1. Special relativity: energy, momentum and mass
Physics 123
1/16/2019
Lecture IX

2. Outline Lorentz transformations 4-dimentional energy-momentum
Mass is energy
Doppler shift
1/16/2019
Lecture IX

3. Lorentz transformations
System (x’,y’z’,t’) is moving with respect to system (x,y,z,t) with velocity v
Lorentz
x=g(x’+vt’)
y=y’
z=z’
t=g(t’+vx’/c2)
Galileo
x=x’+vt’
y=y’
z=z’
t=t’
1/16/2019
Lecture IX

4. Time dilation
Clocks moving relative to an observer are measured by the observer to run more slowly ( as compared to clocks at rest)
Dt – measured in v=0 frame, Dt0- measured in moving frame
Hendrik Antoon Lorentz
Derived time and space transformations before Einstein
1/16/2019
Lecture IX

5. Twin paradox
Two twins: Joe and Jane. Joe stays on Earth and Jane goes to Pluto at v<~c
Joe observes that Jane's on-board clocks (including her biological one), which run at Jane's proper time, run slowly on both outbound and return leg. He therefore concludes that she will be younger than he will be when she returns.
On the outward leg Jane observes Joe's clock to run slowly, and she observes that it ticks slowly on the return run. So will Jane conclude that Joe will have aged less? And if she does, who is correct?
1/16/2019
Lecture IX

6. Length contraction No change in directions perpendicular to velocity
The length of an object is measured to be shorter when it is moving relative to the observer than when it is at rest
1/16/2019
Lecture IX

7. 4-dimensional space – time
Add time to space metric: x1=x, x2=y, x3=z, x4=ict
4- dimensional “length”=interval - Lorentz invariant
AB – real – space-like interval, there exists a frame of reference where the two events happen at the same time (t1=t2 ), but at different places (r12≠0)
AB – imaginary – time-like interval, there exists a frame of reference where the two events happen at the same place (r12=0), but at different times (t1≠t2) x y A B
1/16/2019
Lecture IX

8. Energy, mass and momentum
m0 – mass at rest
Relativistic energy:
Energy at rest
E=m0c2
Kinetic energy:
Relativistic momentum:
4-dimensional Energy –momentum – vector:
(pxc, pyc, pzc, iE)
Lorentz invariant interval:
1/16/2019
Lecture IX

9. Conservation laws
Both energy and momentum are conserved in the relativistic case:
Mass must be considered as an integral component of energy E=gmc2
1/16/2019
Lecture IX

10. Conservation laws Energy could be used to create mass
To conserve momentum electron and positron must collide head on. Then Z-boson is produced at rest.
1/16/2019
Lecture IX

11. Conservation laws Mass could be destroyed and converted into energy
To conserve momentum (zero initially) the photons must be flying in the opposite direction with the same absolute values of momenta
1/16/2019
Lecture IX

12. Mass and energy Mass and energy are interchangeable
Energy can be used to create mass (matter)
Mass can be destroyed and energy released
1/16/2019
Lecture IX

13. Doppler shift Light emitted at f0,l0 In the source’s r.f.
the distance between crests is l0
The time between crests is t0=1/f0= l0/c
Where are crests in the r.f. moving with speed v wrt source’s r.f. (chasing the wave)
l=cDt-vDt=(c-v)Dt
Dt=gDt0=g l0/c
l=(c-v) g l0/c
1/16/2019
Lecture IX

14. Doppler shift
When the source and the observer move towards each other the wavelength decrease (redviolet)
When the source and the observer move away from each other the wavelength increase (violet  red) – Redshift – used to measure galaxies velocities universe expansion (Hubble)
1/16/2019
Lecture IX