How can space travel be like world travel?

[Umbran]

How can something have existed, (however we detect it, now), 15 billion years ago, (or 15 billion lightyears away)? Existence didn't exist 15 billion years ago.

I said 14 billion light years. I then noted that I was rounding up from 13.7+ billion light years.

You were the one who said 15 billion first in this thread, as far as I can tell. Others picked that number up from you.

Getting back to the original question - by the time there were stars to see, the Universe was already very large, with a radius over (likely far, far over) 13.75 billion light years.

Unless the universe is/was at some time expanding at a rate faster than lightspeed?

I've tried to address this before. Let me try another way.

When we look at an object today, we might say it is X light years away. But we are measuring that in our simple 3d concept of flat space, that we get from our local area today. If we assume that our yardsticks and clocks are immutable, and track back to time=0, yes, it looks like we have a problem with lightspeed.

The thing is, your yardsticks and clocks are *not* immutable. That's Einstein's point. The closer to the Big Bang you get, in fact, the more those yardsticks and clocks change!

It is as if we used your height as a measure of distance - a signpost might be 24 Bulllgrits away.

But, the Bullgrit isn't a constant thing, now is it? When you were 5 years old, it was still one Bullgrit, but it was still less than today's Bullgrit. This seems like a paradox - it is still a Bullgrit, but it isn't, because in our heads we are still comparing it to a yardstick that we think of as objective and unchanging. But, and here's the clincher - there is no immutable yardstick!

 

[Umbran]

Woah, where's a 'katty' emote when you need one ?

Getting smartarsed-nitpicky with a physicist on a roll is the rhetorical equivalent of stepping into the street in front of a speeding, fully loaded dump-truck.

 

[Bullgrit]

The closer to the Big Bang you get, in fact, the more those yardsticks and clocks change!

OK, this is the point I was missing.

I said 14 billion light years. I then noted that I was rounding up from 13.7+ billion light years.

You were the one who said 15 billion first in this thread, as far as I can tell. Others picked that number up from you.

Getting back to the original question - by the time there were stars to see, the Universe was already very large, with a radius over (likely far, far over) 13.75 billion light years.

I was using 15 billion just as a simple number one step greater than the age of the universe. 13.75 rounded to 14; 15 is one step greater than that. I could have used 14.1 or 13.76, or 46.5:

The visible universe has a radius of 46.5 billion light years. To reach the edge of it in one year, you need to be able to go 46.5 billion times the speed of light.

46.5 billion is greater than the age of the universe. Considering that the universe isn't over 14 billion years old, the difference between 15 billion [light]years and 46.5 billion [light]years is really what when talking about seeing light from earlier than 14 billion? Isn't this sort of like comparing null to null minus a billion? (Or maybe infinity to infinity plus a billion?)

Bullgrit

 

[Mustrum_Ridcully]

What about time dilation.

While I might still need more than a 100,000 years to go from one edge to the other edge of the Galaxy from an Earth observer, if I fly close to the speed of light, how long would it take me personally - or rather, how fast would it need to take me personally to get there within 2 days (even if the dinosaurs came back twice in during that time on Earth ).

 

[Morrus]

Unless the universe is/was at some time expanding at a rate faster than lightspeed?

As I understand it from my reading of popular science books and podcasts and the like - and a physicist can correct me if I'm wrong - but the short answer is: yes. It did expand much faster than light speed in the early expansion phase, and those things furthest from us (beyond our appoximately 15 billion light year theoretical observable universe ability to detect them) are receding much faster than light speed. We'll never detect them.

However, the English terminology can confuse us. Something can be receding away from us, but that doesn't mean it's physically moving through space at faster than light speed and thus breaking the universal speed limit. It's not that they're moving through space, it's that the space is expanding - or, to put it another way, extra space is being added between us and them. This isn't movement in the way we think of the term movement; it creates the appearance of movement, but that requires motion through space, which is not what they're actually doing.*

*Obviously they are moving through space as well, as nothing is stationary, but that's totally separate to the expansion.

One weird effect of this is that - given that the expansion of the universe is accelerating - things will start to disappear from our view because they're "travelling" (again, the wrong word) away from us faster than the light from them is moving towards us. So the objects in our observable universe will gradually fade from our view, and it will become more and more empty. This is a long way in the future, of course. The ultimate effect of that, ignoring the whole sun-dying-end-of-our-world aspect which will come first, is that eventually the night sky will be empty and there will be no way to detect those objects. If our records don't survive, future astronomers will simply have no way to know they're not in an empty universe because information (in whatever form - light, gravity waves, etc.) cannot move towards them as fast as the objects themselves are being carried away by expansion.

But yes, in short - space can "expand faster than the speed of light" because it doesn't require motion; but the words we use in conversation here can make that confusing. Physicists use equations and stuff to say it accurately, but we normal people don't talk that language and have to rely on our bad analogies and vague terms!

Physicists, feel free to chime in!

 

[Janx]

If we ignored the math and the physics we don't understand yet:

when we look at alpha Centauri, we're seeing it as it looked 4 years ago, because it is 4 light years away. If it blew up tonight, we wouldn't know it until 4 years from now.

