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General Physics:

Basic Physics Questions
Electromagnetic Waves
Particle Physics
Sub-divide Electrons?
Atoms and Empty Space
Coordinates of Time and Space
Cartography and Motion
Motion is Relative
Speed of Light vs Speed of Sound
Properties of Light
Spectrum of Visible Light
Why Do Things Get Smaller When Farther Away?
Wave-Particle Duality
Does Everything Have Wave-like Motion?
Kelvin Convention
Absolute Zero
Energy Cannot Be Created or Destroyed
Grand Unified Theory
String Theory
Size and Scale
More on Size and Scale
Are Atoms in Photons?

  1. Basic Physics Questions

    I am a student, and I have a basic physics question about...

    This question would be better covered by a physics teacher in your school that you can interact with for better understanding. You might get a science fair project out of it, too.

    Dr. Louis Barbier and Beth Barbier
    (October 2002)

  2. Electromagnetic Waves

    Electromagnetic waves are capable of traveling through a vacuum. In order to obtain an electromagnetic wave, you need an electrical field (which requires electrons) and a magnetic field. If there is no matter in space, where do the electrons come from?

    Electromagnetic (EM) waves are composed of electric and magnetic fields (hence the name). They travel freely through a vacuum. You do not need electrons, or any other matter, in a given region to have an electromagnetic wave in that same region.

    Typically, in an antenna for example, moving electrons inside the wire produce an EM wave, which freely propagates away from the antenna -- into space / a vacuum / whatever medium surrounds the antenna. The electrons are confined to the wire, while the EM wave travels out at the speed of light (c) in all directions.

    You might check out some websites, including the Physics 2000 site at the University of Colorado for more help.

    Dr. Louis Barbier
    (January 2008)

    P.S. There is matter in space, including electrons. Learn more about this on our cosmic ray page.

    Beth Barbier

  3. Particle Physics

    I am trying to learn some physics on my own, but I find the descriptions of particles either too technical or too vague. I want a description of the structure of a nucleus and how particles are classified. I need a text that's not too hard, but without sacrificing rigor.

    I'm afraid you can't have a text that's not too hard without sacrificing rigor. If you have a background in physics, a standard undergraduate text book that I would recommend is Nuclei and Particles by Emilio Segre.

    Dr. Louis Barbier

  4. Sub-divide Electrons?

    Can electrons be divided into sub-particles or are they pure energy?

    There is no evidence that electrons can be sub-divided. They appear to be pure, primary particles. Although matter and energy are in some ways different forms of the same thing, electrons have a rest mass and so are considered matter and not pure energy.

    Dr. Eric Christian
    (May 2000)

  5. Atoms and Empty Space

    I understand that atoms are mostly empty space, so is the solidity of objects an illusion? If I am looking at a chair, am I looking mostly at empty space?

    You are right, atoms are mostly empty space. However, we cannot see this because we look at everything with visible light. Light waves have a characteristic size that is bigger than an atom, so they cannot tell us anything about the tiny structure of an atom.

    Light waves passing through atoms behave similarly to water waves passing over pebbles on the beach. The characteristic size of the wave is known as the wavelength. This is the distance between one crest and the next. If a water wave has a wavelength of a few feet, it will be unaffected by pebbles as it approaches the beach. The wavelength is bigger than the pebbles. If we watch these waves, we won't know the pebbles are there. However, if the waves encounter a large boat, they will be diverted around the hull, and we can easily see that.

    The light waves we use to look at everything have a short wavelength, but they are still bigger than an atom. For example, yellow light has a wavelength of about 0.58 micrometers. In comparison, an atom has a radius of about 0.00005 micrometers. People have used X-rays to image crystals, because X-rays have wavelengths about 500-1000 times smaller than light. These images can show the location of the atoms, but not their contents.

    You make a good point about the chair. It appears to be solid, but it is mostly empty space. It resists crushing because the electrons of the chair atoms cannot easily be compressed together. Charged particles repel other particles of the same charge.

    Dr. Mark Popecki
    (February 2003)

  6. Coordinates of Time and Space

    When you look at a computer screen and you move your cursor, the cursor position has a specific coordinate. There are some 20 to 30 thousand positions, but the cursor cannot move in between coordinates. Does the space that we occupy work in the same manner?

    When you move your cursor across the screen, it can only be located at discrete (set fixed) locations because it is made up of individual pixels. The pixels are fixed regions that display the colors. Many discrete things are man-made, such as computer screens.

