At the limits of astrophysics – with Katy Clough

00:55:55
https://www.youtube.com/watch?v=n4RbkTCp16k

Summary

TLDRDr. Katy Clough discusses the intriguing overlap between mathematics and astrophysics in her talk, "At the Limits of Astrophysics." She centers on three ideas: black holes, wormholes, and warp drives, illustrating how such concepts, while appearing like science fiction, are rooted in scientific theory. By explaining spacetime curvature, she elaborates that gravity arises from this curvature rather than being merely a force. Clough uses historical anecdotes and demonstrations to address how theories can evolve and often seem improbable. Ultimately, she encourages a broad-minded outlook toward scientific exploration, highlighting the potential of ideas that can bridge theoretical physics and empirical science.

Takeaways

  • 🌌 Gravity as curvature of spacetime
  • 🌠 Black holes vs science fiction
  • ⚛️ Importance of exotic matter
  • 🌀 Wormholes as theoretical shortcuts
  • ⭐ Warp drives and their challenges
  • 📏 The universe is expanding
  • 🔭 Evidence of black holes
  • 🔑 Understanding spacetime is key
  • 📜 History of scientific conflict
  • 🧠 Open-mindedness in science

Timeline

  • 00:00:00 - 00:05:00

    Dr. Katy Clough addresses the audience at Queen Mary University of London, discussing the limits of astrophysics and presenting herself as a researcher situated between mathematics and astrophysics. She introduces her topic, hinting at the boundary between science and science fiction.

  • 00:05:00 - 00:10:00

    Setting the stage, Dr. Clough presents alternative titles for her talk, focusing on distinguishing between crazy and brilliant ideas in science. She mentions how historical misconceptions have often blurred the lines between the two, leading to unexpected scientific truths.

  • 00:10:00 - 00:15:00

    The discussion shifts to black holes, which Dr. Clough asserts are solid scientific entities despite their seemingly crazy nature. She introduces spacetime curvature, a fundamental aspect of Einstein's theory of general relativity, and illustrates how gravity operates differently in strong gravitational fields.

  • 00:15:00 - 00:20:00

    Using a two-dimensional model to explain spacetime, Dr. Clough elaborates on the nature of gravitational forces, emphasizing that what we perceive as gravitational attraction is actually the effect of spacetime curvature caused by massive objects like stars and planets.

  • 00:20:00 - 00:25:00

    Dr. Clough further illustrates spacetime curvature using an analogy involving two people walking on the surface of the Earth, explaining how their paths would converge due to the Earth's curvature. This analogy highlights the counterintuitive nature of gravity as understood through spacetime rather than traditional forces.

  • 00:25:00 - 00:30:00

    The talk transitions to the expansion of the universe, where Dr. Clough describes the Hubble Expansion. She explains how galaxies move apart due to the curvature of space and time rather than gravitational attraction, revealing insights into cosmic structures.

  • 00:30:00 - 00:35:00

    Dr. Clough dives into black holes, explaining their formation from collapsing massive stars as per Chandrasekhar's theory, which faced initial rejection due to prevailing scientific prejudices. She provides a modern confirmation of black holes through observational evidence, showcasing the first image of a black hole's event horizon.

  • 00:35:00 - 00:40:00

    Following her exploration of black holes, Dr. Clough introduces wormholes, describing them as theoretical shortcuts through spacetime. She discusses the challenges of conceptually and physically creating traversable wormholes due to the requirement for 'exotic matter' with negative energy, which poses several problems in modern physics.

  • 00:40:00 - 00:45:00

    The subject shifts to warp drives as another speculative concept from science fiction. Dr. Clough outlines how they draw from cosmological principles of space contraction and expansion while also facing challenges in terms of the need for exotic matter and potential paradoxes associated with time travel.

  • 00:45:00 - 00:55:55

    Finally, Dr. Clough concludes her discussion on the relationships between black holes, wormholes, and warp drives, reinforcing the importance of maintaining an open mind when approaching unconventional ideas in science, harkening back to the historical dismissal of innovative theories. She invites questions from the audience.

Show more

Mind Map

Video Q&A

  • What is spacetime curvature?

    Spacetime curvature is the concept that space and time are intertwined in a four-dimensional continuum, where the presence of mass can warp this fabric, leading to the effects we perceive as gravity.

  • What defines a black hole?

    A black hole is defined as a region of spacetime from which nothing can escape, characterized by its event horizon.

  • What are wormholes?

    Wormholes are hypothetical passages through spacetime that could create shortcuts for long journeys across the universe.

  • Do warp drives exist?

    Warp drives are theoretical constructs that would allow faster-than-light travel by contracting and expanding spacetime, but they require exotic matter, which has not been proven to exist.

  • What is exotic matter?

    Exotic matter is a type of matter that would have negative energy and density, necessary for stabilizing a traversable wormhole or warp drive.

  • How do black holes form?

    Black holes form when massive stars collapse under their own gravity after exhausting their nuclear fuel.

  • What evidence supports the existence of black holes?

    Recent observations such as the imaging of a black hole's event horizon and the detection of gravitational waves from merging black holes provide evidence for their existence.

  • What is the significance of the Hubble Expansion?

    The Hubble Expansion refers to the observation that the universe is expanding, with galaxies moving away from each other over time.

  • How does time behave near a black hole?

    Time behaves differently near a black hole due to the effects of strong gravity, leading to dilated time for objects at different distances from the event horizon.

  • What historical conflict related to black holes is mentioned?

    The conflict between Chandrasekhar, who argued that massive stars must collapse into black holes, and Eddington, who dismissed this idea, illustrates how scientific biases can delay acceptance of valid theories.

