>> Amy Draves: Thank you so much for coming. My name is Amy Draves and I'm pleased to
welcome Jeff Bennett to the Microsoft Research Visiting Speaker Series. He's here to discuss
his book What Is Relatively, in which Einstein's ideas are presented so that anyone can grasp
the basics. Jeff has worked at NASA headquarters creating research and education programs
for the Hubble space telescope and other NASA missions. He has taught at every level from
preschool through graduate school and has authored several books including award-winning
children's books about Max the dog, you can actually look at those in the back, college
textbooks and three other books for the general public including Math for Life. Please join me
in giving him a very warm welcome. [applause].
>> Jeffrey Bennett: Thank you very much. It's a real honor to be here. This is actually my first
corporate event ever. I talk at universities and elementary schools a lot, but I've never come
and talk at a corporation before and it's especially nice to come to Microsoft since pretty much
every word I've ever written has been done in your software. And my presentation is in your
software. Do we have anyone here that works on WorldWide Telescope? Not in this crowd
right now, but I have a college textbook in astronomy and we use WorldWide Telescope as part
of the online package that's available to students for that, so a lot of connections that I'm very
excited about. I also have a brother-in-law who works for Microsoft, but not here in Seattle.
What we're going to do today is I'm going to talk to you about this question. What is relativity?
I realize you are probably a fairly sophisticated audience and a lot of you probably know quite a
bit about relativity already, but I'm going to pretend that you don't and we're just going to take
it the way that somebody learning about it the first time might hear about it. The reason that
this is a question that draws a lot of attention is because it's a rare thing in science. Everyone
has heard of relativity and Einstein, and yet very few people know anything about what it is or
what he did, and that makes people curious to know what is it. What I'm going to try to show
you today is that you don't have to be particularly sophisticated with science and math to get
the basic ideas. In fact, I think everyone ought to be exposed to the basic ideas. If it were up to
me, this would be a standard part of the curriculum starting in about third grade and continuing
on all the way up the line. And I'll tell you why, because you'll see these subtitles here and why
they matter. Towards the end I'll give you my reasons why I think this is such an important
topic that everyone ought to be learning something about it. The short answer to the question,
what is relativity, is that it is our modern scientific theory of space, time and gravity. Think
about that for a moment. Space, time and gravity, and you realize that's kind of everything.
This really is at the heart of all modern science. Some of you may know and some of you may
not know how deeply it is integrated into all the electronics that we use, and I'll talk a little bit
about that. There's a reason why I'm here now besides the fact that I wrote the book which is
tied to this reason, which is that this is a special year. This is the 100th anniversary of Einstein's
publication of his general theory of relativity. As a result of that there are events going on the
world. The United Nations actually dedicated this year as the International Year of Light
because light is a critical part of relativity. So I thought what can I do for my part of these
International Year of Light activities and I decided that since I had a book and I like to talk to
people I would just go out and talk to people. So I sent out e-mails to people that I knew and
said would you like to have me come and talk? I will even pay my own way and I have had a fair
number of takers here. You might notice that I don't have anything lined up for next fall, so if
you know people in other places who would like to have me come talk, have them send me an
e-mail and I will be happy to do that. They can send it to my e-mail down there
Jeff@bigkidscience.com. In addition to this being the hundredth anniversary of relativity and
making it a good time to talk about relativity, it's also been on people's minds for another
reason. How many of you have seen this movie Interstellar? Good. The movie Interstellar is
actually pretty much about relativity. Kip Thorne Caltech physicists who is one of the world's
leading experts on relativity was an executive producer on this movie, so it's got a lot of good
science and it. And if you want to know which part of science is real and which part is
speculative and which parts are not real, I did write up a little blog post on that that you can
find on my webpage on my blog, so I encourage you to look at that. It doesn't have any
spoilers, so if you haven't seen the movie yet, it's actually a good idea to read the post first so
that you understand some of the science things that are coming out there. I'm going to try to
do now with the rest of our time is go really fast through a little bit more of what relativity
actually is and what it means. I'm going to go really fast because I usually do this talk in about
an hour but I'm going to do it in 30 minutes for you today, so we're going to blast our way
through. One of the key ideas that comes out of relativity and one of the most famous parts of
relativity are these things called black holes. Everybody has heard of black holes. I like to start
talking about relativity with a multiple-choice question. Imagined that the sun magically
collapsed into a black hole retaining the same mass. What would happen to earth? You can
see your options on the screen. I won't read them off. You don't have to commit to an option,
but you probably are aware that if you ask the average person this question, including the
average kindergartner, they will all tell you confidently that Earth would be sucked in to the
black hole. I want to analyze that and see if that is or is not the correct answer, and the way we
can do it is by thinking about gravity. I told you that Einstein's theory of relativity is a theory of
gravity. But it wasn't our first theory of gravity. We had a theory of gravity before that that
came from Newton. You might be thinking why did Einstein create a new theory of gravity.
