Special relativity is best known for letting us know that, as a person's velocity increases, their perception of time slows down. It also creates an interesting little optical illusion that can make objects moving in a straight line appear to twist and turn.

Visualization of special relativity by Joseph Choi

Einstein's theory of special relativity is so popularly known is because it's simple enough for laypeople to understand, but strange enough that we never stop puzzling over it. Two people, one at rest by the side of the road and one in motion on a car, compare watches. The person at rest sees that the person in motion's watch is slow. The person in motion thinks their watch is just fine. They are both right.

Relativity and Physical Changes

Time turning into something that can be stretched out, depending on your particular point of view, is odd enough. Just to confuse people even more, there are things like length contraction. The person in the car will see the car as exactly the same length in motion as it was at rest. To a person sitting by the road, on the other hand, the car will be slightly shorter, and will continue shrinking as it moves faster and faster. The two observers see different things. Again, they're both right.

Except for the fact that the person on the ground doesn't quite see those things. Although length contraction is quite real, it's a logically thought-out consequence of relativity, and not something that is actually observed when it comes to big objects like cars. It's not actually observed, in part, because the person "sitting by the road" would be killed the moment a train going at 80 percent the speed of light came by, and in part because there are so many other weird effects going on with that car that no observer would think to mention the length.

We see because of light, and we see reliably because, when it comes to the world we usually look at, not much has changed between the light leaving its surface and the light smacking into our eyes. When the object approaches the speed of light, a lot can change between light leaving its surface and hitting our eyes, and that makes for some very strange illusions.

The Strangeness of Terrell-Penrose Rotation

Terrell-Penrose Rotation is tough to search for by name. James Terrell and Roger Penrose both published papers about the phenomenon in 1959, and the nomenclature of Terrell, Terrell-Penrose, and Penrose-Terrell rotation has been gently disputed ever since. Whatever it's called, it's a hell of an optical illusion.

Imagine sitting by the side of the road and seeing a car slowly roll past you. All the light you are seeing, including the light from the distant mountains you can see through the car windows, makes the trip to your eye so fast that it doesn't matter how far away the objects you're looking at are. The fraction of a second it takes for the light from the far side of the car to travel across the width of the car and then to you doesn't make a difference.

Once the car gets up to, say, 80 percent the speed of light, that trip across the width of the car does make a difference. There's a reason you never see the back bumper of a car that's traveling towards you, or is just in front of you. Any light that gets bounced from the back bumper towards your eye runs smack into the rest of the car. When the car is going fast enough, this is not the case.