Good evening everybody.
Thanks everybody for coming my name is Dr. Leo Stein, I'm a postdoctoral scholar up here in Cahill and at Caltech.
I'm not giving this up.
There are a lot of people here so probably some of you are new so I'm just gonna make some announcements,
So you know what's going on. This is part of a monthly lecture series.
This is some of the the outreach that we do here at Caltech Astronomy.
We also have another outreach series called Astronomy on Tap that's on Mondays.
Also once a month.
There are some schedules outside on the table. You feel free to pick these up.
The next event is going to be on Monday, September 11th. These Astronomy on Tap events happen at Der Wolfskopf over in Old Town.
So they are only 21 and over, but these events that happen in here are for all ages.
The next event that's in here is going to be on August 21st. That's a Monday. That's for the solar eclipse that's happening.
We'll have some more announcements about that later.
What else do I have to say? Sso the schedule for these events:
We're going to start out with a 30 minute talk. These talks we try to rotate through between graduate students and postdocs and faculty
who are in the department.
After that, we'll have a few questions then we'll have a little break in
For the next 90 minutes after that in here
We're going to have a Q&A session where you can ask experts who work in this building
anything that you want about astronomy or
Really anything about life.
Back on the field behind here
there's going to be stargazing. We're going to have two telescopes and some naked eye observing.
So afterwards if you want to go out to the field
you're gonna go out the front entrance turn right turn, right
walk along here and then turn right at the sign and go through the gates and
the athletic facilities here have been very kind to let us use their field and they're also very strict because they just
Resurface the field so they ask that we don't have any drink or food or pets or high heels that could puncture the artificial turf.
So we're going to try to respect their desire. So they keep letting us use the field.
Okay, so if you need some help at some points there are volunteers wearing badges like this.
There are going to be some in here and then afterwards there going to be some out on the field.
Jessie
Christianson, Dr. Jessie Christiansen is wearing a purple cardigan. She has a bunch of Eclipse glasses for sale that are gonna be a dollar apiece.
There's only about a hundred of them. So if you want to buy them try to find Jessie who's gonna be on the field.
But at the event on August 21st, there's gonna be more Eclipse glasses.
So
There's also a tiny little survey that was on the table. That was outside
please if you want to help us out fill out the survey because this thing is going to help us get some statistics and that's
Going to help us get these events funded more. All right, so you probably didn't come here to hear me ramble.
So we're gonna get on to the main event. So tonight
We're very happy to have professor Sean Carroll
Professor Carroll is an expert in cosmology
He studies foundations of quantum mechanics how philosophy interplays with physics, how complexity arises out of the laws of physics.
Things like how to get information of black holes. He's a prolific author of popular physics books and of textbooks that
people like me who study general relativity have.
He's a recipient of the prestigious Guggenheim Fellowship he's a blogger and a tweeter.
He's a poetic naturalist.
Today we're gonna hear about the origin of the universe and the arrow of time. So please join me in welcoming Professor Carroll.
Thank you, can you people hear me in the back? is it working? Can you hear me? Yes, excellent.
You have no idea how much it warms my heart
To see how many people there are in the world whose idea of a good time on Friday night is
going to a lecture on astrophysics and cosmology and then looking through some telescopes afterwards. So that is wonderful.
I do want to let you know in case you came for the stars,
there will be no stars in this lecture. As a theoretical physicist,
I am interested in things that are smaller than atoms and larger than galaxies and everything in between is kind of noisy bother
to me. So but what I want to do, the universe itself will make an appearance in today's talk
It will be about the theme of what is time and how does time work and the surprising insight will be that
cosmology in the origin of the universe play a big role there and there are important mysteries that we haven't yet solved. So the first
place to start here is just to ask "what is
time?" and this is a fake question in some ways.
This is one of those questions that sounds really deep and profound and difficult. Oh my goodness. What is time? I have no idea.
But you use it all the time, as it were, right? In fact, time is the most used noun in the English language. If
someone says to you "The lecture is at 8:00 p.m."
You don't say "Oh my god. What does that mean?
I have no idea how to use this information." You know exactly what to do.
If someone says the lecture is going to last for half an hour, etc.
You know how to process that. The short version is that time is just a label on the universe. The universe,
big space filled with stuff, there are tables and chairs and people and planets and stars
scattered throughout space and
This arrangement of stuff happens over and over again. There are copies of it
They're just a little bit different from each other and we label them by this number we call time.
It's very much as Einstein pointed out like space
We find things in the universe by labeling where they are in space you say the lecture that 8 p.m. It 8 p.m. Right?
Yes, it is at Cahill building at Caltech--that locates it in space. That helps us find ourselves in the universe.
But there's also a crucial difference
Between space and time, which is that space does not have a direction.
Here in the room space has a direction. There's a difference between up and down. Down is the direction things fall in but you know,
Deep in your heart, that that's not a reflection of any fundamental nature of reality.
That's the earth beneath us pulling on things. If you go to outer space,
Where the pull of the earth can be ignored, there would be no difference between up, down, left, right, forward, backward.
There is no directionality to space. All of the different directions
You can point yourself in in space are more or less the same, and that's true
Both in our everyday lives and in the deep down laws of physics.
Time has a direction.
We all used to be younger and we will be older. It's not just me.
I feel it sometimes, but all of us used to be younger, and we'll be older and it's not just a convention, right?
I mean you might say well,
Sure, you define being younger as what was happening in one direction and being older as the other, but there are differences
There are systemic objective differences between being younger and being older that are
Universal among all people and that is a reflection of the fact
That there is a difference between the past and future. Time has a direction, which we call the arrow of time.
The arrow of time is just the difference between the past and the future. There's somehow a
Directionality that time has that space does not.
So what do I mean by that? How does it manifest itself other than the evolution of kings of rock and roll? Well,
you can remember the past. You can't remember the future. I hope there's no one here who can remember the future.
But we have it's not as something specific about human beings or their biological brains or anything like that. There are
records. There are artifacts. There are physical objects,
whether they are paintings or photographs or fossils, that tell us something specific about what was going on in the past
Where it comes to the future, nobody has a photograph of the future.
There's a famous Mitch Hedberg joke.
He says:  "I don't know why people are always telling me here's a picture of what I looked like when I was younger. All
pictures are what you look like when you were younger."
Right. There are no pictures of what you look like when you were older. We can predict the future,
but we might be wrong in ways that an accurate record of the past doesn't let us be. So here's a question
You should be asking yourself, which maybe you never have asked yourself
Why are there these records of the past?
Why is it possible take a picture of the past and not the future is the short version of that question.
It's a more difficult question than you might think.
But it goes beyond that.
We think that there's a fundamental
Feature of how reality works that there is cause and effect, right?
There's an effect first, and then a cause. I drop a pebble into the
pond and ripples come out. The ripples come out because I dropped the pebble in.
Nobody would ever say: "Oh,
I dropped the pebble in
because there weren't going to be ripples and there had to be something to cause them." Right, the cause always comes before the effect.
Free will--you have the ability to make choices about the future. You could say right now:
"I've changed my mind. This is a really boring lecture. I'm gonna leave." Okay?
You cannot say right now: "I will choose not to have come to the lecture."
You cannot make a choice that
influences things in the past, but you have the ability to do that toward the future. Why is that?
Where does this asymmetry this arrow of time come from?
You might think well, that's just the way it is. Like in the good old days
You were Aristotle
If you were, you know, ancient philosophers or scientists
You wouldn't even have asked this question because it's just a feature of how time works. Of course,
the past is different from the future. But modern physics teaches us something different.
Modern physics by which I mean Isaac Newton and thereafter.