The reason is because Newton's theory of gravity, it works really, really well. It explains why
things fall to the ground. It explains why the planets orbit the sun. We can use Newton's
theory of gravity and nothing else to send a spaceship off and land on other planets. That's
pretty amazing. It works great, but it turns out that there are a few places, particularly when
the gravity is really, really strong where it starts to break down and so we needed a broader
theory that would put all that together and include those other areas. But out here where the
earth orbits the sun, we are pretty far from the sun and gravity is not that particularly strong
and so there is nothing wrong with Newton's theory of gravity, so we don't even need relativity
to answer this question. We can just use Newton's theory of gravity. Newton's theory of
gravity tells us how objects orbit under the influence of gravity. Generally, when you think of
orbit you think of ellipses like this to go round and round and round. Technically, an orbit is
anything governed by gravity and it's also possible to have orbits where things come in from
afar, go back out and return again and those can be in the shapes of parabolas and hyperbolas.
This is what Newton's theory of gravity tells us about orbits. You will notice that sucking is not
on the list of orbits which brings us to an important key point about black holes. They don't
suck. In fact, they are really kind of amazing. If you read the book, which I know a lot of you
have now, in the first chapter I take you on an imaginary voyage to a black hole where you see
the effects of relativity as they would happen to you on your journey and when you get there
and what you observe and so on. Today I just want to focus on what is relativity, so I won't
have time to answer that question, or to go into all of that. But I do very briefly want to
mention the movie Interstellar again. Again, not a spoiler here. In the movie Interstellar they
actually did a nice job of this. They go to this super massive black hole in another galaxy, and
what do they find there? They find planets that are orbiting the black hole. They're not getting
sucked in. They've done it correctly. The same thing with the spaceship, they are in their
spaceship. They orbit around and they don't get sucked in. The only place in the movie where
they make a minor error is when the Matthew McConaughey character, I forget what he's
called, when he goes diving into the black hole one of the other people says something about
he got sucked in. That is not really true and the way you can understand why it's not true that
he got sucked in is if you imagine that you were in a kayak and you are paddling down a river
and that happens to be a waterfall ahead of you and you see it and then you paddle harder and
go for it and go over. Don't blame it on the pool at the bottom. It didn't suck you down. It was
your own fault for paddling to the waterfall. Same thing here. If you get in a spaceship and you
dive into the black hole, you're going in, not because you got sucked in. You chose it. So what
is relativity? We've got a little bit of an intro there are that is going to tell us something about
black holes besides the part that we already have learned is that they don't suck. Relativity
actually has two parts. We usually say the theory of relativity, but it was actually published in
two parts by Einstein. The first was in 1905, his special theory of relativity, 1915, our 100th
anniversary year, general relativity. Why? Why did he publish the theory into parts? There are
a couple of reasons, but one reason is that this first part special, and by special we mean the
special case in which we ignore gravity, was easier than to include gravity. Mathematically, it's
much, much easier. More importantly, he was actually trying to solve some fundamental
questions in physics that had already come up with the laws of electricity and magnetism. This
paper on special relativity was actually called on the electrodynamics of moving bodies. He was
solving these known problems in physics. When he went on to general relativity, he was doing
something a little bit different. Historically it's interesting because historians of science
generally will tell you if Einstein had not published special relativity when he did in June of
1905, it's very likely that someone else would have published basically the same thing the same
year. People were so close to it because it was an idea whose time had come. They needed to
understand these existing problems in physics. General relativity, Einstein really went out
ahead of everybody. A lot of historians will tell you that if he had not published general
relativity in 1915, and might have been decades before anyone else into the same idea as.
What was different In the way, Einstein thought about it? The difference was even though he
was solving these known problems in physics he was thinking about it in a little bit different
way. Going back to when he was a teenager he said that when he was 15 or 16 years old he
used to try to imagine what would it be like to ride on a beam of light. We started to think
about that hard he started to come across all these paradoxes, things that weren't quite making
sense. So he was trying to make sense of them, and special relativity did make sense of most of
them, but not quite all of them and, therefore, even though the other physicists were like hey.
You solved all the problems in physics, he needed to keep going to resolve the rest of the
paradoxes. The words theory of relativity have two unfamiliar words in them for most people,
theory and relativity, so I want to explain those briefly. Theory, in everyday life people think it
means kind of a guess, but in science it means something that has been very well established
and tested. In fact, I would tell you that a scientific theory has been so well tested that really it
can't be wrong. It could be incomplete, not the whole story, but take Newton's theory of
gravity, for example, it works really well. We can land spaceships on other planets with it. It's
not wrong. It's just not the whole story. Einstein's theory broadens that story. And it's very
likely that in the future we the have another theory of gravity that broadens it further than
what Einstein's does today. But Einstein's theory will still be right in the sense that it works
really, really well. The second part of the term theory of relativity is the word relativity. A lot of
people have this misconception that Einstein said everything is relative. He did not say that.
