Does Absolute Velocity Exist? It seems everyone is on the same page about there being no "absolute velocity" due to the fact that everything is relativistic. However, this leaves me confused. This seems to be disconnected with the concept that the speed of light is in fact an unchanging constant. But to state that all speed is relative to an object is to essentially say, "speed does not exist, but is only relative to an observer". But yet, the speed of light is a calculable constant that never changes. How then, can we say, there is no such thing as absolute speed?

If I throw a flashlight in space that is turned on 50mph, does the speed of the emitted light from the flashlight increase by 50 miles per hour in the direction I threw it? Of course not. We know this. But if this is truly the case, how then can there not be an absolute velocity which could be calculated by determining the speed of light in relevance to the object we're measuring (since the speed of light is constant)?

Please help because this is bothering me.
Chris, you are actually on the verge of understanding how special relativity works. You're very close. You only need to take one extra step.


  to state that all speed is relative to an object is to essentially say, "speed does not exist, but is only relative to an observer"


This is correct. Absolute space, as in newtonian physics, does not exist. Same for absolute time. Both are just relations between objects, and between events.

As long as you hang on to the notions of absolute space and absolute time, relativity cannot be understood. If you let go of them, relativity actually seems natural.



Einstein simply noticed this:

A) All previous experiments (like Michelson-Morley) have shown that whenever you measure the speed of light, you always get the same result, no matter what.

B) Maxwell's equations for electromagnetic fields (such as light) include the speed of light, and the value of that speed is frame-independent. Which means, they always show the speed of light to be the same, no matter how the observer is moving.

Both A and B tell the same thing: speed of light is always the same. But how can that be, when different observers are moving through absolute space at different speeds?

The answer is - absolute space is not a thing. Space is just a background for a special type of relation between objects, called "distance". Time is just a background for a special type of relation between events, called "duration". But neither space nor time are things, separate from objects and events. Only when you put together a whole lot of "distance" type of relations, space seems to appear.

The really important thing is speed of light. It's a fundamental constant of nature, like the Planck energy, etc. This universe is built in such a way that speed of light and other constants like it remain the same no matter what.

But space and time are derivative notions. They are not primordial realities like the speed of light. So they depend on the observer. For me, the bundle of distance relations we call "space" looks a certain way. For you, the same bundle looks a bit differently. This is okay, because space is not absolute; it's derived from a lot of other things.

When you throw the flashlight, the speed of light from it is the same for everyone. But how can that be, since different observers are moving in different ways? Simple: they all see the bundles of distance relations differently; those differences are such that speed of light always remains the same.



And that's special relativity in a nutshell.