Reality is much crazier than that. Some of the distant galaxies are causally disconnected from us: they're receding faster than the speed of light (by one out of the several definitions), and will (probably) continue to do so, so no information can travel between us. No causal interaction is possible. It's as if they were practically no longer a part of this universe -- they can't affect anything here, and nothing here can affect them.

(I'm not certain on this point, but I think some of the high-redshift galaxies we can see in deep field images are now causally disconnected. We're receiving some of their old light, but we can't send anything back anymore -- they've faded out into infinite redshift).

edit: Here's an article about this idea by Ethan Siegel, and its HN discussion thread (thanks 'petilon for helping me remember the author):

https://medium.com/starts-with-a-bang/the-disappearing-unive... ("The Disappearing Universe")

https://news.ycombinator.com/item?id=7896776 (64 comments)

But if we can image it, then there IS an effect.

The photons emitted by that galaxy effect us, here in ours because we can see them.

It's the ones we can't see that are disconnected.

used to have an effect. They may no longer.

If you could capture that photon with a photovoltaic to trip a switch that kicks off a Rube Goldberg machine—isn’t that an effect?

Yes, that's an effect, just like capturing the photon and using it to build up an image is an effect. You're thinking along the wrong axis, though. The issue isn't "effect" vs "no effect". The issue is "then" vs "now".

The light that we see today left them 10 billion years ago. Any light that leaves them now[1] will never reach us. And any light emitted by us now will never reach them.

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[1] Yes, I know, "now" is not really a useful concept when dealing with large velocities in Special Relativity. I don't think my overall point is destroyed by that, though. If you can find a more rigorous way to state what I am trying to say, please do.

I think "now" is an underrated concept in physics, as in a universal ordering of events, regardless of what order position and relative velocity cause them to be observed. If there is indeed a universal "now", then faster-than-light travel need not lead to time travel.

Can you provide a link with this info? I’m only a casual follower of this stuff but it seems hard for me to believe

The basic principle is that space itself is expanding—the empty stuff between all those stars, planets, and galaxies. Not only that, but the expansion of space itself? It’s accelerating. And the further away something is, the faster it’s accelerating. Allow this play out over time and with acceleration unbounded, space itself starts to expand faster than the speed of light. And since the speed of light is our universal speed limit… that means the far reaches of universe will forever be out of reach. Eventually our own night sky will go dark (minus the stuff closest to us like the sun, Milky Way, and Andromeda) as more and more objects continue their acceleration away from us.

More at https://en.wikipedia.org/wiki/Expansion_of_the_universe

One consequence of which is that if intelligent life arises in the Milky Way again after that point, they will likely have no way of knowing that there ever were other galaxies beyond the local group.

We're somewhat lucky to be around to see the universe when it's comparatively young.

Can't that be true for us, though? If we had JWT 4 billion years ago, who knows what we would have seen.

Another similar take: https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_....

He’s referring to the cosmic event horizon

“ In an expanding universe, the speed of expansion reaches — and even exceeds — the speed of light, preventing signals from traveling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves, which travel at the speed of light.”

https://en.m.wikipedia.org/wiki/Event_horizon

I was just going to ask about gravity. TIL that gravity travels at the speed of light. Thanks!

Or light travels at the speed of gravity but for a different order of discovery and convention.

Massless particles all travel at the speed at which causality can propagate through the universe (though they themselves do not experience speed because they do not experience time). The fact we reference it as the speed of light is just scientific idiom

Imagine you’re swimming in a magical pool with some friends and the entire pool doubles in size over the course of a second.

Your friend right next to you who was one foot away is now two feet. The absolute distance just grew by 1 foot, a rate of 1 foot/second.

Your friend across the pool who was 10 feet is now 20 feet away. Absolute distance just grew by 10 feet, or a rate of 10 ft/sec.

You probably mean the pool quadruples in size, meaning it doubles in length.

No, I mean doubles across all axes. 1 unit cubed becomes 2 units cubed.

It's like a balloon. Take two distances when balloon is empty. Let's say furthest points of each other (10cm) and halfway points (5cm) When you inflate the balloon the distance of furthest points increased to 20cm. But the 5cm is 8cm and not 10cm. The same in space. the further you go the faster it expands. The event horizon is the point where the expansion is faster than the speed of light. Any photon emitted will never reach us. The crazy thing is that according to some theories even the space between atoms will be beyond the event horizon.

