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Time does not have a frequency, it is a completely different concept. If you could warp gravity, then, inside that warp, time would run at a different rate. Time is not synchronized, two events that are simultaneous in one reference frame are not necessarily simultaneous in another reference frame. There are things like atom oscillations that can be used as meters for the local passage of time.

Dr. Eric Christian

You have brought up a very important issue in physics with very profound ramifications in engineering. This is why engineers need to apply sophisticated securing techniques for screws in systems that are exposed to extensive strong vibrations. In particular, every satellite and space instrument needs to be tested under severe vibrations before it is considered safe to be put on a rocket.

Indeed one could blame the behavior that you describe mostly on the 2nd Law of Thermodynamics, in the sense that the technical systems will ultimately settle for the most likely arrangement, and that is disorder. However, in the examples that you have mentioned, the systems also settle for the arrangement that contains the lowest amount of energy. Thus we have two reasons that contribute to this often unwelcome result.

Let us discuss the issue in two steps:

1) Connection of loose screws with the 2nd Law of Thermodynamics:

The only requirement for the 2nd Law of Thermodynamics to be applicable is that several objects interact and can move relative to each other by chance. In such a case, the objects will always settle into an arrangement that is more likely to occur without ordering hands involved. In other words, usually their arrangement will be less orderly after shaking the system than before. This is not restricted to the molecular or atomic level.

For instance, arrange a few marbles in a rectangular box on a perfectly horizontal table so that they are neatly lined up, say, in two rows along one of the sides. These two rows of marbles do not fill the area of the box. Now shake the box! Most probably these marbles will end up in a random arrangement. Shake again, and another random arrangement will emerge. It is extremely unlikely that the marbles will ever end up in the neatly stacked two rows again that you started with. There are so many possible arrangements.

Translating this picture to our screws, there are many more different arrangements in which a screw/nut combination can end up other than the tight connection, after bolting them together. Therefore, it is so much more likely to end up loose rather than tight.

2) Loose screws and minimization of energy:

In nature everything strives to reach a state of lowest energy. Stones fall down. Springs release their tension when not compressed by force. This also applies to screws. When tightened, the threads of a screw act like little springs, which are compressed. If this pressure on the threads could make the screw turn, it would do so in the direction so that the screw would unwind. To see, what I mean, push down on a wedge that lies on a smooth surface. It will slide away so that your finger slides down to the tip of the wedge. Therefore, under vibration when the compression may repeatedly tighten and slightly loosen occasionally, the screw will slide each time by a tiny amount, but inevitably in the direction where it loosens its grip further. This is the direction in which the screw reduces its energy.

In conclusion, both the 2nd Law of Thermodynamics and the tendency to end up with as little energy as possible inevitably lead to loose screws. To prevent this we either have to turn the screws very tight or we need to apply more or less sophisticated screw locking methods.

Dr. Eberhard Moebius
(December 2004)

You can check out the AIP web site for more information.

Dr. Louis Barbier
(October 2001)

Interesting question. Let's see if we can't kill 2 or 3 birds with one stone. First, there aren't any stupid questions in physics. This is hard stuff and asking questions, putting ideas together, and searching out the answer is how we learn. We often learn more from our mistakes than our successes because we work harder on those, so making mistakes or putting 2 things together incorrectly isn't a problem -- it's an opportunity. So put that worry aside.

Before I answer your question, let me tell you about a couple of physics principles. Isaac Newton said that for every action there is an equal and opposite reaction. That means you (or the big magnet) can't push on something smaller without feeling an equal push on yourself in the opposite direction. Imagine you are on skates and you push someone away from you -- it makes you move backwards until you brace your skate against the ice or the ground. This is going to happen with the magnets. The big one pushes the little one, but is pushed itself in the opposite direction. The little one pulls the big one, but it is pulled itself toward the big one.

Mass just means that force is converted into acceleration and motion disproportionately. The little magnet accelerates faster and moves farther than the big magnet, but they both move in opposite directions as they exert force on one another.

So pushing and pulling, pulling and pushing, the two magnets never actually get anywhere and in the end they settle into whatever configuration eliminates the forces acting on them. This brings us to the second physics principle: You can't get something for nothing. It's the second law of thermodynamics, but it shows up all the time. If someone tells you they have a perpetual motion machine, doubt them. Everything you do costs energy in some form. Two magnets that push themselves apart lose potential energy within their combined magnetic field as they gain the kinetic energy associated with motion.

They can keep moving in whatever direction they start out in, but they can't reverse themselves without tapping into some other energy supply. But where will they find it?

So for every action there is a reaction, and you can't get something for nothing. Pretty good rules. They seem to work in life, too.

Dr. Charles Smith
(February 2003)

First of all, one thing that is important to realize is that in the case of particle annihilation, the end result is always TWO photons. Imagine you were sitting in the center of momentum frame as these two particles annihilated. If only one photon could be produced, then it would be at rest in your rest frame. This is impossible since Einstein said that a photon must be moving at c = 300,000,000 m/s (186,000 miles/sec) in EVERY rest frame. If two photons emerge from the annihilation, this fact is not violated (the two photons can leave your rest frame in opposite directions at the speed of light).

Now, where does this momentum come from? As you may know, for regular particles that have mass, momentum, p is equal to mass times velocity. However, since a photon has no mass, momentum is redefined to be its energy divided by the velocity of light. This is a product of the Special Theory of Relativity. In fact, as a particle with mass moves faster and faster (and approaches the speed of light) its momentum gets closer and closer to its energy divided by the velocity of light. So for photons, energy and momentum are very closely related.

