3 ms·
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 o
by perihelions 4y ago
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-universe-d7447467c63a https://medium.com/starts-with-a-bang/the-disappearing-unive... ("The Disappearing Universe")
https://news.ycombinator.com/item?id=7896776 https://news.ycombinator.com/item?id=7896776 (64 comments)
- joelcollinsdc 4y agoCan you provide a link with this info? I’m only a casual follower of this stuff but it seems hard for me to believe
- tambourine_man 4y agoHe’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 https://en.m.wikipedia.org/wiki/Event_horizon
- davidvarela_us 4y agoI was just going to ask about gravity. TIL that gravity travels at the speed of light. Thanks!
- weego 4y agoOr 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
- petilon 4y agohttps://www.forbes.com/sites/startswithabang/2018/05/12/ask-ethan-how-many-galaxies-have-already-disappeared-from-our-perspective/?sh=7a0b6a7c5100 https://www.forbes.com/sites/startswithabang/2018/05/12/ask-...
- thehappypm 4y agoImagine 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.
- sph 4y agoYou probably mean the pool quadruples in size, meaning it doubles in length.
- thehappypm 4y agoNo, I mean doubles across all axes. 1 unit cubed becomes 2 units cubed.
- btmiller 4y agoThe 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 https://en.wikipedia.org/wiki/Expansion_of_the_universe
- oldjavacoder 4y agoAnother similar take: https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe#:~:text=The%20accelerated%20expansion%20of%20the,supernovae%20to%20measure%20the%20acceleration https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_....
- caf 4y agoOne 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.
- alex_sf 4y agoCan't that be true for us, though? If we had JWT 4 billion years ago, who knows what we would have seen.
- perihelions 4y agoHere'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-universe-d7447467c63a https://medium.com/starts-with-a-bang/the-disappearing-unive... ("The Disappearing Universe") https://news.ycombinator.com/item?id=7896776 https://news.ycombinator.com/item?id=7896776 (64 comments)
- holoduke 4y agoIt'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.
- Pokepokalypse 4y agoBut 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.
- jtsiskin 4y agoused to have an effect. They may no longer.
- bradgessler 4y agoIf you could capture that photon with a photovoltaic to trip a switch that kicks off a Rube Goldberg machine—isn’t that an effect?
- AnimalMuppet 4y agoYes, 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. --- [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.
- largbae 4y agoI 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.