I am intrigued by your assertion that the speed of light is the fastest speed in…

I am intrigued by your assertion that the speed of light is the fastest speed in the universe. It seems to me that we wouldn’t be able to determine the fastest speed achievable in the universe, just as we can’t find the final number in math. When we’re counting, there will always be x+1 so why would calculating the speed of objects in our universe be any different? — GL

Your comparison between the limitless counting numbers and the limited speeds in the universe is an interesting one because it points out a fundamental difference between the older Galilean/Newtonian understanding of the universe and the newer Einsteinian understanding. The older understanding claims that velocities can be added in the same way that counting numbers can be added and that there is thus no limit to the speeds that can exist in our universe. For example, if you are jogging eastward at 5 mph and a second runner passes you traveling eastward 5 mph faster, then a person watching the two of you from a stationary vantage point sees the second runner traveling eastward at 10 mph. The velocities add, so that 5 mph + 5 mph = 10 mph. If the second runner is now passed by a third runner, who is traveling eastward 5 mph faster than the second runner, then the stationary observer sees that third runner traveling eastward at 15 mph. And so it goes. As long as velocities add in this manner, objects can reach any speed they like.

At this point, you might assert that velocities do add and that objects should be able to reach any speed. But that’s not the case. The modern, relativistic understanding of the universe says that even at these small speeds, velocities don’t quite add. To the stationary observer, the second runner travels at only 9.9999999999999994 mph and the third runner at only 14.9999999999999988 mph. As you can see, when two or more velocities are combined, the final velocity isn’t quite as large as the simple sum. What that means is that the velocity you observe in another object is inextricably related to your own motion. This interrelatedness is part of the theory of relativity—that observers who are moving relative to one another will see space and time somewhat differently.

For objects traveling close to the speed of light, the failure of velocity addition becomes quite severe. For example, if one spaceship travels past the earth at half the speed of light and the people in that spaceship watch a second spaceship pass them at half the speed of light in the same direction, then a person on earth will see the second spaceship traveling only four-fifths of the speed of light. As you can see, relativity is making it difficult to reach the speed of light. In fact, it’s impossible to reach the speed of light! No matter how you combine velocities, no observer will ever see a massive object reach or exceed the speed of light. The only objects that can reach the speed of light are objects without mass and they can only travel at the speed of light.

So while the counting numbers obey simple addition and go on forever, velocities do not obey simple addition and have a firm limit—the speed of light. The additive counting numbers are an example of a mathematical group that extends infinitely in both directions, but there are many examples of groups that do not extend to infinity. The group that describes relativistic, real-world velocities is one such group. You can visualize another simple limited group—the one associated with walking around the surface of the earth. No matter how much you try, you can’t walk more than a certain distance northward. While it seems as though steps northward add, so that 5 steps north plus 5 steps north equals 10 steps north, things aren’t quite that simple. Eventually you reach the north pole and start walking south!

I am a mentor to a 7th grader who is doing a report on Einstein. How do I explai…

I am a mentor to a 7th grader who is doing a report on Einstein. How do I explain his theory in a way that will be relevant to her? — MG

The basis for Einstein’s theory of relativity is the idea that everyone sees light moving at the same speed. In fact, the speed of light is so special that it doesn’t really depend on light at all. Even if light didn’t exist, the speed of light would still be a universal standard—the fastest possible speed for anything in our universe.

Once we recognize that the speed of light is special and that everyone sees light traveling at that speed, our views of space and time have to change. One of the classic “thought experiments” necessitating that change is the flashbulb in the boxcar experiment. Suppose that you are in a railroad boxcar with a flashbulb in its exact center. The flashbulb goes off and its light spreads outward rapidly in all directions. Since the bulb is in the center of the boxcar, its light naturally hits the front and back walls of the boxcar at the same instant and everything seems simple.

But your boxcar is actually hurtling forward on a track at an enormous speed and your friend is sitting in a station as the train rushes by. She looks into the boxcar through its window and sees the flashbulb go off. She watches light from the flashbulb spread out in all directions but it doesn’t hit the front and back walls of the boxcar simultaneously. Because the boxcar is moving forward, the front wall of the boxcar is moving away from the approaching light while the back wall of the boxcar is moving toward that light. Remarkably, light from the flashbulb strikes the back wall of the boxcar first, as seen by your stationary friend.

Something is odd here: you see the light strike both walls simultaneously while your stationary friend sees light strike the back wall first. Who is right? The answer, strangely enough, is that you’re both right. However, because you are moving at different velocities, the two of you perceive time and space somewhat differently. Because of these differences, you and your friend will not always agree about the distances between points in space or the intervals between moments in time. Most importantly, the two of you will not always agree about the distance or time separating two specific events and, in certain cases, may not even agree about which event happened first!

The remainder of the special theory of relativity builds on this groundwork, always treating the speed of light as a fundamental constant of nature. Einstein’s famous formula, E=mc2, is an unavoidable consequence of this line of reasoning.

Is it true that a person in space doesn’t get as old as if he was on the earth?

