The Edges of Our Universe | Hacker News Reader

The Edges of Our Universe

arxiv.org

107 pointshenning49 comments

This is such a cool paper!

One of the neat ideas it communicates is that the longer we wait, the less of the universe we can access:

  Delay - Galaxies lost
  1 million years ~ 0.02%
  10 million years ~ 0.2%
  100 million years ~ 2%
  1 billion years ~ 20%
  10 billion years ~ 80%
  150 billion years ~ 99.9999997%

Or this,

"For example, dropping matter into black holes via their accretion disks converts 5.7% of its rest mass into light, which far exceeds the efficiency of the thermonuclear reactions which power stars, and might be a much more efficient way to gain access to the mass-energy stars contain (though it clearly comes with its own engineering obstacles)."

"A related question is not how much matter (or energy) could be reached, but what is the greatest amount a civilisation could secure in one gravitationally bound location, so that it could be used for some unified long-lasting project during the time of isolation"

Maybe we can pinpoint where advanced alien civilizations would be based on an understanding of their energy needs.

Spoiler alert on Hubble Extreme Deep Field photo: "This image thus spans the edge of our affectable universe, with most of the places it shows being forever beyond our reach. "

I find all of this incredibly beautiful.

I haven't read the paper, but knowing a little about cosmic expansion, even if we "don't wait" and manage to launch 2 different teams to 2 different galaxies, at some point those galaxies will become unobservable from each other.

It always seems we have a certain bias when it comes to imagining alien civilizations more advanced than our own and that their energy needs would somehow be the same, similar, or greater than our own. Ignoring gravitational differences of various potential origin planets and the energy requirement differences to get into orbit based on a given planet’s gravity fields...what if the beings themselves were many orders of magnitude smaller or larger (or both for that matter). Even here on earth imagine some other beings with all the capabilities as us but could essentially stretch their arms upward and place little old us into orbit without the need for propulsion, or other beings so microscopic we could never be capable of detecting them even if they had the capability of building craft that could escape Earth’s gravity field.

Are you referring to Dragons egg or just imagining the plot independently?

The only assumption made here is ambition, and it is explicitly stated.

An ambitious civilization will figure out a use for whatever amount of harvestable energy is available. Computation, if nothing else. The paper is about upper bounds; whether any civilization can (or wants to) actually reach anywhere near them is another question for another paper.

This sounds a lot like the topics discussed by SFIA https://www.youtube.com/channel/UCZFipeZtQM5CKUjx6grh54g

For example the 'civilisations at the end of time' series https://www.youtube.com/playlist?list=PLIIOUpOge0LvHsTP5fm8o...

Off-topic, but does anyone else feel claustrophobic when reading about the vastness of the universe? I enjoy astronomy related topics but when reading articles about the scale or the expansion of the universe, etc I start feeling physically uncomfortable and need to take a break.

You're looking for agoraphobic.

We're forever stuck inside a thin layer of the Earth surface. It makes me kind of claustrophobic.

Definitely not forever.

It's only my perceptions.

Yes, several hundreds of people had already been in space. Space travel could be the norm in the next few decades. Man could terraform Mars in the next millennium, or conquer the milky way galaxy five million years later.

However, I still feel like "we're stuck forever".

The sense of smallness of the self is probably what causes the claustrophobia. The last time I read of agoraphobia was in an Ian Fleming novel!

Try and "step back" and observe the universe observing itself through your mind. Your "you" is local chunk of universe coded with a capacity to consider the smallest and largest scales of itself. Your mind can easily encompass the universe in thought, summing up everything known as easily as you take a breath. It's an incredible ability, cherish it!

This reminds me of a physics teacher I had in elementary school:

-But is there anything faster than light?

-??? ...

-Yes! Our thoughts. In my thoughts I can be on the moon instantly.

Elementary school physics classes?

... on Earth?

Denmark, in the 1960s. Elementary school grade 1 to 7, no kindergarten.

I try to explain this to people all the time, especially on topics like panpsychism.

Is the universe conscious? Well, parts of it certainly are! And those parts are learning about itself as a whole.

Yesssss! An amazingly small skin of matter (folds in spacetime) on this rock floating in space has geometry encoded with information about the deepest pores of subatomic reality, the massive scales of space and time above and beyond ourselves. It's pretty fucking incredible. [EDIT: You are a small entry lane in a highway interchange in the universe, that is the busiest fucking information nexus in the observable universe.]

You might feel like you're a speck floating above kilometers of deep dark ocean, but that's just your matter. If you think about your (our!) information, we're almost more like the water itself in our permeance -- or like some sort of aerogel -- compared to anything else floating in this thing :)

In that case you should never look at the Pale Blue Dot image

I can recommend Universal Paperclips as treatment. You start making paperclips by hand and end by converting the entire universe into paperclips. The universe seems big until that counter at the bottom of your screen says "0.0000000001% explored" and starts rapidly going up.