Newton's laws,, which we still teach and torture our students with today are a way of looking at the universe
that does a really good job.
If you have simple systems bumping into each other like billiard balls scattering off of each other, imagine
these are physicists' billiard balls by which I mean, there's no friction. There's no noise.
There's no messiness or anything like that. A classic problem is to take these two billiard balls moving a certain direction with a certain velocity, and
tell me after they scatter what direction they're gonna go in and how fast.
Okay, but I could
tell you how fast they were moving and in what direction at the end of that process and you could play the movie backwards.
You could figure out where they came from.
The Newtonian laws of physics are perfectly reversible.
If you take the earth moving around the Sun or a
pendulum swinging back and forth and you made a movie of that and you played it backward in time,
no one would be able to tell. The behavior is the same
going forwards and backwards in time, and this feature of fundamental physics that Newton introduced
hasn't gone away.
It's just as true for Einstein, for super string theory, for all of our best ideas about how physics works at a deep level.
So that's the mystery: why is there on the one hand
no difference between past and future in the fundamental laws of nature as we currently believe them, on the other hand, an
obvious difference in our everyday lives that is one of the most blatantly important facts about the universe?
Part of the answer can be found in the idea of entropy. We've heard of entropy. All young children have heard of entropy
because
entropy is a measure of the disorderliness of the universe. And
entropy--there's a law for the second law of thermodynamics. The first law just says that energy is conserved.
It's boring.
The second law says that entropy increases over time in a closed system. If
you have a room, and you leave it by itself, you don't work at cleaning it, it will get messy.
This is why young children love this law, because they say:
"Mom, my room is messy because it's a fundamental law of nature that that's gonna happen."
There is a loophole of the law that says if it's not a closed system, if it's an open system, if you go in and
clean your room, that's okay. That is compatible with the laws of physics. You can lower the entropy of a system.
You can lower the entropy of something by doing work on it, but secretly
if you really want to think about it deeply kids,
you're sweating and making noise and so forth, and you're increasing the entropy of the universe by cleaning your room.
The overall entropy of things always goes up.
So that's not an explanation of anything by itself. That's a feature.
Once you go from simple systems,
like two billiard balls or a pendulum going back and forth, to complicated systems with many many moving parts,
there's this idea called entropy which seems to be able to tell the difference between the past and the future.
Entropy was lower in the past, higher in the future. The universe was more organized in the past,
will be more messy and disorderly in the future, and a cube of ice will melt into a cup of water,
but a glass of water sitting up there by itself will not
form an ice cube all by itself. You can mix cream into coffee, but you cannot unmix cream in coffee.
You can break an egg and make scrambled eggs. You cannot take scrambled eggs and reassemble them into an egg. In every single case,
it's because entropy has gone up in the process that actually happens.
So then the question is: Fine, as I made this transition from simple systems with just a couple moving parts to
complicated systems with many moving parts, what changed?
What was different? Why can't I just think of
coffee and cream or an egg as a whole bunch of atoms and they all obey Newton's laws of physics?
What's the difference between a big system and a small system that entropy becomes so important? And
The answer was given to us by this guy, Ludwig Boltzmann.
He was an Austrian physicist
in the 19th century. I always tell my students
Boltzmann succeeded at what every physicist should aim to do.
He got an equation on his tombstone when he died, right?
"What will your equation be?" is what you should be thinking in the back of your mind someday?
Boltzmann's equation is the definition of entropy.
Entropy is a weird thing. The second law says that entropy increases over time.
But the second law was invented before the concept of entropy was invented, and the concept of entropy was invented before we defined it. So
Boltzmann finally came along said: "Ahha,
I know what entropy is."
The point is that when I look at a system with many many moving parts: an egg, cup of coffee, or whatever
I don't see every detail of
the system. Sure, maybe the egg or the cup of coffee are made of atoms and molecules,
but I don't see those atoms and molecules, right?
I see some coarse features: where the cream is, where the coffee is, where the egg yolk is, etc.,
where the books are stacked in my room, there are details that I miss, and
Boltzmann's equation in mathematical terms represents the fact that entropy is just a way of saying
how many ways are there to arrange the fundamental parts of the system that basically look the same to us?
If you have the air in this room
There are atoms. Atoms and molecules of oxygen and nitrogen and carbon dioxide and so forth. You don't see all the atoms.
You don't know where they all are. You see like the temperature, and the pressure, and the velocity of the air in the room.
There are many many arrangements of atoms that would look exactly the same to you.
Boltzmann says that's what entropy is. 
 Entropy is just how many ways can we arrange things? And from that perspective,
it's perfectly clear why entropy goes up.
There are more ways to be high entropy than to be low entropy. And Boltzmann was very proud of himself.
He thought he had proven why the second law of thermodynamics is true. Why does entropy go up?
You just let a random system operate all by its lonesome. It's entropy will tend to go up.
There are more ways to be high entropy than low entropy. There are more ways for your room to be messy then for it to be clean.
It won't clean itself up all by itself.
Now that's a cheat. I'm not going to tell you what the cheat is right now. Boltzmann clearly cheated.
Let's see if you can think of it--how he cheated in answering this question: Why does entropy go up over time?
Here's just an illustration of what we mean here is mixing cream into coffee. Okay? Cream on top, coffee on the bottom at the left.
Over the course of time, either if I mix it or if you just let it sit there, the cream and coffee blend into each other, and
then they're all mixed together so you can tell obviously I but there's a big blue arrow in the bottom saying time goes from left
to right, but I didn't need that arrow, right?
You knew the one on the left came first, and the one on the right came second, because the entropy went up.
Because there's a certain number of ways I could rearrange the cream molecules and coffee molecules in the first picture,
but if I start mixing them together,
it'll start looking at that like the last picture.
There's a lot more ways to organize
and arrange the molecules in the third picture than there is in the first one. It has a much higher entropy.
It's not just cups of coffee, the universe
does the same thing, and this is the part where Boltzmann cheated.
What Boltzmann taught us was why entropy
increases toward the future if you assume the entropy was lower in the past.
He never taught us why the entropy was low to begin with. If you want to know why the entropy of the universe was lower
yesterday than it is today. Why was the universe just a little more orderly yesterday than it is today?
It's because it was even lower entropy the day before yesterday
Well, so why was it lower entropy the day before yesterday?
Can you guess? I'm not gonna let you guys there are too many people here. Because there's even lower entropy the day before that, and
this chain of logic goes on 14 billion years to the Big Bang.
The reason why entropy is lower in the past in the future is because it started out
Incredibly low
14 billion years ago when the universe started. Why did it do that? We have no idea.
We don't know. That's a job for smart young people
I'm too old to answer these questions now. Like there's a rule that young people get to answer these questions.
So I'm hoping that one of the young people in the audience will someday answer this question:
"Why did the early universe have a low entropy?" They will win the Nobel Prize and they will remember this talk,
and thank me in their Nobel Prize acceptance speech.
That's the best I can do. Okay?
So I know you have a question. We're gonna wait till the end to get the questions. Okay. So here's a picture
Here's a little movie,
a still movie, of the evolution of the universe. This is what the universe started out looking like one second after the Big Bang.
We sometimes portray the Big Bang as an explosion in an empty universe, but that's not right.
The Big Bang was everything all over in the entire universe all at once. It was a moment in time,
not a location in space. it looked the same everywhere.
If you were standing in the universe one second after the Big Bang it would look
white everywhere because it was glowing and hot and dense. This is not an actual photograph. This is a white
rectangle that I made in PowerPoint and I tilted it.