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  • 00:00:00
    (techno-style music)
  • 00:00:04
    (audience applauding)
  • 00:00:14
    - Wow.
  • 00:00:17
    Hello. Thank you.
  • 00:00:17
    And thank you especially for the promotion to professor.
  • 00:00:22
    I am sadly only a doctor, but, yes, I'm nevertheless
  • 00:00:26
    very happy to be here this evening.
  • 00:00:28
    So yeah, as Hassan said, I'm Dr. Katy Clough.
  • 00:00:31
    So I am an Earnest Rutherford fellow
  • 00:00:33
    at Queen Mary University of London,
  • 00:00:36
    which is just down the road.
  • 00:00:37
    And this evening, I'm gonna be telling you
  • 00:00:39
    about the limits of astrophysics.
  • 00:00:41
    That's what I've promised to tell you.
  • 00:00:43
    So the title, "At the Limits of Astrophysics,"
  • 00:00:46
    so that's the title of my talk,
  • 00:00:47
    but it's also a bit of a description of me and my research.
  • 00:00:52
    So you'll notice from my slide,
  • 00:00:54
    that I'm actually from the maths department
  • 00:00:56
    at Queen Mary University.
  • 00:00:57
    We've got some mathematicians in the audience.
  • 00:01:01
    And so, actually,
  • 00:01:04
    all the mathematicians think I'm an astrophysicist,
  • 00:01:07
    but all the people in the astronomy unit
  • 00:01:09
    who are real astrophysicists, think I'm a mathematician.
  • 00:01:12
    But I actually sit in this very interesting and rich space
  • 00:01:15
    that exists somewhere between the two things,
  • 00:01:18
    especially for Einstein's theory
  • 00:01:20
    of general relativity for gravity
  • 00:01:22
    that I'm gonna tell you a bit about this evening.
  • 00:01:24
    It's somewhat of a mathematical theory,
  • 00:01:27
    and yet it has consequences in our universe,
  • 00:01:30
    so it sits somewhere between the two things.
  • 00:01:33
    So as Hassan was saying, this is actually the first
  • 00:01:35
    in a series of talks that are gonna be given
  • 00:01:37
    by Queen Mary researchers.
  • 00:01:39
    And when I got the list
  • 00:01:42
    of all of everyone else's abstracts and titles,
  • 00:01:44
    I thought, "Wow, this is really amazing."
  • 00:01:48
    So there's things like supernovae, exoplanets,
  • 00:01:52
    beginning of the universe,
  • 00:01:53
    dark energy, dark matter, all this kind of stuff.
  • 00:01:56
    And I thought, "That sounds really impressive."
  • 00:01:59
    It actually sounds like science fiction.
  • 00:02:02
    And I thought, you know, if I wasn't working in this field,
  • 00:02:05
    I might find it really difficult to know where the line is
  • 00:02:09
    between science and science fiction.
  • 00:02:12
    You know, if I can have, you know,
  • 00:02:13
    all of these crazy supernovae
  • 00:02:15
    and exoplanets and stuff like that,
  • 00:02:17
    you know, why can't I have wormholes and warp drives
  • 00:02:19
    and things like that?
  • 00:02:20
    And so I thought it would be fun this evening,
  • 00:02:22
    to try and explore with you where that boundary lies,
  • 00:02:26
    which is also, as I say, kind of related to this space
  • 00:02:28
    that I exist in for my research,
  • 00:02:30
    somewhere between mathematics and astrophysics.
  • 00:02:34
    Okay, so let's get started.
  • 00:02:36
    So an alternative title for my talk is,
  • 00:02:39
    "What is the Difference Between a Crazy Idea
  • 00:02:41
    and a Brilliant One?"
  • 00:02:44
    So it turns out, of course,
  • 00:02:46
    you know, if you know anything about the history of science,
  • 00:02:49
    that we're not very good at telling the difference
  • 00:02:52
    between these two things.
  • 00:02:53
    So often, we've thought that things were very good ideas
  • 00:02:56
    and they turn out to be completely wrong, and vice versa,
  • 00:02:59
    you know, things that we thought couldn't be true
  • 00:03:01
    actually turn out to be reality,
  • 00:03:03
    you know, the reality of the universe that we live in.
  • 00:03:06
    And so I'm gonna focus on three crazy ideas
  • 00:03:10
    for this evening,
  • 00:03:11
    and I've sort of ranked them in order of craziness.
  • 00:03:16
    So actually for the second and the third one,
  • 00:03:18
    it's a bit of a matter of taste.
  • 00:03:20
    Like it depends, you know, how you feel about these things,
  • 00:03:23
    and it depends on the specific type of wormhole
  • 00:03:26
    or warp drive that you wanna have.
  • 00:03:28
    But definitely the first one, black holes, do exist.
  • 00:03:31
    So black holes are really good science
  • 00:03:33
    and not science fiction,
  • 00:03:35
    which is surprising because they do, as I say,
  • 00:03:38
    on the face of it, sound kind of completely mad.
  • 00:03:41
    So before I tell you about these three types of objects,
  • 00:03:45
    I have to tell you a bit about this really crazy
  • 00:03:48
    but super brilliant idea, which is spacetime curvature.
  • 00:03:52
    So I think for me, this is one of the most brilliant
  • 00:03:55
    but also most crazy ideas in the history of physics.
  • 00:03:58
    This is one of the things where when you first encounter it,
  • 00:04:01
    you just can't accept it.
  • 00:04:03
    Like deep in you, you're like,
  • 00:04:05
    "No, that's not how the universe is."
  • 00:04:07
    But it is, right?
  • 00:04:08
    It's just that you don't have the right intuition
  • 00:04:11
    because you were born and lived most of your life on,
  • 00:04:14
    well, all of your life hopefully, on Earth.
  • 00:04:18
    And, you know, you've never flown to a black hole
  • 00:04:21
    or traveled at the speed of light.
  • 00:04:22
    You know, as far as I know,
  • 00:04:24
    you've all been here for your lives.
  • 00:04:26
    So you just don't have the right intuition
  • 00:04:28
    for these very strong gravity environments
  • 00:04:31
    that you have, for example, around black holes.
  • 00:04:35
    So at the core of this idea,
  • 00:04:36
    is this idea of spacetime curvature,
  • 00:04:38
    so let me break it down for you a bit.
  • 00:04:41
    So the first part is spacetime.
  • 00:04:43
    So this idea of spacetime is that somehow,
  • 00:04:47
    space and time are not separate things,
  • 00:04:52
    they're not independent, they somehow depend on each other,
  • 00:04:55
    so they're part of this bigger thing called spacetime.
  • 00:04:58
    So in this picture, this is probably how you,
  • 00:05:00
    normally, we would think about space and time.
  • 00:05:04
    So along the bottom, there's space,
  • 00:05:06
    and going upwards, that's moving forward in time.
  • 00:05:09
    And you would think that, you know,
  • 00:05:10
    you flew off from your planet to a nearby black hole or star
  • 00:05:14
    and, you know, everyone agrees
  • 00:05:16
    that you left at seven o'clock in the evening
  • 00:05:19
    and you arrived at 7:00 AM the next day
  • 00:05:21
    in time for breakfast,
  • 00:05:22
    and then you turn around and you fly back to your planet.
  • 00:05:25
    And in this picture of time that we have,
  • 00:05:27
    everyone agrees on the time
  • 00:05:30
    which these events happened, right?
  • 00:05:32
    There's like some big clock in the sky
  • 00:05:35
    that everyone agrees on.
  • 00:05:38
    But the reality is
  • 00:05:39
    there is no such big clock on the sky, right?
  • 00:05:41
    To measure the sequence of these events,
  • 00:05:44
    you need to be the person at each one of them.
  • 00:05:47
    You need to be the person traveling in the rocket,
  • 00:05:49
    or sitting on the planet, or sitting at the black hole.
  • 00:05:52
    And what you see on your watch
  • 00:05:54
    will differ depending on where you are,
  • 00:05:57
    and which of these events you are at,
  • 00:05:58
    and how you're traveling through the space and the time.
  • 00:06:02
    And that is, as I say, very counterintuitive,
  • 00:06:04
    but it is really how our universe works.
  • 00:06:08
    The second thing that you need to know, and this is,
  • 00:06:10
    again, it's probably deep in you
  • 00:06:12
    because it's one of the first things you learn
  • 00:06:14
    when you're at school.
  • 00:06:15
    Like I think really, the first science lesson you have,
  • 00:06:17
    you have to learn forces, and you learn magnetism,
  • 00:06:20
    and you learn gravity.
  • 00:06:21
    So you say, "Name a force," and you say, "Gravity,"
  • 00:06:24
    but that's wrong, okay?
  • 00:06:25
    We shouldn't actually teach children this.
  • 00:06:27
    I actually think we should just teach them
  • 00:06:28
    spacetime coverture directly,
  • 00:06:30
    but I'm apparently a minority view here.
  • 00:06:33
    So, yeah, we teach them it's a force, which is okay,
  • 00:06:36
    you know, it works on the surface of the Earth.
  • 00:06:39
    But, you know, what is a force?
  • 00:06:41
    So the idea of a force is that it's something
  • 00:06:43
    that changes your speed or direction.
  • 00:06:46
    So in this picture particularly,
  • 00:06:48
    I'm thinking about a change of direction.
  • 00:06:50
    So I'm flying my spaceship past a planet
  • 00:06:53
    and it gets somehow pulled off course
  • 00:06:56
    by this gravitational force.
  • 00:06:58
    And we think of this as being like,
  • 00:06:59
    you know, a kind of like a big magnet that sort of attracts
  • 00:07:02
    the spaceship towards the Earth.
  • 00:07:07
    But what I wanna tell you
  • 00:07:08
    is that that's not what's really happening.
  • 00:07:10
    So in fact, we would say actually,
  • 00:07:12
    that there is no change of direction.
  • 00:07:15
    So the picture that we should have is something like this,
  • 00:07:18
    that it's not a force that's pulling us off course,
  • 00:07:21
    it's spacetime curvature.
  • 00:07:23
    So I have it up on this slide,
  • 00:07:25
    but Hassan told me, "You have to do a demo."
  • 00:07:29
    And I said, "There's a reason
  • 00:07:30
    why I'm in the mathematics department
  • 00:07:32
    and not in the engineering department anymore."
  • 00:07:34
    But, okay, so he's responsible
  • 00:07:36
    for anything that would happen.
  • 00:07:38
