The Fundamental Forces of Nature

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1 Gravity

2 The Fundamental Forces of Nature There exist only four fundamental forces Electromagnetism Strong force Weak force Gravity Gravity 2

3 The Hierarchy Problem Gravity is far weaker than any of the forces! Why?!? NO ONE KNOWS!!! Gravity 3

4 Newton s Law of Gravity Newton first told us about gravity Newton s gravitational force Gravity 4

5 This force holds together the BIGGEST objects in the Universe! Stars Gravity 5

6 This force holds together the BIGGEST objects in the Universe! Nebulae Gravity 6

7 This force holds together the BIGGEST objects in the Universe! Galaxies Gravity 7

8 This force holds together the BIGGEST objects in the Universe! Clusters of Galaxies Gravity 8

9 This force holds together the BIGGEST objects in the Universe! Filamentary structure GRAVITY CONTAINS THE BIGGEST MYSTERIES IN THE UNIVERSE! Gravity 9

10 Einstein s Theory of Gravity Einstein changed Newton s theory of gravity. Newton said that gravity was a force Careful experiments showed problems with Newton s theory! Gravity 10

11 Einstein s Theory of Gravity Gravitational lensing Gravity 11

12 Einstein s Theory of Gravity Einstein said space is curved! Gravity 12

13 Einstein s Theory of Gravity Einstein said gravity is much more complicated! What else does Einstein say? Gravity 13

14 Remember the Escape Velocity Throw a ball into the air By conservation of energy Gravity 14

15 Remember the Escape Velocity This is the escape velocity of the Earth For Earth it s about 11 km/sec! Gravity 15

16 How High Can the Escape Velocity Go? We can make the velocity higher by increasing the mass. For the sun, v esc ~ 618 km/s Or, we could keep the mass fixed, and squeeze it into a ball of smaller radius! The biggest the escape velocity can get is fixed by the cosmic speed limit, the speed of light c! Suppose we want to squeeze a body of mass M into such a small ball that its escape velocity is equal to the speed of light. The Schwarzschild Radius Gravity 16

17 Even Light Can t Escape! If the radius of the star is less than the Schwarzschild radius, then not even light can escape its gravitational pull! The event horizon! All of this has been done assuming Newtonian gravity! Einstein said Newton was wrong! If you do it right using the correct theory of gravity, you find exactly the same answer! This was shown by Karl Schwarzschild not long after Einstein discovered General Relativity. So, we really do have a solution describing a region that s so dense that not even light can escape - Gravity 17

18 Even Light Can t Escape! By looking at Einstein s theory we can learn so much more! So, we really do have a solution describing a region that s so dense that not even light can escape - a black hole! Gravity 18

19 Black Holes Let s look at black holes a little more closely. What happens at the Schwarzschild radius? What happens at the center? The black hole contains a singularity in its core! To completely understand the singularity we need a quantum theory of gravity, which is still completely unknown! Gravity 19

20 Properties of Black Holes Black holes are not cosmic vacuum cleaners! You can have a stable orbit around a black hole (as long as you re further than 3 times the Schwarzschild radius)! Suppose we watch a helpless astronaut falling down into the black hole. What would we see? Time stops at the event horizon! We would see the astronaut frozen on the horizon forever! (That s not quite right ) The astronaut would see nothing special as he passed through the horizon! BOTH ARE RIGHT! Gravity 20

21 Black Holes Aren t Completely Black! According to Einstein s General Relativity, black holes are one-way tickets - things enter, but never leave. Einstein never believed in quantum mechanics. Quantum mechanics allows for particle pair creation! Hawking reconsidered black holes using quantum mechanics. Einstein was wrong! Gravity 21

22 Black Holes Evaporate! Particle pairs are created in the strong gravitational field around the event horizon. Once in a while one of the particles falls into the black hole while the other escapes. The hole looks like a hot object with a certain temperature! To conserve energy, the hole must lose mass! It has a finite lifetime! Gravity 22

23 Black Holes From the Collision of Particles If we start with two point particles, and smash them together so that they are squeezed into a space smaller than their Schwarzschild radius, we ll make a black hole! Ordinary 4D gravity is much too weak, the particles would have to get within ~ cm for electrons to make a black hole! This would require energies ~ times greater than we have now! String theory says that higher-dimensional gravity is stronger! We might have a chance! In some theories the next particle accelerator could make as many as one per second! Gravity 23

24 Gravity at the LHC The most powerful particle accelerator ever built has just turned on (again) at CERN near Geneva in Switzerland. It has a circumference of 27 kilometers, and is between meters underground. It s main task is to search for a particle called the Higgs, which is believed to be the origin of mass. Gravity 24

25 If Extra-Dimensional Gravity Exists, the LHC Will Have the Best Chance of Finding it! With any luck, we ll be able to see energy leaking out into the bulk after collisions in the form of gravitons. Or maybe we might see tiny black holes! Either way - it s a great time for physics! Gravity 25

26 The Expansion of the Universe There is one more very interesting aspect of gravity to discuss, dealing with the expansion of the Universe. If we look out at distant galaxies they are almost all moving away from us! The BIG BANG! Gravity 26

27 The Cosmological Constant If gravity is always attractive, then the expansion of the Universe should be slowing down. There are other possibilities... Gravity 27

28 The Cosmological Constant, λ Einstein added this to his equations to get a static Universe. After it was found that the Universe was expanding he banished it from his equations, calling it his biggest blunder! For ~ 60 years it was believed to be zero Gravity 28

29 The Cosmological Constant, λ Observations found it! Observations differ from theory THE BIGGEST MYSTERY IN PHYSICS!!! Gravity 29

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