Spacetime: just when you thought it was

it throws you a

€

F =Gm1m2

r2

€

F = ma

On one hand mass seems to encourage acceleration.

On the other hand, it resists it.

Another conundrum Einstein just could not ignore.

PRINCIPLE OF EQUIVALENCE

No experiment in an isolated space can distinguish between a gravitational field and an equivalent uniform acceleration.

No experiment would help you distinguish between being weightless far out in space and being in free-fall in a gravitational field.

GR POSTULATES

• PRINCIPLE OF EQUIVALENCE: IN THE VICINITY OF ANY POINT, A GRAVITATTIONAL FIELD IS EQUIVALENT TO AN ACCELERATED FRAME OF REFERENCE IN THE ABSENCE OF GRAVITTATIONAL EFFECTS

• THE LAWS OF NATURE HAVE THE SAME FORM IN ANY FRAME OF REFERENCE, WHETHER ACCELERATED OR NOT

A new way to look at why the Earth rotates around the Sun.

Two people are walking in parallel lines at the equator. They continue along lines of constant longitude, until, much to their surprise, they meet.

•They conclude they must have been attracted by a force.

•You see that they were actually walking on a curved surface.

Lensing

Gravitational Lensing

Gravitational Lensing

Why do we think there is dark matter?

• Isn’t obvious that most of Isn’t obvious that most of the matter in the Universe the matter in the Universe is in Stars?is in Stars?

Spiral GalaxySpiral Galaxy

Clusters produce distinctive tangential patterns

Distortions of our view of the sky

Gravitational Lensing

Measuring the energy in the UniverseMeasuring the energy in the Universe

• We can measure the mass We can measure the mass of clusters of galaxies with of clusters of galaxies with gravitational lensinggravitational lensing

• These measurements give These measurements give massmass ~0.3 ~0.3

• We also know (from the We also know (from the primordial deuterium primordial deuterium abundance) that only a abundance) that only a small fraction is nucleons small fraction is nucleons

nucleonsnucleons < ~0.04 < ~0.04 Gravitational

lensingGravitational

lensing

Dark Matter

Another Consequence: GRAVITATIONAL WAVES

• Particular kind of phenomena (e.g. orbiting stars) produce ripples in the space-time curvature…

• Ripples travel at speed of light through space• These are called gravitational waves.

Early instrumentation to detect gravity waves,, “Weber bars”, were pioneered by UMD professor, Joseph Weber!

Go Terps!

LIGO - Laser Interferometer Gravitational Wave Observatory

Like Michelson Morley, but each arm is kilometers long!