# The odd thing about gravity . . .

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The odd thing about gravity . . .
General Relativity The odd thing about gravity . . . The force of gravity is proportional to the mass of the object it acts on M F m The acceleration is the same, no matter the mass you drop r Suppose you are in an elevator Now, we cut the cable You fall Other things fall at the same rate To you, it looks like you are weightless This is why astronauts are weightless - they are always falling They are not far from the Earth

The equivalence principle
Are there any other force formulas that are proportional to mass? Suppose you are on a merry-go-round Centrifugal force Like gravity, everything “accelerates” equally You can make it go away by letting go Can we create “gravitational” effects through acceleration? Put the person out in space in the elevator Attach to an accelerating rocket The effects are indistinguishable from gravity The effects of gravity are indistinguishable from the effects of being in an accelerated reference frame

The metric again We will start by reexamining the metric – the distance formula Or in this case, the proper time formula This formula works if you move in a straight line What if you don’t move in a straight line? You can divide any longer path into several short segments Then, find the distance formula for each one We won’t be using this formula Interestingly, you can show that the straight line has the longest proper time path – called a geodesic An object with no forces acting on it will always follow a geodesic, which is the longest proper time path between two points in spacetime

Changing Coordinates It is important to be able to change coordinates to a different system Let’s change to spherical coordinates: (x,y,z,t)  (r,,,t) Similarly,

Moving in curved coordinates
The geodesic principle works in any coordinate system An object with no forces acting on it will always follow a geodesic, which is the longest proper time path between two points in spacetime It is possible, starting from the metric, to find equations that describe geodesic motion

Moving in curved coordinates (2)
The effects of moving in curved coordinates looks like acceleration y But it really isn’t It is moving as straight as it can in curved coordinates You can always eliminate this apparent acceleration, simply by returning to “flat” coordinates It is possible, starting from the metric alone, to prove that spacetime is really just flat x

Curved Space It is possible to live in a spacetime that is inherently curved It’s not just the coordinates, it’s spacetime itself that is curved Consider the surface of the Earth Think of it as a 2D object Imagine two explorers setting out from the North Pole in “straight lines” (geodesics) At first they are traveling away from each other When they reach the equator, they will be traveling “parallel” to each other; their distance is no longer increasing They then start traveling towards each other They meet at the south pole The curvature is real It is space itself that is curved, not the coordinates only

Curvature How can we tell, looking only at the distance formula (the metric), if the curvature is real or a consequence of our coordinate choice? The Riemann Tensor tells you if it is curved If the Riemann tensor is zero, space is not curved; if it is non-zero, it is curved Changing coordinates doesn’t make it go away Some other measures of curvature: The Ricci tensor: The Ricci scalar: The Einstein tensor:

The Stress-Energy Tensor
V What causes gravity? Energy and momentum The presence of matter, or mass density, is the cause of gravity Mass density is proportional to energy density If energy makes a difference, why not momentum as well? Momentum density also contributes to gravity The flow of energy and momentum also causes gravity Another way of looking at momentum density is the transfer of energy It is like power flowing through an area You can also transmit momentum across a boundary This is what forces do Pressure is a good example A more general example is called the stress tensor P A F Gravitational effects in general relativity are caused by the energy density u, the momentum density vector g, and the stress tensor

Einstein’s Equations The central idea of Einstein:
Gravity looks just like acceleration, except When you have a source of gravity, parallel doesn’t remain parallel This tells you gravity has to do with curvature M a m a m The stress-energy tensor Tµ must be related to the curvature Einstein found a relationship that worked, now called Einstein’s equations It may look simple, but it isn’t This is really 16 equations (since and  each take on 4 values) The expression on the left contains hundreds of terms It is highly non-linear

Matter tells space how to curve, and space tells matter how to move
Geodesics in curved spacetime Why do particles curve under the influence of gravity? Because spacetime itself is curved! An object with no non-gravitational forces acting on it will always follow a geodesic, which is the longest proper time path between two points in spacetime Matter tells space how to curve, and space tells matter how to move

Time slows down in proximity to a mass!
The Schwarzschild Solution Assume you have a spherically symmetric source of gravity Doesn’t matter how it’s distributed You have to be outside it The solution you get for the Metric is called the Schwarzschild Solution M If you let M = 0, you get the same solution as before (flat spacetime) The factors of 1-2GM/c2r are the only change The change in the radial term (dr2) tells you that the relationship between radius and circumference isn’t the obvious one C  2r ! The dt2 term tells you that time runs more slowly when you are near a mass! Time slows down in proximity to a mass!

