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# Oct.11 - 2B03.docx

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McMaster University

Astronomy

ASTRON 2B03

Laura Parker

Fall

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Oct. 11 – 2B03 11/10/2011 9:33:00 AM
Principle of Equivalence:
You can’t tell the difference between the acceleration from a rocket
and acceleration from gravity
Rocket-ship ex.
Newton vs. GR
Gravitational time dilation
o Before – moving clocks move slower – special relativity
Special vs. general relativity
Big difference is acceleration
Light deflected in (by) a gravitational field
Spacetime and Gravity
Einstein speaks about 3 dimensions of space, and time. 3+1 grid –
curved around massive objects.
Gravity just describes curvature on spacetime about massive
objects.
We can measure this deflection on large scales – light rays passing
by sun ex.
Gravitational redshift
Extra kind of redshift from general relativity (beyond the one we
talked about before, Doppler)
Light has to use energy to leave curved space-time
o Energy lowered = red shift. More massive the object, the
more the light is redshifted. In black hole, redshifted so much
that is doesn’t have enough light (energy) to get out .
o In a black hole, light is infitnitely redshifted.
o Black hole – you need something faster than the speed of
light to escape – nothing can escape a black hole.
Black Holes
Time dilation in gravitational fields – in gravity, slower the clocks
move. Stronger the g field – stronger the time dilation.
o Gravity of black holes mad strong – clocks run real slow.
Black holes don’t suck
Spacetime and gravity
Distance bigger – gravitational force gets smaller
Bottom floor – closer to centre of earth – more of a gravitational
pull – clock runs more slowly.
Space-time
Grid can be stretched, or “warped” – the more massive the objects,
the more warped or distorted the fabric of space time.
Main point – light appears to be bent through the time – but, it
follows most direct route.
o Milions of observations have confirmed this bending.
Gravitational Lensing
The stretched ones (arks) are having their light bent.
Gravity Waves
not completely confirmed – ind

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