So, what is time? Let’s start by looking up the definition of a second:
Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0K…
So, a second is nine billion periods of radiation. Now, what’s a period? We know that radiation is basically light, so let’s have a look at frequency:
Frequency = 1 / T and Frequency = v / λ
So frequency is the reciprocal of the period T, and also velocity v divided by wavelength λ. No problem. Flipping things around, we see that period T is wavelength λ divided by velocity v. We know that a wavelength is a distance, a thing like a metre:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second...
And we all know that velocity is a distance divided by a time. So a period is a distance divided by a distance divided by a time. The result is another period of time. This definition of time is circular and tells us nothing.
Actually, it's just consistency. You should be very worried if you'd derived that periods were not units of time, starting from the assumption that they were.
And for things to change, something, somewhere, somehow, has to have motion. You don’t need time to have motion. You need motion to have time.
Well, time would be useless to physics if nothing varied over it, and could therefore disappear as a parameter. That's hardly a revelation.
So why do we say things like Clocks slow down as if a clock is something that moves like a car?
"Fast" and "slow" (and their synonyms) are used to describe the rates of processes in general - not just motion. You can talk about a liquid evaporating quickly, or an object absorbing heat slowly. Granted, motion is involved in both these examples, but in neither case is it of direct interest.
Anyway, there's no point arguing about terminology.
That's three Dimensions, with a capital D because we have freedom of movement in those dimensions.
I don't know of a definition of "dimension" that includes "freedom of movement".
The thing you should measure is temperature, which used to be considered a dimension, before the word changed from “measure” to “Dimension” under your feet.
Actually, temperature would still be called a dimension in the sense it used to be. The term has
two definitions in common use:
One (in mathematics), denotes the cardinal of any basis of a given vector space (it also appears in related contexts). When physicist talk about "three dimensional space" or "four-dimensional space-time", it means they're modelling space and space-time with three and four dimensional vector spaces, respectively.
The other (in physics) loosely means "quantity" - basically anything with a unit. Length, time, temperature, intensity, energy, volume, mass, acceleration, and power are all dimensions in this sense.
Special Relativity tells us that your relative velocity alters your measurement of space and time compared to everybody else. You increase your relative velocity and space contracts while time dilates by a factor of √(1-v2/c2). If you travel at .99c, space contracts to one seventh of its former size.
Special relativity tells us that the laws of physics are Lorentz invariant. Inevitable consequences of this are that moving processes will slow down and contract in the direction of motion with respect to stationary ones. What space and time do is subject to your definitions of the base units of space and time.
So your trip to a star seven light years away only takes you a year.
It takes seven years as seen in one reference frame, and one year as seen in another.
But physics is about the universe, and in that universe it took you seven years.
You aren't implying a preferred reference frame, are you?
Later Einstein struggled with the Twins Paradox in 1918. He used acceleration from General Relativity as the explanation, but this explanation was erroneous and didn’t account for passing clocks.
Um, the twin "paradox" is a result of faulty logic. It doesn't need an explanation - just the flaw pointed out (in short, the contradiction is reached by applying properties specific to inertial reference frames to a non-inertial reference frame).
When you read the history you can see a slow evolution from the postulate that says the laws of physics are the same in all inertial frames of reference.
Evolution to what?
The problem with reference frames is that all our observer velocities are zero
Why is it a problem that all observers are stationary with respect to themselves?
and if you don’t take care the sun goes round the earth.
In a reference frame attached to the Earth and rotating with the Earth, the Sun goes round the Earth once every 24 hours. Why is this a problem?
They don’t explain why the speed of light is always the same.
A consequence of the Lorentz invariance of the laws of physics.
Anyway, no matter what theory anyone comes up with, you can always ask why things are the way they are. Accusing a theory of not explaining and only describing is pointless; every theory has its axioms.
The “speed of light” was always the problem. And it was always the problem because time was always the problem. Because at the speed of light there’s no time left for anything else to happen. It’s why c isn’t really a speed, because you run out of time trying to get there, and if there’s no time, there’s no speed because speed is distance over time.
You're mixing frames. The speed of an object relative to an observer is the distance travelled in the observer's reference frame per unit of time elapsed, again, in the
observer's reference frame. You are measuring the distance travelled over the
proper time. This gives the space-like component of the Minkowski
four-velocity of a particle (which does become infinite for anything travelling at the speed of light).
Velocity is prime. It defines your metres and your seconds. We should talk of it as a fraction of c like in the equations, or by degrees, but not by the things it itself defines.
The definition of length isn't derived from velocity in general - just the motion of light. There's nothing circular about defining the velocity of, say, a car to be the distance it travels divided by the time taken. The unit of distance is not defined by the motion of the car. You're right that, technically, you can substitute the definition of distance into the definition of velocity, but this is of no practical use and doesn't actually say anything new about velocity. Defining the metre as "the distance travelled by light in 1/299792458 seconds" is only done for the convenience of giving
c an exact integer value. There aren't any particularly remarkable conclusions to be drawn from this.
We don’t travel in time at one second per second.
For every second that passes, one second certainly passes. I'll let you decide for yourself whether you want to call that "travelling" or not.
they collide at the same location and at the same time whatever their faces say is local time. Local time.
Local time as opposed to what, exactly?
To travel backwards in time we'd need to unevent events, we’d need negative motion. But motion is motion whichever way it goes. You can’t have negative motion.
1) What's "negative motion"? Definition (preferably precise and mathematical), please.
2) How did you conclude that travelling backwards in time would require something that, according to you, is impossible by definition?