The word relativity is talking very specifically about relativity of motion. From relativity of
motion we will derive as we'll see some consequences to relativity of time and space, but in
motion is the key. What do we mean by the idea that motion is relative? What we mean is that
you can see things moving and say how they're moving but you'll get different answers
depending on how you're looking at it. I'll give you a simple example. Imagine that you get into
an airplane in Nairobi Kenya. You take off and you fly at a speed of 1670 km/h, because I like
the speed and you fly and leave in Quito Ecuador. How fast did you go? You're thinking you
just told us. You're going 1670 km/h and yes, you are. But what if you are you watching from
let's say the moon, like it would kind of look to you in this picture? From the moon, you would
be looking at the earth and remember the earth rotates and the earth actually, I picked this on
purpose. The earth rotates in the opposite direction that you're flying and it rotates at exactly
the same speed that you are flying. Therefore, what would you see as you watched this plane
trip from the moon? You would see the airplane lift off the ground in Nairobi and just sit there
while earth rotated underneath of it, and when Quito arrived you would set down. According
to you from the moment, the airplane goes the speed of zero. It goes nowhere the earth
rotates underneath of it. Which answer is correct? Did you go 1670 kilometers an hour, or did
you go zero? Both are equally correct as long as you specify what you're measuring it relative
to. If you don't say what you are measuring relative to then it's not going to have any meaning.
It's relativity of motion that gives the theory its name. In fact, the real heart of the theory
comes not from the relativity of motion, but from two things that Einstein said in 1905 paper
are actually absolute in nature. In a lot of ways they would be a lot less public confusion if the
theory had been called the theory of the two absolutes rather than theory of relativity. What
are those two absolutes? The first one is that the laws of nature are the same for everyone.
You will measure the same laws of nature, discover the same laws of nature here in Seattle as
we will where I live in Boulder Colorado or someone on another planet or on another star. The
laws of nature are the same for everyone. In fact, this is not new at the time Einstein asserted
this. It was generally assumed since the time of Galileo. Part of the reason he had to assert it is
going back to remember I told you it had to do with electricity and magnetism. Those laws
seemed like when you played with the equations they changed and we different depending on
how people were moving and part of what Einstein did with special relativity was to restore
Galileo's idea that yes, indeed, no matter how you're moving the laws of nature are the same.
But other than that little caveat this is not a surprise and most people accept it pretty easily.
It's the second absolute that is surprising. The second absolute tells us that the speed of light is
the same for everyone. Why is this surprising? Imagine that I'm in an airplane right now and
I'm flying by you at 500 mi./h. I have a ball in my hand and I tossed the ball to a friend a few
rows up at a speed of 10 mi./h. What will you say from the ground the ball was doing? Before I
even throw it you say the ball is going 500 mi./h. I throw it at 10 and now it's going 510 mi./h
as you see it. It makes perfect sense. But what if instead I have a flashlight in my hand? I turn
on the light. I would measure the beam of light going out in front of me at the speed of light,
300,000 km/s. You expect that on the ground you would say the speed of light plus the 500
mi./h for the airplane, but Einstein said no. You will still see the exact same speed of light. Why
would he say something like that? For him, it goes back to that vision of riding on a light beam.
He recognized, and I don't have time to go through it, but in the book I lead you through the
thought experiments that show you why. Actually, the world doesn't make any sense unless
this is the case, that the speed of light is the same for everyone. But more importantly, in
science we like things to make sense, but they don't always, but what's more important is
experimental evidence. Experimental evidence demonstrates that the speed of light really is
the same for everyone. It is not affected by motion. The first demonstration of that was
actually done a couple of decades before Einstein published his theory. The Michelson Morley
experiment. They just didn't quite realize the implications of what they were seeing. Einstein
realized it. But it's been measured since many, many other times and, in fact, if you just think
about telescopes, even though we don't do this routinely, we have like entering hours
telescopes from stars and galaxies. They're all moving relative to us. Some of them distant
galaxies are moving away from us at very high speeds, close to the speed of light, and yet if you
take the light entering any telescope and measured speed as it comes in no matter what galaxy
or start coming from you always find the same exact speed of light. Is an experimentally
measured fact. Is from this surprising fact that you can derive all of the basic consequences of
special relativity just with some simple thought experiments. I don't have time to go through all
of them, but I want to do one for you and I'll pick the one that is everyone's least favorite
consequence of relativity. You've probably seen shirts like this right 300,000 km/s, not just a
good idea, it's the law. You cannot go faster than this. Why is this everyone's least favorite
consequence of relativity? Because we are human and we don't like being told what we can
and cannot do. We want to go faster than the speed of light. I'm going to let you for the
moment imagine that there is no limit on speed. You can do anything you want as long as you
are consistent with Einstein's two absolutes. Imagine I'm in the spaceship and I have a
spaceship that can go as fast as anyone can imagine possible. You might think that would be
any speed, but imagination might have limits. Let's see. I get into my spaceship and I go really,
really fast, very close to the speed of light. Then I put it into second gear and I go even faster
and third gear and boom I am going so fast you can barely imagine it. I better not be on the
earth because if I'm going anywhere as near the speed of light I would crash into something
really fast. So I have to be out in space. In space it's dark. I don't want to crash into anything
so I better be able to see where I'm going, so I better have headlights on my spaceship. If you
don't like the headlights, you can just go with the infrared radiation that my body emits
anyway, but we'll use the headlights. I've got headlights of it I can see where I'm going. I look
at my headlight beams and they are going out ahead of me how fast? The speed of light,
300,000 km/s. You're watching me from the ground. What do you see? You see me going as
fast as you can imagine possible with my headlights shining out in front of me. How fast on my
headlights going according to view? The same speed of light. And what are my headlights
doing? They are going out in front of me. They are beating me. Therefore I am going less than
the speed of light. You cannot get around this logic. I know some of you are trying. You can't
do it. You cannot. If the speed of light really is the same for everyone then there is not a logical
possibility of getting up and going to a faster speed than that or even reaching because we
always have to see the same speed of light no matter how we are moving. This is so clear for
people who have looked at it and have tried to go through all of the logical arguments that you
will notice not even science fiction writers try to break this rule. You won't find science fiction
writers or movies where they try to go faster than the speed of light through space. What they
do is they look for loopholes. They try to bend the universe with warp drive or go into
hyperspace where they believe the universe and pop back in or go through wormholes. Those
don't involve travel through space at a high-speed, because you can't do it. As I mentioned, you
can do the same kind of thought experiments to drive all of the other famous consequences of
relativity. I don't have the time to go through them, but I'll just give you a couple of examples.