Gravitational time effects
Suppose you are standing still (dr = d = d = 0) near a mass Time runs more slowly f0, 0 This can be measured directly by comparing atomic clocks in airplanes vs. atomic clocks on the ground Next generation atomic clocks: measure which floor you are on GPS wouldn’t even work if they didn’t take GR into account f,  This has an important effect when studying spectral lines from high mass/small radius stars The frequency f we observe away from the star’s gravitational field is increased compared to f0 Since wavelength is inversely proportional to frequency, the wavelength observed is longer, “red shifted”

Gravitational Forces time fast
Why do objects accelerate in a gravitational field? time medium An object with no non-gravitational forces acting on it will always follow a geodesic, which is the longest proper time path between two points in spacetime I want to toss a ball to myself – release it and catch it one seconds later, at the same place What path should I take Goal: maximize the proper time the ball experiences time slow Strategy #1 – hope it levitates in place Bad – it is stuck near the Earth, where time runs slowly Strategy #2 – toss it very high in the air Bad – high speed causes time to slow down too much Strategy #3 – toss it moderately fast into the air Good – a compromise – it gains effect of going away from Earth without too much speed

Gravitational Deflection of Light
Gravity affects all objects, because it is a curvature of spacetime The effect is small, for the Sun, and almost impossible to measure except from space or during eclipses For objects like galactic clusters, the effect is large and can be used to measure the mass of the cluster

Black Holes There is a radius where the equation looks like it goes crazy Called the Schwarzschild radius This radius is inside the physical radius of the Earth, Sun, and all planets in the solar system About 3 km for the Sun The formulas are no longer valid here If you managed to squeeze the Sun to this radius, something remarkable would happen The Sun would become a black hole

Precession of the Perihelion
Newtonian gravity and General Relativity have almost the same predictions Some slight discrepancies Newtonian gravity says planets orbit the Sun in ellipses General relativity says the direction of the ellipse slowly changes with time Mercury This effect is called precession of the perihelion Depends on the speed Discovered, but not explained, before Einstein For Mercury, it works out to 43 arc-seconds per century This effect has since been seen in other systems Spacecraft Orbiting pairs of neutron stars Sun

Gravity Probe B Four superaccurate gyroscopes that orbited the Earth from Accurate tests of special and general relativity Earth gyroscope Two effects it is looking for: As the object orbits, its motion and Earth’s gravity combine to make the direction of the axis tilt in the direction of motion This motion is called the “geodetic effect” April 2007: effect matches expectation at the 1% level September 2009: effect matches expectation at the 0.1% level The Earth is simultaneously rotating on ITS axis It drags spacetime along with it. This effect is called “frame dragging” September 2009: effect matches expectation at the 15% level

Gravity Probe B

Gravitational Waves Distortions of spacetime are produced by masses
Just like electromagnetic fields are produced by charges If you have accelerating masses, you can make gravity waves Just like accelerating charges make electromagnetic waves These distortions affect everything that feels gravitational forces i.e. everything These effects are truly tiny Motions ~ 0.01 fm Several gravity wave detectors are looking for these effects No positive results yet

Gravitational Wave Detection
The direct way to detect gravity waves is via interferometry Make light interfere after going along two LONG paths Extremely sensitive method LIGO – Laser Interferometer Gravity Wave Observatory Livingston, Louisiana Hanford Washington Indirect detection – If two objects are circling each other, they must lose energy They must move closer Frequency of orbit increases Detected in Pulsar pair PSR Hanford, Washington

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