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Society of Physics Students Lecture Einstein, Black Holes and Gravity Waves, oh my! Prof. David Toback Texas A&M University Mitchell Institute for Fundamental Physics and Astronomy February Feb 2016 David Toback,2016 SPS Lecture Outline • Quick overview of what happened • Backtrack and learn some of the physics, especially General Relativity – Gravity, Black holes and Gravity waves • Back to the discovery of Gravity Waves • Why it’s important in many ways, and why it’s not that new in others Feb 2016 David Toback, SPS Lecture 2 50 Word Summary • About a billion years ago, two 30MSun black holes spiraled in together to create a 60MSun black hole • This merger was so violent that it emitted a huge amount of gravity waves over a short amount of time that were recently discovered here on Earth by the LIGO experiment Note: This is NOT the discovery of the century (IMHO), it is the discovery that took a century – Although it IS pretty awesome Feb 2016 David Toback, SPS Lecture 3 Einstein in the 1910’s • In the early 1910’s Einstein was thinking about some recent experimental results that didn’t make any sense to him using Newton’s theories • Decided we need new ways of thinking about space, time and Gravity • Einstein says that Newton’s Laws aren’t really quite right… • Einstein’s theory is known as the “General Theory of Relativity” Feb 2016 2nd year Grad School Course David Toback, SPS Lecture 4 Newton vs. Einstein Newton: The Earth moves around the Sun because of “the force of gravity” is pulling it Einstein: There is no “force” of Gravity, the Earth moves in a “straight line” around the Sun in the curved space-time created by the Sun This is a VERY different way of thinking about things… Feb 2016 David Toback, SPS Lecture 5 Curved Space? An analogy is to think of curved spacetime as looking like one of those gravity wells you’ve seen downstairs Feb 2016 1st floor of the Mitchell Physics building (MPHY) David Toback, SPS Lecture Another Weird Thing: Mass Curves Space-Time Think of each of the heavy things in the universe (stars, planets etc.) like a ball in the middle of a taut rubber sheet that represents space-time The weight of the ball will make it sink into the rubber sheet, creating a cone shaped dent around it Feb 2016 David Toback, SPS Lecture Mass Curves Space The heavier the ball, the bigger the dent in space-time! Feb 2016 David Toback, SPS Lecture 8 Mass in curved space-time A mass moves in a “straight line” in curved space-time In this example, this straight line in curved space-time makes the path of the small ball look like something is pushing it toward the big ball in 3dimensions Feb 2016 David Toback, SPS Lecture 9 The Earth and the Sun Newton: The Earth moves around the Sun because of “the force of gravity” is pulling it Einstein: There is no force, the Earth moves in a straight line in four dimensions, but the curved space around the Sun makes it go in an orbit in the three space dimensions Feb 2016 David Toback, SPS Lecture The way the Planets go Around the Sun in General Relativity Feb 2016 David Toback, SPS Lecture 11 From Curved Space Time to Gravity Waves • If I have a star, or black hole, just sitting in space then other things know where it is because of the curvature of space-time • If its just sitting there (or moving with a constant velocity) space-time isn’t changing • If it accelerates, then this information is passed through space-time at the speed of light as gravity waves – Similar to if an electric charge was accelerated, or if I dropped a rock in water (or dragged my finger through it). Feb 2016 David Toback, SPS Lecture 12 First Indirect Evidence of Gravity Waves • In 1974, Taylor and Hulse discovered two neutron stars (one of which was a Pulsar which is VERY bright) orbiting each other • They measured the period (how fast they orbit) and saw that they were CLEARLY slowing down • Why were they losing energy? They were emitting gravity waves exactly as predicted by General Relativity – Nobel Prize 1993 – Been looking for DIRECT detection ever since Feb 2016 David Toback, SPS Lecture 13 Two Black Holes Colliding/ Gravity Waves What it would Look Like if you could see it (note the gravitational lensing) https://www.youtube.com/watch?v=I_88 S8DWbcU From the Warp of Space-Time Point of view https://www.youtube.com/watch?v=qfdyg f0TL2U Feb 2016 David Toback, SPS Lecture 14 Gravity Waves Moving Through Space A good set of short clips that help tell the story, including how the gravity waves got to us and how they were detected https://www.youtube.com/watch ?v=FlDtXIBrAYE Feb 2016 David Toback, SPS Lecture 15 Feb 2016 David Toback, SPS Lecture 16 Feb 2016 David Toback, SPS Lecture 17 What the Data Looked Like Ripples In Space-Time that reached us from two ~30MSun Black Holes as they spiral into each other about a billion light-years