Understanding Special Relativity

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As I understand it the speed C allows for distances in spacetime to be agreed upon by all observers. I am finding this confusing as C is not invariant to all observers, unless observed in a vacuum.

Also as I understand it, sitting here writing this I am moving entirely along the time axis and therefore I am moving through spacetime at the speed C. However other observers can rightly say that I am moving quite rapidly along the distance axis.

Another point. If by travelling at very close to speed C a apce ship would shrink the distance between stars or a Muon can shorten the length of the track it is speeding around, why does light not do the same and get here quicker? I pressume this has a lot to do with mass?

As you can see I have some grasp of the concepts but clearly not the fumdemntal ones.

I apologise if I am being somewhat thick and not understanding the answers given so far.

c IS the speed of light in a vacuum. It never refers to the speed of light anywhere else but in a vacuum.

For example, the speed of light in water is .75 c. You would never refer to the speed of light in water as c.



Bearing in mind that being on the surface of the earth, you aren't in an inertial reference frame, if you are in a train at 70 mph relative to the countryside, someone watching the train go by would see themselves as standing still and the train as going 70 mph. At the same time, someone on the train would see themselves as standing still and the countryside going by at 70 mph.



Remember that there are relativistic effect to time as well as to distance and mass. The observer in the moving spacecraft will see time go by less rapidly relative to the observer. Thus, he will feel a shorter travel time than the other observer.

I understand time dilation. This is not what I referring to. If I were traveling alongside a muon at close to the speed of light I will experience it living for 2.2 nano seconds. However speeding around an accelerator observers not in my frame of reference would see it live for 29 times this. Also they would see it do far more circuits of the accelerator. I would see it live for.2.2 nanoseconds but would observe the same number of laps as the "stationary" observers. To explain this physicists say that for both I and the muon the distance around the circuit has shrunk in direct correlation to the time dilation. By the same token, a space craft travelling at close to C would get to its destination quicker because the distance.has shrunk in relation to the time dilation. Unlike the muon and space craft a 3.9 metre car will, if travelling at the correct speed shrink so that it.will fit (for the breifest of.moments) into a 3.6 metre garage.So in the first two examples space is shrunk and the last it is the speeding object that shrinks. This seems nuts as according to this concept we could travelled.to Andromeda in around 40 years yet light travelling just a fraction faster would take 2.5 million years.

You have to remember that observers see different things depending on who is measuring what and in which reference frame. In the case of the muon, remember first of all that the muon is not in an inertial reference frame because it is accelerating (not because it's moving faster and faster but because it's moving around in a circle and a change in direction counts as acceleration). However for the sake of argument, let's consider that the muon is traveling in a straight line and instead of considering the number of "laps", let's consider that there are markers at regular distance intervals (where the distance between two markers is 1 "lap"). In the frame of the muon, the distance between any two markers will shorter than the distance between them as measured by the stationary observer due to Lorentz contraction such that the muon will see the same number "laps" as the stationary observer. If we attach an ideal clock to the muon traveling at constant speed, then the stationary observer will see that clock tick slower than his own clock but an observer in the muon's frame would actually be seeing the clock in the stationary observers frame to be ticking slower.

With respect to the issue of traveling to Andromeda in 40 years, it will only be measured as 40 years by the occupants of the spaceship but it will still be millions of years as measured by observers on Earth. Yes, according to observers on Earth (and in fact any observer not travelling at the speed of light), a photon of light will take 2.5 million years to reach Andromeda but to the photon, travel time is instantaneous. In fact, to any observer in a reference frame traveling at the speed of light, the time taken to get anywhere in the universe is instantaneous. Unfortunately, anything with mass cannot travel at the speed of light.

Ok . I get it now. Thanks for the tip about acceleration I had missed that in relation to particle acellerators. Even though it is clearly in the name I have also gone back over the book I have been listening to and written out the equations as they go. Writing them down has made it easier to see how and why C is the calibrating speed. Thanks to all for your help so far. Watch this space as no doubt another concept will boggle my brain

Grasping all this really shows the folly of people who talk of "common sense" and self evident "truths" arrived at by philosophical logic.

Is spacetime even a valid concept? Space is where something isn't, and time is just a quantification of relative motion, thus spacetime is relative motion where something isn't. In other words: Einstein was a troll.

^ I would love to know how you come to your conclusions. And in particular your concept of what space is, in relation to special relativity and going back to Galileo and his rejection of absolute space?

I've already clarified the definitions.

so in other words you are using your own interpretation of the term SPACE in relation to physics, and in doing so deducing the worlds physicists have it all wrong

Maybe just maybe you should check your definitions before making such spurious statements.

That's a somewhat peculiar interpretation of what I wrote. I'm merely questioning the validity of the concept based on rather uncontroversial definitions as they are commonly used in the relevant context. When ascribing attributes to something that isn't, you are either crazy, or trolling, and I'm pretty sure Einstein was sane as a fruit. He never discarded the aether. It just got rebranded: Spacetime. c is merely the propagation speed of electromagnetic radiation in a medium, much like the speed of sound in air. The phenomenon isn't a very important part of physics, but it has evoked much interest, controversy, and wonder among those who are conceptually bewildered.