But by a few hundred thousand years after the Big Bang, then the universe becomes
transparent. It cools off, the universe expands and cools and becomes transparent. That is a picture. That is data
that's what the universe looks like in the Cosmic Microwave Background, the leftover radiation from the Big Bang, and you see the universe went from
being perfectly smooth to not quite as smooth. It was never perfectly smooth as it turns out there were tiny
tiny tiny little differences in the universe, from place to place, and gravity increases the contrast
of the universe.  Dense regions become much denser. Empty regions become emptier until you get to today.
Tens of billions of years after the Big Bang where you'd have big patches of empty space and bright little galaxies in between.
That's the universe right now.
What will happen in the future, if you go to ordinary cosmology lectures, they stop at today,
but here at Caltech we're gonna tell you what's gonna happen in the future, okay? So eventually the stars will die.
One quadrillion years from now 10^15 years from now the last star will stop burning and the universe will become completely dark.
It'll be full of nothing but black holes and rocks--
rocks being dead stars, planets, etc. And guess what?
Those rocks are gonna fall into the black holes,
if you give them long enough time, and Stephen Hawking taught us in 1970s that even black holes aren't forever.
Black holes give off radiation, lose mass, and evaporate. So ultimately if you wait long enough,
10^100 years,
the last black hole will disappear and there'll be nothing left in the universe, in the observable universe,
but empty space.
So you see, it's hard to see this. So you've got to take my word for it. I have a PhD.
If you go through the math, this initial state when everything was very hot, and dense, but incredibly densely packed,
that was a low entropy state.
Entropy increases in every single step, from the left to the right, as the universe expands, and this I claim
explains why we remember the past and not the future.
Every difference I'm going to argue between the past and the future is because the entropy used to be lower.
So, why do you remember the past and not the future? Again, nothing to do with your brain. Here is a memory.
Here's my favorite example of a memory: It's an artifact--anything that tells you something
specific about the past is a memory.
Okay?
So you're walking down the street one day and come across a broken egg on the sidewalk, and you ask yourself--
you're in a reflective mood--you ask yourself: "I wonder what the past of that egg was like?"
Okay? You say, well...
probably there used to be an unbroken egg, and someone dropped it, and now it's broken.
That's pretty much the only thing that could possibly have happened, right? But what is the future of the egg hole?
There are many possibilities: it could just sit there;
Someone could come clean it up; a dog could come by and eat it; it could be washed away in a rainstorm.
There are many many possible futures for the egg and only one very specific past.
Why is that?
Again, you should be asking yourself these questions. Why are there so many more possible futures than pasts?
And you know what? It's not because of the laws of physics.
The laws of physics tell us that given the information we have about the egg,
we don't know all of its individual atoms and molecules, but we know the basic features of what it looks like.
There are many different futures, but there are equally many different pasts.
There are exactly the same number, as far as the fundamental laws of physics are concerned.
Why do you and I think we know something about the past of that egg that we don't know about the future?
Because secretly since we were born, we knew that there was a big bang that had a low entropy.
And we applied that knowledge to
specify some
possible past
trajectories at the expense of others.  As you go through
life, and you look at books and photographs and memories in your head, and you believe that they tell you something specific about the past,
the only reason that's possible is because there's an orderliness
imposed on the universe by the fact that it began with a low entropy. There's an asymmetry because there's no
corresponding condition on the future. It's only the past that has this special feature. That's why we remember the past and not the future
Now there's something that is like a little aside that is worth talking about here. Because once it sinks in,
what I'm telling you is that everything you know about the progress of time is that the universe is becoming messier--
becoming more disorderly. It's becoming higher entropy. That's the story of time passing--entropy is increasing.
At some point, you're gonna say yeah, but I'm not that messy.
I personally am kind of like a exquisitely organized biological system, right?
How do things like people, and Internets, and Institutes of Technology, come into existence,
if all that happens as the universe expands and cools is that entropy is increasing?
So the answer is that there's a completely different story for
complexity versus simplicity, than there is for entropy versus order.
We tend to connect them in our minds,
but the truth is not quite that simple. In the cup of coffee example: on the left there was low entropy, on the right it's high entropy.
In the middle,
it's middle entropy--medium entropy.
But think about it, the picture on the left: all the cream on the top and all the coffee on the bottom is very simple.
Right? To describe what you see, you just say cream on top, coffee on bottom, and you're done.
The picture on the right--high entropy--is also very simple.
Everything's mixed together. Now you're done. It's the picture in the middle, if the lights were dimmer you would see there's sort of some
complicated fractal pattern, tendrils of cream and coffee mixing in with each other. It's in between
simple and simple that complexity arises.
It's not
whether entropy is high or low, that lets complexity exist.
It's because entropy is increasing that lets complexity exist. The complex structures
you see here come into being because
entropy is increasing, and that's not just
cups of coffee, the universe is exactly the same way. The low entropy past was very very simple;
the high entropy future will be very very simple; the medium entropy present is
when the universe looks very complex. You were fortunate enough to live in the fun go-go exciting
Complex intricate part of the history of the universe this Friday night, right now. That's not really true. We don't know about that.
But this era of the universe's history, this is when the interesting things are happening, and that goes not just to
galaxies and planets, cream and coffee, but life itself.
There's a professor at JPL, up the street here in Pasadena and named Michael Russell,
who has proposed a theory of the origin of life.  We don't know how life began so there are many things
we don't know about that. We know less about how life began than about how the universe began.
But he proposed a theory, he said look the early Earth,
before there was life, had many environments which were low entropy.
The problem was entropy wanted to increase but it didn't know how.
I sat next to Michael on the plane once. He said oh, I know the purpose of life. It's to hydrogenate carbon dioxide.
That's usually not what people say when you ask them.
The point is that in this early environment, there was too much carbon dioxide, which is a low entropy way to be.
Those carbon dioxide's wanted--those carbon atoms wanted to get rid of their oxygens and get hydrogens instead and become methane.
But doing that, the whole process increases entropy,
But individual intermediate steps would decrease entropy first. So there's a blockage, there was no way to get there.
The clever invention that allowed entropy to increase in this circumstance,
according to Russell's theory, is called life.
A living organism
can take these low entropy things and make them into high entropy things. You and I call that eating.
That's what we do. This is a wonderful picture drawn by the physicist Roger Penrose.
Think about what happens on the earth. We get light from the Sun right? You say to yourself: "What good is it?
Why do we get light from the Sun? What do we use that for?" You might say: "Well we get energy from the Sun." Right?
True as far as it goes.
But the truth is the earth radiates the same amount of energy into the universe as we get from the Sun.
The net amount of energy on earth is staying the same.
What's important is that we get visible light from the Sun.
What you see out there on days when there's no solar eclipse, and then we radiate infrared light into the universe.
Which means that for every one photon of light we get from the Sun,
the earth radiates back
twenty photons,
but with on average one twentieth the energy each.
Quantitatively that means the earth
increases the entropy of that solar radiation by a factor of twenty and again what you and I refer to that as is:
photosynthesis,
eating, chewing our cud,
eating the bacon,
using that energy to write books, and take physics courses, and then it all gets radiated back into the universe.
So the fact that you and I are complex intricate systems is not
despite the fact that entropy is increasing. It's only possible
because we're in such a low entropy state and our existence ,and sustenance, and
continuation over time,
relies on the fact that we increase the entropy of the universe.
So that was an aside, your entire existence is just a way for the universe to become messier. Sorry.
But on the way, you can do useful work like answer important cosmology questions. Here's an important cosmology question:
"why did the universe start out with low entropy?" Here's another way of portraying the whole history of the universe.
We have about 14 billion years between the Big Bang and today.
We think we had infinity years between now and the future, but that's a puzzle for a different day.
Why this asymmetry? Why did the early universe start out with such a low entropy?