    People usually experience space and time as being continuous (non-discrete). Pretend you are in a very large room, and there are large tiles on the floor. As you walk across the room, you could stop at any time. Your right foot may land such that the tip of your big toe just touches a line between tiles. (Really though, you could have stopped such that the middle of your right foot was on the line. Or you could have stopped such that the heel of your foot was on the line.) We experience a continuous world, but man often makes discrete devices to make our lives easier.

    I should mention though that on really, really, really small scales, such as those smaller than the size of an atom, particles can behave in a more discrete way. For example, electrons have a high probability of only being at certain fixed distances from the center of an atom. The center of an atom is where the protons (positive charge) and neutrons (no charge) are. The small mass, negatively charged electrons orbit around the center. The electrons are highly likely to be at some fixed distance, which is often referred to as a level. They might be found at a position that is not one of these levels, but the probability (likelihood) of finding an electron in-between fixed levels is very low.

    Dr. Heather Elliott
    (January 2003)

  7. Cartography and Motion

    My daughter asked me this question after looking at the map in an airplane magazine: "Why doesn't the plane fly in a straight line to London from Phoenix?"

    Actually, most planes do fly in straight lines on the 3-dimensional sphere that is the Earth. It is only on a flattened 2-dimensional map or image that the routes appear curved. Maps are not perfect representations of the 3-D Earth, and there are actually a bunch of different ways you can make a map (what are called map projections). If you look at your route on a globe, you'll see that it is pretty straight. It's a good question.

    Dr. Eric Christian
    (April 2005)

  8. Motion is Relative

    What is meant by the statement "motion is relative"?

    The term "relative" usually refers to Einstein's theory of Special Relativity. Einstein's theory states that the laws of physics are the same in any frame that is not experiencing some kind of acceleration (called inertial frames), independent of position or velocity. The basic idea is that there is really no absolute frame from which we can measure the absolute velocity or position. We can merely only measure "relative" positions and velocities.

    For instance, the Earth is rotating and we could measure everything with respect to this rotation. But this is not the "absolute" rest frame, since the Earth is also revolving around the Sun. After accounting for the Earth's orbital motions, we could take it one more step and attempt to measure all motion relative to the solar system's motion around the galactic center. Even this latter attempt does not get us to a "preferred" absolute reference frame, since the Universe itself is expanding. So, as you see, we cannot determine the absolute motion and position of objects.

    Consider another analogy: A train is moving at a fixed velocity relative to an observer on the ground. Another observer on the train would see the person on the ground moving with the same fixed velocity. In addition, if a person on the train were to now throw a ball inside the train, the observer on the ground would see the total velocity of the ball as equaling the initial velocity of the ball as it is being thrown plus the velocity of the train. The observer on the train would simply see that the ball was moving at the speed that she initially threw the ball. The velocity of the ball, then depends, on the observer.

    While positions and velocities are relative, the speed of light, for example, is not. This is one other important observation of Einstein's theory, that the speed of light is constant in any reference frame. In other words, the speed of light is not greater when making the observation on a moving train.

    Dr. Georgia de Nolfo
    (January 2004)

  9. Speed of Light vs Speed of Sound

    How do sound and light arrive at the same time when light travels faster than sound?

    This is a good and interesting question that is rooted in everyday experience. Indeed, it appears that sound and light arrive simultaneously when we attend a play in a theater or watch a movie. The reason is that we are very close to the stage or screen and loudspeaker. This may already be different during a concert in a stadium: For somebody in the distant rows the sound of the band and its movement is out of synch. The speed of sounds is approx. 300 m/s (1000 ft/s), while the speed of light is 300,000 km/s (approx.190,000 miles/s). Therefore, the sound arrives about 1/3 of a second after the corresponding movement, i.e. starts to be recognizable. In a theater the time difference is too short to be recognized.

    The most striking experience of the different speed of light and sound is during a thunderstorm. Unless the thunderstorm is exactly upon us, a silent lightning is followed by a substantially delayed thunder. Yet both originate the same place and time. In fact, one can estimate the distance of the thunderstorm from the difference by counting .twenty-one, twenty-two, etc.. starting with the lightning. Saying each number takes about one second. The distance of the thunderstorm in miles is the number of seconds counted dived by five. Thus a thunderstorm is a great time to find out without instrumentation that sound is indeed much slower than light.

    Dr. Eberhard Moebius
    (November 2003)

  10. Properties of Light

    Does light have weight? Does light make sound?

    Good question, and the answer is "no/yes and sometimes." It's a complicated subject, and it took Einstein to figure it out.

    First, what is weight? Stand on the bathroom scale and you get your weight. Take that same scale to the Moon and your weight changes (it's lower). Gravity is weaker on the Moon.