If you are taking a trip from Earth to a star that is 25 light years away, imagine that you could do
that trip at 99 percent of the speed of light the entire right. Of course this would be very bad to
do because if you accelerate from 0 to 99 percent of the speed of light real quickly leaving Earth
you would be smushed. And the same thing, when you turned around you would be double
smushed. It's not a good idea to actually do this, but if you could, what would happen? From
the point of view of people on earth you're taking a 50 light year round trip and you're going
almost as fast as light does and therefore it will take you a little bit longer than light does, about
50 1/2 years for the trip. So you leave this year 2015 and you would be back in 2065. You get
into your spaceship instantly up to 99 percent of the speed of light. What will you see? You
thought it was supposed to be a 25 light your trip, but now you measured again and it's actually
only 3 1/2 light years to the star. Therefore, since you are going almost the speed of light that
only takes you about 3 1/2 years to get there, 3 1/2 years from an you're back in seven years.
You have seven years worth of meals, seven years worth of supplies, you will be totally fine.
You have seven years worth of heartbeats. If you're 30 years old right now, you'll come back
age 37, but it will be the year 2065. That is not an illusion. That is real. How do we know? First
of all and I should have mentioned this first, what we're seeing is that space and time can be
measured differently by different observers depending on how you are moving. This is what is
relative about space and time. However, they are intertwined together as this four dimensional
thing that we call space-time and space-time, the four dimensional space-time is the same for
everyone. It's just that different observers see different parts of the space and time depending
on how they're moving and that's how we get results like this. What makes us think this is
true? In science it comes down to experiments. We cannot travel at speeds close to the speed
of light, but we can make particles do it, subatomic particles in collider's like the large hadron
collider. And one thing you'll notice about these if you know anything about colliders like the
large hadron collider, this year it's coming online with a lot more energy than it had a couple of
years ago and that has a lot more energy than what previous colliders had and so on. Each time
we dump way more energy in, how much faster to the particles go? They were already going
decades ago to 99.9 percent the speed of light. Now we get a few extra nines on there. We
never get all the way to the speed of light. Why not? Because it can't be done. You just get
closer and closer. Sometimes some particles will fall apart, decay in characteristic lifetimes
when you make them at very high speeds, they last longer than they do at low speeds, showing
that. And we have lots more evidence. E=mc squared is part of the theory of relativity and we
know that's true because it explains why the sun shines, nuclear power and so on. I mentioned
that relativity is involved in electronics, so every time you turn on your radio or cell phone you
are testing special relativity. It works. The only hard part about understanding any of this is
that when you first encounter it it seems like how can that be that different people will
measure space and time differently? The complaint that most people have about it is that it
violates your common sense. But it doesn't. It doesn't violate your common sense. The reason
it doesn't violate your common sense is because when it comes to these ideas you, sorry to say,
have no common sense. What do I say that? Because common sense by definition means
things that you commonly experience and you don't commonly experience speeds at close to
the speed of light where these effects are noticeable. The fastest spaceship we've ever built,
the New Horizons spacecraft on its way to Pluto that will go past in July, 50,000 km/h. That's
really, really fast, but it's one 20,000th of the speed of light. We don't have any common
experience. The problem is you're trying to take your common experience from low speeds
and assume that it should work for high speeds too, but it doesn't. You just have to get used to
that and you will be used that if you try and you think about these ideas long enough. A lot of
people think that's going to be hard, so I like to point out that as hard as it might be, you've
done a before successfully, so there's no reason you can't do it now. How have we done a
before? Here's audience participation. Everybody point up. Everybody point down. That's
common sense, right? It's good common sense. It explains why you shouldn't jump off the roof
of the building and it allows you to play basketball, but one time, you probably don't remember
it too well, but if you think that when you are in first or second grade probably, this common
sense created a crisis. The crisis was that the teacher showed you a globe. You applied your
common sense to it and you go on my gosh. Those poor people are going to fall off. So what
was wrong? The problem was you were taking your common sense that came from your
common experiences in small rooms and assuming it should apply to the whole world, but it
doesn't. You developed a different common sense for the whole world. In the same way
you've got common sense that is perfectly fine for slow speeds, but for high speeds you are
going to need to develop a different one. That is all I will say about special relativity because I
want to go quickly to general because I'm taking longer than I expected and I want to get to the
end part, so I'm going to have to go really fast. Einstein, why did he keep going to general
relativity? A number of reasons, but one was you can ask this question did Newton's theory of
gravity make any sense. You got, for example, the sun here, the earth here orbiting each other
because of gravity, but they are 150,000,000 kilometers apart. They have no eyes, ears noses.