Even Boltzmann thought that this was an issue to be addressed. So the short answer is we don't know.
I told you that already.  Let me give you one guess.
In physics we call this a theory.
A theory that a graduate student and I put forward some years ago, and we think is one promising way to think about this question.
We know that the early universe, although it was low entropy, it was small.
So here's the thing: ask yourself
"What would a high entropy universe look like?" The right way to ask it is: "why aren't we in a high entropy state?"
Right? So what does a high entropy universe look like? Well, I already told you it looks like empty space.
Because of general relativity, because of that subject that is explained in the textbook that Leo held up that you can buy on amazon.com right now,
Space and time are not static.
They're dynamic. They move, and they breathe, and they change when they're pushed around by matter and energy in the universe.
Therefore there's a special thing in general relativity if you have a system, you say well, can I increase its entropy?
The answer is always. Yes.
Namely by making space bigger.
That gives you more places to put stuff and therefore the entropy goes up.
So the highest entropy state the universe can be in,
is one where space is just as big as it can be, and it's completely empty, and there's nothing there.
So the question is: why don't we live there? This is the fundamental question of cosmology. Why aren't we in empty space?
You might say well I couldn't exist in empty space but fine.
So why does the universe have more stuff in it than just you? or more stuff in it than just our solar system?
So here's our theory. We know that there's not only general relativity, which says that space and time are curved and dynamical.
There's also quantum mechanics.  Quantum mechanics says many things, it'd be a whole other lecture, which I'll give someday.
But one of the points of quantum mechanics is that there can be fluctuations.
You can sit quietly for a long time like you're a neutron.
You're a neutron just sitting there. Neutrons have a finite lifetime. If you let them out of their cages, a neutron will eventually decay.
Okay, it will undergo a change from one system to another.
If you marry the idea that space and time are flexible and dynamic, as general relativity tells us, to the idea that there can be quantum fluctuations,
which quantum mechanics tells us, then maybe
space-time itself can fluctuate.
Maybe it can bubble off? So here is a region of completely empty space and
just by the random jigglings of quantum mechanics, we imagine that space itself bends just a little bit, temporarily.
And almost always these bendings would just go away. They would come and they would go away, no big deal.
But occasionally, it would bend a lot. It would pinch off into a little teardrop shape.
There'd be some space here and then a wormhole,
connecting us to a whole other bit of space.
And sometimes that little bit would pinch off and go its own way. This is what we call making a baby universe.
We have our big mommy universe,
through the miracle of quantum fluctuations, giving birth to a baby universe.
So if this is possible, then that little baby universe can start out
small and dense and low entropy because it wasn't the beginning.
It was just a way for the whole universe to increase its entropy, to make one more universe.
But then what that baby universe does is it expands and cools and becomes like our Big Bang.
So we could be a baby universe from a bigger universe
sometime in the past.
And what that means if it's true,
which we have no idea, is that the Big Bang was not the beginning of the universe. When I said the universe was
14 billion years old what I mean is our observable part of the universe is 14 billion years old.
How old is the whole universe? We have no idea.
This idea is saying that if we just had empty space and do nothing to it that empty space is restless.
It doesn't sit there quietly.
It buds off extra little universes, and those universes will eventually grow and cool and become parent universes of their own.
Now if you've been following along if you're you're on your toes right now, you're thinking yourself. Wait a minute,
why is that story different toward the future than the past?
Right? If the fundamental laws of physics are the same moving one direction in time or the other, then if I have an empty universe,
whatever it does toward the future should be the same as what it does toward the past.
And you're right.
So this story leads us to imagine that there is a multiverse that is symmetric in time.
That you have a background universe, the parent
just sitting there. It's empty and occasionally
there's a baby universe, which spits off by a quantum fluctuation,
grows and become a universe all of its own, and in that baby universe there's an arrow of time pointing upward and
This happens an infinite number of times. There's no limit to how often this can happen over the course of eternity.
But it also happens
backwards in time. If you start from the middle and evolve both directions in time,
there's no difference between past and future in the fundamental laws of physics. The same story should be told.
So there are other universes where the arrow of time goes this way.
Those people think that we live in their past.
We think that they live in our past. It's completely symmetric.
The multiverse looks the same in the far far future, and in the far far past.
But almost all of it has the feature that things are changing locally in one direction or another.
So the answer in this picture as to why we see entropy increasing
is because entropy can always increase, just make more universes.
And in any one of those universes, there will be an arrow of time. Now do we know if this is true? Certainly not.
I'd be paid a lot bigger salary if we knew that that was true.
But I think that it is one of the most promising scenarios
for understanding why we see such a difference between past and future even if it's not built into
the fundamental laws of physics.
It's saying that the reason we see a difference is because we see such a tiny little part of the history of the universe.
We're getting a very very non-characteristic picture.
OK? So what we want to do, it's easy to draw pictures like this,
what's hard to do is know whether you're right.
And that requires two different little pieces of work. One piece of work is be better theoretical physicists.
There's a lot of guesses and hopes and crossing your fingers in this picture and we would like to really turn it into a quantitative
rigorous equation-centered picture of reality. And if we were able to do that, then you could move to phase two,
Which is, be good experimental physicists and observational astronomers. Use this kind of scenario to make predictions.
Remember that Cosmic Microwave Background picture we took. That's a remnant of the Big Bang.
If this picture is real, this kind of dynamics should make predictions for what we see in the Cosmic Microwave Background.
We haven't been able to do that yet.
That's the kind of thing that we try to do here at Caltech in our spare time.
Actually, no in a professional time in our spare time. We look at stars in the backyard.
But the overall lesson here is that
Even though there's much we don't know about time. It's an everyday thing, right? We use time every day.
We know what it means. We make use of it. We take it for granted.
But there's a lot that we don't know about it. Once you start thinking about it at a deep level, you're led to these wonderful
mind-boggling speculations about what things might be, and I'm hopeful that a hundred years from now when we come back here to
reunite I'll be able to tell you which of these speculations is actually the right one. Thank you very much.
Alright did everybody have their minds blown?
Okay, so let's let Sean take questions for about five or ten minutes and then I'll make some more announcements and then we'll do some
setup, so
How can something come out of nothing?
There are two answers to that.
Here nothing did come out of nothing. No something's came out of nothing. There was always a something. So this is a scenario--
in cosmology--there's a lot of unanswered questions. The simplest one of which is: is the universe eternal or finite in time?
if you go to less careful cosmology talks,
You might get the impression that the universe is 14 billion years old and there was a beginning, in which case something--
the universe--came out of nothing, in some sense. In my picture, the universe always existed.
It is eternal and infinite was always there. So there's no time when something came out of nothing. Now, there's an equally good question:
Why did the universe exist forever?
Okay. I don't know the answer to that one
I suspect that at some level when you ask these questions. The answer is that's just the way it is.
There is no deeper explanation of the thing.
It's also possible that something can come out of nothing.
It's also possible that our universe
had a beginning: a first moment in time. What we call the Big Bang really was at the beginning.
There, the important thing is that our everyday notion of
how to make something just don't apply to the universe as a whole.
Every time we make something, every time we make a something literally, it is in the context of the pre-existing universe.
The question changes when you apply it to reality as a whole.
What you should be asking is not "why did something come out of nothing," but
"Can the universe have a first moment in time in ways that are compatible with the laws of physics?"
We don't know the answer that one either but we have no reason to believe the answer is no. We have speculations about
what the laws of physics might be that say yes.
You can run the clock
backwards in the universe and you can hit a first page of the book of the universe and you would call that the Big Bang.
We just don't know.
Other questions?
Good so the black holes give off radiation, that's the Hawking radiation named after Stephen Hawking.
