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Physics Forum

P301 Lecture 1• Welcome to P301; syllabus is available through

ONCOURSE • DVB will be using the CALM system to employ

what has become known as “Just in Time Teaching” (JiTT). One or two questions with short answers due before each lecture to give me a gauge of how the class is grasping certain things and to encourage you to read the text BEFORE the lecture.

CALM system

CALM system(question for Wednesday’s lecture)

Physics ca 1900• Consider the state of Physics at the turn of the 20th

Century as reflected in a few quotes and observations:

• “The more important fundamental laws and facts of physical sciences have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote. Our future discoveries must be looked for in the sixth place of decimals”, A. A. Michelson 1894

• “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” Lord Kelvin 1900

• Today we take the existence of atoms for granted, but just 100 years ago some distinguished scientists still disputed their reality! What a difference 100 yr makes

Early hints of a new world order

• The previous quotes show that in the late 19th century even very distinguished scientists thought that Physics was essentially complete, that there were only a few minor loose ends to be cleaned up. Among those loose ends one might include:

• 1827: Brownian motion of pollen grains in water.• 1860: Characteristic elemental emission spectra.• 1865: Maxwell’s equations

– Great advance, but troubling that they look different if applied to a moving reference frame (i.e. They are not “invariant under a Galilean transformation” as are Newton’s laws).

• 1887: Michelson-Morley fail to find the ether• 1887: Hertz sees the Photo-electric effect

– (later elaborated by Elster, Geistel, Lenard and others)

Still more puzzles• In the late 1990’s and early in the 20th century, a

number of other remarkable discoveries opened new cracks, and led to two revolutions (that in many ways continue to this day):

• 1895: Roentgen discovers X-rays• 1896: Becquerel discovers radioactivity• 1896: Zeeman observes changes in spectra for

atoms in magnetic fields.• 1897: Thomson “discovers” the electron (really,

measures e/m; building on work of Faraday, Perrin, and others)

In this course, we will study these (and many more) revolutionary results and the understanding that grew from them.

Gimli Glider (1983)http://www.wadenelson.com/gimli.html

An Air Canada 767 had faulty fuel gauges before take off from Montreal and the crew computed their fuel load by converting the fuel volume (11,500 liters) measured by the ground crew to mass and came up with 20,400; a sufficient amount, so they thought. Unfortunately they needed 20,000 kg of fuel, but the conversion factor they used was 1.77 lb/liter so they had 20,400 lb (only 9,200 kg). They ran out of gas at 41,000 ft half-way across Canada, and had to glide to a landing on an abandoned WWII runway (used in 1983 as an amateur drag strip)

UNITS MATTER!!

Units for the sub-atomic world• Always check your units!! • In P301 we will use several non-MKS units because

they can be much more convenient and provide a natural check on results (e.g. eV and mec2=511keV as units of energy). – Chemical energy scales are on the order of eV/atom,

nuclear energy scales are on the order of MeV/nucleon etc.• It will always be possible to use MKS units, but in

many cases it will be quicker (and less prone to silly errors) to use combinations of constants and non-MKS units in your calculations.

• In contemporary physics, authors often use units in which h=c=1, and it is worth your while to get used to figuring out where they belong in order to get results you can plug numbers into.

Noether’s Theroem• The most important result that we almost never tell

you in elementary physics courses (most texts make you wait ‘till field theory where you have the tools to prove it; needless to say, we won’t prove it here):

• E. Noether:– Conservations laws in dynamical systems are a

consequence of symmetries in the underlying physics (DVB more or less)

• E.G. – Independence from reference frame velocity leads

to conservation of linear momentum– Symmetry under uniform rotations is behind

conservation of angular momentum.– Symmetry under reversal of time leads to Energy

Conservation ...

CALM• In P221 you learned that the speed of sound in a fluid has a fixed value

of vs=(B/ρ)1/2, and that the speed of light in a vacuum has a fixed value of c = 1/(εoμo)1/2. Why is it that the constancy of the speed of light has consequences that are so much more dramatic (as seen in relativity) than the constancy of the speed of sound?

– The first obvious difference between the speed of light in a vacuum and the speed of sound in a fluid is that the speed of light is so much faster. (~5 focused on this)

– The speed of light has more dramatic consequences because its constancy is the result of the properties of space itself (its permeability and permittivity), not just of something that occupies space (~5 answers)

– The speed of sound and the speed of light are both constants; however, they are constants in two very different ways. (THIS IS THE KEY! ~ a dozen of you caught this at some level, but few answers were exactly succinct)

– c is constant independent of the (uniform) motion of the observer, vs is only constant with respect to the medium within which it is propagating.

Cosmic Ray induced muons(data from Quarknet high-school teachers, 2001)

http://cosmic.lbl.gov/SnowMass/main.html