PCS 581 Lecture Notes - Lecture 7: Integral, Galaxy Merger, Gravitational Wave
Lecture 7:
Black Holes
Tidal Forces
Why does the Earth have tides?
Differential Forces
oHence: two high tides
Earth is pulled apart, and squeezed
Because of the moon. High tide would be closest to the moon and the opposite side
Moon will pull that side of the ocean towards it. Why the other side? Not just the moons
gravity – It’s the difference on how moon is pulling
If moon pulled on the entire earth equally then no tide
Gravity gets weaker as you go farther away. Gravity strongest on the near side of the
moon, it will be weaker on the center om f the earth – near side pulled harder from the
middle (that raises the high tide)
When the middle is being pulled out by moon and therefore the exact same high tide on
both side
Elastic band – its stretchy everywhere
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Not moon pulling earth it’s the difference of how moon pulls on the entirety of the
earth
When pull tides – stretched both ways and get compressed from the bottom and the
top
Low tide – is it is actively being compressed
Earth is pulled apart (by high tide) and squeezed (by the low tide)
And the earth is still rotating – here in Canada we get high tides twice a day every 12
hour (6 am and 6 pm)
Spaghettification
Gravity is strong near the black hole
oTorso pulled harder than head
oFeet pulled harder than torso
Tidal forces stretch him, squeeze him, rip him apart
oMillions of times stronger than Moon-Earth tidal forces
oWeaker for a supermassive black hole – you could make it in
Not relativity – you will feel this – physical thing happening to you, so you can feel it and
you will see you rib cage squeezing and able to see your organs as you are being ripped
apart
You die – million times stronger than how the high tides are pulled
Because its bigger you will die before you reach, and the tidal force is weaker –
supermassive black hole
What you see
Your friend becomes:
oSlowed, eventually completely frozen
oReddened, eventually invisible
oStretched & squeezed, eventually ripped apart
What does he see?
oBecome slower and slower and eventually completely freeze in event horizon –
because nothing enters event horizon
oIf small BH stretched apart – if big not
oSees everything go faster and bluer
Black Hole Gravitational Lensing
Gravitational lensing happens around anything with mass – light bending around mass
With black holes there would be very dramatic versions
Light warps so much you see the entire black hole – front and the back
Universe becomes brighter and brighter as you enter the event horizon – when at the
event horizon infinite light
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The Photon Sphere
Light at this distance has to orbit the black hole
Closest anything can get with a stable orbit (for rockets, infinite energy required)
About ½ sch. Radius – orbital seed required at that distance is the speed of light
Any rocket can do that
Light around it orbits it
3 Sch. Radius is how much anything can stay stable
Adding to Black Holes
Stuff do falls into the black hole
Even stars would have spaghettification
Black hole feed on companion star and increase mass and energy
Black holes can change
Extreme space times – simulation of what it would like in lensing when two black hole
merges
BH can merge but we haven’t observed that – very first observation was from merging
of space time can’t really see but it altered space time and we were able to pick it up
Seeing a Black Hole
They’re black... so we can’t see them
o Hawking radiation is minimal
But we can see their gravity
That’s how we know there’s a black hole at the heart of the Milky Way
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Differential forces: hence: two high tides. High tide would be closest to the moon and the opposite side. Moon will pull that side of the ocean towards it. Not just the moons gravity it"s the difference on how moon is pulling. If moon pulled on the entire earth equally then no tide. Gravity gets weaker as you go farther away. Gravity strongest on the near side of the moon, it will be weaker on the center om f the earth near side pulled harder from the middle (that raises the high tide) When the middle is being pulled out by moon and therefore the exact same high tide on both side. Not moon pulling earth it"s the difference of how moon pulls on the entirety of the earth. When pull tides stretched both ways and get compressed from the bottom and the top. Low tide is it is actively being compressed.