    But, okay, so the idea is this...
  • 00:07:40
    Oh, let me start with a small one.
  • 00:07:42
    So the idea is basically,
  • 00:07:43
    that you have to imagine that this two dimensional surface,
  • 00:07:47
    this sheet here, is spacetime.
  • 00:07:50
    So that's the first thing that's quite tricky
  • 00:07:52
    in these pictures.
  • 00:07:53
    So people always show this kind of picture
  • 00:07:55
    when they explain spacetime.
  • 00:07:57
    But this sheet has two dimensions,
  • 00:08:01
    I can describe it by two coordinates,
  • 00:08:04
    but it's supposed to represent spacetime,
  • 00:08:06
    this unified space and time thing that I was talking about,
  • 00:08:10
    and so it should really be four dimensional.
  • 00:08:13
    So already, something three dimensional,
  • 00:08:15
    you know, like a ball,
  • 00:08:16
    is, okay, you can imagine that,
  • 00:08:18
    but four dimensional, maybe not, right?
  • 00:08:22
    It's okay, right?
  • 00:08:24
    You don't have to imagine four dimensional spacetime.
  • 00:08:26
    I actually challenge anyone
  • 00:08:28
    who says that they can imagine four dimensional space time,
  • 00:08:31
    I don't think they can, but it's okay.
  • 00:08:34
    As I say, it's just kind of schematic,
  • 00:08:35
    so just take it as being schematic.
  • 00:08:38
    So if I...
  • 00:08:39
    I mean, this is my rocket, this marble.
  • 00:08:42
    If I roll it across spacetime,
  • 00:08:44
    because it's quite light, it's quite small,
  • 00:08:47
    it doesn't curve spacetime very much,
  • 00:08:50
    and so it just travels in a straight line.
  • 00:08:53
    So this is like in a space
  • 00:08:54
    where I'm traveling through empty space, right?
  • 00:08:56
    And I just always move in a straight line.
  • 00:09:01
    But what if I then put a star
  • 00:09:04
    in the center of my spacetime?
  • 00:09:06
    Oh, that's not enough. I have to do a bigger star.
  • 00:09:10
    Here's a bigger star.
  • 00:09:13
    Okay, now my spacetime has been curved.
  • 00:09:17
    So the nice thing about this picture is it tells you
  • 00:09:19
    where does this curvature,
  • 00:09:21
    this spacetime curvature, come from?
  • 00:09:23
    It comes from energy and matter.
  • 00:09:26
    So things like stars or planets
  • 00:09:28
    curve the spacetime around them.
  • 00:09:31
    And so then when I come in with my little rocket,
  • 00:09:33
    what I can do if I...
  • 00:09:35
    Well, so if I try and go in a straight line,
  • 00:09:37
    I get deflected, right?
  • 00:09:39
    I get pushed off course.
  • 00:09:41
    And so we might describe this as being a force
  • 00:09:44
    that's pulling this marble towards, whoop,
  • 00:09:48
    towards the star.
  • 00:09:50
    But you can see that's not really what's happening, right?
  • 00:09:52
    So actually what it's doing
  • 00:09:53
    is it's following the contours of the surface.
  • 00:09:56
    And because it's a curved surface,
  • 00:09:58
    those contours are not straight lines, right?
  • 00:10:01
    They are curved.
  • 00:10:02
    So there's no magnet in this ball
  • 00:10:05
    pulling this ball towards it,
  • 00:10:07
    it's really just following the curvature of my spacetime.
  • 00:10:10
    So if I do it nicely... I'm not very good at this.
  • 00:10:13
    If I make it do it like this,
  • 00:10:15
    I can actually get it to go round in an orbit.
  • 00:10:17
    And so this is actually what's happening
  • 00:10:19
    with the Earth around the sun,
  • 00:10:20
    and the planets around the sun, okay?
  • 00:10:22
    We don't in-spiral so fast, don't worry.
  • 00:10:26
    It takes a very long time for us to in-spiral
  • 00:10:28
    towards the sun, and so we're all good.
  • 00:10:33
    But, yeah, so this is the concept of spacetime curvature,
  • 00:10:38
    and this is gonna be fundamental to all of the objects
  • 00:10:40
    that I'm talking about.
  • 00:10:41
    So I'm gonna show you lots of these
  • 00:10:42
    two dimensional pictures, which you have to remember
  • 00:10:45
    are kind of schematic representations
  • 00:10:47
    of four dimensional spacetime.
  • 00:10:50
    Sorry, I don't need my next two slides
  • 00:10:52
    because I did it for you.
  • 00:10:55
    Okay, but I have another picture that I really like to show.
  • 00:10:59
    This is the one that I really like
  • 00:11:01
    when I imagine spacetime.
  • 00:11:04
    And the reason is that actually,
  • 00:11:06
    most of the curvature is in the time direction,
  • 00:11:10
    it's actually not in the space direction.
  • 00:11:12
    So to quite a good approximation,
  • 00:11:15
    it's time that's curved and not space.
  • 00:11:18
    Okay, I said they were part of the same thing,
  • 00:11:20
    so it's a bit subtle, but let me explain with my picture.
  • 00:11:24
    So here are Sam and Lliebert,
  • 00:11:27
    these are my two PhD students from Queen Mary University.
  • 00:11:31
    And so I decide...
  • 00:11:33
    Let's pretend that we live a long time ago, okay?
  • 00:11:35
    Where we don't know if the surface of the Earth
  • 00:11:37
    is flat or curved.
  • 00:11:39
    And I decide to send my two PhD students to Africa
  • 00:11:43
    to do an experiment that I can't be bothered to do myself.
  • 00:11:47
    That's what you do with PhD students.
  • 00:11:50
    And so I send them to Africa and I tell them,
  • 00:11:54
    "Okay, we're gonna do an experiment.
  • 00:11:56
    You need to start at the equator
  • 00:11:59
    and you're gonna line yourselves up,
  • 00:12:01
    so you're going parallel, right?"
  • 00:12:03
    So one here, one here, and I tell them,
  • 00:12:06
    "You're gonna walk in a straight line," right?
  • 00:12:08
    "You're gonna just fix your eyes on the horizon
  • 00:12:11
    and you're gonna head north," right?
  • 00:12:12
    "You're just gonna walk always in a straight line.
  • 00:12:15
    It doesn't matter if you're being chased by lions,
  • 00:12:17
    you have to walk in a straight line."
  • 00:12:18
    Okay, so off they go.
  • 00:12:21
    And if the Earth was really flat,
  • 00:12:25
    then this picture is what would happen, right?
  • 00:12:27
    They would stay separated,
  • 00:12:29
    they would stay a certain distance apart
  • 00:12:31
    as they traveled north.
  • 00:12:32
    And when they got to Europe, they would find
  • 00:12:34
    that they were still the same distance apart.
  • 00:12:37
    But of course you know,
  • 00:12:38
    I hope you know that our Earth is in fact a ball
  • 00:12:42
    and therefore, that's not what would happen.
  • 00:12:44
    If they both kept going north, eventually they would find
  • 00:12:47
    that they were getting closer together.
  • 00:12:49
    They would actually find
  • 00:12:50
    that when they got to the North Pole,
  • 00:12:51
    they would actually meet there, right?
  • 00:12:53
    They would find that they'd got together at the North Pole.
  • 00:12:58
    And so they might sort of be kind of confused about that
  • 00:13:03
    if they thought that the Earth was really flat.
  • 00:13:05
    So the reason that we thought for a long time
  • 00:13:08
    in the history of people, that these pictures were the same
  • 00:13:11
    is because we live on this very tiny patch
  • 00:13:13
    of the planet, right?
  • 00:13:15
    So in a very small box, these two pictures look the same,
  • 00:13:18
    people stay the same distance apart.
  • 00:13:21
    It's only when you start to travel on distances
  • 00:13:23
    that are similar to this curvature scale
  • 00:13:26
    of the surface of the Earth,
  • 00:13:28
    that you start to see differences
  • 00:13:30
    that tell you that it's a curved surface.
  • 00:13:33
    So, okay, here's a more schematic picture
  • 00:13:36
    of what I just told you.
  • 00:13:37
    You've got north in space,
  • 00:13:40
    and Sam and Lliebert are going north.
  • 00:13:44
    And in the left-hand picture, this is flat Earth,
  • 00:13:47
    and in the right-hand picture
  • 00:13:48
    is what would happen on the actual curved Earth.
  • 00:13:51
    And if they were...
  • 00:13:53
    But if they were really determined,
  • 00:13:55
    you know, to hang onto this picture of a flat Earth,
  • 00:13:59
    what they might say is,
  • 00:14:00
    "Well, maybe there was some kind of mysterious force
  • 00:14:04
    that pulled us off course.
  • 00:14:06
    So we didn't go in a straight line,
  • 00:14:08
    we got somehow pulled off course
  • 00:14:10
    and that's why we met at the North Pole."
  • 00:14:14
    So that would be wrong, but they could...
  • 00:14:17
    You know, PhD students, what can you do?
  • 00:14:21
    But, you know, they might reasonably conclude that,
  • 00:14:24
    and in fact, they could probably build a model
  • 00:14:26
    where these two things look the same.
  • 00:14:29
    So what I'm sort of getting at indirectly, eventually,
  • 00:14:33
    is that this is exactly what's happening
  • 00:14:35
    with spacetime and gravity.
  • 00:14:37
    So this mysterious attractive force, we call that gravity,
  • 00:14:42
    but actually it's the fact that we are moving
  • 00:14:44
    now through time, and time is curved.
  • 00:14:48
    So instead of going north,
  • 00:14:50
    I've replaced this north axis with time.
  • 00:14:55
    So as I say, that's quite a hard concept to think of,
  • 00:14:58
    time being curved, but that is indeed what happens.
  • 00:15:02
    And so you might ask me at this point,
  • 00:15:05
    "You know, okay, Katy, this is all very interesting,
  • 00:15:09
    spacetime curvature, but to be honest,
  • 00:15:11
    I've got to this point in my life,
  • 00:15:13
    and I've never needed the concept of spacetime curvature.
  • 00:15:16
    Like, just thinking of gravity as a force has worked okay,
  • 00:15:20
    and frankly, what's wrong with just thinking of it
  • 00:15:23
    as a force, if in fact they're just equivalent?"
  • 00:15:27
    But the problem is that in some situations,
  • 00:15:30
    they're not equivalent.
  • 00:15:31
    In some situations, they're different.
  • 00:15:33
    And in particular, in some situations,
  • 00:15:36
    you just can't describe what you're seeing
