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Transcript of IB Physics – Relativity Relativity Lesson 1 1.Galilean Transformations (one frame moving relative...
IB Physics – Relativity
Relativity
Lesson 11. Galilean Transformations (one frame moving relative to
another) Michelson Morley experiment– ether.
2. Speed of light constant
3. Simultaneity.
4. Inertial Observers. Frames of Reference.
5. 2 Postulates of Special relativity – c is constant and all laws are the same for all inertial observers.
6. Light Clocks and how they work; Pythagorean treatment
Click for good background site
IB Physics – Relativity
Inertial frames of reference
• An inertial frame of reference moves at a constant velocity.
• It is not accelerated
0
39
6
Lightning strikes at x = 60 m and t = 3 s
x = 0 x = 10 x = 20 x = 30 x = 40 x = 50 x = 60 x = 70
y
x
Tsokos, 2005, p553
An Event
IB Physics – Relativity
Absolute Rest
• There is no such thing as absolute rest.
• In an inertial frame of reference there are no experiments you can do which prove you are moving. (Unless you look outside!)
• Newton’s Laws consider zero and constant velocity to be identical
IB Physics – Relativity
Galilean Transformations
yy
vuu
tt
vtxx
Relatively easy?
Consider a stationary frame and a frame moving at velocity v in the x direction.
zz
IB Physics – Relativity
Use of the Galilean Transforms
Tsokos, 2005, p553
y
xx
velocity, v with respect to ground
origins coincide when clocks at both origins show zero
0
39
60
39
6
y
vt vtxx yy
x
0
39
6
0
39
6
x
the train has moved away; when the clocks show 3 s, lightning strikes at x = 60 m
Calculate and if v = 15ms-1x t
IB Physics – Relativity
Calculating relative velocity
A ball rolls on the floor of the train at 2ms-1 (with respect to the floor). The train moves with respect to the ground at (a) 12 ms-1 to the right and (b) 12 ms-1 to the left. Find the velocity of the ball relative to the ground.
xAn object rolling on the floor of the ‘moving’ frame appears to move faster as far as the ground observer is concerned
x
y
vt
v v
tux
u
Tsokos, 2005, p553
y
IB Physics – Relativity
Nature of Light
Oscillating magnetic and electric fields at right angles to each other.
Maxwell’s equations predict that the speed of light is independent of the velocity of the light source.
00
1
c
IB Physics – Relativity
Conflict with Galilean relativity.
Observer in the train measures the speed of light = C
Observer on the ground measures the speed of light as C + V
C
V
IB Physics – Relativity
The Michelson-Morley Experiment
Read in Kirk p149
Explain how the null result from this experiment shows that the speed of light is unaffected by the motion of the Earth.
Link to Michelson-Morley powerpoint
Link to Michelson-Morley explanation
IB Physics – Relativity
Modified equations
2
2
1cv
vtxx
21cuvvu
u
2
2
1c
vtt
IB Physics – Relativity
Postulates of Special Relativity
• The laws of physics are the same for all inertial observers.
• The speed of light in a vacuum is the same constant for all inertial observers
Animations showing time dilation and length contraction.
IB Physics – Relativity
Postulates of special relativity video
IB Physics – Relativity
The laws of Physics are the same
IB Physics – Relativity
Electromagnetism and relativity
IB Physics – Relativity
Speed of light is constant video
IB Physics – Relativity
Maxwells equations lead to constant c
IB Physics – Relativity
Simultaneity
• Two events which happen together are simultaneous.
• If two events are simultaneous in one frame of reference they may not be in another.
• See train example in Kirk p143
IB Physics – Relativity
Simultaneity video
IB Physics – Relativity
B
A
Light clocks
S
0x
B
A
L
B
A
B
A
S
0x
x = 0 x = v tvt
IB Physics – Relativity
Time dilation video
IB Physics – Relativity
Relativity
Lesson 21. Time dilation
2. Lorentz Factor
3. Proper time
4. Lorentz contraction
5. Proper length
6. Twin paradox and symmetric situations
7. Muon decay; evidence for time dilation
IB Physics – Relativity
Time dilation
IB Physics – Relativity
Time dilation
2
2
1cv
tt
Think about what this means?
????
IB Physics – Relativity
Proof of formula
The proof of the time dilation formula is a standard requirement in the exam.
Carefully work through the proof using Pythagoras’ Theorem.
Make sure you understand each step.
