Tag: other

How does gravity bend visible light? — AHM, Pasadena, California

According to the concept of inertia, established by Galileo and Newton several hundred years ago, an object that’s not experiencing any pushes or pulls will continue to move in a straight line at a steady pace—in short, it travels at a constant velocity. This observation can also be stated simply as an object in motion continues in motion and an object at rest remains at rest.

When Newton formulated his theory of gravity, he viewed gravity as exerting forces on objects—it pulled them toward one another so that they no longer followed their straight inertial paths. That’s why a ball arcs through the air, gradually turning toward the ground as the earth’s gravity pulls it downward. This interpretation of gravity was very successful and remains extremely useful to this day.

However, there is a second interpretation of gravity: the one offered by Einstein in the general theory of relativity. According to this interpretation, concentrations of mass/energy warp space-time so that objects that are following inertial paths—called geodesics—no longer travel in simple straight lines. In effect, a ball arcs through the air because it is following a curved geodesic path and not because it is experiencing a force. While this exotic interpretation for gravity isn’t all that useful for slow moving objects like balls—Newtonian gravity is much more practical in that case—it’s important when dealing with fast moving objects like light. Light also follows geodesics, but because it travels so quickly its geodesics tend to be rather straight. Even light passing just above the surface of the sun bends only just enough to measure. Still, one of the most important confirmations of general relativity came during a total solar eclipse when light from a star was found to bend slightly as it passed by the sun’s obscured surface.

Finally, I should say that you can also interpret the bending of light in terms of Newtonian gravity—that because light contains energy, it acquires a weight when gravity is present and this weight causes its path to bend. However, this Newtonian observation omits so much of the intrigue and beauty that comes with the bending of space-time that I prefer the more modern interpretation.

We know that high speeds cause time to distort. We also have found wormholes in space that connect two distant points. Therefore, by entering a wormhole we can travel through time. How can we create a wormhole and control its destination point? — JB, Union, New Hampshire

Near some large concentration of mass/energy, the equations of general relativity do admit solutions that have two open ends and that could be interpreted as being wormholes. However, there is no widely accepted interpretation of these solutions and no evidence that such solutions are actually realized in our universe. While there are some physicists and astrophysicists who remain hopeful that wormholes will ultimately be found, the only ones I’ve ever heard about are in science fiction stories.

Even if such exotic structures do exist, there is also no evidence that people could traverse the severely distorted space-time between the two open ends without being destroyed and without having an infinite amount of time pass in the rest of the universe while they were en route. If all of these issues aren’t enough to discourage you, let me add that the possibility of engineering wormholes to connect specific regions of space-time is extraordinarily remote. Working with a wormhole would be at least as difficult as working with a black hole and I, for one, hope never to encounter such a destructive and dangerous object.

How does cathodic protection work? — MM, Dominican Republic

The rusting of damp steel is an electrochemical reaction in which iron atoms in the steel are converted into positively charged iron ions (Fe²⁺) in the water. However, each iron atom that becomes an ion releases two negatively charged electrons and rusting can only continue if there is a suitable destination for these electrons. Normally, the electrons pass through the steel metal and are used together with oxygen molecules to form negatively charged hydroxide ions (OH^–) in the water. Overall, the rate at which the steel rusts is limited by how quickly hydroxide ions can be formed to use up the electrons.

Cathodic protection is a scheme in which a piece of reactive metal, typically magnesium, is connected to the steel to form an electrochemical cell. Magnesium ions (Mg²⁺) form more easily than iron ions and enough electrons are given up by the magnesium atoms as they become positive ions to completely dominate the hydroxide ion formation process. With nowhere for their electrons to go, the iron atoms can’t become iron ions and rusting can’t proceed. As long as the magnesium metal, often called the “sacrificial anode”, remains intact and connected to the steel, the steel won’t rust significantly.

As an alternative to this approach, some companies use a power supply to pump negative charges onto the steel to prevent it from rusting. Pipeline companies often do this and that action has led to some interesting complications: metal objects that are brought into contact with such a pipeline can be protected against rusting as well. For example, when people chained their bicycles to protected pipelines, the bicycles became part of the protected materials. This may have been good for the bicycles, but it confused the pipeline companies who found that they needed to pump extra charge onto the pipelines to handle the increased load. It was particularly bad when the bicycles accidentally grounded the pipelines and allowed the negative charges to escape.