    Weight is force. In physics we talk about mass as the property an object possesses that is the same under all magnitudes of gravity. If you know the strength of gravity at a particular place and you know the mass of the object, you can calculate its weight.

    Light is made up of particles called photons, and photons have no mass, so they have no weight. However, a strong gravitational field can bend light and that sounds like they do have mass. Why is that? Einstein says that mass and energy are equivalent. Light has energy, so its path can be changed by gravity.

    We say it has no mass and no weight because it has no energy associated with zero velocity. We call this the "rest mass", and for photons the rest mass is zero. Ordinary matter like protons and electrons have a rest mass, so we say they have nonzero mass and nonzero energy associated with zero velocity. Protons and electrons respond to gravity the way we expect particles with mass to respond, and they are the reason each of us has mass and weight.

    Sound, unlike mass, is a characteristic of the medium. If you can get air molecules to move together to transport momentum through the air, that motion is sound. Moving air molecules is what the speaker in your radio or phone is designed to do, and electromagnetic fields working with a magnet make the speaker move. Photons are the carriers of electromagnetic fields, so in that sense they can create sound by interacting with objects having mass. The photons we think of as light would be harder to harness in this way, but you can bet that somewhere someone is building a very small device to do just that.

    Dr. Charles Smith
    (May 2003)

  11. Spectrum of Visible Light

    The Sun's visible spectrum consists of seven colors: red, orange, yellow, green, blue, indigo, and violet. Between green and blue there is also a blue-green color that is clearly visible. Why is this not counted as a separate color?

    Light coming from the Sun includes light in the visible spectrum (as well as other wavelengths). You are in fact correct that there is a blue-green color in addition to indigo. The monochromatic colors (i.e. colors that are composed of a single wavelength as apposed to a mixture as is the case with most colors in our everyday surroundings) consist of red, orange, yellow, green, cyan (blue-green), blue, indigo, and violet. However, spectral colors form a continuous spectrum (as you can easily see), and how one divides up the actual colors is somewhat arbitrary.

    Our interpretation of color also depends on the intensity of the light (as an example, low-intensity orange-yellow looks brown) as well as on the physiology of our retina. You can read more about this on the Wikipedia entry about The Physics of Color.

    Dr. Georgia de Nolfo
    (October 2003)

  12. Why Do Things Get Smaller When Farther Away?

    Why do things get smaller the farther they get away from the observer?

    This is easy to see with a picture. Draw a box a certain distance away from your eye. Light reaches your eye from the two extreme corners of the box, and those light rays make an angle with respect to one another. Now redraw the box 5 times further away, and draw the same two light rays from the corners. You'll see that the angle between the two lines has changed. Now think about what would happen if you keep moving the box further away... that angle between the two light rays will keep getting smaller and smaller. Try it for yourself. I hope that helps.

    Dr. Louis Barbier

  13. Wave-Particle Duality

    Do physicists still believe in wave-particle duality? I just read QED by Richard Feynman, and it is his assertion that particles are the way things work on the sub-atomic level.

    The wave-particle duality is still valid. On different scales, and in different circumstances, particle properties are observed or wave-like properties are observed. This may reflect the reality of nature or perhaps our lack of a complete understanding. Perhaps one day another person like Feynman will come along and make it clearer for us all. (One of my teachers preferred not to use the terms 'particle' or 'wave', he used 'wave-icle'.)

    Dr. Louis Barbier
    (December 2000)

  14. Does Everything Have Wave-like Motion?

    If everything we sense is has a wave-like motion, then is everything a wave? If so, does everything have to behave as a wave, as in music theory?

    Your question sounds like you've extended quantum mechanics too much. Yes, everything has some wave-like properties, but it is only for the very small (electrons, etc.) that this has any real consequence. The wave properties of a human body, for example, are completely negligible.

    Dr. Eric Christian

  15. Kelvin Convention

    My chemistry teacher wanted to know today why there's no "degree" if you write Kelvin. Is there any physical reason why you say "10 degrees Centigrade", but you write 283 K?

    The reason is that "Kelvin" was defined as a unit that means "degrees of temperature on the Kelvin scale", which was not done for "Fahrenheits" or "Centigrades" or "Celsiuses". The other three had "degrees of temperature on the X scale" shortened to "degrees X", but that was it. So you can say it is just convention, but it is convention that has been codified by the International Committee on Weights and Measures. Kelvin is also an absolute scale (zero is the temperature where molecular motion is zero).

    Dr. Eric Christian

  16. Absolute Zero

    At absolute zero, does atomic motion cease as well as molecular motion? In other words, would there be a theoretical point at which subatomic particles also cease to have kinetic energy?