Hints
The “prime” notation refers to measurements in the ‘moving’ frame
The speed of light is the same for all observers.
IB Physics – Relativity
The Lorentz factor
2
2
1
1
cv
v 0.1c 0.2c 0.3c 0.4c 0.5c 0.6c 0.7c 0.8c 0.9c c
For what values of v is significant ?
Lorentz Factor
0
1
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1 1.2
v/c
Work out below
IB Physics – Relativity
Example of time dilation
tt If the train passengers measure a time interval of t1 = 6 s and the train moves at a speed v = 0.80c, calculate the length of the same time interval measured by a stationary observer outside the train standing on the ground
IB Physics – Relativity
Atomic clocks prove time dilation!
IB Physics – Relativity
Proper time
A proper time interval is the time separating two events that take place at the same point in spaceobserved time interval = x proper time interval
Note that the proper time interval is the shortest possible time separating two events
IB Physics – Relativity
Examples of proper time
1. The time interval between the ticks of a clock carried on a fast rocket is half of what observers on Earth record. How fast is the rocket moving with respect to the Earth? What are the two events here?
2. A rocket moves past an observer in a laboratory with speed = 0.85c. The lab observer measures that a radioactive sample of mass 50 mg (which is at rest in the lab) has a half life of 2.0 min. What half-life do the rocket observers measure? Again, what are the two events?
3. In the year 2010, a group of astronauts embark on a journey toward Betelgeuse in a spacecraft moving at v = 0.75c with respect to the Earth. Three years after departure from the Earth (as measured by the astronaut’s clocks) one of the astronauts announces that she has given birth to a baby girl. The other astronauts immediately send a radio signal to Earth announcing the birth. When is the good news received on Earth (according to the Earth Clocks)?
In each case first suggest in which frame the proper time is directly measuredTsokos, 2005, p562
IB Physics – Relativity
Length ContractionAnother consequence of the invariance of the speed of light is that the distance between two points in space contracts according to an observer moving relative to the two points.
The contraction is in the same direction as the relative motion.
0L
L Measured by observer who is stationary with respect to the object
Measured by observer in a moving frame with respect to the object A paradox on length contraction
IB Physics – Relativity
Proper Length
The proper length of an object is the length recorded in a frame where the object is at rest
Any observers moving relative to the object measure a shorter length (Lorentz contraction);
lengthproper
length
IB Physics – Relativity
Examples
1. An unstable particle has a life time of 4.0 x 10-8 s in its own rest frame. If it is moving at 98% of the speed of light calculate;
a) Its life time in the lab frame
b) The length traveled in both frames.
2. Electrons of speed v = 0.96c move down the 3 km long SLAC linear accelerator.
a) How long does take according to lab observers?
b) How long does it take according to an observer moving along with the electrons?
c) What is the speed of the accelerator in the rest frame of the electrons?
Tsokos, 2005, p566
Kirk, 2003, p145
IB Physics – Relativity
The twin paradox
Link to twin paradox
Read about this in Kirk p 146.
•The paradox arises because both twins view a symmetrical situation. Explain why?•Explain why it is not a paradox.
IB Physics – Relativity
The muon experiment
This offers direct experimental evidence of time dilation
Key points
Muons have an average lifetime of 2.2 x 10-6 s in their own rest frame.
They are created 10 km up in the atmosphere with velocities as large as 0.99c.
Show that without special relativity muons are unlikely to be detected on Earth.
Muon decay explanation
IB Physics – Relativity
Relativity
Lesson 31. Velocity addition
2. Rest mass and relativistic mass
3. Use of E = mc2 as total energy
4. Acceleration of electrons by a p.d.
5. Use of MeVc-2 and MeVc-1
IB Physics – Relativity
Galilean Velocity addition
u
S
ground stationary
S
train stationary
vu
vuu Easy!
IB Physics – Relativity
What if approaches c ?uvuu
u would be bigger than c
….and my special theory of relativity does not allow this
IB Physics – Relativity
Relativistic velocity formulae
21c
vuvu
u
Just a small correction needed
prove the inverse formulae from this;
21cuvvu
u
Notice the similarity to Galileo's formulae with a small correction
IB Physics – Relativity
Now try this1. An electron has a speed of 2.00 x 108 ms-1 relative to a rocket,
which itself moves at a speed of 1.00 x 108 ms-1 with respect to the ground. What is the speed of the electron with respect to the ground.