  • 00:15:38
    with gravity being a force.
  • 00:15:41
    So a really good example of this
  • 00:15:42
    is that the universe is expanding.
  • 00:15:45
    So this is called the Hubble Expansion,
  • 00:15:47
    perhaps you've heard of it,
  • 00:15:50
    but what happens is that if you take two galaxies,
  • 00:15:54
    so these two galaxies are something
  • 00:15:55
    like a mega parsec apart.
  • 00:15:57
    So maybe you don't know a mega parsec,
  • 00:15:59
    but it's a really long distance.
  • 00:16:01
    So this is such a long distance
  • 00:16:03
    that they're individual curvatures,
  • 00:16:05
    so if I come back to here,
  • 00:16:09
    so the fact that they're individually curving the space,
  • 00:16:12
    they're not affecting each other,
  • 00:16:14
    so they're not being pulled together anymore
  • 00:16:16
    by this sort of what we would call
  • 00:16:20
    the gravitational attraction, the force,
  • 00:16:22
    but it is really down to the curvature
  • 00:16:24
    around each of these galaxies.
  • 00:16:26
    So they're really very far apart.
  • 00:16:28
    And what you'd find if you looked at these galaxies,
  • 00:16:31
    is if they started a mega parsec apart,
  • 00:16:34
    one second later,
  • 00:16:36
    they would be one mega parsec plus 70 kilometers apart.
  • 00:16:41
    So they're actually moving away from each other.
  • 00:16:44
    So that's really weird, actually.
  • 00:16:48
    It's really hard to explain.
  • 00:16:50
    Like, how do you explain that using a gravitational force?
  • 00:16:53
    These objects are normal galaxies,
  • 00:16:55
    they have positive mass and matter in them,
  • 00:16:58
    and so they should, if anything, attract each other.
  • 00:17:02
    Or, you know, if they're so far apart,
  • 00:17:03
    they should just kind of stay where they are,
  • 00:17:05
    they should just stay a certain distance apart in time.
  • 00:17:09
    But what Pablo showed is that in fact,
  • 00:17:11
    they are moving apart in this way.
  • 00:17:14
    And the reason that they're moving apart
  • 00:17:16
    is because the universe is curved in the time direction.
  • 00:17:21
    So we never say this,
  • 00:17:23
    we always say the universe is expanding.
  • 00:17:25
    I would like people to say,
  • 00:17:26
    "The universe is curved in the time direction,"
  • 00:17:28
    but it's not really caught on.
  • 00:17:31
    So the point is that if you see it in this way,
  • 00:17:35
    suddenly things make a bit more sense.
  • 00:17:37
    So the interesting thing which you'll notice immediately,
  • 00:17:40
    is somehow, the universe as a whole
  • 00:17:42
    is curved in the opposite way
  • 00:17:44
    to how it's curved kind of locally around objects.
  • 00:17:48
    So this is completely consistent with Einstein's theory.
  • 00:17:51
    It can happen this way.
  • 00:17:54
    But the universe is somehow curved such that,
  • 00:17:57
    you know, it's like it being curved up instead of down,
  • 00:18:00
    so instead of things sort of moving together,
  • 00:18:02
    they move apart over time.
  • 00:18:04
    But it's still curvature in the time direction.
  • 00:18:07
    But as I say, if you think of it in this way,
  • 00:18:09
    it's really nice because a lot of the questions
  • 00:18:12
    that people often have about cosmology
  • 00:18:13
    and about the universe, you start to think of them
  • 00:18:16
    in what I think is kind of more the right way.
  • 00:18:20
    You start to ask the right kind of questions.
  • 00:18:23
    So for example, one question that people always ask
  • 00:18:26
    is what is the universe expanding into?
  • 00:18:29
    You know, if the universe is expanding,
  • 00:18:31
    what is it expanding into?
  • 00:18:34
    But this is like asking,
  • 00:18:36
    what's the Earth's surface expanding into?
  • 00:18:39
    Or, you know, would you describe the Earth's surface
  • 00:18:41
    as somehow expanding as you move north?
  • 00:18:45
    I mean, you kind of could describe it in this way,
  • 00:18:48
    you know, somehow, the Earth kind of gets bigger
  • 00:18:50
    as you get north, but it's not really expanding,
  • 00:18:54
    it's like it's...
  • 00:18:55
    You know, it's already there
  • 00:18:56
    and we are just moving on this curved surface.
  • 00:18:58
    So this is in a way,
  • 00:18:59
    how you should think about our universe,
  • 00:19:01
    that it's just this kind of curved surface.
  • 00:19:03
    So a more interesting question is to ask something about,
  • 00:19:06
    you know, can I go round the universe, right?
  • 00:19:08
    Like, I can go round the Earth,
  • 00:19:11
    can I sort of fly to the outages of the universe
  • 00:19:14
    and go back on myself and come round again?
  • 00:19:19
    Then another question that people always ask is,
  • 00:19:21
    what came before the start of time?
  • 00:19:25
    And this is like asking
  • 00:19:26
    what is further south than the South Pole, right?
  • 00:19:30
    So if I rewind time in my picture
  • 00:19:32
    of the sort of curved Earth, that's equivalent to saying,
  • 00:19:37
    you know, what happens when you get back to the South Pole?
  • 00:19:41
    And you know that on the Earth,
  • 00:19:42
    you know, all people would come to the South Pole,
  • 00:19:44
    there'd be nowhere else for them to go, right?
  • 00:19:46
    You can't go any further south than the South Pole.
  • 00:19:49
    So of course, you can start asking questions about,
  • 00:19:51
    well, somehow then you need to go off the surface
  • 00:19:54
    of the Earth, right?
  • 00:19:55
    You need to go into a higher dimension.
  • 00:19:57
    So then, you know, that makes string theorists very happy,
  • 00:19:59
    you know, they think about higher dimensions
  • 00:20:01
    and things like that.
  • 00:20:02
    But, you know, maybe that's the right question
  • 00:20:03
    to be asking, right?
  • 00:20:04
    You know, did this come from some higher dimension?
  • 00:20:07
    The two dimensional surface of the Earth
  • 00:20:09
    is somehow embedded in sort of three dimensional space.
  • 00:20:15
    Okay, so let me get on to black holes,
  • 00:20:20
    wormholes, and warp drives.
  • 00:20:23
    So, starting with black holes.
  • 00:20:25
    So I think the really interesting question
  • 00:20:28
    to ask yourself...
  • 00:20:28
    So I hope actually, everyone knows what a black hole is.
  • 00:20:31
    I kind of assume that if people sign up to talks like this,
  • 00:20:33
    they've at least watched one science fiction film
  • 00:20:35
    where there's a black hole in it.
  • 00:20:38
    But maybe I shouldn't assume knowledge.
  • 00:20:40
    Sorry?
  • 00:20:41
    "Star Trek"?
  • 00:20:43
    As long as people have watched "Star Trek,"
  • 00:20:45
    you're definitely okay.
  • 00:20:47
    You're gonna really enjoy the end of the talk.
  • 00:20:50
    So black holes, how do I make a black hole?
  • 00:20:53
    So yeah, a black hole, as I say,
  • 00:20:55
    the sort of definition of a black hole
  • 00:20:57
    is a region of space and time
  • 00:20:59
    that somehow separated from the rest of the universe.
  • 00:21:03
    So the idea is that you can't get out of a black hole.
  • 00:21:06
    So there's some surface that once you've crossed it,
  • 00:21:09
    you would need to travel faster than the speed of light
  • 00:21:11
    to escape from it, and therefore,
  • 00:21:14
    you know, since as far as we know,
  • 00:21:16
    nothing can travel faster than the speed of light,
  • 00:21:19
    everything gets trapped in a black hole.
  • 00:21:22
    But the interesting question that I want to ask today
  • 00:21:25
    is how do I make a black hole?
  • 00:21:27
    Because that's really a question about,
  • 00:21:29
    you know, can one of these things really exist?
  • 00:21:31
    How real is it?
  • 00:21:32
    And so, you know,
  • 00:21:34
    I can now use again my little demonstration.
  • 00:21:37
    What I wanna do is I wanna make this deeper, right?
  • 00:21:41
    So if I want things to get trapped in it,
  • 00:21:43
    somehow I want this to make it go down.
  • 00:21:46
    So I can put my heavier object,
  • 00:21:50
    and you see it, it's getting deeper, right?
  • 00:21:52
    So it becomes more difficult for something to escape if I...
  • 00:21:56
    Ooh!
  • 00:21:57
    This is why I said
  • 00:21:58
    I shouldn't be trusted with a demonstration.
  • 00:22:01
    Ooh. It's really quite addictive.
  • 00:22:03
    So they said that they will leave it outside
  • 00:22:05
    once we've finished the lecture
  • 00:22:06
    and everyone can play with it.
  • 00:22:07
    And I tell you, it is massively addictive.
  • 00:22:10
    But I need something more, right?
  • 00:22:12
    So, here we go.
  • 00:22:15
    What have I got here?
  • 00:22:18
    Oh!
  • 00:22:21
    They said two hands, two hands.
  • 00:22:23
    Bend your knees.
  • 00:22:26
    Okay.
  • 00:22:27
    So what I'm doing, right,
  • 00:22:29
    what I'm doing is putting more and more stuff
  • 00:22:33
    into one place.
  • 00:22:34
    So it's not enough...
  • 00:22:36
    So if you think about it,
  • 00:22:37
    it's not enough just to make the ball bigger.
  • 00:22:39
    So obviously, making it bigger
  • 00:22:41
    isn't gonna sort of give me this kind of tube
  • 00:22:44
    that I'm really looking for.
  • 00:22:46
    I need to somehow contract this stuff into a small space,
  • 00:22:49
    so I need something that's very dense.
  • 00:22:51
    So this ball is very dense.
  • 00:22:52
    So having a ball of the same size but a lower weight,
  • 00:22:56
    wouldn't work, right?
  • 00:22:58
    So this is the thing we have to do.
  • 00:23:02
    So actually, what we have to do
  • 00:23:04
    is we have to get all the mass that's in the sun,
  • 00:23:07
    so all the matter that makes up our sun,
  • 00:23:10
    and we have to sort of squash it into a region
  • 00:23:13
    that's only a kilometer across, right?
  • 00:23:16
    So kilometer is like, I don't know,
  • 00:23:18
    here to Green Park Station.
  • 00:23:19
    If it's not, it's a bit further.
  • 00:23:23
    No, okay, I've got no sense of distance.
  • 00:23:26