I always keep this diagram of the scales of universe [1] close at hand. It feels liberating to look at it when I feel overwhelmed. To me, it shows that it is completely preposterous to worry about.. anything, really. The Earth is negligible.

[1] https://en.wikipedia.org/wiki/File:Location_of_Earth_(9x1-En...

Not being dismissive but a serious answer: go outside? Walk a trail? Just do something to get out of your head. Maybe then you could consume some audio content about astronomy without feeling overwhelmed (or just find a place to read outside).

I get the sense that being in a room (which is always pretty small) makes the abstract claustrophobia feel more real.

Do you mean agoraphobic?

I think it can be re-framed like a feeling of freedom and possibility. For instance, if you managed to live one million years and visit all the galaxy, you would have not even started.

Claustrophobic as for being afraid closed space? Universe produces exactly opposite effect on me - head-spinning feeling of a tiny speck floating in the turbulent never-ending massive storm of gigantic proportions, at mercy of titanic forces all around. Kind of vertigo when standing on the edge of the roof of super-tall building, looking down, and barely seeing the ground below, and open air all around.

It is always a very humbling sensation, which is good. No place for arrogance when you are nothing and mean nothing from certain scale of things.

Initially Reading your comment I thought not claustrophobic but I really identify with the last part of your description “physically uncomfortable.”

Truth is I have never been able to describe that particular physical feeling, so maybe claustrophobic is not exact but perhaps a similar type of physical sensation/experience.

For me it’s actually very difficult to put myself in that state, it’s somewhere between extremely deep thought and meditation...I’ll go so far as to say even things I don’t necessarily believe in such as remote viewing and/or some type of outer body experience.

I’ll also add that while it’s not something I actively practice by any means ( and I otherwise engage in breathing exercises/meditation - mostly to help my running) I really like that uneasy feeling and find I can only maintain it for short periods.

There isn't really any sort of distinct boundary between "you" and the "rest" of the universe.

I used to believe this, but it turns out not to be true, and in a very profound way. If you're a living thing, i.e. a self-reproducing system, then you have no choice but to draw some sort of boundary around yourself, i.e. the system that is doing the reproducing, and the "rest of the universe" which contains the resources you need to exploit in order to reproduce. These boundaries are manifest at multiple layers of abstraction: the membrane around the cell nucleus in eukaryotes, cell walls, your skin, families, species, city-states, nation-states. The boundaries may not be "distinct", but they definitely and necessarily exist.

> The Total Perspective Vortex was a machine built with the intention of showing beings the infinity of creation, which became used as a method of torture.

https://hitchhikers.fandom.com/wiki/Total_Perspective_Vortex

Our galaxy is huge but we don’t really notice because it is so bright, and moreover has been bright for a very long time.

The light we see is old. It’s of interest for studying the science of bright old things but for anything contemporary it is useless. If you ignore our own star and count out the ancient starlight, the sky is completely dark to us.

It is as if we are searching for someone to talk to by rummaging through rocks, looking for fossils. It applies to humans too. Anyone we sent to another star would be gone for years. They would be physically gone, but fundamentally we just wouldn’t be able to talk to them at all for years on end.

Unless something about the speed of light changes, Earth and her inhabitants will always be completely and fundamentally alone.

Our galaxy is "only" 50,000 lightyears across. 50k years is an eyeblink in cosmological terms. Behaviourally modern humans have been around about that long and it's too short a time for fossils to even form, except in extraordinary conditions. It's a thousandth of the time to the dinosaurs. I think that's reasonably up to date in evolutionary terms.

Andromeda at 2.5m ly is a bit more of a stretch, but it's still a fairly short period in the history of life on Earth. It all depends on how long we think technological civilizations last on average. If they can reach sustainability it could be at least as long as their solar systems remains habitable, maybe hundreds of millions of years, even without interstellar travel.

"The light we see is old. It’s of interest for studying the science of bright old things but for anything contemporary it is useless. If you ignore our own star and count out the ancient starlight, the sky is completely dark to us."

Even sunlight is really old.

The photons from our own star, the sun, were produced 100,000 years ago. That's how long they take to make their way to the surface from center of the sun where the nuclear fusion occurs.

True, but then it's just 8 minutes from the sun's surface to the earth's.

You mean the energy from the core of the sun took 100k years to reach the surface and be emitted as photons.

With some form of hybernation or life extension Milky Way could be open for exploration even without ftl travel.

Gotta disagree on that one. Well maybe with the small change to "Earth and her current inhabitants".

