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RELATIVIDAD ESPECIAL, Apuntes de Física

CLASE DE RELATIVIDAD CON EJERCICIOS. TRANSFORMACIONES

Tipo: Apuntes

2025/2026

Subido el 18/02/2026

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Review Modern Physics, Ph 311
It was found that there was no displacement of the
interference fringes, so that the result of the
experiment was negative and would, therefore, show
that there is still a difficulty in the theory itself
- Albert Michelson, 1907
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Review Modern Physics, Ph 311

It was found that there was no displacement of the interference fringes, so that the result of the experiment was negative and would, therefore, show that there is still a difficulty in the theory itself…

  • Albert Michelson, 1907

to

of our

modern economy !!!

Inertial Reference Frame

◼ A reference frame is called an inertial frame

if Newton’s laws are valid in that frame.

◼ Such a frame is established when a body, not

subjected to net external forces, is observed

to move in rectilinear motion at constant

velocity.

The Galilean Transformation

For a point P ◼ In system K: P = ( x , y , z , t ) ◼ In system K’: P = ( x ’, y ’, z ’, t ’) x K P K’ x’-axis x-axis

The Inverse Relations

Step 1. Replace with. Step 2. Replace “primed” quantities with “unprimed” and “unprimed” with “primed.”

Results of Maxwell's electrodynamics ◼ Visible light covers only a small range of the total electromagnetic spectrum ◼ All electromagnetic waves travel in a vacuum with a speed c given by: (where μ 0 and ε 0 are the respective permeability and permittivity of “free” space)

An Absolute Reference System

◼ Ether was proposed as an absolute reference

system in which the speed of light was this

constant and in all frames moving with

respect to that frame, there needed to be

modifications of Maxwell’s laws.

◼ The Michelson-Morley experiment was an

attempt to figure out Earth’s relatives

movement through (with respect to) the ether

so that Maxwell’s equations could be

corrected for this effect.

  1. AC is parallel to the motion of the Earth inducing an “ether wind”
  2. Light from source S is split by mirror A and travels to mirrors C and D in mutually perpendicular directions
  3. After reflection the beams recombine at A slightly out of phase due to the “ether wind” as viewed by telescope E.

The Michelson Interferometer

As the direction of the ether wind is unknown, the apparatus has to be turned around by 90 degrees to see a shift (starting form many different initial settings)

The Lorentz-FitzGerald Contraction

◼ Another hypothesis proposed independently by both H. A. Lorentz and G. F. FitzGerald suggested that the length ℓ 1 , in the direction of the motion was contracted by a factor of …thus making the path lengths equal to account for the zero phase shift. ❑ This, however, was an ad hoc assumption that could not be experimentally tested. It turned out to be “less than half of the story”

Length contracted for the moving muon, it’s own life time just 2.2 micro seconds Life time of the muon delayed for observer on Earth so that it can travel the whole distance as observed from Earth Great thing about special relativity is that one can always take two viewpoints, moving with the experiment, watching the experiment move past, the observations need to be consistent in both cases They move with about 98 % the speed of light

Mary has a light clock. A suitable clock is just any periodic process, the time it takes for one cycle of the process is the period, its inverse is the frequency. Tom watching Mary go by figures that her time is delayed (dilated) due to her moving in a straight line with a constant high velocity with respect to him.

No simultaneity if not also at the same position, just a consequence of the Lorentz transformations

Einstein’s Two Postulates

With the belief that Maxwell’s equations (and with

it all of the known physics of the time) must be

valid in all inertial frames, Einstein proposes the

following postulates:

1) The principle of (special) relativity : The laws

of physics are the same in all inertial systems.

There is no way to detect absolute motion, and

no preferred inertial system exists.

2) The constancy of the speed of light :

Observers in all inertial systems measure the

same value for the speed of light in a vacuum.

Relativistic Mechanics

Relativistic dynamics can be derived by assuming that mass is increasing with velocity. The Lorentz factor gets larger when velocities get larger and so does mass apparently as we can see from the relativistic momentum equation. Einstein derived relativistic dynamics that way. His derivations are sure correct, but the foundations are somewhat shaking as there is no really good definition for mass.