Hacker News Reader

Comment by lisper | Hacker News Reader

> We can only see a finite volume—a finite number of galaxies. That’s essentially because there’s a horizon, a shell around us, delineating the greatest distance from which light can reach us. But that shell has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean.

No, that's not true. You can rise above the surface of the ocean. Barring some major unforeseen revolution in our understanding of physics, you cannot transmit information faster than light. The oceans horizon is grounded in a technological limit, but the cosmological horizon is grounded in a fundamental physical limit.

View on HN

According to current cosmological thought, the laws of physics at that shell are very close to if not identical to the laws of physics here. And yet, that shell is retreating from us faster than the speed of light. The shell is ever expanding, but we shall never see what that current shell will look like when it is 10 billion years.

How can that be, you ask? It is quite simple. The speed of light is a limit on how fast light can travel through the universe. But the universe is expanding. Light trying to get to us is like a bug crawling over an expanding balloon. If the balloon is expanding faster than the bug is crawling, it can crawl forever but never reach the point it is trying to crawl to.

So there is a fundamental epistemological limit - we can't actually know that our current models are correct. But within existing theory, the location of "the farthest we can see" is not particularly meaningful physically.

> within existing theory, the location of "the farthest we can see" is not particularly meaningful physically

Except that you can always put a finite upper bound on that distance. That is what matters with regards to the topic under discussion.

Which means that it is exactly as meaningful as the event horizon of a black hole. The last point where the external observer (ie us) can see what happens. But in a different coordinate system, not particularly special at all.

Yes, that's right. Different observers have different light cones. Yours is a little different from mine. But that is missing the point, which is that every observer has a light cone beyond which they cannot see -- not even by collaborating with other observers with different light cones.

It's also worth noting that, in a sense, the shell is actually shrinking. Every moment, more of our observable universe disappears "beyond the veil", so-to-speak, as the universe on our side of the edge transitions from near-light-speed to beyond light-speed outside the shell. (I know that's a sloppy way to use the speed of light, but it works for me.)

Small nitpick, that shell---the Hubble sphere---is shrinking due the accelerating expansion of space. Constant expansion cooresponds to a constant Hubble radius, and decelerating expansion with an expanding radius.

Good point, thank you.

it only has physical significance from our perspective.

Well, yeah, but our perspective is the only one that can possibly matter for us. There may be unicorns and leprechauns out beyond our light cone for all we know, but what possible reason would there be to care?

What happens on the far side of the moon doesn't matter for us, either, but we still looked.

> What happens on the far side of the moon doesn't matter for us, either,

How do you know? Maybe the far side of the moon is chock-full of exploitable resources. You can't know until you look.

> but we still looked.

Sure, because we could. We cannot look beyond our light cone.

For now we can just by waiting. With every second we see just a little further.

Tomorrow we might look out at the stars and see the tips of giant teeth. But, probably not.

You don't quite understand the concept of a light cone.

Yes, we get new data all the time, and the extent of our horizon is growing. But it is always finite. You cannot look backwards past the big bang singularity, and you cannot look forwards beyond "now" for any value of "now".

We can’t

But 200 years ago we “didn’t need” relativity and then we did

Asking questions and wondering about the things we don’t know is how we now know about many valuable things that, in the past, didn’t appear to be valuable

You may want to temper your claims with "based on the currently accepted Standard Model" or "with high probability." Because paradigm shifts occur.

See my OP: https://news.ycombinator.com/item?id=18094991

"Barring some major unforeseen revolution in our understanding of physics..."

No, you fail to understand a light cones end at finite points. If I travel 1 light year, then I observe a different light cone and it takes 1 year for somone that stayed behind to receive information from me. This effectively means you can see something before the light cone at your current location would observe it.

If you consider a light cone as all points out to infinity then that’s the entire universe. Instead at each point in time you have a different light cone that extends to the age of the universe.

In theory we can never see past a second limit based on the expansion of the universe, but that’s yet to be observed speculation without any consistent theory of what’s going on.

Is the only reason to investigate the universe to look for ways to exploit it?

The argument is it's beyond our observable universe. It's not possible to be investigated as we can receive no information.

Ah, but if someone on a distant planet were to record their perspective, e.g., data about their visible universe, and transmit it to us, it would be consistent with our perspective by the time it reaches us. For instance, stars that they can see but we can't, would become visible to us as their message reached us.

They couldn't transmit such a message; the part beyond "the wall" is moving away faster than light: the data would never reach us. From our perspective, their earlier era is "frozen" in time: red-shifted to zero. We cannot receive any "new" signal from them. Therefore, the seeming "faster than light" violation is not really the case. Relativity "fixes" it by mucking around with time.

From a practical perspective, the red-shift horizon is a pretty solid "wall". The Orange Dude would love it. There could be a million Darth Vaders or run-away planet-eating nanobots on the other side, and they can never touch us. Maybe that's a good thing. The Egyption Empire lasted an extra long time largely because it was so hard to invade. It might even be the Anthropic Principle "protecting" us from really dangerous things.

> the part beyond "the wall" is moving away faster than light: the data would never reach us

To nitpick, the wall itself is moving away at about one light-year per year. The photons that are not moving from our perspective because they are receding away from us at the speed of light will soon find themselves in a space where they are again moving towards us.

"we routinely observe galaxies that have, and always have had, superluminal recession velocities" -- Davis & Lineweaver.

Are you sure about that? While there may be a "neutral zone" where that's the case, it's only a small percent of the stuff that's beyond our senses.

Any information that gets to us via some alien friends will have to travel even further than photons from those stars. By the time we got the alien signal, we would have seen photons from those stars already.

i don't understand what you are "no, not true"-ing. it's just analogy. you pointed out a flaw if one happens to take the analogy too far, but i don't see the point when the original analogy was clear enough what they are getting at.

I am disputing the claim that "that shell has no more physical significance than the circle that delineates your horizon if you’re in the middle of the ocean".

>Barring some major unforeseen revolution in our understanding of physics, you cannot transmit information faster than light.

There's been a significant amount of research done in the recent years into quantum entanglement. Although we're not there yet, it wouldn't surprise me if we eventually manage to pass information via entanglement.

Nope, not possible:

https://en.wikipedia.org/wiki/No-communication_theorem

Reply on news.ycombinator.com