    So, yeah, what I need to do is sort of compact the sun
  • 00:23:33
    into a region that's very small.
  • 00:23:35
    And so on the face of it,
  • 00:23:36
    initially people thought black holes sounded crazy
  • 00:23:38
    because they said, well, you know, how did that happen?
  • 00:23:42
    That seems like it would be really hard to do, right?
  • 00:23:44
    Like, how do you get the sun and squash it?
  • 00:23:46
    So initially, people thought that black holes
  • 00:23:49
    were just a mathematical curiosity.
  • 00:23:53
    So yeah, as I said, they have...
  • 00:23:55
    So actually, the point I wanted to bake,
  • 00:23:59
    I was gonna do it with slides as well,
  • 00:24:00
    but then they made me do it with this.
  • 00:24:03
    So what I wanted to say is that actually,
  • 00:24:05
    technically speaking, black holes are this event horizon.
  • 00:24:10
    So it's quite interesting,
  • 00:24:11
    we don't actually describe black holes
  • 00:24:14
    by what they're made of.
  • 00:24:15
    So if you think all the other things
  • 00:24:16
    that we have in astrophysics, like a star or a planet,
  • 00:24:19
    we generally describe them by what they're made of.
  • 00:24:22
    But with black holes, we actually don't do that,
  • 00:24:24
    we actually define them as just being this surface
  • 00:24:28
    beyond which nothing can escape.
  • 00:24:30
    And one of the reasons that we do this
  • 00:24:32
    is because we really can't know what's in them.
  • 00:24:36
    So there's actually no way, if I fly up to a black hole,
  • 00:24:40
    for me to tell what it's made of.
  • 00:24:42
    So was it made of stars? Was it made of bowling balls?
  • 00:24:46
    Was it made of televisions that got thrown in?
  • 00:24:49
    You know, whatever I throw into a black hole
  • 00:24:51
    and make this black hole,
  • 00:24:52
    once it's formed this event horizon,
  • 00:24:54
    somehow I can't tell anymore what it's made of.
  • 00:24:57
    So it doesn't really make sense to define a black hole
  • 00:25:00
    as what it's made of.
  • 00:25:05
    But you might really wanna know,
  • 00:25:06
    you know, what is this object?
  • 00:25:07
    So there has to be something in there, right?
  • 00:25:09
    Like there has to have been something that fell in
  • 00:25:11
    that curved the spacetime that causes the black hole,
  • 00:25:14
    the spacetime to be so curved.
  • 00:25:17
    And so we would really like to know,
  • 00:25:19
    you know, what is this object that's curving the black hole?
  • 00:25:23
    So at this point, usually, people sort of...
  • 00:25:26
    Well, naively, people sort of think,
  • 00:25:27
    "Well, couldn't you somehow fly into the black hole
  • 00:25:32
    and then look at what's in the black hole, and then..?"
  • 00:25:36
    You know?
  • 00:25:37
    But then, no, you can't, right?
  • 00:25:38
    So there's actually so many reasons
  • 00:25:40
    that flying into a black hole is a bad idea.
  • 00:25:44
    It's actually, you know...
  • 00:25:45
    Always when I give this talk at schools and stuff,
  • 00:25:47
    all children want to fly into black holes.
  • 00:25:49
    They're like, "I could fly into the black hole."
  • 00:25:50
    I'm like, "No, don't fly into black holes."
  • 00:25:53
    Like, fortunately they don't have any way to do it,
  • 00:25:55
    so it's okay, but I'm always telling them like, "No."
  • 00:25:57
    So obviously, firstly, once you go in,
  • 00:26:00
    you can't get out again.
  • 00:26:02
    So it's pointless, right?
  • 00:26:03
    Like even if you went in and you looked
  • 00:26:05
    and you found that the black hole was made of stars,
  • 00:26:08
    you couldn't go out again and tell anyone, right?
  • 00:26:11
    So somehow, that knowledge is trapped
  • 00:26:13
    in the black hole with you.
  • 00:26:16
    Then in addition, it's deadly to fly into a black hole.
  • 00:26:20
    So at some point...
  • 00:26:22
    So because it's very curved,
  • 00:26:24
    so it's actually quite nice in this picture,
  • 00:26:26
    the part of you that's closer to the black hole
  • 00:26:29
    will be kind of falling into the black hole
  • 00:26:32
    much faster than the part further away.
  • 00:26:34
    So somehow, the gravitational force on it is different
  • 00:26:37
    on the part of you that's closer to the black hole
  • 00:26:39
    than further away from the black hole,
  • 00:26:41
    and so it ends up stretching you.
  • 00:26:43
    And this tidal stretching force
  • 00:26:45
    will actually kill you at some point.
  • 00:26:48
    So it doesn't actually necessarily happen
  • 00:26:50
    as you cross the event horizon.
  • 00:26:52
    A lot of people think
  • 00:26:53
    it's like a property of the event horizon.
  • 00:26:55
    So some very super massive black holes,
  • 00:26:58
    you could still cross the horizon
  • 00:27:00
    without being torn apart by tidal forces.
  • 00:27:03
    But eventually, as you got closer and closer
  • 00:27:05
    to the center of the black hole, you would die.
  • 00:27:08
    So as I say, it's really not a good idea.
  • 00:27:12
    And the final reason why it's really not a good idea
  • 00:27:14
    to fly into a black hole, or even very close to one,
  • 00:27:18
    is this point about time that I mentioned earlier,
  • 00:27:22
    that time changes.
  • 00:27:24
    So if you go very close to a black hole
  • 00:27:27
    and then you fly back to your planet,
  • 00:27:29
    what you'll find is that time hasn't passed as much for you
  • 00:27:34
    as it has for everyone else on your planet,
  • 00:27:36
    who is a long way from the black hole.
  • 00:27:38
    And so actually, when you get back home,
  • 00:27:41
    you know, it's a bit sad, everyone you ever knew is gone,
  • 00:27:43
    and, you know, maybe, I dunno,
  • 00:27:45
    your planet's not even theirs, being taken over by aliens.
  • 00:27:48
    So as I say, don't fly into a black hole, not a good idea.
  • 00:27:53
    But of course, black holes do exist,
  • 00:27:56
    I've told you they do exist in nature.
  • 00:27:58
    So how do they form? How are they made?
  • 00:28:00
    How do we crush something the size of the sun
  • 00:28:03
    into something a kilometer across?
  • 00:28:06
    Well, it turns out that actually, nature does it for us.
  • 00:28:09
    So this was first realized by Chandrasekhar.
  • 00:28:13
    So Chandrasekhar actually gave a talk
  • 00:28:16
    just down the road from here,
  • 00:28:17
    at the Royal Astronomical Society,
  • 00:28:20
    roughly a hundred years ago.
  • 00:28:22
    And he gave this really fantastic and interesting talk
  • 00:28:25
    on some research he'd been doing about massive stars.
  • 00:28:29
    So what he'd realized
  • 00:28:31
    is that if you have a very massive star,
  • 00:28:33
    when it comes to the end of its life
  • 00:28:35
    and it's burnt all of its fuel, it will start to collapse.
  • 00:28:40
    And people had said,
  • 00:28:41
    "Well, even though it starts to collapse at some point,
  • 00:28:44
    it will be supported again by electrons
  • 00:28:47
    in the matter of the start moving around very fast."
  • 00:28:51
    But what Chandrasekhar realized is
  • 00:28:54
    that's actually at some point, not enough.
  • 00:28:56
    So the electrons would start to have to move
  • 00:28:59
    faster than the speed of light
  • 00:29:00
    to be able to support the star against further collapse.
  • 00:29:04
    And since we know that things can't move faster
  • 00:29:06
    than the speed of light, he concluded that therefore,
  • 00:29:08
    it must continue to collapse.
  • 00:29:10
    And actually, this would end up creating something
  • 00:29:13
    that was compact enough to form a black hole.
  • 00:29:16
    And so he said, "A star of large mass
  • 00:29:18
    cannot pass into the white dwarf stage,
  • 00:29:21
    and one is left speculating about other possibilities."
  • 00:29:24
    So he realized this was quite controversial,
  • 00:29:26
    and so he put it in this slightly polite way,
  • 00:29:29
    you know, "speculating about other possibilities,"
  • 00:29:31
    but he understood what he was saying.
  • 00:29:35
    So unfortunately for Chandrasekhar,
  • 00:29:37
    the next person to take the stage was Arthur Eddington.
  • 00:29:41
    And Eddington...
  • 00:29:42
    So Eddington was an expert in general relativity.
  • 00:29:46
    So actually, Eddington was the person who proved
  • 00:29:48
    that Einstein was correct about general relativity
  • 00:29:51
    by measuring the bending of light around the sun.
  • 00:29:55
    But he really didn't like this idea.
  • 00:29:57
    He said, "I think there should be a law of nature
  • 00:30:00
    to stop a star behaving in this absurd way."
  • 00:30:03
    But notice the language he uses, right?
  • 00:30:05
    "I think there should be a law."
  • 00:30:09
    But there isn't one, right?
  • 00:30:10
    So he was actually wrong and it was just his idea
  • 00:30:14
    that this should not happen, that this was a crazy idea,
  • 00:30:16
    this couldn't possibly be true,
  • 00:30:18
    that led him to completely discount
  • 00:30:20
    the perfectly valid scientific result
  • 00:30:23
    that had been presented to him by Chandrasekhar.
  • 00:30:27
    And it was really unfortunate because actually,
  • 00:30:30
    of course Chandrasekhar was right,
  • 00:30:32
    but because Eddington was so well known and influential
  • 00:30:36
    and considered so clever, everyone listened to him,
  • 00:30:38
    and so the whole scientific community
  • 00:30:40
    just rejected this idea that Chandrasekhar had.
  • 00:30:43
    And it was 40 years later, 40 years,
  • 00:30:47
    before this idea was kind of revived
  • 00:30:49
    and shown to be correct.
  • 00:30:50
    So it was a kind of a big setback for science.
  • 00:30:54
    And of course, you know, you can actually speculate a bit
  • 00:30:57
    about the fact that Chandrasekhar being an Indian,
  • 00:31:02
    you know, was there an element of racism
  • 00:31:04
    in this dismissal of his idea?
  • 00:31:06
    I think that's certainly a possibility,
  • 00:31:08
    and it was certainly something that he felt was the case,
