Relative Speed of Objects

albert einstein

This undated file photo shows famed physicist Albert Einstein.

Reference: Spacetime 1: Speed and Frequency

We note that the units of space and time are fixed only for matter and not for light. Therefore, the concept of speed does not really apply to light.

Michelson–Morley experiment actually determines the speed of earth relative to light.

The Dutch astronomer De Sitter showed that the “speed of light” does not depend on the speed of the source of light. In this case the speed of the source of light is determined relative to earth. Light seems to serve as the ultimate reference point for speed of stars.

All stars and planet are traveling at the “speed of light” relative to light.

Light seems to have sort of an independent and absolute existence. All objects seems to behave the same way relative to it. The universal constant of “speed of light” needs to be reinterpreted as follows.

The speed of all mass objects relative to light is the constant ‘c’.

This gives us a new perspective on the speed of objects relative to each other.


We can no longer dream of objects moving at the speed of light, because they already are doing that. That dream had actually to do with our desire to travel to distant corners of this universe that are light years away.

Can a rocket travel at the “speed of light” relative to earth? This question suddenly acquires very different implications. Speed ‘c’ comes about in reference to light that has zero inertia.

What limiting speed comes about when an object with considerable inertia is used for reference?

The concept of inertia is associated with the resistance that mass puts up to being moved or disturbed. Gravity seems to be a reaction to disturbance that has already taken place.

All mass with inertia seems to be fighting to come back together as a single entity.

Einstein’s General Relativity models mass, momentum, pressure and stress as curvatures within a space measured by meter rods.

This may put a limit to the speed that two mass objects may attain relative to each other.

An understanding of that limit would require a better understanding of inertia, and how it relates to curvature of space.


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  • vinaire  On March 25, 2016 at 8:15 AM

    I have revised this article to clarify the role of mass, gravity and inertia.

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