Gravity
The Fudametal Forces of Nature There exist oly four fudametal forces Electromagetism Strog force Weak force Gravity Gravity 2
The Hierarchy Problem Gravity is far weaker tha ay of the forces! Why?!? NO ONE KNOWS!!! Gravity 3
Newto s Law of Gravity Newto first told us about gravity Newto s gravitatioal force ~F = G N Mm r 2 ˆr Gravity 4
Gravity holds together the BIGGEST objects i the Uiverse! The Solar System Newtoia Gravity
This force holds together the BIGGEST objects i the Uiverse! Stars Gravity 6
This force holds together the BIGGEST objects i the Uiverse! Nebulae Gravity 7
This force holds together the BIGGEST objects i the Uiverse! Galaxies Gravity 8
This force holds together the BIGGEST objects i the Uiverse! Clusters of Galaxies Gravity 9
This force holds together the BIGGEST objects i the Uiverse! Filametary structure GRAVITY CONTAINS THE BIGGEST MYSTERIES IN THE UNIVERSE! Gravity 10
Eistei s Theory of Gravity Eistei chaged Newto s theory of gravity. Newto said that gravity was a force ~F = G N Mm r 2 ˆr Careful experimets showed problems with Newto s theory! Gravity 11
Eistei s Theory of Gravity Gravitatioal lesig Gravity 12
Eistei s Theory of Gravity Eistei said space is curved! Gravity 13
Eistei s Theory of Gravity Eistei s said gravity is much more complicated! R µ 1 2 g µ R = 8 G N c 4 T µ What else does Eistei say? Gravity 14
Remember the Escape Velocity Throw a ball ito the air By coservatio of eergy 1 2 mv2 = G N Mm R E Gravity 15
Remember the Escape Velocity This is the escape velocity of the Earth For Earth it s about 11 km/sec! Gravity 16
How High Ca the Escape Velocity Go? We ca make the velocity higher by icreasig the mass. For the su, v esc ~ 618 km/s Or, we could keep the mass fixed, ad squeeze it ito a ball of smaller radius! The biggest the escape velocity ca get is fixed by the cosmic speed limit, the speed of light c! Suppose we wat to squeeze a body of mass M ito such a small ball that its escape velocity is equal to the speed of light. The Schwarzschild Radius v = q 2G N M R Gravity 17
Eve Light Ca t Escape! If the radius of the star is less tha the Schwarzschild radius, the ot eve light ca escape its gravitatioal pull! The evet horizo! All of this has bee doe assumig Newtoia gravity! Eistei said Newto was wrog! If you do it right usig the correct theory of gravity, you fid exactly the same aswer! This was show by Karl Schwarzschild ot log after Eistei discovered Geeral Relativity. So, we really do have a solutio describig a regio that s so dese that ot eve light ca escape - Gravity 18
Eve Light Ca t Escape! By lookig at Eistei s theory we ca lear so much more! So, we really do have a solutio describig a regio that s so dese that ot eve light ca escape - a black hole! Gravity 19
Black Holes Let s look at black holes a little more closely. What happes at the Schwarzschild radius? What happes at the ceter? The black hole cotais a sigularity i its core! To completely uderstad the sigularity we eed a quatum theory of gravity, which is still completely ukow! Gravity 20
Properties of Black Holes Black holes are ot cosmic vacuum cleaers! You ca have a stable orbit aroud a black hole (as log as you re further tha 3 times the Schwarzschild radius)! Suppose we watch a helpless astroaut fallig dow ito the black hole. What would we see? Time stops at the evet horizo! We would see the astroaut froze o the horizo forever! (That s ot quite right ) The astroaut would see othig special as he passed through the horizo! BOTH ARE RIGHT! Gravity 21
Black Holes Are t Completely Black! Accordig to Eistei s Geeral Relativity, black holes are oe-way tickets - thigs eter, but ever leave. Eistei ever believed i quatum mechaics. Quatum mechaics allows for particle pair creatio! Hawkig recosidered black holes usig quatum mechaics. Eistei was wrog! Gravity 22
Black Holes Evaporate! Particle pairs are created i the strog gravitatioal field aroud the evet horizo. Oce i a while oe of the particles falls ito the black hole while the other escapes. The hole looks like a hot object with a certai temperature! To coserve eergy, the hole must lose mass! It has a fiite lifetime! Gravity 23
Black Holes From the Collisio of Particles If we start with two poit particles, ad smash them together so that they are squeezed ito a space smaller tha their Schwarzschild radius, we ll make a black hole! Ordiary 4D gravity is much too weak, the particles would have to get withi ~ 10-55 cm for electros to make a black hole! This would require eergies ~ 10 27 times greater tha we have ow! Higher-dimesioal gravity is stroger! We have a chace! I some theories the ext particle accelerator could make as may as oe per secod! Gravity 24
Gravity at the LHC The most powerful particle accelerator ever built has recetly restarted at CERN ear Geeva i Switzerlad. It has a circumferece of 27 kilometers, ad is betwee 50-175 meters udergroud. It s mai task is to search for (ad fid!) a particle called the Higgs, which is believed to be the origi of mass. Gravity 25
If Extra-Dimesioal Gravity Exists, the LHC Will Have the Best Chace of Fidig it! With ay luck, we ll be able to see eergy leakig out ito the bulk after collisios i the form of gravitos. Or maybe we might see tiy black holes! Either way - it s a great time for physics! Gravity 26
The Expasio of the Uiverse There is oe more very iterestig aspect of gravity to discuss, dealig with the expasio of the Uiverse. If we look out at distat galaxies they are almost all movig away from us! v = H 0 d H 0 70 km/s/mpc The BIG BANG! Gravity 27
The Cosmological Costat If gravity is always attractive, the the expasio of the Uiverse should be slowig dow. There are other possibilities... Gravity 28
The Cosmological Costat, λ Eistei added this to his equatios to get a static Uiverse. After it was foud that the Uiverse was expadig he baished it from his equatios, callig it his biggest bluder! For ~ 60 years it was believed to be zero Gravity 29
The Cosmological Costat, Observatios foud it! λ Observatios differ from theory THE BIGGEST MYSTERY IN PHYSICS!!! Gravity 30