Author: Lou Bloomfield

Why does the ground move slowly when viewed from a jet?

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When you traveling in a jet plane, why do objects on the ground look as though they are still or moving slowly? — K, India

When you watch something move, what you really notice is the change in the angle at which see you it. Nearby objects don’t have to be traveling fast to make you turn your head quickly to watch them go by so you perceive them as moving rapidly. An object that is heading directly toward you or away from you doesn’t appear to be moving nearly as quickly because its change in angle is much smaller.

When you watch a distant object move, you don’t see it change angles quickly so you perceive it as moving relatively slowly. Take the moon for example: it is moving thousands of miles an hour yet you can’t see it move at all. It’s just so far away that you see no angular change. And when you look down from a high-flying jet, the distant ground is changing angles slowly and therefore looks like it’s not moving fast.

Does color affect baking speed?

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If I were to heat up a brownie and a white piece of cake, would the brownie heat up faster by radiation transfer because of its darker color? — B

In principle, the brownie would heat up faster by radiation in a hot environment and cool off faster by radiation in a cold environment. A black object is better at both absorbing thermal radiation and emitting thermal radiation, so the brownie would soak up more thermal radiation in the hot environment and give off more thermal radiation in the cold environment.

In practice, however, most of the radiation involved in baking these desserts and letting them cool on a kitchen counter is in the infrared and it’s hard to tell just what color a brownie or cake is in the infrared. It’s likely that both are pretty dark when viewed in infrared light. Basically, even things that look white to your eye are often gray or black in the infrared. Thus I suspect that both the brownie and cake absorb most of the thermal radiation they receive while being baked and emit thermal radiation efficienty while they’re cooling on the counter.

How can light travel through vacuum?

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How can light “travel” through a vacuum when there were no “particles” in the vacuum on which it could “transmit” its charge? — DC

Light has no charge at all. It consists only of electric and magnetic field, each endlessly recreating the other as the pair zip off through empty space at the speed of light.

The fact that light waves can travel in vacuum, and don’t need any material to carry them, was disturbing to the physicists who first studied light in detail. They expected to find a fluid-like aether, a substance that was the carrier of electromagnetic waves. Instead, they found that those waves travel through truly empty space. One thing led to another, and soon Einstein proposed that the speed of light was profoundly special and that space and time were interrelated by way of that speed of light.

Can microwaves measure food’s nutrition?

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For my industrial design project, I am redesigning the microwave oven and adding some extra functions. Is it possible for microwaves to somehow measure food properties such as calories, sugar, salt, vitamins, and fat content? How can I translate those readings onto an LCD display so that the user can see them, and can they also be transferred to a computer via Bluetooth? — IB

What you propose to do is far more difficult than you imagine. Determining the chemical contents of food is hard, even with a well-equipped laboratory and permission to destroy the food in order to study it. The idea of analyzing a casserole in detail simply by beaming microwaves at it is science fiction. Think how much easier airport security would be if they could chemically analyze everything that came in the front door just by beaming microwaves at it.

That said, however, let me make two comments. First, the question quickly turns to computer interface issues, as though the chemical analysis part is trivial in comparison to computer presentation part. Physical science and computer science are truly different fields and not everything in the scientific domain can be reduced to a software package. Physics and chemistry haven’t disappeared with the advent of computers and there will never be a firmware upgrade for your microwave oven that will turn it into a nutritional analysis laboratory. As a society, we’ve gone a bit too far in replacing science education with technology education, particularly computer software.

Second, while remote chemical analysis isn’t easy, it can be done in certain cases with the clever use of physics and chemistry. One of my friends here at Virginia, Gaby Laufer, has developed an instrument that studies the infrared light transmitted by the air and can determine whether that air contains any of a broad variety of toxic or dangerous gases in a matter of seconds. Air’s relative transparency makes it easier to analyze than an opaque casserole, but even when you can see through something it’s not trivial to see what it contains. Gaby’s instrument does a phenomenal job of fingerprinting the gas’s absorption features and identifying trouble.

Note added: a reader informed me that there are now microwave ovens that can read bar codes and adjust their cooking to match the associated food. A scale in the base of the oven can determine the food’s weight and cook it properly. Another reader suggested that a microwave oven might be able to measure the food’s microwave absorption and weight in order to adjust cooking power and time. While that’s also a good possibility, ovens that sense food temperature or the humidity inside the oven can achieve roughly the same result by turning themselves off at the appropriate time.

What does it mean to coast?

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If something is coasting or moving at a steady pace, is it experiencing a net force of zero? — NP

That’s exactly right! Coasting and zero net force go hand-in-hand: when an object is experiencing zero net force, it doesn’t accelerate and thus it coasts. A coasting object is an inertial object, meaning that it moves at a steady pace along a straightline path. And if the coasting object is at rest, it stays at rest.