Answer; u = 2.45 x 108 ms-1
The trick is to clearly sort out your reference frames.
ground
s
rocket
s
8101v
8102u
Tsokos, 2005, p567
IB Physics – Relativity
And this; a bit harder
2. Two rockets move away from each other with speeds of of 0.9c to right and 0.8c to the left with respect to the ground. What is the relative speed of each rocket as measured from the other. Answer ± 0.988c
Again the trick is to be very careful with your frames of reference
0.8c 0.9c
A B
Tsokos, 2005, p567
IB Physics – Relativity
rocket B
s
ground
s
cv 9.0 rocket A
cu 8.0
u is then the velocity of A with respect to B.
Write down then check the velocity of B with respect to A
Learn this solution
IB Physics – Relativity
Apply the same approach for the question in Kirk on p147
By now you should have realised that at low velocities the relativistic formulae approximate to the Galilean equations.
Check this out.
Show also that the relativistic formulae do not allow objects to have velocities greater than c
............. but F = ma allows velocities > c so we might have to look at mass again!!!
IB Physics – Relativity
Galileo and Newton allow v > c
t
v
c
constant acceleration
decreasing acceleration as speed approaches c
but I don’t allow this
IB Physics – Relativity
Inertial mass
Defined as m = F/a
For a constant force Einstein predicted that as v approaches c the acceleration will decrease to zero. This ensures that speed can never exceed the speed of light
This means the inertial mass must be approaching infinite.
IB Physics – Relativity
Rest Mass
The rest mass, m0, of an object is the mass as measured in a frame where the object is at rest
If mass is measured in a frame moving with respect to the object;
m = m0
Lorentz Factor
0
1
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1 1.2
v/c
m/m0
IB Physics – Relativity
Try these
1. Find the speed of a particle whose relativistic mass is double its rest mass. Answer = 0.866c
2. A constant force, F, is applied to a particle at rest. Find the acceleration in terms of the rest mass, m0. Find the acceleration again if the same force is applied to the same particle when it moves at a speed 0.8c.
3. Find the rest energy of an electron and its total energy when it moves at a speed equal to 0.8c. Give values in MeV.
Tsokos, 2005, p571
IB Physics – Relativity
Mass - Energy
The energy needed to create a particle at rest is called the rest energy given by;
E0 = m0c2
If this particle accelerates it gains kinetic energy and the mass also will increase so total energy;
E = mc2 or E = m0c2
IB Physics – Relativity
Proper energy; mc2
IB Physics – Relativity
Kinetic energy (T) and momentum (p)
The formulae; Ek = ½ mv2 and p = mv no longer work at high speeds.
Total energy of a particle;
E = Ek + m0c2
So total energy is the kinetic energy plus the rest mass energy
IB Physics – Relativity
Now try this
An electron is accelerated through a p.d. of 1.0 x 106V. Calculate its velocity; (use the data book to find the electron rest mass and charge). Answer = 0.94c
Hint; First calculate the energy gained then find the rest mass energy. This will give you a value for .
***??**
Kirk, 2003, p148
IB Physics – Relativity
Relativistic formulae
At high speeds you must use the following formulae;
p = m0v for momentum
E = Ek + m0c2 for kinetic energy
E2 = p2c2 + m02c4 for total energy
E = m0c2
the derivations of these are not required.
IB Physics – Relativity
Useful units
The rest mass of an electron = 9.11 x 10-31 Kg
This is equivalent to energy E0 = m0c2
= 9.11 x 10-31 x 9 x 1016 = 8.199 x 10-14 J
This is more conveniently quoted in MeV.
so E0 = 8.199 x 10-14 / 1.6 x 10-19 = 0.512 MeV
Working backwards we know E0 /c2 = m0
So mass can be measured in MeVc-2
electron rest mass = 0.512 MeVc-2
Similarly KeVc-2 and GeVc-2 etc. may be used
IB Physics – Relativity
Similarly MeVc-1 may be used for momentum
and problems are much easier to solve.
IB Physics – Relativity
So try these using the new units
1. Find the momentum of a pion (rest mass 135 MeVc-2) whose speed is 0.80c. Answer 180 MeVc-1
2. Find the speed of a muon (rest mass = 105 MeVc-2) whose momentum is 228 MeVc-1. Answer = 0.91c
3. Find the kinetic energy of an electron (rest mass 0.511 MeVc-2) whose momentum is 1.5MeVc-1. Answer = 1.07 MeV
Work through the example in Kirk on p.150
Tsokos, 2005, p581
IB Physics – Relativity
Relativity
Lesson 4
General Relativity (without the maths)
1. The equivalence principle
2. Spacetime
3. Gravitational redshift
4. Black holes
IB Physics – Relativity
The Nature of Mass
Gravitational Mass
mg = W/gInertial Mass
mi = F/a
I do not think so!