So as these two particles annihilate, the photons get their momentum (and energy) from these the particles. The faster these two particles are moving when they hit each other, the greater the energy of the two resultant photons.

Lauren Scott
(October 2003)

The term "spooky particles" refers to quantum entangled photons.

Photons are the massless "particles" that make up light (and gamma-rays and x-rays) - they always travel at the speed of light. In more technical terms, photons carry the electromagnetic force.

Quantum mechanics is the theory of how particles behave at the atomic and subatomic level. Quantum mechanics says that particles have properties that are quantized (have fixed values) - such as spin and charge. The values of these properties are referred to collectively as the "state" of the particle. It also says that any given particle at any time can be described as a combination of states with different properties. (In quantum mechanics this combination is called a superposition.) For example, imagine a weather vane that can only point in the 4 cardinal directions (north, south, east, west) - but can't point anywhere in between. (Of course it's not of much use!) Anyway, in quantum mechanical terms you could describe the "state" of the weather vane (its position at any time) as a combination of the four states: north, south, east, and west, each with some associated probability. When you aren't looking at the weather vane, it's in all four states simultaneously. It's only when you look at it that you force it into one of the four positions. That's what this means in quantum terms.

Now entangled particles are ones (say pairs of particles for simplicity) such that, measuring one property of one of them affects the second particle. For example, with entangled photons, measuring the spin of one of them will affect the measured spin of the other. (Aside: the photon is a "spin-1" particle. It has a spin of either +1 or -1. That is, its spin is either parallel (+1) or anti-parallel (-1) to its direction of motion.) Another example is when the pi-0 - pronounced "pi-zero" - (a neutral charge, elementary particle) decays into an electron and a positron (its antiparticle). The electron and positron are entangled.

Quantum entangled photons - or spooky particles - are being used in the design of quantum computers. The information contained in the entangled particles is called a "qubit", analogous to the bits used in ordinary digital computers. They are also being used in making better, more precise atomic clocks.

Two web sites you can check out are:

Spooky Atomic Clocks
Spooky bits propel quantum computer

A Google search for "spooky particles" will turn up many sites. Happy reading!

Dr. Louis Barbier
(February 2004)

The device you are referring to is called a "radiometer". If the glass bulb is completely evacuated, photons are absorbed on the black side and reflected on the white side. The reflection of photons transfers twice as much momentum as absorbtion, so the radiometer will rotate with the black side leading (the white side has gotten more of a push). If there is air in the bulb, the radiometer spins in the opposite direction. This is because the momentum transferred by the gas molecules is much more than the photons, and the gas will rebound with more momentum on the hotter black side than the cooler white side, as some heat is transferred.

Drs. Louis Barbier and Eric Christian

As you may have gathered from our website, I'm not really an expert in interpreting the medical hazards of radiation exposure, but hopefully, I can help to answer a few basic questions about radiation detection.

Power transmitters and receivers will definitely produce some electromagnetic background, but this background has not been known to cause any harm, particularly with respect to cancer. Scientists' general understanding of radiation exposure which leads to cancer is that the radiation damages human cells or even their DNA, causing significant changes in cell behavior. The implicit understanding is that the radiation must impart enough energy to the cell to cause some damage, and this is true of x-ray and gamma-ray electromagnetic radiation but not true of photons with less energy (such as radio waves).

Now I also see that you are concerned about local background radiation. This can be a significant problem depending on the location and arises from the various sediment/rock formations in the area. The most common forms of background radiation are from radon (which is generally inhaled and "may" lead to lung cancer) and also radioactive nuclides such as potassium (which is found in very low abundances in the ground and even in bananas and your body!) and carbon, as well as more damaging, but very rare, isotopes such as uranium, thorium, and radium. These isotopes emit radiation in the form of alpha particles, gamma-rays, and electrons and positrons. So there is a possibility that the positrons may annihilate to produce gamma-rays or x-rays.

We can at least alleviate one of your concerns, which is thatthe gamma-rays (or x-rays) that are directly emitted or produced subsequently from background radiation sources, are easily detected with conventional technology. Background radiation studies are conducted around the world and even in our own labs here regularly. It is unlikely that radiation sources in high enough concentrations to produce serious health hazards to individuals in a working place would go undetected.

Now, that being said, it is entirely possible that ambient electromagnetic radiation has adverse effects on humans in ways which scientists do not yet understand, although the evidence is not in favor of this. Scientists are learning more about the effects of background radiation all the time.

As an interesting example, we know that pilots flying at high altitudes have a higher risk of cancer due to exposures from another type of background radiation that comes from outer space, cosmic rays. Cosmic rays are much more intense at altitudes where planes fly than they are on the ground. So scientists are learning everyday about our environment and how humans are connected to it!

You might find the Wikipedia article on background radiation helpful.

Dr. Georgia de Nolfo
(March 2007)

There is no way to make a light saber. To a physicist, a light saber looks like a square wave of light, but it would require a nearly infinite combination of wavelengths and a very high energy density, and we just don't have any way to generate that.

Then could I use plasma to make the blade?

You would have to have some way of containing the plasma, or it would just dissipate. A strong magnetic field would do it, but even if you could generate the properly shaped magnetic "bottle" external to the light saber handle, the magnetic bottle would interact with matter (walls and such) in a way that light sabers don't. Plus the light sabers would give off a tremendous amount of heat that wouldn't be stopped by the magnetic bottle. A light saber is fiction and will be for the foreseeable future.

Dr. Eric Christian