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In special theory of relativity, time and time rate in two inertial references are treated as different. If t and t' be the time recorded for a given event in two inertial references, then t may not equal to t'. We shall return to this topic again.

Absolute and stationary reference

There is no preferred inertial reference frame. This idea predates special relativity. It means that there is no absolute reference frame. Had there been an absolute reference, then we would have a fixed universal space in which all other objects (references) would be considered to be either in rest or moving. But the concept of space is a variant. In other words, the perception of space changes from one reference (say ground) to other reference (say moving train). If we drop a stone from a train, then the trajectory of the stone is a straight vertical line for an observer on the train. The same stone, however, is seen to follow a parabolic trajectory for an observer on the ground. Referring to these trajectories for a single motion of a stone, Einstein questioned the very concept of fixed space.

Trajectory of motion

Trajectory of motion

Though there is no absolute reference, but the notion of a stationary reference is a powerful idea which stems from our life long perception of stationary objects in Earth's reference. Despite the fact that Earth is moving at about 107,278 km/hr (29.8 km/s) around Sun, we are generally not aware about it unless we make detailed observations about celestial objects. But as the concept of stationary system is ingrained in our perception, we employ this concept with great effect in the study of relative motion. We refer either of the moving systems as stationary in which an observer is making the measurements. Consider for example two spacecrafts moving with uniform velocities. We can refer either of spacecrafts as stationary and other as moving with a velocity which is measured from the referred stationary reference (spacecraft). There is no preference. In the case of a motion of a train, for example, we consider Earth as stationary and train as moving reference. There is no bar though that we consider moving train as stationary reference with the observer and Earth as moving reference in the opposite direction to the moving train.

The "rest" is local concept. The object like a house is at rest in Earth's reference. But the same object as seen from a spaceship is not stationary.

Constancy of speed of light

Constancy of speed of light has two different considerations in the study of relativity. In the first place, it is the central idea of special relativity. But besides this consideration, the constancy of light has other important consideration in that it is one of the measuring standards which can not be challenged in any inertial reference. This aspect is important as meaning of space and time in different inertial frames is not very explicit. We shall see subsequently that they are in fact entangled. Further, the distance (space) and time are relative and are therefore very subjective in conception and measurement.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
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Source:  OpenStax, Electricity and magnetism. OpenStax CNX. Oct 20, 2009 Download for free at http://cnx.org/content/col10909/1.13
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