cosmic forces - cornell universityhosting.astro.cornell.edu/.../2cosmicforces.pdf · cosmic forces...
TRANSCRIPT
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Lecture Two
Cosmic Forces
FROM ATOMS TO GALAXIES
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Curiosity Demands that We Ask Questions
• "Nothing exists except atoms and empty space - everything else is opinion“
Democritus of Abdera 430 B. C. Greece
• Matter is made of ATOMS.
• Different atoms for different ELEMENTS.
• What are Atoms?
• Atoms have a NUCLEUS of:
• PROTONS p+ Positive Electric Charge
• NEUTRONS n Neutral Charge
• The Nucleus is Surrounded by:
• ELECTRONS e- Negative Electric Charge
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The Simplest Atom
Carbon
• Uranium – heaviest natural atom: 92 protons, 143 neutrons, 92 electrons
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Atoms are VERY Small
• ~ 1 to 2 x 10-8 cm
• 10-8 cm = 1 ANGSTROM = 1 Å
• Size of Nucleus = 10-13 cm
• Your Thumb is made up of ~ 1024 ATOMS
Structure of Matter
• All things are made of ATOMS
• Little particles in perpetual motion
• Take water for example:
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Fundamental Laws of Nature
• Is there only one?
• Are the laws universal in Space and Time?
• The Test of Knowledge is always EXPERIMENT
• "My ambition is to live to see all of physics reduced to a single
formula so elegant and simple that it will fit on the front of my T-shirt"
Leon Lederman, 1993
The Scientific Method
• Observations
• Hypothesis
• Verifications
• “LAW”
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Evolution of Knowledge
• Laws are accurate... until they fail!
• DISASTER? NO!
• The discovery of a more accurate law.
• Example:
– Mass of an object is constant (Newton)
– Mass increases with velocity (Einstein)
Force: F = ma or a = F/m
• A force is that which can change the velocity of an object (either speed or direction).
– 1 dyne = g-cm/s2
– 1 Newton = kg-m/s2 (after Isaac)
= 105 dynes
• Forces cause acceleration!
• Example forces: Gravity
– Earth’s gravity: 980 g-cm/sec2
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Cosmic Forces
Name Relative Strength
1 Nuclear 1
2 Electromagnetic 1/137
3 Weak 10-13
4 Gravitational 10-39
Future: Grand Unified Theory (GUT) – 1+2+3
Theory of Everything (TOE) – 1+2+3+4
“Theory of Everything”
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Theory of Everything • Unite gravity with the other
fundamental forces. – Merging of gravity with quantum
mechanics and other forces.
• We don’t have a theory yet but the most promising ones involve “STRING THEORY” and “higher dimensions”
• String Theory suggests there are 11 dimensions (10 spatial + 1 time).
Gravity
• "Isaac Newton and the Apple" A.D. 1667
• F = Force of attraction (Newton)
• G = 6.67 x 10-11 m3kg-1sec-2
• m1 ; m2 = Masses (kg)
• d = Distance between objects (m)
2
21
d
mGmF
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Electromagnetic Force
• 1037 times stronger than Gravity
• Not all things attract
• Unlike particles attract - (+, -)
• Like particles repel - (+, + or -, -)
Electromagnetic Force
• ELECTROMAGNETIC FIELD AROUND THE PARTICLES
• Emitted energy propagates through this field
– Like a wave in the water
– With the speed of light
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Electromagnetic Energy
Atoms and molecules emit energy or "waves" of different "length" (or different energy).
Nuclear (Strong) and Weak Force
• Strong Force: Binding Force of Atomic Nuclei
– Keeps protons and neutrons together
– (A strong glue - "GLUONS")
• It has a VERY short range
• When released by nuclear interactions, vast amounts of energy are emitted.
• Weak Force: Gives rise to radioactivity
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New Revolution In Physics (1900 – 1930)
• Albert Einstein - Theory of Relativity
– No longer 3 dimensional space but 4 – Space-Time
– No longer a flat space but curved
• Quantum Mechanics
– Worked much better (more accurate) but predicted new unexpected results!
• Classical Physics is Rigidly Deterministic
– A given cause leads to a given result always and everywhere.
• In Quantum Mechanics, a given cause can lead to any of several results.
– Individual cases cannot be determined with certainty.
– The only laws are statistical ones for large numbers of instances.
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QM: The Uncertainty Principle
• The uncertainty principle of QM states that we cannot know both where something is and how fast it is moving.
• Thus we cannot predict exactly what will happen in a given experiment.
• The ‘Measurement’ problem
Uncertainty Principle
• For subatomic particles: Very important.
• However, the uncertainty becomes very small for larger things.
– Launch a rocket on a predictable path-- without worrying about subatomic particles.
– The behavior of an electron around the nucleus of an atom is more "Probabilistic"!
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Orbitals rather than orbits
• We don’t really see the electron in “orbit” around the nucleus.
• We have a probability of detecting an electron in a given location.
Humans have been able to explore the UNIVERSE from the very very small ATOMS to the very very large GALAXIES