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Einstein postulated that the speed of light in free space is the same for all observers, regardless of their motion relative to the light source, where we may think of an observer as an imaginary entity with a sophisticated set of measurement devices, at rest with respect to itself, that perfectly records the positions and times of all events in space and time.

It is really exciting to understand Einsteins thoughts. I have a doubt with respect Einsteins above postulate,

  • Is there any significance in saying an observer as an imaginary entity? I thought light could be called as some thing imaginary, rather than calling observer as an imaginary entity.
  • What does it mean one being at rest with respect to one self? I think it mean that, observer is not under motion due to its own virtue, but the observer may be under motion, if considered inside a moving frame of reference. I don't know whether I am right with respect to this, if I have misunderstood, please explain.
  • Why is speed of light considered same for all observers (imaginary entity which is at rest with respect to itself) regardless of their motion relative to the light source? I thought, if we consider observer inside a moving frame of reference, speed of light should be different for the observer, due to relative motion with respect to light.
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Is there any significance in saying an observer as an imaginary entity?

Yes. From Wikipedia:

Physicists use the term "observer" as shorthand for a specific reference frame from which a set of objects or events is being measured. Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is experiencing events, but rather it is a particular mathematical context which objects and events are to be evaluated from. The effects of special relativity occur whether or not there is a sentient being within the inertial reference frame to witness them.

Thus, we must be careful to understand that what is observed, as the term is used in the context of special relativity, is not to be confused with what is seen by a localized being.

What does it mean one being at rest with respect to one self?

It means that one is not moving relative to one's reference frame. This is a tautology.

speed of light should be different for the observer, due to relative motion with respect to light.

Empirically, the two-way speed of light is the same for all observers. It turns out that the one-way speed of light is not physically meaningful. This is because two spatially separated synchronized clocks must be used in one-way measurement.

Einstein's invariance of light speed postulate is, operationally, the selection of an synchronization convention. Again, from Wikipedia:

It is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again) is frame-independent, because it is impossible to measure the one-way speed of light (for example, from a source to a distant detector) without some convention as to how clocks at the source and at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition.[14][16] The special theory of relativity explores the consequences of this invariance of c with the assumption that the laws of physics are the same in all inertial frames of reference.[17][18] One consequence is that c is the speed at which all massless particles and waves, including light, must travel in vacuum.

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  • $\begingroup$ Thank you for the answer and valuable time. Is Einstein considering stationary frame of reference (which is at rest with respect to itself) with sophisticated measuring set of devices, and is he arguing that, for any frame of reference within that stationary frame of reference, speed of light is same regardless of the relative speed with respect to light? Or is there any thing which I have misunderstood here. $\endgroup$
    – Sensebe
    Commented Dec 9, 2013 at 18:17
  • $\begingroup$ @VINAY, I think you need to read some more and refine your concept of reference frame. There is no absolute rest so there isn't a stationary reference frame. Two reference frames may be stationary or moving with respect to each other. Einstein's postulate is that light emitted in one reference frame (the reference frame of the source) will propagate with speed c in that reference frame as well as all relatively moving reference frames. $\endgroup$
    – Alfred Centauri
    Commented Dec 9, 2013 at 20:10
  • $\begingroup$ Thank you for the comment. I will try to learn about reference frame clearly. $\endgroup$
    – Sensebe
    Commented Dec 9, 2013 at 20:49

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