One of the most mind-blowing revelations from science in the past two hundred years came out of a concept so simple that a sixth-grader could understand it.
You've all observed that the motion of objects is relative. Picture a train with glass sides (only so you can see into it from outside). The train is moving forward at 5 kilometers per hour, with an observer standing next to it watching it roll past. At the same time, a guy is walking toward the back of the train, also at 5 kilometers per hour.
From the point-of-view of anyone on the train, the walking man is moving at 5 kilometers per hour. But from the point-of-view of the stationary observer outside the train, it appears like the man on the train isn't moving -- he's just walking in place while the train slides out from under him. This is what is meant by relative motion; the motion of an object is relative to the frame of reference you're in. We don't observe the motion of the Earth because we're moving with it. It, and us, appear to be motionless. In the frame of reference of an astronaut poised above the plane of the Solar System, though, it would seem as if the Earth was a spinning ball soaring in an elliptical path around the Sun, carrying us along with it at breakneck speed.
With me so far? Because here's the simple-to-state, crazy-hard-to-understand part:
Light doesn't do that.
No matter what reference frame you're in -- whether you're moving in the same direction as a beam of light, in the opposite direction, at whatever rate of speed you choose -- light always travels at the same speed, just shy of 300,000,000 meters per second. (Nota bene: I'm referring to the speed of light in a vacuum. Light does slow down when it passes through a transparent substance, and this has its own interesting consequences, but doesn't enter into our discussion here.)
It took the genius of Albert Einstein to figure out what this implied. His conclusion was that if the speed of light isn't relative to your reference frame, something else must be. And after cranking through some seriously challenging mathematics, he figured out that it wasn't one "something else," it was three: time, mass, and length. If you travel near the speed of light, in the frame of reference of a motionless observer your clock would appear to run more slowly, your mass would appear greater, and your length appear shorter. (Where it starts getting even more bizarre is that if you, the one moving near light speed, were to look at the observer, you'd think it was him whose watch was running slow, who had a greater mass, and who was flattened. Each of you would observe what seem to be opposite, contradictory measurements... and you'd both be right.)
All of this stuff I've been described is called the Special Theory of Relativity. But Einstein evidently decided, "Okay, that is just not weird enough," because he did another little thought experiment -- this one having to do with gravity. Picture two people, both in sealed metal boxes. One of them is sitting on the surface of the Earth (he, of course, doesn't know that). The other is out in interstellar space, but is being towed along by a spacecraft at an acceleration of 9.8 meters per second (the acceleration due to gravity we experience here on the Earth's surface). The two trapped people have a communication device allowing them to talk to each other. They know that one is sitting on a planet's surface and the other is being pulled along by a spaceship, but neither knows which is which. Is there anything they could do, any experiment they could perform, anything that would allow them to figure out who was on a planet and who was being accelerated mechanically?
Einstein concluded that the answer was no. Being in a gravitational field is, for all intents and purposes, exactly the same as experiencing accelerated motion. So his conclusion was that the relativistic effects I mentioned above -- time dilation, mass increase, and shortening of an object's length -- not only happen when you move fast, but when you're in a strong gravitational field. If you've seen the movie Interstellar, you know all about this; the characters stuck on the planet near the powerful gravitational field of a black hole were slowed down from the standpoint of the rest of us. They were there only a year by their own clocks, but to everyone back home on Earth, decades had passed.
Maybe you're thinking, "But isn't the Earth's gravitational field pretty strong? Shouldn't we be experiencing this?" The answer is that we do, but the Earth's gravity simply isn't strong enough that we notice. If you travel fast -- say on a supersonic airline -- your clock does run slow as compared to the ones down here on Earth. It's just that the difference is so minuscule that most clocks can't measure the difference. Even if supersonic seems fast to us, it's nearly standing still compared to light; if you're traveling at Mach 1, the speed of sound, you're still moving at only at about one ten-thousandth of a percent of the speed of light. The same is true for the gravitational effects; time passes more slowly for someone at the bottom of a mountain than it does for someone on top. So on any ordinary scale, there are relativistic effects, they're just tiny.
But that's what brings the whole bizarre topic up today -- because our ability to measure those tiny, but very real, effects just took a quantum leap (*rimshot*) with the development of a technique for measuring the "clocks" experienced by a cluster of atoms only a millimeter long. A stack of about 100,000 strontium atoms that had been cooled down to near absolute zero were tested to see what frequency of light would make their electrons jump to the next energy level -- something that has been measured to a ridiculous level of accuracy -- and it was found that the ones at the bottom of the stack (i.e. nearer to the Earth's surface) required a different frequency of light to jump than the ones at the top. The difference was incredibly small -- about a hundredth of a quadrillionth of a percent -- but the kicker is that the discrepancy is exactly what Einstein's General Theory of Relativity predicts.
So Einstein wins again. As always. And if you're wondering, it means your feet are aging slightly more slowly than your head, assuming you spend as much time right-side-up as you do upside-down. Oh, and your feet are heavier and flatter than your head is, but not enough to worry about.
All of this because of pondering whether light behaved like someone walking on a train, and if someone being towed by an accelerating spaceship could tell he wasn't just in an ordinary gravitational field. It brings home the wonderful quote by physicist Albert Szent-Györgyi (himself a Nobel Prize winner) -- "Discovery consists of seeing what everyone has seen, and thinking what no one has thought."
My dad once quipped about me that my two favorite kinds of food were "plenty" and "often." He wasn't far wrong. I not only have eclectic tastes, I love trying new things -- and surprising, considering my penchant for culinary adventure, have only rarely run across anything I truly did not like.
So the new book Gastro Obscura: A Food Adventurer's Guide by Cecily Wong and Dylan Thuras is right down my alley. Wong and Thuras traveled to all seven continents to find the most interesting and unique foods each had to offer -- their discoveries included a Chilean beer that includes fog as an ingredient, a fish paste from Italy that is still being made the same way it was by the Romans two millennia ago, a Sardinian pasta so loved by the locals it's called "the threads of God," and a tea that is so rare it is only served in one tea house on the slopes of Mount Hua in China.
If you're a foodie -- or if, like me, you're not sophisticated enough for that appellation but just like to eat -- you should check out Gastro Obscura. You'll gain a new appreciation for the diversity of cuisines the world has to offer, and might end up thinking differently about what you serve on your own table.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]