What Is Time?



NOTE: I don’t actually know anything about physics, and I didn’t do much research before writing this stuff because I enjoy the exercise of just thinking through it. Consequently, everything that follows is probably wrong.

Einstein proved that there’s no giant clock continuously ticking, determining what time it is everywhere in the universe. Rather, time is relative, and affected by speed; if you leave the earth in a spaceship and approach the speed of light and then come back, you can be younger than your children. That’s just a thought experiment, of course — but if relativity is a law, that means it applies in ALL circumstances. Even if, rather than approaching the speed of light, you’re driving down a highway at 65mph.

Say I get in my car and drive across town to see a movie. I take the highway, so I’m driving 65mph all the way there, and I’m driving 65mph all the way back. According to relativity, when I get back, I’m infinitesimally younger than my neighbor Bob, who stayed at home. So far, so good.

Now let’s assume Bob and I meet for coffee, and we both reach forward to shake hands. In that moment, our hands are traveling faster than the rest of our bodies are, which means when we’re done shaking hands, our hands are infinitesimally younger than the rest of our bodies. Our elbows are also younger than our bodies, but not as much younger as our hands, because our elbows didn’t travel as fast.

What I’m getting at is this: Everything is moving at different speeds all the time, which means that time, rather than being a straight line, is a huge, insanely twisted mess. Things on Earth constantly travel at different speeds, becoming younger than or older than other things. And because of the handshake effect, even the idea of things breaks down — in reality, time is warping on a much smaller scale, an atomic scale. Every atom, in other words, has a temporal relationship — a degree of being older-than or younger-than — with every other atom in the universe, based on the speeds they have traveled relative to one another.

So what does it mean if, at a given moment, atom A has a different idea of what time it is than atom B?  This question doesn’t even make sense, because it assumes the existence of universal time, the big ticking clock that you compare everything against. A more meaningful question is, what does it mean if I am perceiving these two atoms that have different ideas of what time it is?

This is where my head explodes. So the thing that is coercing all of these atoms into sitting next to each other when they have different ideas of what time it is is ME?

Obviously that’s not the case (at least, I don’t THINK it is), because atoms with totally different ideas of what time it is can still bump into each other. But it begs the question: If there’s no universal ticking clock, and every atom has a different idea of what time it is, then what determines what “now” is?

I think I’m asking the wrong question — atoms don’t have different ideas of what time it is, they just have different ideas of how old they are. The big ticking clock actually does exist, it just doesn’t work in quite the way I thought it did. But the question makes me wonder: What would the universe look like if you perceived it in such a way that every atom was always the same age?

This one is tough. For starters, Imagine if you were able to figure out the exact age of every atom, and you could ask, “I want to know where these two atoms were when they were so many microseconds old.” It’s possible (albeit unlikely) that they could both have occupied in the exact same position in space when they were the exact same age. (It’s only our human perception that would declare that they were in that position at different points in time.)

So what would the universe look like if you saw everything through a lens that perceived all atoms as being synchronized by age? I’m having a real hard time getting my brain wrapped around this one. In a universe perceived from this perspective, not only could multiple things occupy the same spot, causality would look really strange. For example, assume a plane lifts off from a runway, flies for a few hours at high speed, and then lands on the same runway five seconds younger than the runway. Further, let’s say it crushes a beetle under its wheels. The atoms of the beetle would be five seconds older than the atoms in the plane wheel at the time of the crushing, which would mean that the beetle would not get crushed until five seconds after the plane rolled over it.

That’s a simplified example, of course; I can’t picture what things would really look like in my head. But I feel pretty sure that they would look very strange indeed.

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Fullness and Emptiness

As a person travels closer and closer to the speed of light, his or her perception of time changes. To that person, everything else in the universe seems to be happening very quickly. To an observer, time seems to be passing very slowly for the traveler; it might take them a year to have a sip of coffee. The faster the traveler goes, the more pronounced this effect becomes.

Photons travel at the speed of light, which effectively means that time does not exist for them. More precisely: to a photon, everything else seems to be happening infinitely fast, and to an observer, time for the photon seems to be passing infinitely slowly. If you could ride on a photon from here to Alpha Centauri — or anywhere else in the universe — the ride would literally take no time at all. You’d get on, and bip, you’d be there.

If you could ride on a photon, you’d have no shortage of rides, because the universe is full of them. How full? Step outside at night and pick a star. You can see that star because photons from that star are entering your eyeballs. Now, think about that for a second. You can see that star only because photons are continuously streaming from that star across trillions of miles of empty space to your pupil, which is about a quarter-inch in diameter. Even though those photons are fanning out from the star, getting further and further from each other with each mile they cross, there’s still enough of them hitting the quarter-inch circle that is your eye for you to register an image.