So what happens to it? Well?
Space is expanding
all along so individual photons of radiation get stretched, which means they get lower and lower in energy.
Until eventually you just can't detect them anymore.
So when I say that we go to empty space, what I mean is the probability of detecting a particle in any region of space
goes gradually down to zero over time. So space gets emptier and emptier even if there are,
technically there's always a nonzero chance of seeing something, that chance becomes arbitrarily small.
 What is this space?
I Just gave you a whole talk about time and you want me to tell you what space is. I don't think this is fair.
You know space is, well now that you know, what time is space is a lot like time.
It is
the place where things happen, of course, there's a deeper question of
Quantum mechanics and quantum gravity and is space emerging from something more fundamental?
We don't know the answer to any of those questions. At the level of general relativity,
Einstein's theory of gravity and curved space-time,
space-time is a fundamental concept. It is the starting point.
You don't explain it in terms of something else. You say there's a
four-dimensional mathematical structure called space-time and that's where things happen. To different observers,
they would divide it up differently into: this is space; this is time, etc.
It has a geometry which you can calculate and affects things but that is just an axiom. It's not something that comes out of something deeper.
All right
Yes
Ah
Is there possibility after the black holes disappear? The universe could have a second Big Bang?
As a rule of thumb as a professional cosmologists when someone asks the question, is there a possibility the answer is always yes.
There are many things we don't know the answer to.
In some sense, it depends on what you mean by a second Big Bang, you know in some sense, this is, exactly that.
The universe empties out. Like maybe I didn't say this explicitly, but but maybe this is what you're getting at
This parent universe has an infinite number of babies both toward the past and toward the future. Well, what about its children?
Do they have their own babies? Yes, every baby universe has an infinite number of its own babies
So if this is true after our universe expands and empties out,
there will be a certain chance--small, but nonzero--that in empty space
there's a quantum fluctuation that makes a baby universe. So in some sense, that's another Big Bang.
There's a whole other sense of that question,
Which is could the universe which is now expanding and cooling after the Big Bang, could the whole universe re-collapse and crunch?
There are some people who think that that might happen, but those people are crazy.
All right one more question right here yes
Good you're ready for graduate school here at Caltech. This is what we are trying to answer.
It turns out that even empty space has an entropy
Why is that? It's because of
How can I simplify this so without just using the jargon? You know what? I like Jargon--let me use the jargon.
It's because of quantum field theory
Okay
So what does that mean? You know the light
from these light bulbs.   You may have heard, this is electromagnetic waves. Right? At every point in this room,
there's something called an electric field. It has a value.
It's like a little arrow pointing here or whatever and there's a magnetic field and
A field is just something that has a value at every point in space.
Okay, an electromagnetic wave is
when a field, in this case
the electric field and the magnetic field,
oscillates back and forth the electric field bounces up and down the light from these light bulbs comes because there's an electron somewhere
Which has an electric charge and it's being shaken
By the current running through the light bulb and that shaking
Electron has a makes the electric field around it vibrate and we see that as an electromagnetic wave.
Why am I telling you this? Because we now in modern physics, think that
Everything is a weight at some level.
It's not that hard to think that the gravitational field is a field that the electromagnetic field is a field.
But you say well, what about the particles that make up the table? What about the
electrons and the protons or the quarks underneath? Those are also
little vibrating quantum fields. Okay? So rather than the universe being made of empty space
with particles in it. We have empty space
But at every point in space, there's a whole bunch of fields that have their values.
There's the electromagnetic field, the gravitational field, the electron field, the up quark field, the neutrino fields, etc
Okay? At every point in space.
So when I divide up space into this region and the outside,
even if they're, in some sense, is nothing inside that region, even if it's empty space, there are still fields there
doing their little quantum jiggling. And those fields come equipped with an entropy, and what that means is that in fact
counter to all of your intuition the highest entropy state that we know of is empty space.
Yeah, and that's an excellent place to end thank you very much.
Okay, if you are here for the Q&A session then we're gonna get started
So let's just go through our panel and have them introduce ourselves
Okay, so I'm Anders Thygesen. I'm a postdoc here and I know stuff about
star and
chemical chemical evolution of the milky way and like general stuff about the Milky Way and
Yep stuff about telescopes as well. Observing. Yeah
Hi, I'm Astrid. I'm a postdoc here. I do mostly theoretical work and simulations about
Stars mostly in binary systems and things that emit very energetic light like gamma rays and I also work on the intergalactic medium
which is the things between galaxies basically the black part of the sky the things that we don't see I
am Leo Stein I work on Einstein's theory of general relativity and
how to test it by simulating black holes smashing together and the gravitational waves that they make
I'm Lee Rosenthal. I'm a first year grad student and I've become an expert in
galaxies telescopes supernovae and star formation
Okay, so let's just open it up if anybody has any questions
So the gentleman, sorry
So gentleman was asking about quantum space-time
Granularity which and its effect on the propagation of gamma rays, which are very high-energy light. I
Must confess. I have no idea. I don't know if anyone
Panel has thoughts on this
So
The okay, so the Cobb the big caveat about these claims that people make
is that
There's no like precise theory that they're working under the there's like a kind of general
principle but
There isn't actually a detailed theory of quantum gravity
That says you know, here's the parameter
That's the size of the granularity and that tells us how much you know, which
Energies of gamma rays should come earlier in which energies of gamma rays should come later and how much it would pull
Rotate the polarization or anything like that instead. They're working under the principle that
if there is if space-time is quantum mechanical
Then there would be a shortest distance and that the the presence of a shortest distance breaks what's called Lorentz?
invariance
so Lorentz invariance is something that we have in Einstein's special theory of relativity that
Basically says that the that the universe that physics in the universe is the same in every frame and doesn't matter how fast you're going
so that basically
one of the core one of the results of that of Lorentz invariance is that
Light always travels at the same speed no matter what the energy of the light is
So if you have a really distant gamma-ray burst then all of the different
Energy. Gamma rays are gonna get to you at the same time if there's no intervening matter
But so the the principle is that if you break it if you make space-time have some shortest length scale
Then you broke Lorentz invariants
So for some way that people don't actually have a way to calculate the different energies would arrive at different times
The problem is that there isn't actually a details theory that's used to calculate that and in fact
There are some theories like loop quantum gravity
that
Do have a shortest?
Not really a short. Well, yes kind of a shortest length and yet they still
maintain Lorentz invariants, so it's really hard to understand how that's possible and
I don't understand it because I haven't like tried to do calculations in this but you know
I looked at like the reviews that say here are the results
So yeah
There is a shortest length and there is a smallest area and there's a smallest volume
But it doesn't tell you that the you know, like the area has to be like a square
It could be like that same square maybe in another frame
It could be really narrow in one direction and really long in another direction
so so actually for some reason this theory still maintains Lorentz invariants not really clear how
so I I would put a lot of caveats on these types of claims, but those are my my thought is basically
have caveats
That sounds really exciting
So, I guess the questions sorry, let's just say that the question first was
What is given that like recently? Apparently this is exciting there have been discarded
recorded or catalogued discoveries of a bunch of black holes
What are the size distribution of them like how many black holes of us like?