away Feb 2016 David Toback, SPS Lecture 18 Conclusion • The first direct observation of gravity waves is very exciting, and many years in the making • This allows us to see things we’ve never seen before that don’t produce light • Lots of fun questions still remain to be answered – How often does this happen in the universe? – Where did 30 solar mass black holes COME FROM? – What other new and exciting things will come from this powerful new technology? • Enjoy! Feb 2016 David Toback, SPS Lecture 19 Interested in learning more? • Physics department offers a course entitled “Big Bang & Black Holes” (ASTR/PHYS 109) – Covers Stephen Hawking’s “Brief History of Time” – Origin and Evolution of the Universe – How do stars form? – What is Dark Matter? Dark Energy? – What are Black Holes? – More on General Relativity, Quantum Mechanics and Particle Physics – Has a lab (if you want) – There is an option to take is an Honors class http://people.physics.tamu.edu/toback/109/ Feb 2016 David Toback, SPS Lecture End of Lecture Feb 2016 David Toback, SPS Lecture 21 Why Gravity Next? • By looking at photons/light we can learn about things here on Earth as well as about inner/outer space • What else do we need to describe/understand the Universe? Quantum Mechanics and Gravity • To understand the answer to these questions we need to learn more Feb 2016 David Toback, SPS Lecture 22 The Two Great Theories • General Relativity: Chapter 6 (Gravity) Predictions about the very large, from sizes of a few meters to the size of the universe (1024 miles across) Chapter 7 • Quantum Mechanics: Predictions about the very small (atoms, particles,<10-10 m) Feb 2016 David Toback, SPS Lecture Overview of Gravity for the Course 1. What’s so important about Gravity? 2. Newton’s Theory of Gravity 3. Einstein’s morecorrect version –Curved SpaceTime, and evidence for it with Dark Matter David Toback, SPS Lecture Feb 2016 Gravity: Why do we care? • Gravity: The great attraction in the Universe • Gravity is the theory that predicts the attraction and the motion of BIG things over large distances: – Planets – Suns – Galaxies – How Galaxies form etc. Feb 2016 David Toback, SPS Lecture 25 Newton and Gravity 1. Everything moves in a straight line unless acted upon by a force 2. Gravity is a force – Every object in the universe attracts every other object in the universe – The further the distance between the objects, the smaller the attraction – The bigger the mass, the Physics 201 bigger the attraction or 218 • Light is massless not affected by gravity Feb 2016 David Toback, SPS Lecture Large Number of Scales Kinda amazing! Gravity covers the attraction between – An apple near the Earth – The Earth and the Moon – The Earth and the Sun – The Sun and our galaxy – Our galaxy and the universe – Every particle in the universe and an apple – The Earth and you – Bevo and Reveille Feb 2016 David Toback, SPS Lecture Gravity continued… The force of Gravity makes the Moon “fall” towards the Earth Call this an orbit – Does a great job of explaining how the planets move around the Sun Feb 2016 Without gravity, the Moon would fly off into space and ignore the Earth completely David Toback, SPS Lecture What’s next? Tell you the different, and surprising, way that Einstein describes space, time and gravity Then tell why his description of why the Earth goes around the Sun is better than Newton’s Feb 2016 David Toback, SPS Lecture 29 Observational Fact Light *ALWAYS* moves at the speed of light to all observers So what? Feb 2016 David Toback, SPS Lecture 30 What happens if I’m driving a car moving at half the speed of light and I turn the headlights on? Feb 2016 David Toback, SPS Lecture 31 Two observers get different answers The light is one foot ahead of that car! The light is two feet ahead of me! Feb 2016 David Toback, SPS Lecture 32 Einstein’s Answer Feb 2016 Einstein says both observers are correct Space and Time are more related than we thought 33 David Toback, SPS Lecture Start with Space-Time Can’t think of Space and Time as separate – Space (measured with a ruler) – Time (measured with a clock) Single combined entity which we call four dimensional space-time – If the four dimensions are related, unexpected things that we’re not used to can happen Feb 2016 David Toback, SPS Lecture 34 Other Weirdness • Space and time are not only more related than we thought, Space can Curve • Need to talk about what we mean by Curved Space-Time Feb 2016 David Toback, SPS Lecture 35 We’ll start by using the strange “new” words of General Relativity in an example about why we care, and THEN explain them a bit more Space-Time and Gravity Feb 2016 David Toback, SPS Lecture 36 Moving in Curved Space: Analogy • Let’s say my friend and I are at the equator and we both start working due North – Exactly parallel to each other • We will notice that we “mysteriously” are getting