How do the earth and sun even know each other are there? In fact, you might say something
like this, that one body may act upon another at a distance through a vacuum and force may be
conveyed from one to another, is to me so great an absurdity that I believe no man who has a
competent faculty in thinking can ever fall into it. Here we have the speaker saying that
Newton's theory is absurd. You might be saying Einstein, that's great that you came up with
this new theory, but did you have to be so rude about it? But it wasn't Einstein who said that.
There was Newton himself who said that about his own theory. Why? Let's take a little
example, we already explained why. Let's take an example to see how Einstein solves this
problem. Imagine you lived a long time ago. You think the earth is flat. You hire to explorers
and you say let's do some exploring. You go that way and you go that way and don't come back
until you find something really amazing. Sometime later they both come back. What do they
tell you they discovered? They ran into each other. You really think the earth is flat you would
be shocked, right? They went off in opposite directions. Are we shocked? No we're not
shocked. Why not? Because we understand the earth is not flat. It's curved. You're going to
meet on the other side. Imagine that you're floating in space somewhere and you send off two
probes in opposite directions, one that way and when that way. Suppose that sometime later
they run into each other. Will you be surprised? You shouldn't be. We don't do this regularly,
but we could. Imagine that you're floating in the international space station. You send one
probe that way and one probe that way. What are they going to do? They are going to orbit
around the Earth and meet on the other side. Why? Because of the magical force of gravity. In
essence, Einstein looks at this and he says wait a minute. A moment ago you showed me a
picture and you said the reason that they met was because Earth was curved. Now you're
telling me that it's because of this magical force that makes no sense is involved. One of you
just say they meet because space is curved, and that is what Einstein said. Gravity arises from
curvature of space-time. We cannot picture the curvature of four dimensional space-time
because we live in only three dimensions. We can't picture. We can use two-dimensional
analogies like this and here you see if earth is curving space-time changing its shape, you send
one probe that way and one the other way, of course they are going to meet on the other side.
Not because of any magical force, just because that's the actual shape of space-time. Again,
general relativity has been very well tested. Here we see examples of how light is bent by
gravity because gravity curves space-time. Time runs slower in gravitational fields. These
things called gravitational waves are emitted when objects change, like collapse into a black
hole or two objects orbiting very fast around one another, and you can curved space-time so
much that you can do the equivalent of poking a hole in space-time and that is what we call a
black hole. And I would love to tell you way more about all of these things, but you'll have to
ask the questions afterwards because now I want to very briefly tell you the four reasons why I
think this is important for everyone to know. First, the science reason. Since this is our modern
theory of space-time and gravity if you want to understand modern science you are going to
need to do something about it. It's practical. It explains nuclear power of the sunshine. It also
is used in your GPS navigation systems, those corrections because time runs different for those
satellites than here on the ground must be taken into account or you would actually be lost.
Second reason, all of us want to believe that we have a perception of reality that is at least in
some sense accurate. If your ideas about the universe are based on a belief that Earth is the
center of the universe, these days that's probably not a very good way of going about thinking
about your place in the universe because we know it's not the center of the universe. In the
same way if you are thinking that space and time or absolute the way most people think about
it, then you do not have an accurate perception of reality that we live in. I think most people
would want to have that. Third reason, human potential, relativity is really an amazing idea
when you think about it. Particularly the fact that Einstein came at it almost from a pure
thought perspective before the experimental evidence showed that it was really correct. It
shows what we are capable of as human beings when we put our minds to the positive instead
of to the negative. I think that's a good thing for everyone to realize. And the fourth and last
idea is a little bit more subtle but remember I told you even though space and time can be
different for different observers depending on how you are moving? The space-time reality is
the same for everyone. If you could imagine hypothetically some kind of being, four
dimensional being that could move through space-time the way we can move through just plain
space, then you could in a sense go to different points in time. In the movie Interstellar at the
end that's what he's doing in that bookcase like thing moving around through time as well as
space. If any of you have read Kurt Vonnegut's novel Slaughterhouse Five, he talks about his
four dimensional beings who can do this. You all look like very nice people, but maybe when
you were young you had a little bit mean streak going, say may be in first grade you whacked
the kid sitting next to one day, terrible, terrible thing. But the good news is even though the
kids screamed and the teacher said what's going on? You said I have no idea and you got away
with it. A four dimensional being could in principle at any moment, whatever a moment means
to a four dimensional being, I don't know. But they could go to that point in space-time, right
there. Oh there you are, whacking the kid. You didn't get away with it. You just got caught
because it's permanent. It's part of your permanent record. Every action you have ever taken
is there permanently in space-time, so I will close that out with the last few sentences from my
epilogue. Your life is a series of events and this means that when you put them all together you
are creating your own indelible mark on the universe. Perhaps if everyone understood that we
might all be a little more careful to make sure that the mark we leave is one that we are
product. So maybe I'm being a good little naïve, but I actually think that if everyone learned
about relativity, we would treat each other a little bit better. With that, we'll just about ready
for the questions, but because I like this other project so much even though it has very little to
do with relativity, I'm going to tell you about it. A few years ago I got a phone call and they told
me about this idea they had. Imagine astronauts reading stories in space to children and
families around the world on a program that combines literature and science education. It was
actually an astronaut on the phone and he said they had this idea. They wanted to read some
books from space. Why are you calling me? They found by books that you see in the back of
the room and they decided those were the ones that they wanted to read. Last January, a little
over a year ago my five children's books were launched through the space station. I have a
little clip for you.
