Space and Astronomy in general

My fantasy of a twin planet system where they are so close that they share an atmosphere and you can fly from one to the other doesn't seem so crazy anymore.

Yonder wrote:

My fantasy of a twin planet system where they are so close that they share an atmosphere and you can fly from one to the other doesn't seem so crazy anymore.

That is a cool fantasy, but holy f*ck would they have be circling each other super fast to avoid being brought together by gravity.

A contact binary of two Earth-size planets would rotate in about one hour.

CaptainCrowbar wrote:

A contact binary of two Earth-size planets would rotate in about one hour.

I think if they were Earth-sized, they'd break up from being under the Roche limit.

Maybe I'm experiencing a frequency illusion, but I'm seeing more and more references to Deccan volcanism causing the KT extinction.

https://www.theatlantic.com/magazine/archive/2018/09/dinosaur-extinction-debate/565769/

This has always struck me as a "Porque No Los Dos?" situation. tidal waves and atmospheric particles are a real real bad short to medium term survival hit, with the deccan flood basalt events perpetuating the harsh conditions longer than they would ordinarily persist.

That was a really interesting read.

A shame to see how scientists can be such sh*tty people. Tend to think of aspiring to Star Trek ideals, but wow such virtiol.

Stele wrote:

That was a really interesting read.

A shame to see how scientists can be such sh*tty people. Tend to think of aspiring to Star Trek ideals, but wow such virtiol.

Scientists are people, with all the personality variation implicit in that. It can get particular nasty when someone specializes and builds their identity around a particular theory, which is not something good scientists should be but of course happens way too often. It means a contradictory theory isn't just new info, but an attack on a person's identity. I think academia can encourage that type of behavior as well (I went into practical genetics and not research for a reason)

Ain't nobody got time for that.

Given the scale of the universe, it does seem rather peculiar that the absolute speed limit is as low as it is.

BUT

Entanglement. And wormholes.

BadKen wrote:

Given the scale of the universe, it does seem rather peculiar that the absolute speed limit is as low as it is.

Simplifying assumptions made to reduce the computational requirements of the simulation, along with having things like a minimum temperature and a few other limits. And by only storing the position or velocity of a particle, but not both, cuts storage requirements in half.

I can see that the size of the universe is in part dependent on the speed of light, but I can't see why the relationship would go in the other direction. The speed of light has not changed meaningfully with the expansion of the universe, that we know of.

The reason is that the speed of light is part of what makes up (among other constants) the fine structure constant, which, if it changed significantly, would render the universe very different to what it is. For example, a 4% change would mean that stellar fusion no longer makes carbon. We would definitely notice that, so it's safe to say that the ideas put forth by Creationists and others that the speed of light was faster in the past are not viable. While there is the possibility that the speed of light and other constants vary over time, the variation is probably around 0.00072 +/- 0.00018 % over the last 6-10 billion years. That's not meaningful at our scale, and it does not support Creationism either. (Not that you implied that question, I just throw it in because it often comes up.)

BadKen wrote:

Given the scale of the universe, it does seem rather peculiar that the absolute speed limit is as low as it is.

BUT

Entanglement. And wormholes.

The scale of the universe is so huge compared to the speed of light because it's been around for 14 billion years. If it was only 6000 years old the speed of light would seem positively zippy because the universe would probably be less that 15k light years across. (Not entirely sure how much expansion would increase the naive 12k you get from light expanding out in a sphere without those effects)

I think that's one concept I still don't quite grasp. Universe is x billion years old. Observable space is y light yeas to us. Size is z light years, something bigger.

Like light speed is max speed, so that's as far back as we can see, but how can things be farther apart than the age when things can't move faster than light?

Something about the big bang that just doesn't fit right to me, somebody explain it please.

EDIT: A quick google asks the question I think I needed: How Can the Universe Expand Faster Than the Speed of Light?

EDIT2: Well, reading that was part of it. But still. I'm confused.

Okay, so imagine that we come from a group of atoms that are split in the early universe. They move on a vector exactly opposite to ours, for the same amount of time. We can only see as far back as the start, but they are an equal distance in space and time *beyond* that, from our perspective. Not only can't we see them, they are as far along in their travels as we are.

Make sense?

Not only that, but while 'stuff' can only move as fast as the speed of light, the expansion of the universe itself isn't 'stuff', and has no such constraints. The further things are away from us, the faster they appear to be moving, because there is more space between us and them, and hence it is stretching faster. So really far away things (and in space stuff is REALLY far away. It still makes me dizzy thinking about it, and I've been into astronomy for as long as I can remember) move faster than the speed of light, from our point of view.

This one is a bit complicated, but let me try.

Imagine a rubber band. Make two marks on that rubber band. Those are two galaxies.

Stretch the rubber band. The galaxies move away from each other. So far, so good.

Now, put an ant on the rubber band on one of the marks and convince it to go to the other mark. (This bit is left as an exercise for the student.) Now, stretch the rubber band again.

You can stretch the rubber band faster than the ant can walk, so even though it is heading for the other mark, it keeps getting further away. That's light and the expansion of the universe.

Edit: Also, and here's where it gets really weird, it isn't that nothing can travel faster than the speed of light; it's that information can't travel faster than the speed of light.

Suppose I have a super-powerful flashlight and someone creates a light-years long screen. I can point the flashlight at one end of the screen, then in a fraction of a second flip it to the other side of the screen. The spot of light will move all of those light years in a fraction of a second, but since I can't convey any information that way, it works.

Rallick wrote:

So really far away things (and in space stuff is REALLY far away. It still makes me dizzy thinking about it, and I've been into astronomy for as long as I can remember) move faster than the speed of light, from our point of view.

This was what that article explained. And I think the rubber band example covers that too.