To clarify the term “net force,” note that when an object is experiencing several separate forces, it doesn’t accelerate in response to each one individually. Instead, it accelerates in response to the sum of all the forces acting on it: the net force. Remember that forces have directions associated with them (forces are vector quantities), so when you sum them you must consider their directions carefully. The proper force to consider in Newton’s second law is actually the net force on the object. If you know both the net force on the object and the object’s mass, you can predict the object’s acceleration. And if the net force is zero, then the object doesn’t accelerate at all — it coasts.

Can you put a microwave oven in the landfill?

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Can or should a microwave be disposed with the normal trash, what if any are the environmental impacts of the magnetron or other parts sitting in a landfill? — DNR

I figure that some day, we’ll turn to our landfills as resources for precious elements like copper and gold. That assumes, of course, that we survive global warming. In the meantime, we’ll just keep throwing stuff out.

Despite the scary title “microwave radiation,” a microwave oven is basically just another household electronic device. It is an extremely close relative of a convention cathode-ray-tube television set. If you’re OK with putting CRT televisions and computer monitors in the landfill, you should have no problems with putting microwave ovens there, too. Even when the microwave oven is on, all it has inside it is microwave radiation and that’s just not a big deal. The instant you turn it off, it doesn’t even have those microwaves in it. It’s just boring inert electronic parts and they’ll sit in the landfill for generations, rusting and decaying like every other abandoned electronic gadget. I’d rather see it go to a recycling center and have its precious materials returned to the resource bin, but as landfill junk goes, it’s not all that bad. Given that toxic chemicals are the primary concern with landfills, microwave ovens are probably rather innocuous. They have no radioactive contents and although the high-voltage capacitor might have oil in it, that oil can no longer be the toxic PCBs that were common a few decades ago. Even when that oil leaks into the environment, it’s probably not going to do much.

So there you have it, microwave ovens go to their graves no more loudly or dangerously than old televisions or computers or cell phones.

In fact, I might start calling cell phones “microwave phones” because that’s exactly what they are. They communicate with the base unit by way of microwave radiation. Given the number of people who have cell phones semi-permanently installed in their ears, concerns about microwave radiation should probably be redirect from microwave ovens to “microwave phones.” Think about it next time your six-year-old talks for an hour with her best friend on that “microwave phone.”

Why do deep wells require pumps at the bottom?

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Why do deep water wells need a pump at the bottom rather than one at the top? — LG, Vancouver

While it’s easy to push on water, it’s hard to pull on water. When you drink soda through a straw, you may feel like you’re pulling on the water, but you’re not. What you are actually doing is removing some air from the space inside the straw and above the water, so that the air pressure in that space drops below atmospheric pressure. The water column near the bottom of the straw then experiences a pressure imbalance: the usual atmospheric pressure below it and less-than-atmospheric pressure above it. That imbalance provides a modest upward force on the water column and pushes it up into your mouth.

So far, so good. But if you make that straw longer, you’ll need to suck harder. That’s because as the column of water gets taller, it gets heavier. It needs a more severe pressure imbalance to push it upward and support it. By the time the straw and water column get to be about 40 feet tall, you’ll need to suck every bit of air out from inside the straw because the pressure imbalance needed to support a 40-foot column of water is approximately one atmosphere of pressure. If the straw is taller than 40 feet, you’re simply out of luck. Even if you remove all the air from within the straw, the atmospheric pressure of the water below the straw won’t be able to push the water up the straw higher than about 40 feet.

To get the water to rise higher in the straw, you’ll need to install a pump at the bottom. The pump increases the water pressure there to more than 1 atmosphere, so that there is a bigger pressure imbalance available and therefore the possibility of supporting a taller column of water.

OK, so returning to your question: once a well is more than about 40 feet deep, getting the water to the surface requires a pump at the bottom. That pump can boost the water pressure well above atmospheric and thereby push the water to the surface despite the great height and weight of the water column. Suction surface pumps are really only practical for water that’s a few feet below the surface; after that, deep pressure pumps are a much better idea.

What is the second fastest thing in the universe?

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My eight year old daughter asked me, “If light is the fastest thing in the universe what is the second fastest thing in the universe?” — JPW, Lancaster, PA

Your daughter’s question is a cute one. I like it because it highlights the distinction between the speed of light and all other speeds. The speed of light is unimaginably special in our universe. Strange though it may sound, even if light didn’t exist there would still be the speed of light and it would still have the same value. The speed of light is part of the geometry of space-time and the fact that light travels at “the speed of light” is almost a cosmic afterthought. Gravity and the so-called “strong force” also travel at that speed.