These are completely
different
IB Physics – Relativity
Einstein’s Happiest Thought
Sitting in a chair in the Patent office at Berne (in 1907), a sudden thought occurred to me.
" If a person falls freely he will not feel his own weight”.
I was startled and this simple thought made a deep impression on me. It impelled me towards a theory of gravitation. It was the happiest thought in my life.
I realised that ........for an observer falling freely from the roof of a house there exists – at least in his immediate surroundings – no gravitational field.
The observer therefore has the right to interpret his state as at rest or in uniform motion................... McEvoy & Zarate, 1995, p32
IB Physics – Relativity
The equivalence principal
a
PlanetPlanet
1. An object inside an accelerating rocket in outer space will “fall”.
2. An object in a stationary rocket inside a gravitational field will fall
So both situations are equivalent
Path of “falling” objects
IB Physics – Relativity
and similarly
Planet
v 1. An object inside a rocket in outer space moving at constant velocity is weightless.
2. An object inside a rocket accelerating due to a gravitational field feels weightless
Again both situations are equivalent
Equivalence animations
?????
IB Physics – Relativity
Einstein’s elevator
IB Physics – Relativity
The path of light
v
Observer inside rocket
Inertial observer outside rocket
x y x
y1
IB Physics – Relativity
The bending of light
a
x y x
y1
y2
y2
Observer inside rocket
Inertial observer outside rocket
IB Physics – Relativity
But according to the equivalence principle
a
x yy2
Is equivalent to
Planet
x yy2
So gravity bends light towards the planet
IB Physics – Relativity
Space time
Einstein viewed space time like a rubber sheet extending into the x, y, z and t dimensions
Space time is “curved” by mass.
An object moves along the path of least resistance. This means they take the shortest path between two points in curved space time.
Flat space time.
Objects move in a “straight line”
McEvoy & Zarate, 1995, p32
IB Physics – Relativity
General relativity
Model of a planet in orbit
An object is “just” captured by the depression.
Model of a meteorite crashing into the Earth.
McEvoy & Zarate, 1995, p32
IB Physics – Relativity
Space-time movie
IB Physics – Relativity
So what’s so great about general relativity?
Matter tells space how to curve and then space tells matter how to move ........
The beauty of this simple model is.... we don’t need forces.
“objects move in a straight line in curved space-time”
IB Physics – Relativity
More on space-timeEvents can be given an x, y, z and t coordinate in space-time to describe where and when they occurred.
ct
xspace-time diagram
45o
a bc
1. Why is the time axis multiplied by c?
2. What is the gradient of a line on this axis?
3. What is the velocity of lines a, b and c?
4. What is wrong with d?
d
Think?
IB Physics – Relativity
Gradient of a space-time diagram
x
tcgrad
v
cgrad or
c
v
grad
1
now try the questions on the previous slide
IB Physics – Relativity
Even light bends in space time
General relativity predicts that the path of light is deviated by curved space-time.
“There was to be a total eclipse of the Sun on 29 May 1919, smack in the middle of a bright field of stars in the cluster Hyades.”........Arthur Eddington led an expedition to the island of Principe off the coast of Africa to photograph the eclipse.
Eddington found that the position of the stars appeared different from pictures taken at a different time. He concluded that the light had curved around the sun by the exact amount that Einstein had predicted.
hyperphysics.com, 2006McEvoy & Zarate, 1995, p32
IB Physics – Relativity
Gravitational lensing
Light from objects (e.g.quasars) which are very far away can be bent round massive galaxies to produce multiple images; the galaxy behaves like a lens.
Researchers at Caltech have used the gravitational lensing afforded by the Abell 2218 cluster of galaxies to detect the most distant galaxy known (Feb, 15th 2004) through imaging with the Hubble Telescope.
Spot the multiple images
Wikiepedia.org, 2006
Kirk, 2003, p155
IB Physics – Relativity
Black Holes
r
GMv
2
When a star uses up its nuclear fuel it collapses. If the remnant mass of the star is greater than 3 x the sun’s mass there is no mechanism to stop it collapsing to a singularity. The curvature of space-time near a singularity is so extreme that even light cannot escape.