  • 00:31:12
    which is why he left the UK and went to the US.
  • 00:31:16
    But it was definitely a prejudice against how we think
  • 00:31:19
    that our universe should work, right?
  • 00:31:21
    We have these very strong prejudices about,
  • 00:31:23
    this is how our universe is and it can't behave another way.
  • 00:31:26
    And this led, as I say, to this idea being sadly discounted.
  • 00:31:33
    So even with this idea
  • 00:31:35
    about stars having to collapse inevitably into black holes
  • 00:31:39
    at the end of their life,
  • 00:31:41
    the real proof that a black hole is not a crazy idea
  • 00:31:45
    is experiment, right?
  • 00:31:47
    Is observation.
  • 00:31:48
    Like, ultimately, the real test is, do we see them?
  • 00:31:51
    Do they exist in our universe?
  • 00:31:54
    And so, yes,
  • 00:31:57
    so there's been some really exciting developments recently,
  • 00:32:00
    in black hole observations.
  • 00:32:03
    So this is one of them.
  • 00:32:04
    There's actually been a photo taken of the black hole
  • 00:32:08
    at the center of our galaxy.
  • 00:32:10
    This is the photo.
  • 00:32:11
    And so this is completely consistent
  • 00:32:14
    with what we expect a black hole to look like.
  • 00:32:16
    So obviously, we're not really seeing the black hole,
  • 00:32:18
    so in a way it's a bit of a cheat to say
  • 00:32:20
    it's a picture of a black hole.
  • 00:32:22
    We are really seeing the sort of hot gas and dust
  • 00:32:25
    around the black hole, that's falling into the black hole.
  • 00:32:29
    But what we see is that there's this dark patch
  • 00:32:32
    at the center, right?
  • 00:32:33
    So this dark patch is a part
  • 00:32:35
    where there is gas and dust falling in,
  • 00:32:37
    but somehow the light from it can't escape anymore
  • 00:32:40
    because it's gone past the event horizon.
  • 00:32:43
    So this image is, you know, in a sense,
  • 00:32:45
    the most direct proof we have that black holes exist.
  • 00:32:50
    So unfortunately for the people
  • 00:32:51
    at the event horizon telescope
  • 00:32:53
    who made this very nice image,
  • 00:32:54
    which was obviously a very massive scientific achievement,
  • 00:32:59
    Hollywood made one before,
  • 00:33:01
    and actually only a few years before.
  • 00:33:03
    So I felt really sorry for the people
  • 00:33:05
    at the event horizon telescope
  • 00:33:06
    because when they unveiled their image,
  • 00:33:08
    I think everyone was expecting this,
  • 00:33:11
    and then it was like, you know,
  • 00:33:12
    it was this.
  • 00:33:14
    And so it was a bit of like, "Oh, that's nice."
  • 00:33:20
    Okay, you can't beat Hollywood, right?
  • 00:33:22
    But, you know, this is a Hollywood image,
  • 00:33:25
    so it's from the movie "Interstellar,"
  • 00:33:26
    has anyone seen the movie "Interstellar,"?
  • 00:33:28
    Yeah, of course. Someone said, "Of course."
  • 00:33:30
    I take nothing for granted.
  • 00:33:32
    So, yeah, this is the super massive black hole
  • 00:33:37
    that appears in the movie, "Interstellar."
  • 00:33:39
    And it was really impressive because they actually did
  • 00:33:43
    a scientifically accurate representation
  • 00:33:45
    of what the black hole would look like.
  • 00:33:47
    So this, maybe, I don't know if you can see the reference,
  • 00:33:51
    but it's from a journal, "Classical & Quantum Gravity,"
  • 00:33:54
    so it was actually published as a scientific result
  • 00:33:56
    because no one had done such accurate simulations
  • 00:33:59
    of what this would look like before.
  • 00:34:01
    And, you know, it looks kind of fantastic,
  • 00:34:03
    and indeed, it kind of matches what we actually see.
  • 00:34:10
    So the second really exciting advance in black holes,
  • 00:34:15
    in observations of black holes,
  • 00:34:18
    is this one, is gravitational waves from black holes.
  • 00:34:21
    So this was in 2015,
  • 00:34:23
    we had the first observation of gravitational waves
  • 00:34:27
    coming from binary black holes.
  • 00:34:29
    So it turns out that binary...
  • 00:34:31
    So black holes can exist in pairs, in binaries,
  • 00:34:34
    where they orbit each other over time.
  • 00:34:37
    And because they're omitting these gravitational waves,
  • 00:34:39
    they lose energy.
  • 00:34:41
    So very much like my ball going around here,
  • 00:34:44
    you know, eventually, it will spiral in and they will merge.
  • 00:34:49
    And so when they merge, it's a hugely energetic event
  • 00:34:52
    and there is this ripples in space and time,
  • 00:34:55
    these gravitational waves that are given off,
  • 00:34:57
    and they reach us here on the Earth,
  • 00:34:59
    and we were able to measure them in 2015.
  • 00:35:02
    So this is the image of the wave that was seen.
  • 00:35:07
    This is the signal that was detected.
  • 00:35:10
    And so for me,
  • 00:35:11
    this is quite a kind of personally exciting image.
  • 00:35:16
    So 2015, so I actually started my PhD in 2013,
  • 00:35:20
    so I was two years into my PhD when this discovery was made.
  • 00:35:24
    And my sort of job, my day job certainly during my PhD,
  • 00:35:28
    was to generate these signals, to do computer simulations
  • 00:35:32
    that predicted what these signals would look like.
  • 00:35:36
    And so I was spending all day generating signals like this
  • 00:35:39
    and then suddenly, you know, wow, here they were in reality.
  • 00:35:43
    And we all got called into the seminar room
  • 00:35:45
    and we were sitting and watching the broadcast,
  • 00:35:48
    and it was just such a...
  • 00:35:50
    You know, it was quite emotional
  • 00:35:52
    to see that this had really been detected on Earth.
  • 00:35:58
    But as I say, yeah, this is the ultimate proof, right?
  • 00:36:01
    So black holes are real science,
  • 00:36:04
    they're not science fiction,
  • 00:36:06
    and the reasons are that they don't violate
  • 00:36:09
    any fundamental principles.
  • 00:36:11
    In addition, they really are inevitably,
  • 00:36:13
    the endpoint of super massive stars.
  • 00:36:16
    And then of course, finally,
  • 00:36:18
    the proof is that they've been observed.
  • 00:36:23
    Wormholes.
  • 00:36:25
    Okay, maybe Dan can take away the...
  • 00:36:29
    I can't make a wormhole demonstration.
  • 00:36:31
    So this is why I said I can't do a demonstration,
  • 00:36:33
    because how do you make a wormhole?
  • 00:36:35
    Well, I guess we could.
  • 00:36:36
    So I hope you all know what wormholes are.
  • 00:36:40
    Again, if you've watched enough science fiction,
  • 00:36:43
    you'll probably know.
  • 00:36:44
    We had a "Star Trek" fan, right?
  • 00:36:47
    Good.
  • 00:36:48
    Yeah, so this is an image from "Deep Space Nine,"
  • 00:36:52
    which is in my view, one of the finest "Star Trek" series.
  • 00:36:57
    But I have to say that... Thanks, Dan.
  • 00:36:59
    I have to say that that picture of a wormhole,
  • 00:37:02
    I'm not actually so keen on.
  • 00:37:04
    It's a bit...
  • 00:37:05
    It's not quite right.
  • 00:37:07
    So you can see it looks like a tunnel.
  • 00:37:09
    So as I say,
  • 00:37:11
    again, I kind of assume people know what wormholes are.
  • 00:37:14
    So wormholes are somehow like tunnels
  • 00:37:16
    that take you from one part of the universe to another,
  • 00:37:19
    quickly, you know, a shortcut that stops you having to go
  • 00:37:22
    the long way round through space.
  • 00:37:23
    So in "Deep Space Nine,"
  • 00:37:24
    it takes you from the alpha quadrant to the delta quadrant
  • 00:37:27
    It's very useful.
  • 00:37:29
    So this one is sort of shown like this tunnel,
  • 00:37:33
    it sort of opens up like, "Whew!"
  • 00:37:35
    And then you fly into the tunnel
  • 00:37:36
    and it's got like an entranceway.
  • 00:37:39
    And certainly, they were influenced
  • 00:37:40
    by this picture of wormhole.
  • 00:37:43
    So again, this is one of these two dimensional pictures
  • 00:37:46
    of four dimensional spacetime.
  • 00:37:49
    So this is quite a bad picture because the rocket
  • 00:37:53
    is like not on the surface.
  • 00:37:55
    So in these two dimensional pictures,
  • 00:37:57
    you're supposed to stay on the surface, right?
  • 00:37:59
    You can't go into the higher dimension,
  • 00:38:01
    you're supposed to roll along the surface like a marble
  • 00:38:04
    and then go through this tunnel and come out the other side.
  • 00:38:07
    So this is the kind of two dimensional picture
  • 00:38:10
    of the four dimensional wormhole in space and time.
  • 00:38:15
    And fortunately for us, "Interstellar" made another picture
  • 00:38:20
    that was much better, of an intra-universe wormhole,
  • 00:38:25
    so one that's going from one point to another.
  • 00:38:27
    So this is the image they made,
  • 00:38:28
    and again, it's scientifically accurate in some ways.
  • 00:38:32
    So what a wormhole would look like is this,
  • 00:38:34
    it would look like some kind of soap bubble.
  • 00:38:37
    You know, it doesn't have an entrance, right?
  • 00:38:39
    Like, you can fly around this bubble
  • 00:38:42
    and you can go into it from any direction,
  • 00:38:44
    and once you go in, you'll go through this tunnel
  • 00:38:46
    and you'll come out the other side.
  • 00:38:47
    Or in principle, that's how it works.
  • 00:38:50
    And what you're seeing through the wormhole,
  • 00:38:52
    these things that make it look shiny,
  • 00:38:55
    that's the light from the galaxies and the stars
  • 00:38:58
    on the other side of the wormhole,
  • 00:38:59
    coming through the wormhole to you.
  • 00:39:02
    So this is how it would really look in space.
  • 00:39:08
    Okay, so let's play the game
  • 00:39:10
    like we played with the black hole.
  • 00:39:12
    How do I make a wormhole?
  • 00:39:15
    So this is when we start to immediately have problems.
  • 00:39:18
    So I want a wormhole, but I don't want...
  • 00:39:21
    I want to be able to go through it.
  • 00:39:23
    So a traversable wormhole is one...