OK, so there is actually a multi-way tie for first place in the speed rankings. Your daughter’s question is what comes next? The actual answer is that it’s a many-way tie between everything else. With enough energy, you can get anything moving at just under the speed of light, at least in principle. For example, subatomic particles such as electrons, protons, and even atomic nuclei are routinely accelerated to just under the speed of light in sophisticated machines around the world. The universe itself has natural accelerators that whip subatomic particles up until they are traveling so close to the speed of light that it’s hard to tell that they aren’t quite at the speed of light. Nonetheless, I assure you that they’re not. The speed of light is so special that nothing that has any mass at all can possibly travel at the speed of light. Only the ephemeral non-massive particles such as light particles (photons), gravity particles (gravitons), and strong force particles (gluons) can actually travel at the speed of light. In fact, once photons, gravitons, and gluons begin to interact with matter, they don’t travel at the speed of light either. It’s sort of a guilt-by-association: as soon as these massless particles leave the essential emptiness of the vacuum and begin to interact with matter, even they can’t travel at the speed of light anymore.

That said, I can still offer the likely second place finisher on the speed list. I’m going to skip over light, gravity, and the strong force traveling in extremely dilute matter because that’s sort of cheating — if you take something that naturally travels at the speed of light and slow it down the very, very slightest bit, of course it will come ridiculously close to the speed of light. In real second place are almost certainly cosmic ray particles. These cosmic rays are actually subatomic particles that are accelerated to fantastic energies by natural processes in the cosmos. How such accelerators work is still largely a mystery but some of the cosmic ray particles that reach our atmosphere have truly astonishing energies — once in a while a single cosmic ray particle that is smaller than an atom will carry enough energy with it that it is capable of moving small ordinary objects around. Even if it carries the energy of a fly, that’s a stupendous amount of energy for an atomic fragment. Those cosmic ray particles are traveling so close to the speed of light that it would be a photo-finish with light itself.

Good ideas are harder to come by than fancy equipment

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I have a large commercial superconducting magnet and am looking for a high-value-added product or manufacturing process to pursue with it. Is there anything you have learned in your research that would be worth producing? — PT

As a general observation, the bottleneck in scientific research and technological innovation is almost always the ideas, not the equipment. Occasionally, a revolutionary piece of equipment comes on the scene and makes a whole raft of developments possible overnight. But a commercial superconducting magnet isn’t revolutionary; you can buy one off the shelf. As a result, all the innovations that were waiting for magnets like that to become available were mopped up long ago and any new innovations will take new ideas.

Coming up with good ideas is hard work and if I had them, I’d have gotten hold of such a magnet myself. Although science is often taught as formulas and factoids, it’s really about thinking and observing, and good ideas are nearly always more important than good equipment. Good ideas don’t linger unstudied for long when commercial equipment is all it takes to pursue them.

Eyeglasses bend an object’s light rays so that your eye can focus them properly.

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How do glasses work and what is the physics behind them? — SDM, Missouri

Like a camera, your eye collects light from the scene you’re viewing and tries to form a real image of that scene on your retina. The eye’s front surface (its cornea) and its internal lens act together to bend all the light rays from some distant feature toward one another so that they illuminate one spot on your retina. Since each feature in the scene you’re viewing forms its own spot, your eye’s cornea and lens are forming a real image of the scene in front of you. If that image forms as intended, you see a sharp, clear rendition of the objects in front of you. But if your eye isn’t quite up to the task, the image may form either before or after your retina so that you see a blurred version of the scene.

The optical elements in your eye that are responsible for this image formation are the cornea and the lens. The cornea does most of the work of converging the light so that it focuses, while the lens provides the fine adjustment that allows that focus to occur on your retina.

If you’re farsighted, the two optical elements aren’t strong enough to form an image of nearby objects on your retina so you have trouble getting a clear view while reading. Your eye needs help, so you wear converging eyeglasses. Those eyeglasses boost the converging power of your eye itself and allow your eye to form sharp images of nearby objects on your retina.

If you’re nearsighted, the two optical elements are too strong and need to be weakened in order to form sharp images of distant objects on your retina. That’s why you wear diverging eyeglasses.

People are surprised when I tell them that they’re nearsighted or farsighted. They wonder how I know. My trick is simple: I look through their eyeglasses at distant objects. If those objects appear enlarged, the eyeglasses are converging (like magnifying glasses) and the wearer must be farsighted. If those objects appear shrunken, the eyeglasses are diverging (like the security peepholes in doors) and the wearer is nearsighted. Try it, you’ll find that it’s easy to figure out how other people see by looking through their glasses as they wear them.