>>: Hi. I'm NASA astronaut Mike Hopkins on board the International Space Station and it's one
of my favorite times; it's story time from space. Today's story is Max Goes to the Moon, a
Science Adventure with Max the Dog.
>> Jeffrey Bennett: So the videos eventually, curriculum materials everything are all being
posted at www.storytimefromspace.com. If you don't mind a quick plug here, the reason that
the curricular materials are coming eventually and some of the videos aren't up there yet is
because the program actually has no funding and they are running an Indiegogo campaign right
now, so if you go to www.storytimefromspace.com you'll find the Indiegogo link and I'm sure
they would very much appreciate anything that anyone does. They have some cool perks too,
so you will like that. Those are the five kids books that I have. We have some Spanish editions.
This one is coming out in Spanish this summer. These are my college textbooks. And there is
the book we talked about today and I also have a book about teaching science, how to teach
science, math and how to teach math and this is about the search for extraterrestrial life.
Thank you. [applause]. And I only went over by two minutes. All right. I'm ready for
questions, thanks.
>>: I've always said to teach kids complex things before they realize it's supposed to be hard.
So I love the first page of your chapter where he said, in teaching it to kid you realize that it's
not that relativity is hard. It's that almost as adults because it goes against what we were
taught as kids. We think it's hard so we don't want to share it with kids, but if you go straight to
the source all of a sudden it makes sense to them. Can you expand a little bit more on this as
an educator? Or is this just on science? Do you also think computer science? Perhaps getting
more gender into stem studies or other things? I love that point. How are you getting that
reaction as part of your tour?
>> Jeffrey Bennett: Absolutely, I do think it applies. There are a couple of different things going
on. With relativity, one of the things that you run up against with adults that you don't have as
much with kids is that we have solidified what we think is our commonsense understanding.
With kids, that earth example that I gave you of up and down, they just, that just happened to
them recently, so another thing that's not the way I thought it was. No big deal, but by the
time you get the adult it seems like a bigger deal. Kids actually do absorb these ideas a lot
better because they don't have as many misconceptions to come over. But more generally, I do
believe people capable of more than we usually think. We need to set high expectations and
help people meet those expectations. Without getting too far off topic I mentioned my book
on teaching science. I did a talk on that yesterday and what I always tell people and the
fundamental thing in that book is that there really is one key to all learning. And the key to all
learning is that it requires study and effort. That is so obvious that we forget to say it, but there
is a consequence to forgetting to say it, which is that it gets left out of discussions. So you have,
for example, now common core, next generation science standards, which personally, I think
are really good. They set a higher bar. I think they set a realistically higher bar, but if you are
going to set a higher bar, you need more time for study and effort and nowhere do you see
anyone discussing how they are going to provide teachers and kids with the resources to do the
additional study and effort needed to meet that higher bar. As a result, you get pushback from
parents, from teachers and everyone, so I do think we need to keep that in mind. We can
expect a lot from kids but we have to provide them the resources to meet those expectations.
Yes?
>>: One question I have been thinking from my school days. [indiscernible] who cannot see so
the maximum speed they can actually [indiscernible] listening to sound like the maximum
[indiscernible] signal. Don't they have a theory of relativity where the sound and [indiscernible]
actually meet? How would they [indiscernible] differently?
>> Jeffrey Bennett: The question is whether sound, if you had species that had no visual sense,
would sound perhaps be a similar thing. The answer is no and the reason is because light is
different. Sound is a wave that moves through material. You have to have atoms and stuff in
motion. Light is not. Light travels through empty space, through a vacuum. That is what
actually threw off Michelson and Morley when they did their experiment. They assumed there
had to be this ether thing out there for light to travel through, but there is not. That's why they
didn't get the correct interpretation of their results. If you get more technical about it the
reason light travels at the speed of light is because it has no mass. It is a massless particle.
Other massless particles, for example, gravitational waves I mentioned that are carried in
principle by these principles called gravitons, they also travel at the speed of light because they
have no mass. But anything that has mass cannot ever reach the speed of light.
>>: Typically when you read the explanations a classic thought experiment is two observers
passing each other and one of them sees the length dilation, the measuring sticks. My question
is do they both perceive the other's measuring stick as being a shorter or does one perceive it
as longer than the other as shorter?
>> Jeffrey Bennett: Yes.
>>: The latter or the former?