Here's a link about this idea by Ethan Siegel, and its HN discussion thread (thanks 'petilon for helping me remember the author):

https://medium.com/starts-with-a-bang/the-disappearing-unive... ("The Disappearing Universe")

https://news.ycombinator.com/item?id=7896776 (64 comments)

There are situations where time passes differently like with high speeds and in gravity wells, but except for unusual circumstances, time will pass mostly at the same rate for life on any two planets across the universe. Because on relativistic scales they’ll be in very nearly exactly the same inertial frame.

Like maybe a few nanoseconds over ten minutes difference. One could probably come up with a standard deviation statistic, but whatever it would be would be way below a perceptual difference. (We can measure the difference in the flow of time with a clock on a table compared to another on the floor, but of course that’s way less than could be felt)

> Did a civilization rise and fall somewhere?

Yes. The same civilization! (wait for it)

> or is it one of those things where relativity is so unintuitive that asking a question like this is nonsensical?

The latter. One of the most unintuive aspects of relativity is the relativity of simultaneity. Basically, the order in which events happen in different places in the universe is relative. Imagine event A happening in galaxy A, event B happening in galaxy B, and event C happening in galaxy C.

From the POV of one observer, events might happen in order ABC, while for other they may happen in order BCA.

This doesn't mean that the observers simply see the events in different order because of their distance to them--it means that even when taking distance into account, one event happened before the other.

So yes, a civilization rose while you wrote that comment. It also fell... from the POV of someone else.

[1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity

There's a Star Trek episode addresses this sort of https://memory-alpha.fandom.com/wiki/Blink_of_an_Eye_(episod...

Also I remember a short story where scientists created a mini universe then find that they can take over conscious beings in this reality, and influence them. So a few minutes to the scientists is thousands of years in this simulation, and they use it to get new technology and such.

Hindus imagined this thousand of years ago.

Uncountable universes(multiverse) are born when Supreme consciousness Vishnu breathes out. Inside each universe God Bharma is born with the responsibly of managing the affair. Brahma lives for his 100 years which is roughly 311 billion years. During this time many mini cycles of creation and dissolution takes place. When Bharma finally dies after completing his 100 years,that particular universe dies with him. After another 311 billion years another Brahma is born again and universe restarts. When supreme god Vishnu finally breathes in, multiverse stop existing and creation end till the time supreme god breathes out again.

That's beautiful. I always wondered if there's some lost knowledge from ye olde ancient past where people knew more (about what we consider scinetific facts now) than we thought they did when they came up with these stories.

Hindus were thinking about time ranging from nano seconds to billions of years[0]. As far lost knowledge is concerned Indian civilization have faced numerous attacks especially on its knowledge system and wealth. During one such attack Nalanda university was destroyed, more than 10000 teachers were killed, and library with millions of books was set on fire which raged for 3 months. Invaders themselves recorded these event gleefully. As per them India was a land of idol worshipers.Here is summary article which contains links to primary sources[1]. It's through struggle and luck Indian civilization have survived although with lots of gaps.

I am sure there were others great civilizations. But they are not here any more to tell the tale.

[0] https://en.wikipedia.org/wiki/Hindu_units_of_time

[1] https://www.myindiamyglory.com/2017/09/11/nalanda-9-million-...

I think the Greeks through Anaximander had the concept of "apeiron", or the infinite. Some of them believed the universe to be infinitely old, therefore they must have had the concept of one to infinity, including billions.

Yep ancient civilizations have thought about these concepts for a long time. Some got destroyed by floods, some by famine, but horrific destruction have happened when Abrahmic religions came on the scene with "holier than thou" philosophy which made destruction of others a religious duty.

Hindus were obsessed with zero and infinity and to them it was the same as god itself. Shanti mantra(used to wish well to all living being) from īśopaniṣad says.

> ओं | पूर्णमदः पूर्णमिदं पूर्णात्पूर्णमुदच्यते । पूर्णस्य पूर्णमादाय पूर्णमेवावशिष्यते ॥

> oṃ | pūrṇamadaḥ pūrṇamidaṃ pūrṇātpūrṇamudacyate | pūrṇasya pūrṇamādāya pūrṇamevāvaśiṣyate ||

> That is Whole and this is Whole, the perfect has come out of the perfect; having taken the perfect from the perfect, only the perfect remains. Let there be Peace, Peace, Peace.

