We All Travel Through Spacetime at the Speed of Light

Our speed through spacetime is constant. The more we move through space, the less through time and vice versa.

Lenka Otap
Predict
Published in
6 min readDec 28, 2019

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Image by alan9187 from Pixabay

Everything travels through spacetime at the speed of light. Me, you, the cat (even Schrödinger's cat), the Earth, the sun, bits and bytes, and any particles including photons (light particles).

Note that I said spacetime. Of course, we travel at different speeds through space, we might even be standing still in our frame of reference. This frame of reference is moving relatively in other frames of reference, for example you might be sitting still while reading this, but you are on Earth (I’m guessing), which is spinning at 1,600 km/h at the equator, and Earth is also moving around the Sun (107,000 km/h), and the Sun is moving around the center of the Milky Way (828,000 km/h). Milky Way is also moving in relation to any other galaxy at 600 km/s or 2,160,000 km/h.

But none of the above mentioned, except for photons, travel at the speed of light through space (which is 300,000 km/s or 1,080,000,000 km/h for comparison). Our motion through spacetime, on the other hand, is different.

We all travel at exactly the speed of light through spacetime.

Spacetime

Spacetime is a 4-dimensional concept, the three spatial dimensions, also called space, and one temporal dimension. Our speed is spread between the four components, and since the speed has a direction, we call it velocity. The 4-velocity is the vector U = γ(𝑐,𝑣_𝑥,𝑣_𝑦,𝑣_𝑧) where γ is the Lorentz factor.

It doesn’t matter for the equations, how we spread our movements through the 3 space-dimensions x, y, and z, we only care about the speed, not the velocity with a specific direction, so we say that the movement is broken in two components, space and time.

When we are still, e.g. don’t move through space (as seen in our own frame of reference), then we move through time at the maximum speed. When a photon moves at the maximum speed, c, then it doesn’t move through time at all. For a photon there is no time component.

We could draw the coordinate system like this, with space on the x-axis and time…

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Lenka Otap
Predict

Computer scientist and astrophysicist. Curious about life, the universe, and everything.