The Most Mind-Bending Fact I Learned in Physics

The Most Mind-Bending Fact I Learned in Physics
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Physics is built out of philosophically fascinating ideas. Or, at least, ideas that fascinate us as physicists. We are often moved to reverentially proclaim the beauty of various concepts and theories. Sometimes this beauty makes sense to other people (we're made of star stuff) and other times it's opaque (Frobenius manifolds in psuedo-Euclidean spaces).

I have my own personal favorite idea. It arises from the philosophically fantastic (but mathematically moderate) workings of Einstein's relativity theory. The theory of special relativity holds that time and space are not separate entities, each operating on its own; rather they are intimately and inextricably codependent. We are born, live, and die along "world-lines" through a four-dimensional spacetime.

Here's what awes me: we travel through this 4-D spacetime always at a constant speed: c, the speed of light.

No matter what we do in our momentary lives, we are always truly traveling through our universe in time and space together, always at at the same rate. Let's consider a few facts that follow from this realization.

A man who sits still uses none of his lightspeed to travel through space. Instead he is travelling in time at the speed of light. He ages--in the view of those around him--at the fastest rate possible: light speed. (How's that for a philosophical argument against sloth?)

As we travel about in our daily lives, we use up a miniscule amount of our alotted light speed to move through the spatial dimensions surrounding us. We borrow that speed from our travel forward in time and thus we age more slowly than our sedentary neighbors. You've probably never noticed that fact, and there's a clear explanation why. It's only when you travel at unimaginablly high speeds that the weirdness of time becomes large enough to notice. The mathematical reason for this is that the effect of time dilation at a particular speed "v" is only (v/c)2.

Try putting the fastest you've ever traveled into the top of that equation and then dividing it by the 671 million miles per hour that light travels. Then square that tiny number to make it vastly smaller.

Imagine a strange jet-setter who spends an entire 80-year lifespan cruising at 500 mph on a Boeing 747. When his long flight finally touches down, the watch on his wrist, set to match the airport clock at takeoff, will be only one millisecond behind. However, we can watch a subatomic particle live five times longer at 98% of light speed than sitting still.

Maybe the strangest case of this phenomenon is light itself, the sole thing capable of travel at c. From our point of view, then, a photon is using the entirety of its spacetime velocity to travel through space. It never ages (from our frame of reference, watching)! That's why we see photons will fly through space in a straight line from one side of the universe to the other for all of eternity without changing in any way unless externally influenced. This imperviousness makes them excellent historical records. And here, the deeper general theory of relativity (also courtesy of Einstein) leads us to something more bizarre.

Many of the photons generated at nearly the beginning of the universe are still travelling through space in their birthday suits. But, over the course of their billions of years in transit to us, the space they inhabit along their path through the stars has grown more than 1000 times bigger since they were born. This expansion of spacetime has stretched the wavelength of the photons along with it, like an enormous slinky being pulled apart. Now they are a thousand times longer but still timeless to us.

Spacetime physics, adhering to relativity as we know it, reveals utterly surreal truths. Many of these are posed as famous puzzles and arguments, such as the twin paradox, the ladder paradox, and the failure of simultaneity. But the mere fact that we always travel through spacetime at the speed of light never ceases to stop me in my tracks (metaphorically speaking). I believe it is the most stunning thing I've ever absorbed in a physics class.

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