To the average person, time is a liner progression of events. In the classic model, time is the quantitative result of repeated cause & effect at set intervals. More accurately, the progression of time is not a singular motion, but a component of something much larger. When Einstein revealed spacetime to the world, we gained a greater understanding of the reality we exist in. We’ve learned that while time continues from moment to moment, we as observers may not realize that we’re also moving.
In a classic sense this movement is part physical. Sitting still, we may think no movement is apparent. However, while siting still, the earth rotates one revolution every 24 hours. The earth also orbits the sun ~once every year, and our solar system has a galactic year of ~235 million earth years with respect to the centre of our galaxy. Then there’s the movement of our galaxy in the local group, and finally, the groups movement in the Virgo supercluster and so on. We won’t include the expansion of the universe, as that’s a homogenous expansion. Simply put, we’re moving fairly fast, but not fast enough to really notice.
Spacetime is a reference to the bound relationship of space and time, just like the electric field and the magnetic field, you can’t separate the two parts. It may surprise some that spacetime has a finite limit in relativity. This is because spacetime has only two dimensions, not unlike a drawing on paper. Everything in spacetime has both a velocity in space and a velocity in time. The direction in space is irrelevant, as spacetime cares not of the direction you choose to travel. In spacetime, everything travels at the same velocity. The geometry (or path) within spacetime is where we find the interesting parts.
Those sitting still in their living room chairs are moving fairly quickly. Yes, as noted previously, we travel through the galaxy at quite a speed. Yet, nothing compared to the speed we travel through time. Think of a graph where time is set along the X-axis and space is set along the Y-axis. If you drew a horizontal line along the bottom of the X-axis to represent a single year, you might get an idea of just how fast we’re travelling through time and how slow we’re travelling though space. The limit of spacetime is represented by the length of line you drew along the X-axis. This length is unchangeable. If the top of the Y-axis represents the speed of light, and the end of the X-axis represents 1 year, how fast would you have to travel in order to change the angle of the seemingly horizontal line?
It becomes apparent just how much velocity through space is needed before we see a change in time. Basic geometry will tell you that travelling at equal velocities of space and time will place you at 211,986,245 m/s or ~474.2 million miles/hour. That’s fast enough to get you to Mars in 20 minutes. At that velocity, time will have slowed to 70.711% of normal. Of course, you wouldn’t noticed this as you travel. The only thing you might notice is while everyone on earth will know you took only 20 minutes to reach Mars, the voyage for you will have taken a little over 14 minutes. You will have travelled as fast through time as though space. Increasing velocity through space takes away velocity from time, and vice-versa.
If you think 70.711% of the speed of light is fast, it is. At that speed you can get to Pluto in just under 4 hours. Yet, it will still take you 3 years to reach Alpha Centauri — the closest star to us.
For those hoping scientists will invent a time machine, don’t hold your breath. Curved space is only a relative distortion, and all that implies.
A question I asked Neil: “While the universe goes though its homogenous expansion, what effects might this have on spacetime? If we find a way to move our spacetime velocity more towards space (and less towards time), what might we have to consider if space is expanding? Better yet, what if our measurement of the expanding universe is not a measurement of the expanding universe?… if you get what I mean.”
The implication of my last comment was this: What if the universe is not expanding, but instead, time is accelerating? What if dark energy contributes to a compound effect on time — producing an accelerated view of galaxies further away? Our local frame would appear faster than the rate of time when light from distant galaxies began their journey. Therefore, the light would seem slower (red) to us after millions of years of travel.
Are we really seeing a homogenous expansion between galaxies known as Hubble’s Constant? One thing is for sure, spacetime has a limit. Divide its components as you like, but in the end, spacetime is constant. Perhaps our position on the graph of space and time isn’t that clear. With all things relative, perhaps spacetime is slowly shifting away from space, ever closer towards the direction of time.