Verse above describes God superficially but it also describes zero and infinity[0]. This small text with only 18 verses and yet it is so profound. Similarly Buddhist philosophy which also came out from India is based on Shunya(zero).

[0] https://en.wikipedia.org/wiki/Isha_Upanishad

>>but horrific destruction have happened when Abrahmic religions came on the scene with "holier than thou" philosophy which made destruction of others a religious duty.

Both true and false. Most of the killing and destruction was done by the pagans to themselves. The pagan wars were no joke. Here in the Indian subcontinent itself there was a war called 'Kalinga War', the death and destruction was just so much and complete. Nothing remained standing.

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

This is just to be honest one of many hundreds to thousands of wars in India alone. Paganism was mostly destroyed by its own people.

After Kalinga war Ashoka renounced voilence as per the teaching of Buddhism. Do you have a parallel example from Abrahmic kings?

Having said that every civilization have had destructive infights. People fought for money, power,fame etc.

But that doesn't mean they destroyed each other's knowledge system, temples, and monuments. That's is only true for Abrahmic philosophy which stresses on destroying the culture and knowledge systems of the conquered.

>>After Kalinga war ... Do you have a parallel example from Abrahmic kings?

No, I don't have any example of any king in Europe, Middle East or North Africa who fought a war as destructive as Kalinga war. And that's just one war.

>>Having said that every civilization have had destructive infights. People fought for money, power,fame etc.

Causes for the fall of any civilisation are always internal, eventually you just get take over by somebody better.

>>That's is only true for Abrahmic philosophy which stresses on destroying the culture and knowledge systems of the conquered.

Would love read more upon 'Abrahmic Philosophy'.

> No, I don't have any example of any king in Europe...

Kalinga deaths are recorded to be 150000. I will suggest reading about the crusades and others religious wars to find how many were killed. Please also read about how Bible was used to justify the slavery and subjugation of pagans.

Indian conquered upto Cambodia, Persians conquered a big part of known world yet didn't engage in erasing the native population or the culture of conquered. But in contrast almost entire population of native Americans were annihilated. They practically do not exist.

> Would love read more upon 'Abrahmic Philosophy'.

First 2 commandments of bible summarises the doctrine/philosophy.

Does time move faster the deeper you are in a gravity well?

If a civilization was trying to compete with another in another galaxy, could they coalesce as much matter as they could into a single star, increasing gravity, and relativistically experience more time in their star system than the other civilization?

Yes. You can actually measure this just by the mass difference of a mountain here on Earth, with a sufficiently accurate pair of clocks.

http://www.leapsecond.com/great2016a/index.htm

Yes, it does (from the point of view of observers outside the gravity well), and yes they could. They could also accelerate themselves to relativistic speeds to accomplish the same feat.

I think they’d just need to travel slower through space to experience more time relative to the other civilization. Then that civilization could take more that time to develop a civilization-ending laser or whatever.

Then for gravity, the less gravity that civilization experiences the slower time moves for them relative to their competitor.

So I think the goal would be to send star matter to the other civilization so their time slows down relatively speaking.

I’m no expert, though. Happy to take corrections.

Would that not achieve the opposite effect?

If interstaller thought us anything, the closer you are to a black hole the slower time gets. When they got close to the black hole they only spent a few hours there but lost years compared to our frame of reference.

Time will pass for them very very closely to the same rate it does for us.

But here's something fun: the space between us and them is stretching, so in a billion years when the light arrives at our position, the time between photons will be longer than when they left (so things will appear slower) and the photons will be at a longer wavelength making it more red.

Depends on their velocity relative to Earth. The unintuitive bit of relativity is that you observe the other as experiencing time more slowly - this is the origin of the twin paradox (i.e., shouldn't each twin observe the other as having aged less?) whose resolution comes from the fact that the second twin is in a non-inertial reference frame once he changes directions to return to Earth.

I once asked this question to a scientist and I believe the passage of time seems pretty uniform (in similar gravitational circumstances) The exception is the beginning of the universe where time might have been more fluid for the lack of a better word.

Yes, the two settings (aka "reference frames" in special and general relativity) can experience time at different rates.

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