closer to each other, and will eventually bump into each other • Is there a Force drawing us together? No… moving in curved space time LOOKS like a force Feb 2016 The rules of geometry are different in curved space http://en.wikipedia.org/wiki/Parallel_(geometry) David Toback, SPS Lecture Ok… • Both Newton and Einstein correctly predict how the planets go around the Sun – Actually mostly their predictions for the locations of the planets at any given time are almost identical • How do we decide if one is right and one is wrong? • Do an experiment where the predictions from the two competing theory's are very different!!! Feb 2016 David Toback, SPS Lecture 38 When do General Relativity and Newton predict different things? • Ok… One equation, but I’m guessing you’ve heard this one 2 E=MC Ok… what is this equation saying? Energy and mass may not be the same, but in Einstein’s theory they are equivalent Feb 2016 David Toback, SPS Lecture So what? What about light? It has energy, but no mass To Einstein, its mass doesn’t matter, it is a particle that moves in curved space-time Feb 2016 David Toback, SPS Lecture 40 What happens to light as it goes past the Sun? Path doesn’t curve Path does curve • Newton’s theory: Only things with “real” mass “feel” the force of gravity • General Relativity: Objects move according to the curve of space-time, regardless of whether they have mass or not Feb 2016 David Toback, SPS Lecture 41 Do an Experiment 1st Experiment: Looks like Can see the its over star directly here! If Einstein is 2nd Experiment: right, the Sun Sun in the way curves the path If Newton is of the starlight right, we won’t see the star Question: Where Gross exaggeration of sizes Feb 2016 does the star appear when you do the experiment? David Toback, SPS Lecture The Great Experiment of 1915 Look at a star’s position “behind” the sun as it “passes” in between us and the star during an eclipse to block the glare of the sunlight Feb 2016 David Toback, SPS Lecture 43 Another View Feb 2016 David Toback, SPS Lecture 44 Evidence and More Evidence • The results exactly agreed with Einstein’s predictions – Contradict the predictions of Newton • Over time, even more compelling evidence has come in in favor of General Relativity observations of Dark Matter in galaxies • Next give some of the evidence for dark matter and that the story hangs together Feb 2016 David Toback, SPS Lecture 45 Evidence galaxies contain Dark Matter • Described the Dark Matter in the Universe and in galaxies in Chapter 2 • In order to better understand dark matter in galaxies, its useful to show some evidence that there IS Dark Matter in galaxies • Start by considering the case that there IS no Dark Matter in galaxies Feb 2016 David Toback, SPS Lecture How stars move in galaxies • Laws of gravity accurately predict the orbits of planets and stars as they move around the solar system and galaxy • The brightest region of both places is the center lots of mass there • For the solar system, the data agree perfectly, but for the outer part of galaxies it should look like stars in orbit around a massive center • Problem: This isn’t what the data shows Feb 2016 David Toback, SPS Lecture 47 The Data Feb 2016 David Toback, SPS Lecture 48 Does this work for Stars? Watch how fast a star rotates around the center of the galaxy… Simulation without Dark Matter Simulation with lots of Dark Matter particles in the galaxy Data looks like this http://bigbang.physics.tamu.edu/Figures/StolenAnimations/galrot_anim.gif Feb 2016 David Toback, SPS Lecture 49 Data well explained by lots of “Dark Matter” we can’t see This is where it gets its name In some sense, the name is a statement of almost all we know about it (it doesn’t interact with light, and it has mass) Feb 2016 David Toback, SPS Lecture 50 Another Experiment Maybe there isn’t dark matter and General Relativity is wrong? Look at the gravitational impact on light that travels through the Universe, through a galaxy, and towards us Feb 2016 David Toback, SPS Lecture 51 Einstein Ring’s? Feb 2016 David Toback, SPS Lecture Another view Feb 2016 David Toback, SPS Lecture Einstein Ring The “near” galaxy The light from the “far” galaxy spread out into a ring Amount of lensing explained by extra mass of dark matter Feb 2016 David Toback, SPS Lecture Winner: General Relativity • Many observations indicate that General Relativity correctly describes the motion of light, galaxies and stars in the Universe we live in space and time are related • Space can curve, stretch, change etc. • Curved space-time will be very important in later chapters when we talk about the expansion of the Universe • Dark matter is an important part of the evolution of our universe Feb 2016 David Toback, SPS Lecture 55 • 1.3 billion years ago • Sept 14th, 2015 • Binary black hole merger – Half the speed of light Feb 2016 David Toback, SPS Lecture 56