Let’s say you’re a light year from the star you’re looking at (most stars, of course, are much further away), and you can see the star. The star puts out light in every direction — so no matter where you are, if you’re a light-year away from the star, you can see it. To represent this, imagine a sphere with the star at the center, where the surface of the sphere is a light year from the star. The surface area of such a sphere is over 434 septillion square miles* — and the star is continuously painting every single quarter-inch of it with photons.

Photons, photons, everywhere!

Now multiply that by the number of stars in the sky on a dark night in the country. That’s how many photons are pouring into your eye any time you look at the sky. (During the day they’re overwhelmed by the light from our sun, but they’re still pouring in.) That’s a lot of photons.

Space is full of other things, too. Let’s say you’ve got a cell phone. The fact that you can receive a call in a given place indicates that signals from a cell phone tower are passing through your cell phone’s antenna. And the tower isn’t just transmitting for you; it’s probably handling calls from hundreds of other people, too. All of that information is passing through your cell phone’s antenna continuously. (And your pupil, to boot.)

Maybe you’ve got a radio, too, and you can pick up ten stations. That means radio waves from ten distant towers, possibly dozens of miles away, are continuously passing through your radio’s antenna, your cell phone antenna, and your pupil. A dozen or so channels of television, too, each channel packing thirty images per second of data and two channels of sound, all passing through the same space as everything else.

It all makes space sound pretty crowded, doesn’t it? Well, it’s not. To start with, matter is composed of atoms, and atoms are composed primarily of space. There’s a tiny little nucleus in the middle, and some electrons moving around on the outside at a tremendous distance from the nucleus. To put this in perspective, if the nucleus was the size of a pea, the electrons would be something like three hundred feet from it. And that’s how it is even for the densest elements; they’re mostly made up of space. When you touch something, you would pass right through it if it weren’t for the charges in electrons repelling each other.

That’s matter on the atomic scale. On the cosmic scale, space is even emptier. According to Marshall Brain, if the universe were the size of planet Earth, all of the matter in that universe would take up as much space as a speck of dust.

To summarize: The universe is really crowded, completely empty, and weird as hell.

*Thanks to Eric Schoondergang for the correction on this.

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The weird world of virtual slit photography

Virtual Slit Photography

In standard photography, you open a shutter for some period of time and then close it. The entire image gets exposed in one pop. Anything that’s stationary looks normal. Moving objects may be blurred depending on the length of the exposure.

In slit photography (aka strip photography), rather than opening and closing a shutter, you drag a narrow slit across the film from one side to the other over some period of time.  Again, stationary objects look normal. Moving objects, on the other hand, become distorted because they are in different places as the slit passes over different parts of the film.

In the past, slit photography could only be accomplished by building an elaborate camera rig. In recent years, though, the same effect has been accomplished by affixing a lens assembly to a flatbed scanner, and then scanning the image being projected on to the bed. (This process is referred to as “scanner photography.”)

You can also simulate slit photography programmatically. For example, one way to do so is to make a movie of the subject, and then combine one row or column of pixels from each frame into a single image.  (Each row or column of pixels simulates the effect of a slit passing over that part of the image.) An advantage of this approach is that you can produce moving images.

Regardless of how you produce a slit image, the effect is the same: the image captures a period of time moving in the direction the slit travels. For example, if the slit travels from top to bottom, portions of the image on the bottom are exposed later than portions of the image on the top.

Virtual slit photography diagram

Moving objects in a slit image are distorted based on two things: the direction they are moving in, and the direction the slit is moving in.  If the object is moving in the opposite direction as the slit, the object is compressed.  If both are moving in the same direction, the object is stretched.

Virtual slit photography diagram 2

This relatively simple system can produce some startling results.  For example, consider the following pictures, which were all generated from the same movie of an approaching bus.  Note the differences that result from the various different scan directions.

Virtual slit photography diagram 3

When you start to animate the image, things get even weirder.  The following movie was generated from the same footage using an early version of a virtual slit photography application I wrote.

In addition to being distorted, moving objects can in some cases be reversed by the process.  For example, the following two images were created from the same movie, but one was scanned left-to-right and the other was scanned right-to-left.  (In the movie, the cars are traveling right-to-left.)

Virtual slit photography diagram 4

Again, animation makes things even more interesting. In this movie, cars are passing through an intersection in opposite directions.

Now see what it looks like after processing with the slit image application, with a left-to-right scan.

Note that not only does the process reverse the cars that are traveling from right to left, it also reverses their direction of travel.  (I have yet to figure out why this is.)  Here’s the same clip scanned right-to-left.

Rotating objects are a lot of fun to work with. Here’s a group of bottles rotating on a lazy susan:

Here’s the same clip, scanned bottom-to-top:

One of the strangest phenomena I’ve seen is illustrated by the following clip from a roller derby match.  First, the unprocessed footage:

Now, see what happens when this footage is scanned from left to right:

Who are the doppelgangers skating in from the right side?  Again, I have no idea how this is happening.

Here’s a montage of some of the more interesting results:

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