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### How can any speed be defined as a constant? [duplicate]

We know that the speed of light is a constant, and can therefore be used to calculate many other relative values, but I'm having difficulty understanding how speed can be a constant, seeing as it's ...
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### Why is the speed of light in vacuum a universal constant? [duplicate]

While getting familiar with relativity, the second postulate has me stuck. "The speed of light is constant for all observers". why can't light slow down for an observer travelling the same direction ...
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### Is light relative? [duplicate]

I was wondering, if two cars (A and B) are going at different velocities let's say A=90km/h, B=80km/h then if we add the two speeds we get 170km/h meaning they are relative to each other. But if one ...
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### Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
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### What is the speed of light relative to?

Consider the scenario where you measure the time it takes for light to travel to the left 10 meters and to the right 10 meters. Both measurements will take the same time, even though we are moving ...
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### How can time be relative?

I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is. I have always been told (and thought) that time is basically a ...
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### Time dilation derivation of special relativity

In almost all of the derivations using the postulates of special relativity (SR), we use experiments involving light signals. For example, we make a clock using a light signal or measure lengths using ...
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### What is the universal speed limit relative to? [duplicate]

If all speeds are relative, then what "governing" force is that speed limit relative to? Is there some sort of fixed or absolute grid with locations everything is compared to? Does this also mean ...
1k views

### What is the limiting factor of the speed of light?

Have been really struggling for some considerable time to understand the universal limitation of speed i.e. it is not possible to travel faster than the speed of light. In particular - what is the ...
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### Objects travelling relatively to each other faster than light?

When we say that something is travelling a certain speed, it's really travelling that speed relative to the Earth. When saying the speed of anything, it is, for the most part, relative to something ...
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### Is the speed of light the limit or just everything moves at this speed?

When I was a little kid, I was fascinated by the fact that we are not able to surpass the speed of light. I imagined a giant spaceship trying to catch a light beam like superman tries to catch flash. ...
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### Why is the speed of causality equal to the speed of light?

I heard that the most fundament concept is Causality, and also that it is turn out that speed of light has nothing special in it so that It is ultimate speed limit but because it is happen to be ...
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### Special relativity: where does this naive calculation go wrong?

Inspired by the recent question, "If all motion is relative, how does light have a finite speed?", I tried to work out a little calculation. Where does it go wrong? My friend Buzz is traveling in a ...
An observation more than a question. Take any function $f$ (additional hypotheses may follow) and evaluate it on any two points $x_1, x_2\in\mathcal{D}_f$. Define then the sum of these two points as ...