    No, absolute zero (0 Kelvin) is defined as where all molecular motion stops. The electrons still orbit the nucleus, the nucleons still spin, etc.

    Dr. Eric Christian

    Is there a law that prevents the attainment of absolute zero? Or is it that humans don't have the technology to reach it?

    In order to reach a temperature of absolute zero, a "heat engine" would have to be built that is 100% efficient. This is not possible. We will never build such a device.

    Dr. Louis Barbier

  17. Energy Cannot Be Created or Destroyed

    I understand that matter can not be created nor destroyed. It just changes to different levels and states. Is this true, or has something been found to contradict this? If it is true, what continues to support this?

    Energy (not matter) cannot be created or destroyed. Matter is just one form of energy and can be converted into other forms. You can convert matter into light (electromagnetic energy), for example. This is pretty well accepted as a physical law, because there is NO evidence that this doesn't hold throughout the universe.

    Dr. Eric Christian

  18. Grand Unified Theory

    I'm looking for a quick definition of the Grand Unified Theory, and whether or not current thinking is hedging on ever solving or proving it.

    A Grand Unified Theory is a theory that combines the four known forces (Electromagnetism, Gravity, Strong Nuclear Force, and Weak Nuclear Force) into one big equation. Electromagnetism and the Weak Nuclear Force have already been "combined" (called Electro-Weak Theory), and I think it's possible that all four may eventually be combined. Proving it will require something indirect (such as proton decay) because the length scales where they combine are too small, and the energy scales are too big, for our current technological level.

    Dr. Eric Christian

  19. String Theory

    What exactly is string theory?

    Check out The Official String Theory Web Site for everything you'd want to know about string theory.

    Dr. Louis Barbier

  20. Size and Scale

    I was wondering how the perceptual scale of the observer affects the observations of particle interactions.

    I'm imagining a "gigantic" scientist (as big compared to the Earth as we are compared to a hydrogen atom). The scientist lives to be 15 billion years old, and has a very slow metabolic rate (e.g., 1 heartbeat every 500,000 years).

    If this scientist made observations on the Milky Way galaxy, would celestial bodies still appear to move according to Newton's mechanics, or would they exhibit quantum-relativistic interactions?

    To reverse the question, I'm imagining a scientist about the size of a proton. The scientist lives only a fraction of a second, and has a heart rate near the speed of light. Would subatomic particles appear to exhibit Newtonian mechanics to such an observer?

    No, there seems to be a natural scale to the Universe. The problem is that as something get bigger (more massive) its effective wavelength gets smaller. So subatomic particles (electrons, protons, neutrons, etc.) have a dual wave/particle nature that is observable because the wavelength of the particle is "close" in size to the particle. If you get larger, things get more disparate. A human body has a wavelength that is so small, there is nothing for it to interact with, and a galaxy even smaller.

    Dr. Eric Christian

  21. More on Size and Scale

    If a person were scaled down to only two or three inches tall, and he or she jumped off of a stool approximately a foot and a half in height, would that person suffer the same physical damage as an average-sized person jumping off of a building scaled to match the height of the stool? Why?

    Someone falling a foot and a half will reach about the same speed whether they are 6 feet or 2 inches tall. Actually the 2 inch tall person will probably not be moving as fast because of more air resistance. So the damage will not be "scaled". This is why cats, mice, grasshoppers, etc. can jump or fall much larger distances relative to their size than a man, and still not take any damage. A mouse would scarcely notice a foot and a half fall, and their legs are much thinner relative to their body size than a man's.

    Dr. Eric Christian
    (May 2000)

  22. Are Atoms in Photons?

    Are atoms in photons?

    The main distinction between photons and atoms is that photons are made up entirely of energy (they are weightless) and atoms are made of matter and do have a very small mass. You can touch and hold something that is made of atoms, but you can't "touch" a beam of light.

    Atoms can be stationary or they can move extremely fast, but nothing can move as fast as light does. Light always moves through the air at 186,000 miles per second (it can travel around the world more than seven times in one second).

    Atoms can absorb photons. When this happens, the photon disappears and gives energy to the atom, but does not add any matter to the atom.

    Lauren Scott
    (December 2002)

  23. Radiation

    What causes radiation?

    From your question it's not clear whether you are referring to electromagnetic radiation (the energy spectrum, including ultraviolet, X-ray, gamma-ray radiation, etc.) or "nuclear radiation."

    You can learn a lot about electromagnetic radiation on the "Imagine the Universe!" site with their Electromagnetic Spectrum Introduction.

    And the EPA has a good description of nuclear radiation at their page: What is Radiation?

    Dr. Louis Barbier and Beth Barbier
    (June 2003)

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