There's current projects that hope to launch small probes to the nearest start (under 5 light years away) at a few % of speed of light powered by large arrays of lasers. Assuming it gets attention and additional funding the probes will get bigger and of course sensors are continuing to improve per unit of mass.

With a better survey of the nearest star, plants, asteroids, comets, and magnetic field we'd have a much better idea for what to send next.

Assuming reasonable progress on fission, fusion, plasma drives, light sails, and related technologies we could look at accelerating on this ends and slowing down at the opposite end. Once we can send small probes they would work on building mirrors to help decelerate whatever we send next. Similarly on this end we could put more solar panels+lasers in orbit to help with launches.

If any of the planets looking promising it doesn't seem insurmountable to eventually send humans (or at least their DNA) to the nearest star.

The interesting thing about the universe is that it appears, as far as can be measured, to be flat. This implies that it’s either infinite or so large that if it’s curved then we can’t detect the curve over 90 billion light years.

Either way, the universe is unbelievably, vastly bigger even than the observable 90 billion light years we live in.

What my mind fails to understand is, if the universe was infinite, how does that work with the "big bang" theory?

Apparently the universe originally (somehow) began as a point of infinite density and expanded outward from this point.

During this process, it would seem to need a defined size in order to be expanding at all. Even with the "blowing up a balloon" way of thinking of the universe, while the individual galaxies are growing further apart, the surface area is measurable (not infinite).

I am likely thinking of this incorrectly but I can't fit "infinite size" in with "expanding universe" in my head.

Unless these theories are incompatible, and to get infinite size you need the steady-state theory.

I think you should read this article "Misconceptions about the Big Bang": http://pages.erau.edu/~reynodb2/LineweaverDavis_BigBang_SciA...

The big bang was an "explosion" of space itself. There was not previously empty space.

> The big bang was an "explosion" of space itself. There was not previously empty space.

I am very familiar with all of this. My question was merely around the opposing concepts of "expansion" and "infinite".

Not infinite size AT the big bang. Infinite size of the current universe.

Technically I think yes, it may have a finite extent. If inflation theory is correct though, it will continue to inflate at extreme rates (if unevenly) for infinite time and so depending on how you think about infinity will have infinite extent in spacetime.

On the other hand if Roger Penrose is correct about Conformal Cyclic Cosmology, and cosmic inflation is false, then ultimately the universe will reach a state where scale ceases to matter and it will become mathematically indistinguishable from a hot dense big bang again.

https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology

Some shapes have finite volume and infinite surface area.

https://en.m.wikipedia.org/wiki/Gabriel%27s_Horn

The size of the universe is governed by an equation like this (this is pure analogy):

   D = d * (t - t0)

Where “d” is space, “t” is “time” and “t0” is “the beginning”. Notice that when t=t0, we get a 0 term.

But if we can only access, interact or see a bubble of 62 billion light years like the paper says, do the rest really exist for us?

I don’t think I’ll ever understand the astrophysics use of the verbiage “causal” and “observable” in lieu of just using “light/ERM” and “spacetime”.

The only argument I could see for the need of these terms in addition to just light/ERM would be the ability to detect the existence of something beyond the furthest edges of the universe from where light reaches us through phenomena such as gravitational pull from beyond the edges on the bodies at the edges.

In the early universe, light could not travel freely, because the ambient temperature was hot enough that the universe was filled with plasma. The end of this period is called "recombination", and the light from that point in time is called the Cosmic Microwave Background.

That light is the earliest light we can observe, but it is not the earliest event to have had casual effects on us, nor is the earliest event we can observe.

In particular, that light was emitted about 370,000 years after the Big Bang, while events within the first second after the Big Bang have had casual effects on us.

So there is an important difference between "light" and "casual" in physics.

Nice, my initial impression of your comment still fits squarely within the door I left open for using causal...except when I was thinking phenomena outside the edges physical space where light reaches us you expand it to outside the edges of time where light still reaches us.

Unfortunately I’m now just sort of stuck thinking about causality, determinism and free will...not exactly knowing where you are on Earth I guess it’s a possibility you have never been and never will in my light cone, yet my comment was the causality of your reply in some sense. And in maybe an HN 1st you’ve made me reconsider my position, and Recognize I need to more deeply consider causality as having good use outside phenomena from outside light at the edges of space and time and spacetime.

Of course they've been in your lightcone -- otherwise it would've been impossible for you two to have a back-and-forth exchange!

I'm not sure I understand. Something can be causally connected through something other than light. Say gravitational waves. So using "light" in this case would be wrong.

Causality refers to the underlying constraints from the theory (nothing travels faster than the speed of light), this is more general than talking about any particular type of particle/field.

The observable universe is the set of points from which information can reach you. That is not the same as spacetime?

Haven't seen a non-TeX paper that looked that good (on arXiv) for a while.

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