  • 00:39:26
    So you might have noticed it kind of looks like
  • 00:39:28
    two black holes stuck together,
  • 00:39:31
    but I don't want it to be two black holes stuck together
  • 00:39:34
    because if it's two black holes stuck together,
  • 00:39:36
    I'll get stuck once I get to their event horizon.
  • 00:39:39
    So you can have it so that, you know,
  • 00:39:41
    if you went through the event horizon,
  • 00:39:43
    you'd end up stuck in this...
  • 00:39:45
    Let me go back to this image.
  • 00:39:46
    You'd get stuck in the middle of this throat
  • 00:39:49
    and you couldn't go out either end,
  • 00:39:51
    and that would obviously not be a useful wormhole.
  • 00:39:53
    I want one that I can go all the way through.
  • 00:39:56
    And I also don't wanna be pulled apart by these tidal forces
  • 00:39:59
    that you get when you get sort of into a black hole.
  • 00:40:04
    So if I want those things,
  • 00:40:07
    then I need exotic matter in order to support the wormhole.
  • 00:40:12
    So what is exotic matter?
  • 00:40:13
    So just broadly speaking,
  • 00:40:16
    exotic matter is something that has negative energy.
  • 00:40:21
    So you could ask, what's negative energy?
  • 00:40:24
    That's a very good question.
  • 00:40:26
    So everything that we know of in our universe
  • 00:40:28
    has positive energy.
  • 00:40:30
    So you, me, stars, planets,
  • 00:40:32
    everything is a positive energy object, right?
  • 00:40:36
    So even things in cosmology
  • 00:40:38
    that we don't understand very well,
  • 00:40:40
    dark matter and dark energy,
  • 00:40:42
    these components of our universe
  • 00:40:43
    that we actually don't understand,
  • 00:40:45
    they also have positive energy,
  • 00:40:47
    and so they're not something that we can build
  • 00:40:49
    a wormhole with.
  • 00:40:51
    But, okay, if we just assume
  • 00:40:53
    that somehow we do get some negative energy from somewhere,
  • 00:40:58
    then can I somehow stuff this negative energy density
  • 00:41:02
    into one place, and punch a hole through spacetime,
  • 00:41:07
    and then have it form a wormhole?
  • 00:41:11
    No, is the short answer.
  • 00:41:15
    So it turns out that that is just forbidden
  • 00:41:18
    by the laws of physics as we know them.
  • 00:41:22
    So the really difficult thing is this punching a hole.
  • 00:41:24
    So if I wanna go from the image on the left
  • 00:41:27
    to the image on the right,
  • 00:41:28
    I wanna go from this sheet to this wormhole thing,
  • 00:41:31
    what I need to do is fold it over,
  • 00:41:33
    and that's okay,
  • 00:41:35
    but then I need to somehow punch a hole, as I said,
  • 00:41:38
    in the top and bottom, and then stick them back together.
  • 00:41:41
    And it's this punching a hole and sticking back together
  • 00:41:44
    that seems to not be possible.
  • 00:41:46
    So forming a wormhole in this way,
  • 00:41:49
    so building one from scratch, is somehow not allowed.
  • 00:41:53
    However, if one already happened to exist in the universe,
  • 00:41:58
    if say during the big bang,
  • 00:42:01
    the universe formed with lots of connections
  • 00:42:03
    between different points in spacetime,
  • 00:42:06
    then maybe I could get hold of one of these
  • 00:42:09
    and like, you know,
  • 00:42:10
    reinforce it with some additional exotic matter,
  • 00:42:13
    and then use it to go through.
  • 00:42:15
    So this is mining wormholes.
  • 00:42:18
    So we want to mine wormholes,
  • 00:42:19
    we don't want to build them from scratch.
  • 00:42:23
    So Matt Visser compares this to a recipe for dragon stew.
  • 00:42:27
    First find a dragon, right?
  • 00:42:30
    Like, you know, it kind of seems...
  • 00:42:32
    So it's not like black holes, right?
  • 00:42:34
    So with black holes, we have a sort of a natural way
  • 00:42:37
    for them to form in our universe,
  • 00:42:39
    whereas with worm holes, we kind of have to say
  • 00:42:41
    that they're already there.
  • 00:42:46
    The thing that really sort of does it I think,
  • 00:42:48
    for wormholes, is that they allow time travel.
  • 00:42:51
    So has everyone seen "Back to the Future,"?
  • 00:42:55
    I don't know, because I showed this
  • 00:42:56
    to one of my younger collaborators the other day
  • 00:42:59
    and they were like, "What's 'Back to the Future'?"
  • 00:43:01
    And I was like, "No, this is not possible.
  • 00:43:03
    You must know what 'Back to the Future' is," right?
  • 00:43:05
    Okay, anyway, so for young people who are, you know,
  • 00:43:08
    it's not on TikTok, so don't know,
  • 00:43:12
    "Back to the Future," Marty uses a flux capacitor...
  • 00:43:16
    Okay, it's not real science.
  • 00:43:18
    Uses a flux capacitor to go back in time,
  • 00:43:21
    and accidentally prevents his parents from getting together,
  • 00:43:25
    thereby, you know, stopping himself from existing,
  • 00:43:28
    and so he has to fix it before he goes back home.
  • 00:43:30
    So already you see, there are lots of problems, right?
  • 00:43:33
    When you you start to have time travel,
  • 00:43:35
    we've all watched films with time travel in,
  • 00:43:38
    and it's always a mess, and it's always unsatisfactory,
  • 00:43:40
    because somehow it doesn't all join up correctly.
  • 00:43:43
    So really, this is what in physics,
  • 00:43:47
    we call this concept of causality, right?
  • 00:43:50
    So causality is really at the sort of the base
  • 00:43:53
    of every physical theory.
  • 00:43:55
    And it's this idea that somehow,
  • 00:43:57
    you know, things have an ordering,
  • 00:43:59
    and so if I have a fire and that fire spreads over time,
  • 00:44:04
    and then it sets some factory on fire,
  • 00:44:07
    then there's an explosion at that factory.
  • 00:44:10
    That explosion cannot have started the fire, right?
  • 00:44:13
    Oh, so I can't have a loop in time, right?
  • 00:44:18
    I can't have the thing that happens in the future,
  • 00:44:21
    causing the thing in the past.
  • 00:44:22
    And this seems so obvious that it's like,
  • 00:44:24
    you know, we hardly ever say it.
  • 00:44:26
    It's always like a, you know,
  • 00:44:28
    zeroth more minus one law of physics,
  • 00:44:30
    that there should be this idea of causality.
  • 00:44:35
    And wormholes, as I say, wormholes potentially break that.
  • 00:44:39
    So in order to make
  • 00:44:41
    these closed time-like curves from wormholes,
  • 00:44:44
    you have to take the two wormholes
  • 00:44:46
    and you have to make them move relatively to each other
  • 00:44:48
    so that the two mouths of the wormhole,
  • 00:44:52
    you have to move them relative to each other,
  • 00:44:54
    and then you have to kind of bring them together.
  • 00:44:57
    And at some point when they get close enough,
  • 00:44:59
    they will form one of these closed time-like curves,
  • 00:45:01
    these loops in time,
  • 00:45:03
    and allow you to travel backwards into the past.
  • 00:45:08
    So obviously, people really don't like this,
  • 00:45:10
    like this is really bad.
  • 00:45:12
    So there are some solutions,
  • 00:45:14
    saying that they're solutions is probably a bit strong.
  • 00:45:18
    I don't think they've really solved the problem,
  • 00:45:20
    they are all conjectures.
  • 00:45:22
    So how could I have wormholes
  • 00:45:25
    and still have things be okay for time travel?
  • 00:45:30
    So most of these are from science fiction
  • 00:45:32
    so I don't really need to explain them,
  • 00:45:33
    but there's multiple universe timelines, right?
  • 00:45:36
    So when Marty goes backwards in time
  • 00:45:38
    and he stops his parents getting together,
  • 00:45:40
    the idea would be here, that he doesn't affect his timeline,
  • 00:45:45
    there's somehow a branching of a new universe that comes off
  • 00:45:49
    in which his parents don't get together and he's never born.
  • 00:45:52
    So he can still exist
  • 00:45:53
    because he comes from this other timeline.
  • 00:45:55
    But, you know, there are then multiple universes
  • 00:45:58
    kind of branching off
  • 00:45:59
    every time someone does one of these time traveling events.
  • 00:46:02
    And so the problem with this is,
  • 00:46:03
    you know, we have no known mechanism
  • 00:46:05
    for branching the universe into many different time paths.
  • 00:46:09
    As I say, that's still science fiction, sadly.
  • 00:46:14
    The Novikov consistency conjecture.
  • 00:46:17
    This is a very like,
  • 00:46:18
    sort of formal sounding name for something that again,
  • 00:46:21
    you probably know from science fiction films, where...
  • 00:46:25
    So what happens here is this is like where I go back in time
  • 00:46:29
    to try and stop my friend being killed,
  • 00:46:31
    but then somehow accidentally,
  • 00:46:33
    I become the person that kills her, you know?
  • 00:46:36
    So somehow what has happened must always happen.
  • 00:46:39
    So despite the fact that I go back in time,
  • 00:46:41
    I can't change things,
  • 00:46:42
    you know, history is set in stone and by going back in time,
  • 00:46:46
    I will only propagate that loop as it has always been.
  • 00:46:51
    And so this is very unsatisfactory
  • 00:46:53
    for the same kind of reasons
  • 00:46:54
    that the films that include it are very unsatisfactory
  • 00:46:57
    because it's somehow very contrived, right?
  • 00:47:01
    Like, it means that we don't have control over what we do
  • 00:47:05
    and it makes things that are very unlikely somehow certain.
  • 00:47:08
    So like, you know, I go back in time
  • 00:47:10
    to try and kill someone,
  • 00:47:11
    it means that the gun must jam at the last minute,
  • 00:47:14
    which would be a very unlikely event,
  • 00:47:17
    but that somehow has to happen
  • 00:47:18
    because I can't be allowed to kill this person.
  • 00:47:22
    So this one, as I say, I find a bit unsatisfactory.
  • 00:47:25
    There's also the "chronology protection" conjecture.
  • 00:47:29
    So this one is, if you think of the wormholes,