>> Jeffrey Bennett: As I go through in the book, I have this experiment in the book. You have
one spaceship racing by you. You are floating weightlessly; they're floating weightlessly. You
look with your super telescope. Then you watch them and you see their time is running slow
and their measurement is different. So you send them a radio message. Hey. Why is your time
running slow over there? In their spaceship they have their super telescope and they are
looking at you and they say it is your time running slow. They see your time running slow. So
radio back and they say no. It is not us running slow. It is you running slow. But wait a minute.
When you play their radio message back, what do you hear? You hear them going noooo.
Iiiiiit's noooot…. Right? They clearly are running slow and yet they are claiming that they are
not. You have a better idea. You take a video of them and you take the video that shows that
their time is running slow compared to your time. You put it in a really fast rocket so it can
catch up to them even though they are going close to the speed of light and you figure when
the rocket gets there they will pull out the video and they will look at it and they will see that
yes. It is our time running slow. Great plan. Just after you send it off guess what happens? A
rocket reaches you coming from them. You pop it, open it up and there is a video and it. And
what does it show? It shows that you are the one with the time running slow from their point
of view. Why does this bother you? Because it seems different. We want to say common
sense, but it's not. And it's the same thing. Right now look outside. The sun is out. You get on
the phone and you call a friend, the sun is out; it's great. And they say, no it's not. It's
nighttime. Are you surprised? Not if they live in Australia. We understand. It can be different
from different viewpoints. There's no disagreement. We're just looking at the same thing in
different ways and that's exactly what's happening with relativity. If you got together, then
your clocks would agree, but of course then you're not traveling relative to each other
anymore.
>>: It's just curious to me that I see asymmetry in that there is a dilation but there doesn't
seem to be a lengthening.
>> Jeffrey Bennett: It's space that is getting shrunk and the time that is expanding. Remember,
it's in four dimensions. Space-time all four dimensions are equivalent. Time is not the fourth
dimension, it's one of the four dimensions. Therefore, it's your motion that determines how
much of the space part and how much of the time part you see. That's why space-time is the
same for everyone.
>>: I want to go back to your Nairobi Quito thing. It seems to me if you're traveling in the same
direction then I would think that you would see that you are just hovering that it would look
motionless. But in the example that you gave where the earth is turning in the opposite
direction, why doesn't it appear that you are going twice as fast? Think about it again. You got
it the opposite way around. If you were going the same direction with Earth's rotation then you
would be going twice as fast, but because you are going the opposite direction…
>>: [indiscernible] relative to what you are hovering. You are going twice as fast but you're
hovering in the same spot if you are traveling at an equivalent speed, aren't you? So you're not
making progress.
>> Jeffrey Bennett: Here's Nairobi. Here's Quito. If you're going this way, so you go like that.
But from the point of view of the moon, liftoff and now it lands. So I didn't have to move at all.
If I was going the same direction that we were rotating, so if I am going this way, then you
would see it, until it catches up it's going twice as fast. It's fun to think about those things when
you're on international airplane flights because they'll explain why you're getting day and night
when you do.
>>: You said you think there will be another theory of gravity after Einstein's. Do you see any
hints of that and what circumstances would make it happen?
>> Jeffrey Bennett: The key behind this, there are a couple of different ways of looking at it.
One is that we have general relativity which explains large-scale universe really, really well and
strong gravitational fields. We also have quantum mechanics that explains the subatomic very,
very well. There is one known place where the two do not meet well and that is the singularity
of a black hole, because the singularity of a black hole is very, very tiny, so quantum mechanics
should apply. But it's also infinitely strong gravity, so general relativity should apply. And the
two basically give you different answers about what should be going on there. We can't
actually test that because we can't see it right now, but the fact that our two very successful
theories give us different answers for the same thing tells us we must be still missing
something. Another part of it is that gravity has some different properties than the other three
forces, strong, weak and electromagnetic that people are trying to unify together. If you saw
the movie about Stephen Hawking's theory of everything, the theory of everything is the theory
that would in principle pull all of these things together. As the movie showed, people like
Hawking and many others have been working on this, but so far without any clear success.
>>: Isn't that what Einstein was working on his deathbed? He was trying to figure it out?
>> Jeffrey Bennett: People have been working on this one for a long time. Somebody asked me
last night whether I thought it would be answered by the end of the century and the only thing I
could say was I have no idea. I hope so.
>>: What are your expectations for the large hadron collider running at higher energy? Is
anything that you're looking forward to being able to discover?
>> Jeffrey Bennett: The large hadron collider going to higher energies is not going to tell us
anything new about relativity. We've already tested all of these predictions of relativity to 15
decimal places, so going a little faster is not going to change that. It's more in the subatomic
physics realm that it's changing things. The Higgs boson was discovered back in 2012 and the
higher energy should give us more information and answer some of the questions about that.
It's not an area that I know much about, but I do highly recommend a movie I watched recently
called Particle Fever which traces the discovery of the Higgs boson and it was really well done.
>>: I have a question from a friend. He says mass bends space-time resulting in gravity, but
mass comes from the Higgs field, so is the Higgs field the reason for gravity?
>> Jeffrey Bennett: That is an excellent question that not only can I not answer, but I don't
think anyone can answer, because I think that would bring us into that place where quantum
mechanics and relativity have not been brought together yet. Tell him to keep working. He's
got a Nobel Prize in his future. [laughter].
