth's ecosystems) that would have the effect of foreclosing the option for interstellar migration by our distant descendants.
So this fills in an additional piece of the puzzle: that the effective limit of communication requires a more thorough explanation than the simple model I'd asked about. What I'd like, but I'm reluctant to ask for short-cuts to working this out for myself, is for someone with the relevant knowledge to just flat-out say: in Q billion years, the radius of effective communication will be R, and in X billion years, the radius will have decreased to Y, with the numbers filled in so I can put together some simple math and estimate reasonable boundaries (in spatial extent and in duration) for a possible interstellar civilization some billions of years hence. Understood that it may be a fool's errand, but none the less interesting to me to work it out.
The scenarios depicted assume, however, that the proton does NOT decay. Maybe long before, I believe it's said around 10^32 years, baryonic matter may simply vaporize itself out of existence.
Then there's also a point beyond which planetary orbits in our solar system cannot be predicted with any degree of certainty.
Anyway, top-notch, thought provoking stuff.
Re: Wow @33. Exactly my point - but even the 'fundamental laws of nature' are actually mathematical (mostly) descriptions of the real world. And since we can't actually calculate those emergent properties, it doesn't make any sense to say that because we know how subatomic particles interact (an amazing feat!) then we've done all the description we really need to!
What I’d like, but I’m reluctant to ask for short-cuts to working this out for myself, is for someone with the relevant knowledge to just flat-out say: in Q billion years, the radius of effective communication will be R, and in X billion years, the radius will have decreased to Y, with the numbers filled in...
From the wikipedia entry on the metric expansion of the universe: "For example there are stars which may be expanding away from us (or each other) faster than the speed of light, and this is true for any object that is more than approximately 4.5 gigaparsecs away from us."
If you want to do the calculation yourself, that same Wikipedia article lists the expansion rate (Hubble constant) as 67 km/s/Mpc. Though I think that may be out of date and the most recent estimates are more like 76 km/s/Mpc.
"but even the ‘fundamental laws of nature’ are actually mathematical (mostly) descriptions of the real world."
Well ANY description of the "fundamental laws of nature" will be "descriptions of the real world", so why is this something to bring up as a problem?
"And since we can’t actually calculate those emergent properties"
Hey, who the hell said THAT was the case? We don't necessarily have to be able to calculate the emergent properties ex nihilo, but
a) that doesn't mean we can't calculate ANY of them
b) that we can't calculate them ex posto facto.
Langton's Ant can have its emergent property stated. You just have to run it for a while to see what they are.
Eric at #41: Thanks for the leads, I'll follow up on those. I copied your post to my notes.
I take it that the language (paraphrase) "there may be stars that are expanding away from each other faster than c," is more properly translated as "expanding away from each other at a rate that places them outside of each others' local universes." That would be, each moving at a sub-c velocity away from some hypothetical midpoint.
This produces the A-B-C scenario where signals from A are delayed reaching B to the degree that by the time they do reach B and B retransmits toward C, the signals can't catch up. Admittedly this is the point at which Einsteinian relativity makes one wish that psilocybin was available on prescription, the easier to visualize things in sufficient detail to explore the apparent paradoxes;-)
I'll try to calculate this stuff based on both the lower and higher numbers for the Hubble Constant. I'm not making any claims of "new insights" here, just seeking to use approximations based on existing science to give some estimate of the future that awaits us if we manage to expand across the galaxy.
Just pondering: after all BHs have evaporated via HR, what happens to the resulting particles (who lost the second half of the pair in the process of radiation from BH)? I suppose they would just roam the otherwise empty spacetime and mostly never meet any other such particle? They would (again, a guess) have mass/energy/temperature, but the spacetime expansion would eventually make it meaningless?
Thanks for another great post Ethan!
"I suppose they would just roam the otherwise empty spacetime and mostly never meet any other such particle?"
Yup, pretty much.
Each particle running away into the void in an expanding universe, never (in any meaningful sense) finding anything else. If they eventually decay into radiation, then the universe will revert to energy instead of the temporary state of matter and cool continually.
Without sufficient temperature to do work from, no intelligence, nor life, can exist.
Re. Wow at #45: "each particle ... never finding anything else..."
This is something I've been wondering about for a while, and have never found a decent answer for:
If time is defined by change in entropy or by relative motion between objects: then in the absence of relative motion, what happens to time?
For example if the stage is reached where each particle is outside the local universe of any other particle, effectively in a local universe unto itself, doesn't that pretty much eliminate the time axis from fourspace? And if that's so, then what happens to the rest of what was previously a four-axis system? I'm inclined to think that the universe then becomes a multiverse of one-dimensional objects (point-particles in individual local universes). And then what...?
"If time is defined by change in entropy or by relative motion between objects: then in the absence of relative motion, what happens to time? "
Then it's either not defined by a change in entropy (but the heat death of the universe will still lose entropy by its expansion, so you need to rethink your implicit assertion), or time as is understood won't pass.
I fail to see what the problem here is.
Time didn't pass before there was a universe. It won't pass after its heat death is no problem either, just like the fact that you weren't alive before you were born shows that there's no inherent problem in not being alive after you're dead.
The problem I'm seeing is that an axis of measurement that was previously a line, becomes a point. That is a description of an alteration of the geometry of spacetime, and it can't be trivial or have no effect on anything else.
However if every particle eventually becomes a local universe due to expansion, then the fact that the expansion is continuing (and increasing) after that stage is reached, should have no effect on each of those particles. In some larger sense there is still relative motion between particles, but it's immeasurable and has no effect on anything.
There's an apparent paradox or partial paradox here (change in spacetime geometry can't not have effects, but, local point-universes have no effect on each other, uh-oh), of a type that suggests there's a large missing piece in my thinking about this. What are the subdomains of knowledge that deal with these types of issues?
G, you need to change things to "EVENTS" in spacetime.
Then you need to consider not where things ARE, but where they WERE when the thing that is responding to it is reacting to it.
Which requires a lot of care that you reform your scenario into the right frame of reference.
However if every particle eventually becomes a local universe due to expansion, then the fact that the expansion is continuing (and increasing) after that stage is reached, should have no effect on each of those particles.
So what? I have no problem with this.
If absolute indivisible things have extent, then eventually there must be a time when the observed horizon is less than the extent of the particle, in which case we *may* have a problem.
Or extent in space as no meaning for indivisible particles, in which case we know something about reality we never knew before: that electrons may have absolutely ZERO extent and therefore indivisible and primary particles.
And therefore other indivisible particles also have zero extent.
Note too that particles that can't be considered interacting often enough to be considered a fluid and indivisible whole have no valid calculation of entropy.
We make laws based on how things react. And that's called "entropy" here.
But if we have one, there's nothing to react to.
But who says the universe gives a shit? It does what it does as it does the things a universe does. We see patterns that help predict what will be the result of what a consequence of what happens and use them.
"But who says the universe gives a shit? It does what it does as it does the things a universe does. We see patterns that help predict what will be the result of what a consequence of what happens and use them."
This is a good one. Last part should be corrected a bit... :) But nice piece of wisdom. *thumbs up
It wasn't necessarily well said, but it was said.
You've gotta say that for it...