100 solar masses are there and how many are there of one solar mass. I don't know but I think someone else here did
So
We haven't well, I haven't heard of this none of us have so presumably this was
something that wasn't true that
Well in
So there are plenty of black holes in the Milky Way
And so we know of black holes in the Milky Way, so there's two different sort of black holes
We have what we call stellar-mass black holes and we have what we call supermassive black holes
So supermassive black holes. There's typically one per galaxy
So there is one in our own galaxy and so we know the distance because it's at the center of our galaxy
it's
about
Eight kiloparsecs is like twenty four thousand light years away
Something like that, and then we have stellar so that one is about more than a million times the mass of the Sun
So it's really really big and then we have stellar-mass black holes
Those are much more common. So we couldn't future definitely have catalogs maybe of thousands of these with I think with microlensing
That's what they're going for
So maybe you have heard of something that is
Proposing to make something like this because it's a plan to have a catalog of a lot of these black holes
and so these have typically a mass of five times the mass of the Sun tend the mass of the Sun and
They can get much closer to us. There is I think the closest one is a few kiloparsec
So it's closer to us than the one at the center so that yeah that but you still need a few thousand light years
Well black holes are like
That so yeah
So the gentleman asked if black holes were wandering in space or if they were at a given location?
How how do they move around there? That's what I understand
black holes are like
Anything that has a mass in a sense that it just reacts to whatever gravity is around it. So
There is very strong gravity if there is a lot of mass so at the center of our galaxy
There's a lot of mass. So the black hole that sits there is just keep sitting there because that's where it's attracted
but if you have a black hole with a star around it it will typically
Turn around the star. So black holes. Just react to whatever is around them
So they don't really want it but they they move just like stars
They they obey gravity the same as every other mass
So in the sense of like obeying the laws of gravity or not
I think like yeah just like I should said like there's nothing different about how a black hole reacts to gravity
versus a star
They're they're both masses. Yeah
Yeah, and as well just so maybe to like
Elaborate a little bit on that. I mean like one of the like the very I
Guess a very common misconception about like hoses that they like kind of like access giant vacuum cleaners and like suck things into them
It's like they don't really do that. I mean if you if you're sitting outside a black hole
You you kind of like move around it. Like we're moving around the Sun. I mean, it's it's the same
It's the same kind of kind of laws that governs how stuff move
outside of a black hole like with with the difference that
from a black hole like the
gravitational field is so strong that
that if I'm pretty close to a black hole like the
Gravitational pull that my feet would feel and my head would feel would be very very different
that would probably be rather unpleasant and but aside from that like things would things would move and
Pretty much as we were at we as say a planet orbiting a star
Yeah, exactly
and of course like the
there's like if you like once you have matter that like crosses the swastika radios of them off the black hole and then there's no
Going back then stuff ends up
In the black in the center of a black hole. Do you think I mean, we don't really know what's happening inside us
Watch your radios, but stuff is episode not coming back out as far as I'm aware
Okay, so she was asking what the lifecycle of a black hole is
so
First stellar-mass black holes. So the smaller ones we
We know that they come from the remnants of massive stars
So they're basically dead bodies of really massive star when the start has
Burned all its fuel it explodes
And the thing you have left is a black hole
So our Sun is too small to make a black hole that will not happen
But if it were much bigger, it wouldn't make a black hole
Now for the really massive ones
We actually don't really know how we make them
Because
They're so big and we
We don't know
We know they can only grow at a certain rate like it
Basically if you feed them gas
they get bigger and bigger and that's how they grow but it's hard to understand how certain black holes can be a billion times the
mass of the Sun and
We don't really understand what has been feeding them. So for the really massive ones we
Don't know where I don't there's theories but I don't really know
That okay so that that's an active field of research where like so we know how small black holes are born
we don't yet know how
giant supermassive black holes are born and
in Einstein's
classic theory of general relativity this this one and
What's in this textbook?
Once you form a black hole then it then nothing really happens to it it could merge
it could eat more stuff and it could merge with other black holes and
Get bigger, but it could never get smaller in classical
Einstein theory of general relativity
but when you add quantum mechanics to the theory
Then it turns out that black holes can actually die
If you have a black hole, that's just sitting there in space with nothing feeding it no gas no electromagnetic
Radiation feeding it if it's just sitting in empty space
then when you add quantum mechanics, it turns out that if you send in
nothing
Then for some reason vacuum fluctuations, which are words that mean certain calculations
Make particles come out
so it kind of takes a lot of
graduate education
Calculations to show why that is so I don't really have a very good explanation for why
Black holes that are just sitting there end up emitting radiation
But according to this calculation from Stephen Hawking they do and it's been done on a lot of different ways. So
It then turns out that how much radiation they're giving off depends on how big they are
So really really big black holes actually make radiation whose wavelength is also really big so that turns out that the temperature is really small
so
Big black holes are really cold and small black holes are really hot
So if you have a black hole, that's just sitting there then it gives off radiation
And then a shrinks and it gets hotter and it gives off radiation
Faster and it shrinks more and it gets even hotter and that thing runs away until it becomes something
That's so small that you would really need a theory of quantum gravity to describe it
But we don't have that theory so we don't really know about the deaths of black holes either
So in so in principle the tiny black holes the at the
The their size when you need quantum gravity would be when they get to the order of the Planck length
Which is around 10 to the minus 33
centimeters
So anyway, they would have a Planck mass which is around 10 to the minus 5 grams, which is about the weight of a mosquito
So that's actually really really really really huge in mass compared to what we think of as
wimps which are one of the candidates for dark matter particles, so
Stable so so one possibility for the the deaths of black holes is that they don't really die that they make something called a stable
Remnant and they just end up sitting there as this thing that has the mass. That's about a couple of Planck masses
That's one possibility people don't really know but that would be too heavy to be a a wimp
because it would be
It would be around
Ten to the fifteen times the masses that we think that wimps are which is what a sense of the fifteen. Is that like
Okay million billion
Trillion quadrillion quadrillion times the mass. Okay. So anyway, it's it would be big
Somebody anybody who hasn't asked any questions
Okay, so the gentleman's question was basically
is gravity
weaker at
Long distances and stronger at short distances. Is that right?
Right
Yeah, well I'm the answer is we don't really know how to describe
Gravity at the Planck scale. That's one of the one of the like big puzzles in
high energy physics nowadays
No, we don't so that's not something that we can probe with particle accelerators
the the highest energies that we can probe in particle accelerators is
Around 14 tera electron volts at the Large Hadron Collider and the
The Planck scale is at 10 to the
19 electron volts which is
10 to the 10 tera electron volt. Oh no, giggle Ektron volts. Yeah
So anyway, it's it's way out of the the realm of possibilities at at the Large Hadron Collider
So not
Sorry, the question was do you still use Newton's law so you don't use Newton's laws and you don't use Einstein's theory of general relativity
Really? There is no theory that we have at that level. There's a bunch of speculation
There's speculation. Yeah speculations like loop quantum gravity or string theory
But basically it's it's all kind of open-ended and people are trying to figure out what things could we calculate?
That we would actually be able to measure and see if those things are true or not
And so far people have been really hard-pressed to find actual
Predictions that we we have access to it's just that such an unfathomably high energy
So if you want to know more about what what Shawn was talking about today
Sorry, the question was do you have any book recommendations for what since inspired what we're thinking about today are just things that are interesting
in science
Shawn wrote a great book a couple years ago
Either called from here to eternity from eternity to here. One of them is a movie and the other one is this book
But it's about it got a lot of it goes over this talk it was
partially a
Summary of it, but he talks about the directionality of the time and goes into detail about it
I really like it because
Sean's a physicist that also is a philosopher and he really likes to think about these ideas deeply and that's good
Jan 11 is a great science writer who wrote a book about about LIGO that came out right around
Gravitational waves were detected called black hole Blues
I haven't read it yet, but I just gave it as a gift because it's supposed to be a great book
When I was in high school, I think I read mishio kaku's book called hyperspace
I
Don't remember
Any other books that that that one got me really interested in you know?