Robear wrote:

Okay, so imagine that we come from a group of atoms that are split in the early universe. They move on a vector exactly opposite to ours, for the same amount of time. We can only see as far back as the start, but they are an equal distance in space and time *beyond* that, from our perspective. Not only can't we see them, they are as far along in their travels as we are.

Make sense?

I think this might be more what I'm getting at. So we see 15b years back to the bang. But there has to be stuff 15b years on the other side of the bang, since expansion is symmetrical?

But if light speed is the max travel speed, then how do things get so far apart in so little time? Shouldn't the max size be less than a sphere with 15b radius?

Or can we see 15b in all directions? Or only towards where the bang started? Or wait do we see it away from the bang as it's still expanding?

I just don't... something just doesn't click about the size of the universe vs the speed of expansion vs light speed vs our location from the center. I'm not sure how the observations add up I guess.

So we see 15b years back to the bang. But there has to be stuff 15b years on the other side of the bang, since expansion is symmetrical?

This is an open question. Though there are, indeed, some theories that there is an equal-and-opposite antimatter, reverse-time universe going the other way through time from the big bang. It's not proven, but the math allows for it (under our current understanding, in some models).

tanstaafl wrote:

Edit: Also, and here's where it gets really weird, it isn't that nothing can travel faster than the speed of light; it's that information can't travel faster than the speed of light.

Suppose I have a super-powerful flashlight and someone creates a light-years long screen. I can point the flashlight at one end of the screen, then in a fraction of a second flip it to the other side of the screen. The spot of light will move all of those light years in a fraction of a second, but since I can't convey any information that way, it works.

This sound fascinating. Is there some further reading you would recommend on this topic?

Stele wrote:

I think this might be more what I'm getting at. So we see 15b years back to the bang. But there has to be stuff 15b years on the other side of the bang, since expansion is symmetrical?

Well, I simplified it (and physicists please step in if I screw it up, I read a lot but this is not my field). As far as we know, the expansion is symmetrical. So yeah, there is stuff behind our visible horizon.

But... There is no "center" to the universe, because that symmetry is a symmetry *of* space, not *in* space. That might be tripping you up. The Big Bang was not an explosion *in* space, it was an explosion *of* the stuff that makes up space.

What all this means is that from any point in space, things appear to be receding or approaching *relative* to that point. If we see a galaxy receding from us at 10,000 kilometers per second, but see our own world as fixed (ie, the measurement is *relative* to us), then people on that other galaxy will see us receding from them at 10,000km/sec, *relative* to them. See that word? This applies to any point in the universe, *relative* to other points, from the viewers *frame of reference*. This is, I think, General Relativity.

But if light speed is the max travel speed, then how do things get so far apart in so little time? Shouldn't the max size be less than a sphere with 15b radius?

It's the max travel speed of information *inside* space, not of space itself. Remember the ant on the rubber band? The ant is limited to a particular speed - all ants are in some way - but the rubber band can stretch much faster. And the ant might not even perceive that speed unless it could measure the motion of the dots *relative* to each other, or to itself. (See how often the word *relative* crops up? It's so cool when you start thinking that way...)

Or can we see 15b in all directions? Or only towards where the bang started? Or wait do we see it away from the bang as it's still expanding?

You can work this out yourself with a little thought. How old is the oldest light we can possibly see? (Light from shortly after the Big Bang, right?) So any light that is within a sphere centered on us, and bounded at a distance equal to the number of years since the Big Bang multiplied by the speed of light, and with an expansion factor added in, that light we can see if it moves towards us long enough. But there are things that are not in that sphere! We won't be able to see their light from where we are, because we are not inside *their* sphere, the one that is *relative* to their location. If they are outside our sphere, then their light will never reach us here, assuming both spheres expand at similar rates.

Oh, and the Big Bang's location relative to us? It's the farthest surface of the sphere. The *entire* inner surface of the sphere. Space has no center, remember? And we know this experimentally, because the entire sky is backgrounded by the remnants of radiation from the period shortly after the Big Bang. Hence, the term "Cosmic Background Radiation".

Does that help? I think I got it right. Someone will let us know if I screwed up.

Man, if I'd had better high school math teachers, I'd be a kick-ass Astrophysicist right now. This stuff is soooo cool.

So you’re saying, I’m the center of the universe?

You're the center of your observable universe, yes.

I watched the Jan 9 episode of Nova, Einstein's Quantum Riddle last night. It mostly centered around a recent experiment that used the light from two quasars to gate the filters for an experiment proving quantum entanglement. This factor of the experiment - how the filter gates were randomly chosen - was one of the few remaining questions in the confirmation of the existence of entanglement.

The episode was mostly the usual quantum discussion that I am pretty familiar with. The narrator kept using the phrase "spookily entangled" which really got on my nerves. I was actually a little disappointed because when I used to watch Nova regularly, I remember learning quite a bit from each episode. I guess maybe I know more about quantum theory from casual reading and curiosity than maybe your average PBS viewer.

Anyway, the reason I bring it up is that the most interesting part of the episode came in the closing minutes. One physicist was explaining that a current theory called the Holographic Universe says that we may have to give up Einstein's understanding of the universe - spacetime - because it may not really exist. In this admittedly radical theory, spacetime is defined by entangled quantum particles on a sphere infinitely far away. These particles are what define reality, and somehow they project that into the universe, creating the reality that we perceive.

At that point my mind exploded and I don't remember the end of the episode.

I understand that the Holographic Principle was brought from information theory to address an issue in String Theory, but that it also is not clear that it does not conflict with other elements of that framework. So... Maybe not.

All this universe talk makes me wonder what is outside the universe? I never see anyone try to address that question.

That'd be a matter for philosophy, not astronomy, wouldn't it? We cannot observe it at all.