1. Does the speed
of light depend on motion of source of light?
No, the motion
of light is not affected by motion of source of light.
2. Is photon a
particle?
The photon is
a particle of light, but it doesn’t possess all essential properties we ascribe
to a tiny ball i.e. photon doesn’t behave as a common sense particle but it has
got some peculiar properties.
3. When we follow
Albert Einstein in developing special theory of relatively, we are developing a
theory of space and time.
4. The principles of special theory of light.
Principle 1:
Colloquial
statement: If we are in unaccelerated vehicle, its motion has no effect
on the way things happen inside it.
Formal statement:
The laws of physics are the same in all unaccelerated reference frames.
Principle 2: The
motion of light is not affected by motion of source of light.
5. The special theory of relativity
Special:
The word special in name arises because we employ only unaccelerated reference
frames, not all reference frames that one can think of. In other words, we
special to the way things appear when observed from uniformly moving reference
frames.
Relativity:-The
word relativity comes from a phrase coined by Henri Poincare, an eminent French
physicist and mathematician.
In 1904, Poincare was invited to address the international
congress of arts and science, held in st Louis to commemorate the 100th
anniversary of Louisiana Purchase. Poincare spoke of a principle of relativity.
If you are in plane on its way from Chicago to phoenix,
another plane making the return flight, over wheat fields of Kansas. A farmer,
looking up, notes that you are flying south west at 500 miles/hr relative to
his wheat fields.
The pilot of return flight notes that the distance between
the two planes is decreasing at about 1000 miles/hr. So far as the pilot is
concerned, you are travelling at about 1000 miles/hr relative to his plane.
The essence is this: statements
about uniform motion relative to a specified reference frame wheat fields or
another air plane are meaningful.
A quantitative statement about uniform motion without
specification of a reference frame is not meaningful. Why? Because our
principle 1 says we cannot discern uniform motion without recourse to some
reference frame.
Take first the colloquial form of that principle if we are
in an unaccelerated vehicle, its motion has no effect on the way things happen
inside it. So by just doing experiments inside the vehicles, we have no way to
assign a velocity to the vehicle. Only if we look out of window and thereby use
wheat fields of Kansas as an outside reference frame. We can decide on velocity
(velocity to that outside reference frame).
Theory: It
appears because principles 1 & 2
are generations from observation and experiment.
6. THE CONSTANCY OF SPEED OF LIGHT
- Observes in all un accelerated reference frames measure the same speed for light ( in vacuum) from any given source.
- They all measure 3*10 8m/sec always for light in vacuum.
- This remarkable property is called “constancy of speed of light”.
Note:-Some
factors other than light may be observed differently in unaccelerated frames.
7. An “event” is anything
that happens at some definite locations at some definite time. Proto typical
examples are your birth, assassination of Abraham Lincoln etc. In contrast, a
forest fire that sweeps across 10000 acres in 5 days does not constitute an
“event” because the fire is spread out in space and time.
The adjective “definite” means “distinct” or “limited” for any one observing the happening.
8. THE RELATIVITY OF
SIMULTANEITY:
- Spatially separated events that are simultaneous in one frame are, in general, not simultaneous when viewed from other reference frame.
- Simultaneity is a relative concept, but not an absolute one.
- The concept of simultaneity between two events in different space points has an exact meaning only in relation to a given inertial system i.e. “Each frame of reference has its own particular time”.
- To measure the length of an object means to locate its end points simultaneously. As simultaneity depends on frame of reference, the length measurements will also depend on frame of reference.
- Thus, “The length i.e. Space is a relative concept, not an absolute one”.
- Thus there is no such thing as an absolute, global “now”.