If alpha C was moving away from us (say at .1c), saying that it is 4LY away from us is not quite precise. The fuzziness comes in at:
did we mean it is 4LY as it appears NOW?
did we mean it is actually 4LY in its current real position, not as seen?
Will we be updating this # next year to 4.1LY?

I assume most of us civilians interpret a statement of Star X is Z light years away as how it appears to us.

So. if you say something is 50 billion LY away from us and the universe is 14 billion years old, and stuff can't go faster than light, then things are a little confusing.

When we hear that the universe is expanding as part of the big bang, we assume you mean, all the particles or coagulation of particles (stars, planets, etc) are flung out from the center, and thus, of course stuff is moving away from each other.

But that's because of their trajectories in 3d space from a center point, described as expansion if you were observing the whole thing.

This is how I think most civilians understand these concepts.

When physicists speak (like umbran), we hear these keywords, but miss that they keep putting in wiggly concepts like yardsticks and clocks that aren't constant. And the idea of adding more space between objects.

So, to the physicists, what the heck is really going on?

 

[Umbran]

Considering that the universe isn't over 14 billion years old, the difference between 15 billion [light]years and 46.5 billion [light]years is really what when talking about seeing light from earlier than 14 billion?

Nobody talks about seeing light from earlier than 14 billion years ago. Or, nobody should - if you think someone has, there's probably been some miscommunication.

No. It is far, far simpler.

Going back to the OP for a moment. Imagine you have a map. It is an old map. You point to a thing on the map, and say, "This is 13.14 billion light years away, I want to go there, today."

I tell you, "That is an old map. If you want to get to that thing today, you'll have to go 45 billion light years to get there."

If you went, today, those 13.14 billion light years, there'd be... nothing there! The map you have, that you get from looking at the sky, is inaccurate. It doesn't give you *today's* positions of those objects. If you want to go to *today's* position, you're going farther than 13.14 billion light years.

I'm seeing light from 14 billion years ago. It shows me an object that 14 billion light years away, as it was 14 billion years ago. If I want ot go to that same object, I'll have to go farther, and I'll see something much different (a star 14 billion years older, quite probably dead) when I get there.

 

[jonesy]

I think you're being a bit too persnickety, here. You know what I meant, and you're not addressing what my question/point. How can something have existed, (however we detect it, now), 15 billion years ago, (or 15 billion lightyears away)? Existence didn't exist 15 billion years ago.

Unless the universe is/was at some time expanding at a rate faster than lightspeed?

I don't know what persnickety is, and I did think I knew what you meant, and I did think I answered it.

We aren't seeing what the universe is like. We are deducing it. All information arrives to you on a time delay. Even looking at something next to your nose is on time delay. You could be looking at a lit lamp, only it isn't really lit at that exact moment because of a power failure, and then a fraction of a fraction later the information that the lamp isn't lit gets to you. It's much much moreso when you look out at the stars.

Here's an example of what I mean: let's suppose that all of the stars suddenly went out, except Sol. You know how long it would take us to find out? We'd be all alone next to the only lit star in the universe and we wouldn't even know, because all of the information coming to us would be outstandingly outdated. As it is already.

I said that we aren't seeing something 15 billion light years away, because the things that light is telling us are there aren't there. Not anymore. They've moved on. And it's not a one-way highway. It's going everywhere. And it's relative from where you are. And that is messing up the image that light is telling us. The universe has been fattening itself for so long it's made it very far indeed.

But let's finally get down to what's really bugging you. Current data puts the age of the universe at ~13.75 billion years. Current data puts the edge of the observable universe at ~47 billion light-years away. Right now the universe is expanding at the speed of ~74 kilometers/second/megaparsec. That means that for every million parsecs of distance the rate of expansion increases ~74 kilometers per second from the observer. 1 parsec is 3.26 lightyears. 1 lightyear is 9,460,730,472,580.8 km. So, for every 30,856,780,000,000,000,000 km (a million megaparsecs) the expansion we see increases by 74km/s. If we are looking at light from a spot 13.75 billion lightyears away the rate of expansion is (421,576,859,283km x 74km)/s, from our point of view. If we did the calcuation there, it would be the same, from there.

But that's only the observable universe.

And if I messed the math at some point I blame the screwy way of different people writing billion a different way.

 

[Bullgrit]

General comment: I fully understand that we are seeing things on a time delay because of distance and the speed of light. I know this. I knew it before this thread. This is not my "issue." But this fact leads to the "issue."

* * *

Nobody talks about seeing light from earlier than 14 billion years ago. Or, nobody should - if you think someone has, there's probably been some miscommunication.

"Observable universe"
"Visible universe"
These terms suggest seeing. And even if it isn't seeing light, it is detecting something farther away and further back in history than the universe has existed.

I'm still reading and digesting some of the information presented since my previous post, but I wanted to say this so people don't think I'm completely dense or obstinate.

Bullgrit

 

[jonesy]

Edit: damn, the frickin edit ate my post.

What was I saying? Something about the whole universe-relatively-expanding-faster-than-lightspeed being a good way to rile up a group of astronomers. There's debate on that.