What is the chemistry involved with natural dyes adhering to surfaces? — AG, Aloha, OR

Unless a chemical reaction binds them permanently in place, dye molecules that are soluble enough to wash into fabrics are equally likely to wash back out of the fabrics later on. To remain in place, the dyes must undergo chemical reactions that attach them to the fibers of the fabric. Some dyes react spontaneously to the fabric molecules but many others need help. The traditional scheme for binding dyes to fabrics involves mordents—relatively colorless chemicals that bind to both fabric and dye, and that hold the two together. Tannic acid and various metal salts have been used as mordents for centuries. They form insoluble compounds that wedge themselves into hollow spaces in the fibers and then bind chemically to the dye molecules. These mordents hold the dye molecules in place in much the same way that technical climbing gear holds rock climbers to the face of a cliff.

How does the telephone work? — JB, Sydney, Nova Scotia

A telephone uses an electric current to convey sound information from your home to that of a friend. When the two of you are talking on the telephone, the telephone company is sending a steady electric current through your telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into your telephone’s microphone, the current that your telephone draws from the telephone company fluctuates up and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice at the microphone.

Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend’s telephone. Thus as you talk, the current through your friend’s telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuations reproduce the sound of your voice. Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior.

What is the relationship between gravitational force and electromagnetic force? — TPC, Foster, OK

As yet, there is no direct relationship between those two forces. Our best current understanding of gravitational forces is as disturbances in the structure of space itself while our best current understanding of electromagnetic forces involves the exchanges of particles known as virtual photons. However, physicists are trying to develop a quantum theory of gravity that would identify gravitational forces with the exchange of particles known as gravitons. How closely such a quantum theory of gravity would resemble the current quantum theory of electromagnetic forces (a theory called quantum electrodynamics) is uncertain. It’s also uncertain whether those two quantum theories will be able to merge together into a single more complete theory. Only time will tell.

If one accepts the existence of black holes, would it be plausible to assume that a “white hole” exists on the opposite end due to captured light by the black hole?

I think not. Depending on your frame of reference, the passage of material into a simple black hole—one that isn’t spinning very fast and that doesn’t have a great deal of electric charge in it—has one of two results. If you are traveling with the material, things proceed more or less normally as you pass the point of no return—the so-called “event horizon” from which even light can’t escape. You accompany the material all the way to the center of the black hole—its “singularity”—and are crushed to infinite density. If instead of traveling with the material, you remain outside the black hole looking in toward it, you see the material approach the event horizon but without ever quite entering its surface. In fact, all of the material that went into forming the black hole in the first place, plus all the material that has fallen into the black hole since its formation, appear to reside forever on the event horizon surface. In effect, the material never quite gets to the black hole. Since the material never quite gets to the black hole, there is no need for it to reemerge elsewhere from a “white hole.”

However, there are more complicated black holes—ones involving angular momentum and electric charge—that have more complicated structures. In falling into one of these black holes, it is apparently possible to miss the singularity. There is some discussion of such material reemerging from the “other end” of one of this black holes but I believe that there are serious problems with such two-ended interpretations of the equations governing such black holes.

What is ink made of? — JD, Langley, British Columbia

Ink is made of light absorbing pigment particles or dye molecules that are suspended in a fluid that contains a dissolved binder chemical. When the ink is deposited on a sheet of paper, the binder’s solvent diffuses into the paper or evaporates into the air, leaving the pigment particles or dye molecules bound to the paper by the binder.

How fast does the earth wobble and why does it wobble? — MF, Tokyo, Japan

The earth’s rotational axis wobbles around in a circle once every 25,800 years because of torques (twists) exerted on it by the moon’s gravity. The moon’s gravity is able to twist the earth slightly because the earth isn’t quite spherical. The earth’s rotation causes it to bulge outward a little around its equator and it is this bulging that allows the moon to exert a torque on the earth.

My husband and I watch Star Trek often. He says that travel at warp speeds (faster than the speed of light) is impossible and that Einstein’s theories prove it. Is this true? — JL, Las Cruces, NM

I’m afraid that travel at or above light speed is simply impossible and that “warp speed” travel is just a Hollywood fantasy. Einstein’s special relativity forbids objects with mass from reaching or exceeding the speed of light and even if there were some way to travel vast distances in less time than it would take light to cover those distances, but without actually traveling at light speed, such travel would violate some important principles of causality—you would be able to meet your own grandparents as children and that sort of thing.

One of the reasons that Hollywood ignores real physics so often is that real physics is almost wilder than fiction. Suppose that you decided to travel to a star 5 light-years away from the earth and that you have a starship that can almost reach the speed of light (another nearly impossible thing, but let’s ignore that problem). If you travel to the star at almost the speed of light, make one loop around it, and head right back to earth, I will have aged 10 years while waiting for you to return. However, you will only have aged days or weeks, depending on just how close you came to the speed of light. During the trip, we will have disagreed on many physical quantities, particularly the times at which various events occurred and the distances between objects. The mixing of time and space that occur when two people move rapidly relative to one another would be so disorienting to movie or television viewers that Hollywood ignores or simplifies these effects.