>>: I wonder about the example where you have the spaceship that travels from Earth to a star
and back and earth ages by 50 years and the starship doesn't. If you imagine you are a little
alien living on the surface of the spaceship, in between the surface and they force field, but say,
because, of course, you have a force field. And what you see, you look out your window and
you are standing on ground and you see the earth flyaway and you look in the other direction
and you see a star come towards you and you get up.
>> Jeffrey Bennett: This is the famous twin paradox. The idea behind the twin paradox is you
get out into your spaceship, go off 99 percent of the speed of light to that star, come back and
it's your time that was slower because you took the trip. But why can't you say no. I didn't go
anywhere. Earth and the star flew off and the star came to me and then they flew back and so
it was them moving at 99 percent of the speed of light. It's called the twin paradox and it
troubles a lot of people, but it really shouldn't. There is not anything really paradoxical about it
once you understand it. Special relativity is what tells us that time is different for movers in
equivalence reference frames. But general relativity comes into play when you are not in a
free-floating weightless reference frame. Whenever you are accelerating, and this is what's
called the principle of equivalence. It's really the heart of general relativity. Einstein said that
gravity and acceleration are actually the same thing. Basically, how do you know if you are
being affected by gravity or acceleration? Because you will feel a force. I am being affected by
gravity. That's why I'm standing on the floor. But if this room was accelerating through space
with the same one g of acceleration that I would feel exactly the same. Acceleration and
gravity, you know you are experiencing them when you feel a force instead of floating
weightlessly. So the difference is when you travel off to that distant star and back you're the
one who feels that force squishing you when you accelerate up to 99 percent of the speed of
light. You're the one who turns around and feels the double force as you go around. The
resolution is yes. You are allowed to say I didn't go anywhere. It was Earth and the star coming
back to me, but in the earth point of view you feel this big acceleration and your point of view
you feel this strong force of gravity and either way general relativity tells us that time runs slow
in stronger gravity. Either way it's you who makes the trip and comes back younger.
>>: Can you explain a little bit more about gravity waves and the difference between gravity
and gravity waves?
>> Jeffrey Bennett: Yes. Gravitational waves are the gravitational equivalent of
electromagnetic waves, photons. The idea is that you get photons emitted when charges
accelerate, so when there's a change in the distribution of charge. It's the same thing for mass.
Just from sitting there there is no gravitational waves, but if an object collapses into a black
hole or if you have two objects, if you think about that two-dimensional analogy, if you imagine
two neutron stars orbiting around each other in a rubber sheet, they are going to be making
that sheet have waves go out through it and they are going to propagate outward. It's kind of
the idea of if those two objects moving around or if a star collapses into a black hole, how does
the rest of the universe know that it happens? It knows because the information moves out
through space-time in the form of gravitational waves. That's what we are looking for. The
reason that they are so difficult to detect is because gravity is so weak compared to the
electromagnetic force tend to the 43rd times weaker. We've never detected gravitational
waves directly. We know indirectly. We have measured the effects of the energy loss from
them being emitted, but directly, they have never been detected. But this year advanced LIGO,
Laser Interferometer Gravitational Wave Observatory is coming online. One branch I believe is
here in Washington and the other in Louisiana. Hopefully, within the next couple of years that
will make the first direct protections of gravitational waves.
>> Amy Draves: We have had several questions online, but I'm going to select this one which is,
there is a lot of distrust of science. How can we as science enthusiasts help improve the
situation of the general population?
>> Jeffrey Bennett: Very good question. What I would say is we need to explain what science is
and what its purpose is are. The purposes that I like to focus on for science are they give this a
way of distinguishing possibilities from realities. Here is a nice possibility. Another nice
possibility is that space and time are always the same for everybody. Which one is real?
Science, by looking at the evidence and measuring, we can figure out which possibility might be
real and which ones not. Secondly, this idea, nobody would have taken this seriously, and yet,
it was accepted very, very rapidly afterward. Why? Because the measurement evidence. The
evidence was so overwhelming that unless you just wouldn't look at it, it was pretty clear that
what Einstein had done was correct. Science gives us tools for coming to agreement on things
which we might not have been able to agree on before. Science is often thought of as this
source of discord in society, like the questioner asked. But, in fact, it's a way of bringing us into
agreement. Once people understand those basic ideas, I think they will see that science is not a
threat to anyone. It's something that we can all look at together in a rational objective way,
and once the evidence becomes strong enough we'll all, if we look at it honestly, see the same
conclusions.
>> Amy Draves: One more, one last question.
>>: [indiscernible] the twin paradox. [indiscernible] not experience any force [indiscernible]
does that mean that we are affected by the time space in the initial part but not in the uniform
[indiscernible] part?
>> Jeffrey Bennett: Actually, when you analyze it carefully it's that initial part but even more
importantly the turnaround part. It works out very nicely because the mathematics of general
relativity is very complex. Luckily, it works out to whatever your average speed is for the trip
you can just use the simple time dilation formula from special relativity, which is just a square
root and really easy to calculate to get what the time affect would be. Thank you. [applause]