Black Holes: the Reith Lectures - Deepstash
Black Holes: the Reith Lectures

Black Holes: the Reith Lectures

Stephen Hawking

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What is a Black hole ?

What is a Black hole ?

Black holes cannot be seen as they don't reflect any light.

The centre of black hole is called singularity - a point of infinite density i.e the space curvature extends to infinity.

According to scientists, billons of black holes exists of which millions are having the abode Milky way galaxy

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Can‘t see , but how do we know their existence

Can‘t see , but how do we know their existence

  • But scientists know they exist because they rip apart stars that get too close to them and because they can send tremors through space.
  • It was a collision between two black holes more than a billion years ago that triggered what are called ‘gravitational waves’.
  • Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity because matter could not be compressed beyond a certain point.
  • The principal exception was the American scientist John Wheeler,emphasized that many stars would eventually collapse, and pointed out the posed for theoretical physics.

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Formation of Black holes

Formation of Black holes

  • During most of the life of a normal star, over many billions of years, it will support itself against its own gravity by thermal pressure, caused by nuclear processes which convert hydrogen into helium.
  • The explosive force of nuclear fusion inside them creates outward pressure which is constrained by gravity pulling everything inwards.
  • Eventually, however, the star will exhaust its nuclear fuel. The star will now contract.
  • For more detail , check out Chandrashekhar limit and Tolmann Volkoff Oppenheimer approximation

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Singularity

Singularity

  • if you take pressure out of the calculation,a uniform spherically symmetric star would contract to a single point of infinite density,I.e,Singularity.
  • A singularity is what you end up with when a giant star is compressed to an unimaginably small point.
  • It refers not only to the end of a star but also to a far more fundamental idea about the starting-point for the formation of the entire universe.
  • All our theories of space are formulated on the assumption that space-time is smooth and nearly flat,so they break down at the singularity.In fact, the singularity marks the end of time itself.

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Einstein‘s View

  1. It was already clear that a uniform spherical star would contract to a point of infinite density, a singularity.
  2. Einstein’s Theory of General Relativity says that objects distort the space-time around them.
  3. At this point of infinite density, one can’t predict the future, which in turn implies that something strange could happen whenever a star collapsed.

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Information

  • All the black hole will remember is the total mass, the state of rotation and the electric charge.
  • A black hole has a boundary, called the event horizon. This is where gravity is just strong enough to drag light back and prevent it escaping.
  • Because nothing can travel faster than light, everything else will get dragged back also.
  • There is a black hole with a mass of about four million times that of the sun at the centre of our Milky Way galaxy.
  • Scientists believe that there are huge black holes at the centre of virtually all galaxies.

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Properties

Properties

  • With no light escaping from the black hole, there is no way that anyone watching from a distance could actually witness your descent. In space, no one can hear you scream; and in a black hole, no one can see you disappear.

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