Fourth dimension and thinking about what is space-time made of and does time travel possible stuff like that
So I can't remember any of the books along the lines of what Sean described but a couple of days ago
Actually, we were having dinner with friends and one of them said okay, I just was reading this book by Neil deGrasse Tyson
It's something like astrophysics for people in a hurry or something and that apparently is a hit
But it's on my to-do to read list
There's another if you're interested in the more physics these side of things like string theory and
These things that Leo was talking about Brian Greene is a writer who's written a bunch of great books
The elegant universe is one that I read in high school
that's out of date at this point, but I mean
hmm
He's written a bunch that are all basically about string theory, but that's a really good one. Yeah
That if you if you go looking though
there'll be a lot of books written by these people and public science is always a great thing to read I
Remembered another one. I think I read Stephen Hawking's the universe in a nutshell. Is that Oh a brief history of time
Yeah, a brief writ. Yeah, okay
This is this is Sean Carroll's textbook if you want to learn general relativity, it's a good one
Yeah
We could we could turn this into like a little book reading if you want
Consider the three killing vector fields of the two spheres show that their commutator satisfy the following algebra
Okay, yeah, so the question was where does one find a wormhole and is it the same thing as an einstein-rosen bridge so
Mainly you find it in physics papers
Probably not so much in the real universe
and the
Reason is that in in people's calculations?
Every time they try to say what would happen
If you could build one of these things it turns out that to build it you would need
Stuff that we have never observed you would need something whose energy density is negative and we don't know what that means
We've never observed anything like that and it seems that it's always unstable
so if you could build it then if you poked it it would collapse and
The second one the second question. Yes, an einstein-rosen bridge is a model of a wormhole
It's one type and there are a bunch of different types of models that you could write down
But they're all basically like what if space-time was shaped like that
what would it mean and then you figure out what kind of matter you need to do that and
Now that was one of the early ones
So she's asking if there is a current explanation for the mass gap between your turn stars and black holes
Okay, so what the
so neutron stars are the leftovers of stars from
intermediate-mass and black holes are the leftovers of stars are really of a bigger mass and
So from what we've been able to measure so far the neutron stars
we know of are typically decide the mass of one time's the Sun or twice the Sun typically and
It doesn't really go above two point two times the mass of the Sun something like that and the black holes must we've been able
To measure the lowest mass we've been able to measure I think it's five times the mass of the Sun and so there's really nothing
between two point two and five and so that's what we call the mass gap and
so
We we're not foolish so
There's two things. We're not fully sure that the mass is really gap is really through because
Maybe there are observational
biases that we have that make it easier for us to detect
Black holes that are a little bit more massive and maybe there are three times the mass of the Sun black holes
but they're just really hard for us to see and
so what may be LIGO will
Enable us to detect black holes that are three times the mass of the Sun. So maybe the
Mass gap doesn't exist. We we don't know
but
Assuming the mass gap exists we have models that explain
Stellar evolution and they at least we can make them in such a way that they make a mass cap
presumably if we found the black hole that of two times the mass of the Sun we can change the models and make them make
Two times solar mass. Black holes, don't worry
We'll we can make sure that the models work but the models we have currently is just because the neutron stars and the black holes
Come from different initial stars. This way we can actually create a mass gap basically
But we would not enter press ensure that the mass gap exists
And
actually one of and I guess one of the one of the reasons I guess that
To go a little bit like this
We might have an observational bias is that if you have a black hole it just like sits there
We can't really see it because it doesn't meet any light so they're kind of hard to
They really have to observe so correct me if I'm wrong, but like most of the stellar-mass black holes, we see or x-ray sources
Right. Yeah
and
I'm actually currently involved in a project where we are searching for
Stellar max mass black holes that are not emitting x-rays at all. So so these
Like stellar-mass black holes that we wouldn't be able to see next race that are not really doing anything that are just kind of sitting
there
At this point we have around
20 some candidates
We were still getting observations and we hope that some of these
candidates will turn out to actually be stellar mass black holes and
So the so the way we the way we do this is actually it's it. It's kind of cool. So we're basically we're looking at
Pulsating stars, so some stars have like
Light variations so and some of them pulsates really really regularly. So like
every
say every like either they have a pulsation period of like maybe maybe 30 minutes so like then
So were they extremely good clocks?
and if we measure like the
the arrival time of this light as it comes toward us then if this star is
Moving around in an orbit and it's moving around something then the distance that the light actually has to travel
to
arrive at our service area changed us a little bit and
And that mean we can actually see if this really really stable clock is moving around
something else that we don't see and
We can get an idea of the mass of this object
So we are currently trying to find stellar mass black holes that I'm not really doing anything
By looking at something that's moving around the black hole and but it's it's active research in in progress. So and hopefully in like
Hopefully like a year time. We should be able to have some results represent on that
Okay, so the the question is what would happen if we were to replace the Sun with a black with a black hole and
so
Yeah, didn't do it. Yeah, that's a--fun on you. You can get that one me
This is an awesome question
So there are a couple ways to answer this
But start by saying that the black hole that you swapped swapped out for the Sun is the same
The same mass as the Sun. It's one solar mass
So in the answer is what happen to the earth?
for the first eight minutes
is nothing because a
The force of gravity that the earth feels from from that black hole is going to be the same since it's the same mass
I don't think that the the earth would be far enough away from the black hole that the weirder
General relativity effects of gravity wouldn't be important. So it would basically still be just a point source
so it orbit at the same speed and the reason I said eight minutes is
Because the earth is at a circular orbit around the Sun and it's eight light minutes away
which means that the distance between
The Earth and the Sun which no judgment. I don't remember what that is
They don't know either
50 million 50 million kilometers 150 million kilometers, so eight light minutes means that it it takes light
When the you know, the Sun is putting out light
it takes the photons from the Sun eight minutes to reach the earth and a very cool thing about gravity is
The force of gravity travels at the speed of light Leo could answer this better than me, but they're making me answer
so the reason is that
Like the speed of light is sort of like a universal speed limit
No particles can travel faster than that and in theory and tell me if I'm wrong
Gravity is the force of gravity is like the fact that the fact that we're all do sitting here and not flying off the face
and all that is
transmitted by particles by gravitons
Like which are basically waves in space-time and they're also massless like light so they move at the speed of light
so when so the tug that that the earth feels
From the Sun is actually eight minutes old
so when you
Swap wait, I just realized that's not important. But it the same thing is true of the photo
So the same thing is true the photons coming from the Sun
So the first eight minutes when you swap them out
You would still see the Sun because all those photons are still coming and the photons that were made at eight minutes ago
Are only just arriving now
But then after eight minutes suddenly the Sun is gonna disappear and then there are other ways fun ways
You could answer this fun ways. You can answer this question. Like what happens when the Sun goes out
Well solar power is dead in the water for one thing
But I mean
All the processes that that I create life would eventually stop
I mean because plant based life with this is gonna get sad but
But
but you know plant based life would would would die because you can't have
Photosynthesis without light and unless we went about building a whole bunch of fluorescent lights, really really quickly
You couldn't couldn't really fix that and also, where would you get the power for those forests and lights, you know
I'd start burning a lot more coal there. I
Think but I know there are a lot of other things you could say it, but I think one of the fun fun
Things about this question is that when you know when you're physicists and you're presented with a problem
There are a whole bunch of routes that you can go down and answer different facets of the question
so I guess the last thing I would say is if you want to see a lot more questions like that and
Answers to them. There's a great blog called them. What if if you look up what if question mark
Online and then there's gonna be weird the letters X K C D after it
There's this writer who used to be a roboticist at NASA who answers questions like this
So the first one he answered was
What happens if someone pitched a baseball?
you at the speed of light and he wrote this hole and you're all this all I say explaining what would happen cuz there actually
Is an answer
But I mean these questions are really fun. So so you should you should check that out. And yeah, good question
So many cool things
The question was what cool things. Can you tell us about the total solar eclipse that's coming I
can say some stuff at first and if I don't think of anything you guys should should say something but
So this is the first one in almost
This is the first total solar eclipse in almost 100 years across
Across North America and what total solar eclipse means is that they're going there's going to be like a path a spot
moving across earth
where if you are standing there at a certain time you will see the
moon pass
100% in front of the Sun and what's really cool about this is purely through chance
The angular size of the moon on the sky is like
Within a percent of a percent of a percent almost exactly the same as the Sun
so
When you're in the path of totality, which is you know, that that path across the u.s
Where if you stand there for a couple of minutes, you'll see the Sun pass, right?