  • 00:47:31
    I had to sort of bring them somehow together
  • 00:47:34
    to form this closed time-like loop.
  • 00:47:37
    And so this one sort of says something,
  • 00:47:41
    sort of maybe quantum physics, quantum gravity,
  • 00:47:44
    you know, waving my hands here,
  • 00:47:46
    something stops me from moving them close enough together
  • 00:47:49
    to form this time-like loop.
  • 00:47:50
    So somehow they will repel each other
  • 00:47:52
    in a way that prevents me from ever forming
  • 00:47:55
    this closed time-like loop.
  • 00:47:59
    And of course, the final one,
  • 00:48:02
    and probably sadly, the most likely one
  • 00:48:04
    is the "boring physics" conjecture,
  • 00:48:05
    which just tells you that wormholes sadly don't exist.
  • 00:48:11
    Okay. Warp drives.
  • 00:48:15
    So, okay, warp drives.
  • 00:48:17
    Warp drives, again, if you're a "Star Trek" fan,
  • 00:48:19
    warp drives are everyday stuff.
  • 00:48:21
    So the idea of the warp drive
  • 00:48:23
    is actually very inspired by cosmology,
  • 00:48:25
    by this idea of our universe expanding.
  • 00:48:29
    So the space in front of my warp ship
  • 00:48:33
    is contracting over time,
  • 00:48:36
    and the space behind is expanding over time.
  • 00:48:39
    And so this somehow drives me forward through space.
  • 00:48:43
    So for me, I just feel like I'm kind of sitting there
  • 00:48:45
    in this bubble, but for someone outside the bubble,
  • 00:48:48
    I'm traveling potentially faster than the speed of light.
  • 00:48:52
    So you don't have to travel faster than the speed of light
  • 00:48:54
    in a warp ship, it could be one that traveled slower
  • 00:48:56
    than the speed of light.
  • 00:48:58
    But, you know, somehow you're being powered forward
  • 00:49:00
    by this contraction and expansion of space time
  • 00:49:04
    behind and in front of the ship.
  • 00:49:08
    So the bad news,
  • 00:49:10
    warp drives, like wormholes, require exotic matter.
  • 00:49:15
    So we have all of the problems
  • 00:49:16
    associated with needing to find some
  • 00:49:19
    sort of negative energy material.
  • 00:49:22
    They also violate causality.
  • 00:49:24
    You can also create these kind of paradoxes
  • 00:49:27
    where you generate time travel.
  • 00:49:29
    And so again, they're slightly unlikeable for that reason.
  • 00:49:35
    A really practical point about warp drives
  • 00:49:38
    is how do you steer one?
  • 00:49:40
    So it turns out that if you are in this bubble
  • 00:49:42
    and you want to send a signal to the front of the bubble,
  • 00:49:46
    you can't, right?
  • 00:49:47
    You actually can't get a message to the front of the bubble,
  • 00:49:50
    and that means you can't turn it off.
  • 00:49:52
    So once you are in the warp drive,
  • 00:49:55
    you just kind of have to keep going, right?
  • 00:49:57
    So that's not, again, very useful
  • 00:50:00
    if you really want to use it as a warp drive
  • 00:50:02
    for traveling through spacetime.
  • 00:50:04
    But okay, let's just set these aside
  • 00:50:08
    and see if we could make gravitational waves with one.
  • 00:50:11
    So this is...
  • 00:50:13
    I have to emphasize, this is a bit of a hobby of mine.
  • 00:50:18
    So I'm very aware that most of the people in the audience
  • 00:50:20
    are probably British taxpayers and you do pay my salary,
  • 00:50:25
    so I want to assure you that I do actually do
  • 00:50:28
    other more useful stuff in my main research.
  • 00:50:33
    Well, you can judge that for yourself. I also do teaching.
  • 00:50:37
    So in my spare time, my hobby is to simulate warp drives.
  • 00:50:44
    So as I say, you know, these objects,
  • 00:50:46
    although they're science fiction,
  • 00:50:47
    they actually have quite good descriptions in physics.
  • 00:50:50
    And so I can actually build one in my computer simulation
  • 00:50:55
    and I can actually see whether I can generate
  • 00:50:57
    gravitational waves from it,
  • 00:50:58
    that maybe could be observed here on Earth.
  • 00:51:02
    So it's an international collaboration.
  • 00:51:06
    So Tim "Jim T." Dietrich is the captain,
  • 00:51:11
    so he's actually an expert in neutron star physics
  • 00:51:14
    from Potsdam University.
  • 00:51:17
    I also have Sebastian "Wrath of" Khan
  • 00:51:21
    who is an expert in gravitational wave data science
  • 00:51:24
    and data analysis from Cardiff University.
  • 00:51:27
    And then there's me,
  • 00:51:29
    and I'm very much like Scotty in the original "Star Trek"
  • 00:51:31
    in that I do all the work
  • 00:51:34
    and these guys literally do nothing.
  • 00:51:36
    So Tim just writes me emails saying,
  • 00:51:38
    "Have you done the simulations yet?"
  • 00:51:40
    And I'm like, "You know, I have a day job
  • 00:51:42
    so there's things I have to do."
  • 00:51:45
    And Sebastian just sends us all "Star Trek" memes
  • 00:51:48
    all the time,
  • 00:51:49
    and episodes of "Star Trek" that he's been watching,
  • 00:51:51
    thinking about warp drives.
  • 00:51:53
    So as I say, I am the one who does all the work.
  • 00:51:57
    And so we've actually done some calculations,
  • 00:51:58
    so this is to prove that we have calculations,
  • 00:52:00
    we have simulations, we even wrote a code to do this.
  • 00:52:04
    So this is our computer, it's available on GitHub,
  • 00:52:07
    so for anyone who wants to download,
  • 00:52:08
    it's publicly available, a warp drive code.
  • 00:52:11
    And I will now share with you our results.
  • 00:52:15
    So in this slightly poor looking movie,
  • 00:52:20
    you can see that there's a blue patch and a red patch.
  • 00:52:23
    And so the blue patch is the part of the spacetime
  • 00:52:26
    that's expanding behind the warp ship,
  • 00:52:29
    and the red part is the part that's...
  • 00:52:32
    Sorry, yeah, the part that's expanding,
  • 00:52:34
    and the red part's the part that's contracting
  • 00:52:36
    in front of the warp ship.
  • 00:52:37
    So like in my previous picture.
  • 00:52:39
    And so the premise for this simulation is
  • 00:52:42
    the containment field for the warp drive has broken down,
  • 00:52:46
    and so the warp drive is gonna...
  • 00:52:47
    The warp bubble is gonna collapse
  • 00:52:50
    and give off gravitational waves.
  • 00:52:52
    Okay, so are you ready? Are you watching the bubble?
  • 00:52:54
    Okay.
  • 00:52:56
    (dramatic music)
  • 00:52:59
    So the thing that took me the longest
  • 00:53:00
    was actually attaching the soundtrack to this movie.
  • 00:53:04
    Doing the simulation was easy,
  • 00:53:05
    but then I couldn't work out how to do the thing.
  • 00:53:08
    So I'll show it to you again 'cause I'm so proud of it.
  • 00:53:11
    (audience laughing)
  • 00:53:12
    I have to go back.
  • 00:53:13
    (dramatic music)
  • 00:53:17
    Thank you. Thank you.
  • 00:53:18
    (audience applauding)
  • 00:53:23
    So what you might have seen is that it's actually very bad
  • 00:53:26
    for the people in the warp bubble.
  • 00:53:28
    So you probably noticed
  • 00:53:29
    that everything kind of collapses inwards
  • 00:53:31
    before it goes out again,
  • 00:53:33
    so actually they would be completely torn apart
  • 00:53:35
    by this like, stretching of spacetime,
  • 00:53:38
    and they would sadly, unfortunately, die.
  • 00:53:41
    But you can also see that there are waves being given off.
  • 00:53:44
    So now we have these gravitational wave signals
  • 00:53:47
    that we've generated,
  • 00:53:49
    we can actually go and look in the data
  • 00:53:51
    from the Ligo gravitational wave detector,
  • 00:53:54
    and maybe in the future,
  • 00:53:56
    you may see this news article saying
  • 00:53:59
    that Einstein's gravitational waves have been seen
  • 00:54:02
    from warp drives.
  • 00:54:03
    Or sadly, maybe not.
  • 00:54:08
    Okay, so that's all I wanted to tell you about black holes,
  • 00:54:13
    wormholes, and warp drives.
  • 00:54:15
    So, you know, I had this...
  • 00:54:16
    "What's the Difference Between a Crazy Idea
  • 00:54:18
    and a Brilliant One?"
  • 00:54:19
    So that was the premise of my talk.
  • 00:54:21
    So when I started writing this talk,
  • 00:54:23
    I actually thought it would be quite easy to say,
  • 00:54:27
    "No, this is where the line is drawn.
  • 00:54:29
    Don't worry guys, we've got it all under control.
  • 00:54:31
    We know what's real and what's not real."
  • 00:54:34
    But actually, some of the issues were quite subtle,
  • 00:54:36
    you know, these questions that I raised about exotic matter.
  • 00:54:40
    You know, there are cases where you have quantum effects
  • 00:54:43
    where you can generate
  • 00:54:44
    small amounts of negative energy density,
  • 00:54:47
    not enough to power a warp ship.
  • 00:54:49
    But, you know, in principle,
  • 00:54:50
    there are aspects of physics that we don't understand well,
  • 00:54:53
    particularly this quantum realm
  • 00:54:56
    where you have very small scale physics
  • 00:54:58
    that interacts with gravity.
  • 00:54:59
    We just don't understand that well.
  • 00:55:01
    So, you know, sometimes,
  • 00:55:03
    I can think, you know, maybe,
  • 00:55:06
    maybe somewhere in the universe,
  • 00:55:08
    someone has figured this out
  • 00:55:10
    and they've figured it out in a way that they can use it
  • 00:55:12
    to power warp drives, for example.
  • 00:55:15
    I think unfortunately, it's probably not the case.
  • 00:55:19
    It's the "boring physics" conjecture is gonna win out.
  • 00:55:22
    But, you know, we should always remember the story
  • 00:55:25
    of Chandrasekhar and Eddington
  • 00:55:27
    and remember to keep an open mind to these kind of ideas.
  • 00:55:32
    And so I will stop there,
  • 00:55:34
    and with fear and trepidation, ask for questions.
  • 00:55:38
    (audience laughing and applauding)
Tags
  • Astrophysics
  • Spacetime
  • Gravity
  • Black Holes
  • Wormholes
  • Warp Drives
  • Exotic Matter
  • Hubble Expansion
  • Einstein
  • Mathematics