Escape velocity For a photon
r
GMc
2
IB Physics – Relativity
Schwarzchild Radius; RSch
r
GMc
2
At a distance, RSch, from a singularity the escape velocity is the speed of light.
2
2
c
GMRSch
At a distance less than RSch from a singularity;
escape velocity > C
Light cannot escape
RSch is also called the event horizon. Everything trapped within the event horizon is not observable in our universe.
IB Physics – Relativity
Try this
1. Calculate the Schwarzschild radius for a star of one solar mass; (M = 2 x 1030 Kg)
Tsokos, 2005, p591
IB Physics – Relativity
Time in gravitational fields
General relativity predicts that time runs slower in places where the gravitational field strength is stronger. For example; the Earth’s field weakens as you go further away from the Earth’s surface.
This means that time runs more slowly at the ground floor than at the top floor of a building.
2
1
rg
IB Physics – Relativity
Gravitational Red Shift
Gravitational time dilation, or clocks running slower in strong gravitational fields, leads us directly to the prediction that the wavelength of a beam of light leaving the Earth’s surface will increase with height.
fc f
c
fT
1 cT
or
but Period
IB Physics – Relativity
Red-shift Explained
As the light gains height so time runs more quickly because the gravitational field weakens.
cTSo as time speeds up, the period increases and hence the wavelength also increases
Planet
Large wavelength
Short wavelength
IB Physics – Relativity
Calculating frequency shifts.
02 hfmcE
Consider a photon of frequency f0 leaving the surface of the Earth. It gains potential energy given by mgh and the frequency is reduced to f.
Photons do not have mass but have an effective mass given by;
20
c
hfm
Conserving energy we get;
Do not get h the Planck's constant confused with h the height gain.
20
20
0
20
0
0
c
hg
f
f
c
hg
f
ffc
hghfhfhf
hmghfhf
IB Physics – Relativity
Blue shift
Similarly, a beam of light directed from outer space towards the Earth is blue-shifted i.e. the wavelength decreases as the gravitational intensity increases. The same formula is used to calculate red-shift or blue-shift.
Examples to try
1. A photon of energy 14.4 KeV is emitted from the top of a 30 m tall tower toward the ground. What shift of frequency is expected at the base of the tower? Ans; f = 1.16 x 104 Hz Tsokos, 2005, p589
2. A UFO travels at such a speed to remain above one point on the Earth at a height of 200 Km above the Earth’s surface. A radio signal of frequency 110MHz is sent to the UFO. (i) What is the frequency received by the UFO? (ii) If the signal was reflected back to Earth, what would be the observed frequency of the return signal? Explain your answer. Ans; (i) f = 1.1 x 108 Hz (shift is v. small). (ii) return signal is the same frequency as the emitted signal
Kirk, 2003, p153
IB Physics – Relativity
Experimental support
In 1960 a famous experiment called the Pound-Rebka experiment was carried out at Harvard University to verify gravitational blue/red-shift. The frequencies of gamma ray photons were measured at the bottom and top of the Jefferson Physical Laboratory tower. Very small frequency shifts were detected as predicted by the theory.
Atomic clocks are very sensitive and precise. Atomic clocks sent to high altitudes in rockets have been shown to run faster than a similar clocks left on Earth
IB Physics – Relativity
An alternative view of black holes
The gravitational intensity near black holes is very strong and approaches infinite at the event horizon. Time effectively stops and any object falling into a black hole will appear to stop at the event horizon according to a far observer. Light emitted from a black hole will therefore be infinitely red-shifted; hence no light is emitted.
Rs x
ct
Space-time diagram of the formation of a black hole
This photon is trapped in the black hole.
This photon will escape
IB Physics – Relativity
Beyond relativity!
IB Physics – Relativity
Bibliography
Kirk, T; Physics for the IB diploma, OUP, UK, 2003McEvoy, J.P. & Zarate, O; Stephen Hawking for beginners, Icon Books, U.K., 1995.Tsokos, K.A.; IB Physics, Cambridge University press, U.K., 2005.
hyperphysics.com, 2006http://en.wikipedia.org/wiki/Gravitational_lensing, Mar 06
©Neil HodgsonSha Tin College
IB Physics – Relativity
Videology
1. Postulates of special relativity
2. Speed of light is constant
3. Simultaneity
4. Time dilation
5. Atomic clocks prove time dilation