You'll you'll see the earth pass
You'll see the moon pass right from the Sun if you if you saw the Sun pass in front of the moon
That would be a different story. That's a problem
But what you'll see is you'll be able to see the corona of the Sun
so the corona is this part of the Sun where you have like the photosphere which is sort of the
The outer the outermost edge of like the Sun as a ball of gas because the Sun is a big spherical ball of gas
that's just held together by its own gravity and
supported by the pressure of fusion
But past the photosphere you have this wild crazy atmosphere basically like I miss fear called
what I
The corona thank you. Um, I called the corona and and it it's like this beautiful shape and it's chaotic
But you can't see it
During the day because these like the Sun itself they're you know behind the photosphere
Is many my orders of magnitude brighter and your eye, you know has contrast so when you're presented with an image
That's a thousand times bright brighter than something next to it. You don't see the fainter thing. But when the Sun
when the moon passes in front of the Sun
Since they're almost exactly the same size
The
The moon totally blocks out like the the photosphere part of the Sun and you can actually for a couple minutes see the corona
And if the moon were even a little bit bigger
you won't be able to see it because it would just
cover the corona and if it were even a little bit smaller you and be able to see it because even just the edge of
The photosphere would be too bright to see the edge
But with all that being said even if you're not in the path of totality which you can look up matte maps
like like go to
Nasa.gov and like look for a map of where totality is gonna be like Oregon is the closest place you can go
It's it's so really cool because
You get to see something passed in front in front of the Sun. But if you do very important, there's a reason that we were
Advertising buying Eclipse classes, it's not just because we want to make a buck off of each of you. It's because
Like if you look at the Sun if if you look at the Sun for too long
At all, you can do serious damage to your eyes
So like I think on Amazon you can buy a 10 pack of these things for five dollars or something
But there are eclipse paper paper glasses. You can buy that have
Filters that will that will make it safe for you to look at the Sun
So
Let me add my cool thing about this, which is that
so the strip of
totality that's only sixty miles wide and
if you're in the strip of totality, I am told I've never been in a total eclipse, but I will be
it's supposed to actually get cooler and you're supposed to actually be able to feel the air get cooler because
you don't have the heat from the Sun warming up the air and
Other fun facts, is that the Earth's this the moon's orbit around the Earth is not a perfect circle
So actually the moon sometimes is a little closer to us and sometimes a little farther away
Even when eclipses happen so sometimes you get an alignment
but the moon is a little closer or a little farther away and if it's a little closer than it looks a little bigger and
If it's a little farther away, it looks a little smaller. So sometimes you get these eclipses that are called
annular eclipses
where the size of the disk of the moon is a little bit smaller than the size of the disk of the photosphere of the
Sun so you end up with kind of like a ring an annulus of it and it looks like
You know a flaming ring where it's dark in the middle
and it's bright on the outside and that would be really cool to see I
Don't have anything. Sorry
I've already seen one in my life
Yeah in Europe
in Europe when I was a kid when I was
12 or something there was one so I'm not driving up to, Oregon
If you're not going to be in the path of totality in August there's another one in seven years on in the US
Which goes the other way it goes from from the from the west most part of Mexico to?
Through upstate New York, so there will be another opportunity
soon I
Guess another cool thing
and is that if when you're in the in the path to Sally you should ask the
shadow of the moon passes and passes
In front of you you should also be able to pick out some of the brightest stars in the sky during the day
That's pretty cool. And I think that's a cool thing. You can actually see stars during the day. Oh
Very cool
Yeah
So so you you'll be able to you'll be able to spot stuff and in the in the sky that you and that
You normally wouldn't see
And and I guess another fun thing
about these eclipses is also that
That the like the orbit of the moon around the earth
is tilted a little bit with the respect to the
orbit of
The earth around the Sun so the the moon is not orbiting in the same plane as the earth is around the Sun
because it and if it was we would have
we would have
Total solar eclipse every month like every time new moon pass around the Sun but because there it's a little a slight angle
That's why this doesn't have them all the time while it's kind of
Kind of rare went because like the alignment has to be just right and that's just that that doesn't happen all the time
So that's like a little
Cool detail as well. Oh
Yeah, also yes important ditional information and we will actually be hosting an eclipse viewing event here at Caltech and
Over on the on the Beckman lon over on on Wilson in front of the Beckman Institute
Will be will be bringing out a couple of a couple of telescopes with some solar filters and some projectors
so and so people can just
Drop by there and then check out the Eclipse scope and we'll start at around
9:30 a.m. And then go on for a couple of hours and we'll be
a good handful of astronomers there, too to
Look at the look at the Eclipse and answer questions about the Sun
We only have like five minutes left before
Security is going to lock the doors over there
So let's let's take like one more question or maybe two if they're really quick in the back
So that's a great question the question was expanding on that other great question before about
what would happen if you swapped the
The the Sun out with a black hole
Let's not pretend instead that you put a five solar mass black hole there and that the earth instead of staying in an orbit
Fell in what would it feel like to be a person?
On earth passing through the event horizon, which is the border of a black hole
within which nothing can escape no information can pass from inside the event horizon to out only stuff in
So the answer is it wouldn't feel like anything?
Wait, wait. Wait, no Leo's gonna say wrong, but before before he does
It's wait. It's because it's it's still massive enough that you don't feel significant. No, never mind. Leo's gonna answer this question
Okay. So well Lee was going to say would be true for a supermassive black hole
so this the the tide which is, you know, the tide is the difference between
How much you feel gravity at one side?
versus the other side right the different
the reason that the earth has tides is because
You know one side of the earth is farther away from the moon than the other side
So there's a difference in how strongly the moon is pulling on water and stuff
so
The same thing is true of like a human body the human body can feel a tide if we're close to something like a black
hole so
the strength of the tide depends on
How far away you are but also how big that object is, but at at the event horizon some things cancel out
so it turns out that at the event horizon of a black hole the tide just
depends on the mass of the black hole and it depends on
one over the mass squared
Which means that for smaller black holes you get a bigger tide and for bigger black holes you get a smaller tide, so
For a supermassive black hole you might be able to fall through the event horizon
Without realizing it in the sense of you didn't get ripped apart
you'd realize it because you would see that like
Suddenly the rest of the universe looked like a shrank into like half of the sky and then into like a little disk
But that's that's a separate issue you you didn't get torn apart right you get torn apart like an hour later
For a stellar-mass black hole for for a black hole that's about the size that's about the mass of the Sun a
One solar mass. Black hole it takes about a microsecond to go from you know, the nearest orbit to
inside the event horizon so
that should give you a sense of like how
Quickly you get torn apart you get torn up
So so I guess you're kind of right that you wouldn't feel anything because you get torn apart too fast to feel anything
So yeah, you're right. You're right. You don't feel anything
So, I think that that's all the time we have today so thanks everybody for coming come next month
