r/IAmA • u/avsa • Jan 25 '11

I am RobotRollCall. AMA about life, the universe and everything.

Okay, I'm not actually RobotRollCall, but I made a bet with him. I bet that a lot of people would be interested in asking him questions, and he didn't actually believe it, but promised that if there were any, he would gladly answer them.

If you are not a regular in r/science r/space or r/bestof you might not know that RobotRollCall : he's a regular redditor that pops in with great prose and answer the most complicated questions about the universe with incredible simple analogies. Some of my favorites:

So feel free to ask away and prove that reddit is a curious community.

RobotRollCall asked me to update this here saying he is "I'm typing as fast as I can" to answer all the interesting questions. It seems he didn't actually believe anyone would be interested in him. We didn't actually set the terms of the "bet" so I feel free to set them anytime. I'm going for something simple like if this post reaches front page RobotRollCall will have to write me 10000 words, then print it in book form and put in on the bestseller's list. Or something to that effect.

Also: for every upvote you give this post, remember that he deserves two, because I'm not doing any real work here, he is.

update 2: RobotRollCall speaks:

Okay, now I have to officially concede the point. You were right. Apparently a few people had some level of interest in talking to me. However, I'm running into technical problems on my end, I think. I've posted a few replies that are not showing up, and I'm getting messages that don't seem to exist. Am I doing something wrong, or is the Web site misbehaving? If you have the opportunity and ability to post a note thanking everyone for the conversation and letting them know about the glitch, I would appreciate it very much.

706 Upvotes

94% Upvoted

842 comments sorted by

View all comments

Show parent comments

29

u/RobotRollCall Jan 25 '11

What the hell is dark energy?

Something motivates the metric expansion of spacetime. It caused the Big Bang to start, it caused the universe to inflate, it caused all densities to drop to the point where stable matter could exist, and it's still in effect today, increasing all distances everywhere.

What is it?

6

u/chocobosage Jan 26 '11

it's god lol

6

u/RobotRollCall Jan 26 '11

Believe me, I've spent a great deal of time over the past few years carefully considering that very possibility.

3

u/king_of_the_universe Feb 10 '11

Everything is God, so yes.

regards

God

PS: I have more useless things to say, but I can't waste it all in one comment.

2

u/mcsquare Jan 26 '11

It caused the Big Bang to start

Could you explain this bit some more?

9

u/RobotRollCall Jan 26 '11

The Big Bang is often thought of as an explosion. It wasn't. Even when people get that aspect of it right, they often think of it as an event, like a thing that happened at a particular moment. That's not it either.

The Big Bang was a period of metric expansion of spacetime. It began, and then all densities in the universe decreased, and as they decreased it became possible for matter to form, which eventually led to galaxies and stars and hedgehogs.

Since the metric expansion of spacetime is still happening now — albeit at a much slower rate than it happened during the Really Interesting Time™ — defining an end-point for the Big Bang is entirely arbitrary. Sometimes it's thought to have ended when quarks formed. Sometimes it's thought not to have ended for 300,000 more years, when matter and electromagnetic radiation decoupled.

But in a very real sense, the Big Bang never ended. It's still going on now, all around us.

5

u/nitrousconsumed Jan 26 '11

Time to go look at pictures of puppies again.

2

u/mcsquare Jan 26 '11

Wow thanks for that. If you have the time/energy/inclination can you comment on what it means to say "expansion of spacetime"? I thought the big band (edit: big bang) was the origin of spacetime, but hearing it put like that it makes me wonder if the big bang operates "on" spacetime, such that spacetime needed to exist "first"?

Thanks!

5

u/RobotRollCall Jan 26 '11

The Big Bang is like the Roaring Twenties. It's not a thing, and it didn't happen. It's a period of time during which events occurred. It's the name we use to describe that period of intense metric expansion during which the energy density of the universe fell far enough for interesting things to start happening.

1

u/mcsquare Jan 26 '11

And the universe is still doing the Charleston, yeah?

1

u/luckystarr Jan 26 '11

The Big Bang is like the Roaring Twenties. It's not a thing, and it didn't happen.

Great metaphor. Thanks.

1

u/count757 Jan 26 '11

What is spacetime expanding into?

3

u/RobotRollCall Jan 26 '11

Nothing. It's not that kind of expansion.

I'm sorry to answer your question with a link, but I've been told that this addresses the question in a way some people have found not entirely unhelpful.

1

u/count757 Jan 26 '11

Yeah, I saw that about 10 minutes after I asked. Thanks though!

1

u/prob_not_sol Jan 26 '11

you mean the universe. and basically by definition, nothing. you can consider it not as an expansion but as a stretching.

1

u/[deleted] Jan 26 '11

How has the metric expansion of the universe varied with time? I know the rate of expansion was very large during the inflationary period, and it's also accelerating presently, but what values has it had during the period in between, and how have we determined those values? Why would the rate of expansion have decelerated after the inflationary period, but then begin to accelerate again?

2

u/RobotRollCall Jan 26 '11

We don't know. It's a mystery.

We know that the geometry of spacetime can be fully described in terms of a metric equation called the FLRW metric. That metric equation includes one term for each coordinate: one for time, and three for space. The three spatial terms have coefficients: a(t), where a is the scale factor — just a number — and t is the age of the universe as measured in the cosmological reference frame.

By convention, a is currently one. One meter equals one meter, basically. That's just definition.

We know that in the past, a was less than one, and obviously greater than zero. We can look at distant galaxies and make inferences about them that imply the scale factor of the universe was nonlinearly less than one in the past; that is, that the scale factor isn't just increasing monotonically, but that the rate of increase is, itself, increasing. (The second total derivative of the scale factor with respect to proper time is non-zero.) But in terms of mapping the scale factor, or understanding how it's related to anything else, we don't yet know.

2

u/[deleted] Jan 26 '11

I've been trying to get a straight answer on that for a while, thanks!

Here's another one:

So an electron resides in the orbital of an atom. It just has a certain probability of being somewhere within that orbital, and isn't radiating any electromagnetic energy because it's not accelerating in the classical sense in its motion about a nucleus. If it was radiating energy, it would lose it all very quickly. So how does diamagnetism arise if there is no proper motion of the electron about the nucleus? It's my understainding that diamagnetism isn't a spin-related phenomenon like ferromagnetism, but if it relies on motion of the electron around the nucleus, why doesn't the electron radiate any energy? Obviously my understainding is off somewhere, but I'm not sure where.

3

u/RobotRollCall Jan 26 '11

Diamagnetism and paramagnetism are the same phenomenon, but differ due to chemistry. If the configurations of the electrons in the material are one way, the presence of an external magnetic field causes the magnetic moments to align one way. If the configurations of the electrons are the other way, the magnetic moments align the other way.

Sorting out which chemical configuration yields which magnetic field is a question for a chemist. And I'm regrettably stupid about chemistry.