Is it true that a person in space doesn’t get as old as if he was on the earth? — ASB, Chiapas, Mexico

The effects you are referring to are extremely subtle, so no one will ever notice them in an astronaut. But with ultraprecise clocks, it’s not hard to see strange effects altering the passage of time in space. There are actually two competing effects that alter the passage of time on a spaceship—one that slows the passage of time as a consequence of special relativity and the other that speeds the passage of time as a consequence of general relativity.

The time slowing effect is acceleration—a person or clock that takes a fast trip around the earth and then returns to the starting point will experience slightly less time than a person or clock that remained at the starting point. This effect is a consequence of acceleration and the changing relationships between space and time that come with different velocities.

The time speeding effect is gravitational redshift—a person or clock that is farther from the earth’s center experiences slightly more time than a person or clock that remains at the earth’s surface. This effect is a consequence of the decreased potential energy that comes with being deeper in the earth’s gravitational potential well.

I’ve heard that there are only four basic forces in nature: gravitational, elect…

I’ve heard that there are only four basic forces in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear. Is this true, and if so, what are the basic differences? — SH, Purdue, Indiana

The number of “basic forces” has changed over the years, increasing as new forces are discovered and decreasing as seemingly separate forces are joined together under a more sophisticated umbrella. A good example of this evolution of understanding is electromagnetism—electric and magnetic forces were once thought separate but gradually became unified, particularly as our understanding of time and space improved. More recently, weak interactions have joined electromagnetic interactions to become electroweak interactions. In all likelihood, strong and gravitational interactions will eventually join electroweak to give us one grand system of interactions between objects in our universe.

But regardless of counting scheme, I can still answer your question about how the four basic forces differ. Gravitational forces are attractive interactions between concentrations of mass/energy. Everything with mass/energy attracts everything else with mass/energy. Because this gravitational attraction is exceedingly weak, we only notice it when there are huge objects around to enhance its effects.

Electromagnetic forces are strong interactions between objects carrying electric charge or magnetic pole. While most of these interactions can be characterized as attractive or repulsive, that’s something of an oversimplification whenever motion is involved.

Weak interactions are too complicated to call “forces” because they almost always do more than simply pull two objects together or push them apart. Weak interactions often change the very natures of the particles that experience them. But the weak interactions are rare because they involve the exchange of exotic particles that are difficult to form and live for exceedingly short times. Weak interactions are responsible for much of natural radioactivity.

Strong forces are also very complicated, primarily because the particles that convey the strong force themselves experience the strong force. Strong forces are what hold quarks together to form familiar particles like protons and neutrons.

Is it physically possible for a baseball player to hit a baseball that has been …

Is it physically possible for a baseball player to hit a baseball that has been pitched 60 ft away at 90-95 mph? If so, why are the highest baseball records between 3 and 4 out of ten?

If the ball was pitched straight and true, the same way every pitch, good batters could hit every one. There is enough time in the wind-up and pitch for the batter to determine where and when to swing and to hit the ball just right. But the pitches vary and the balls curve. That limits the batter’s ability to predict where the ball is going. There aren’t any physical laws that limit a batter’s ability to hit every ball well, but there are physiological and mental limits that lower everyone’s batting average.

When people are able to bend spoons or move tables with their minds (if this is …

When people are able to bend spoons or move tables with their minds (if this is actually possible and not just a hoax), what sort of force is being exerted on the object? Is it possible to create forces with the mind?

I’m afraid that spoon bending is simply a hoax. While there are electrochemical processes going on in the mind that exert detectable forces on special probes located outside the head, these forces are so small that they are incapable of doing anything as demanding as bending a spoon. Spoon bending and all other forms of telekinesis are simply tricks played on gullible audiences.

What is an event horizon?

What is an event horizon? — KRH

An event horizon is the surface around a black hole from which not even light can escape. But to make it clearer what that statement means, consider first what happens to the light from a flashlight that’s resting on the surface of a large planet. Light is affected by gravity—it falls just like everything else. The reason you never notice this fact is that light travels so fast that it doesn’t have time to fall very far. But suppose that the gravity on the planet is extremely strong. If the flashlight is aimed horizontally, the light will fall and arc downward just enough that it will hit the surface of the planet before escaping into space. To get the light to leave the planet, the flashlight must be tipped a little above horizontal.

If the planet’s gravity is even stronger, the flashlight will have to be tipped even more above horizontal. In fact, if the gravity is sufficiently strong, light can only avoid hitting the planet if the flashlight is aimed almost straight up. And beyond a certain strength of gravity, even pointing the flashlight straight up won’t keep the light from hitting the planet’s surface.

When that situation occurs, an event horizon forms around the planet and forever separates the planet from the universe around it. Actually, the planet ceases to exist as a complex object and is reduced to its most basic characteristics: mass, electric charge, and angular momentum. The planet becomes a black hole. and light emitted at or within this black hole’s event horizon falls inward so strongly that it doesn’t escape. Since nothing can move faster than light, nothing else can escape from the black hole’s event horizon either.

The nature of space and time at the event horizon are quite complicated and counter-intuitive. For example, an object dropped into a black hole will appear to spread out on the event horizon without ever entering it. That’s because, to an outside observer, time slows down in the vicinity of the event horizon. By that, I mean that it takes an infinite amount of our time for an object to fall through that event horizon. But the object itself doesn’t experience a change in the flow of time. For it, time passes normally and it zips right through the event horizon.

Finally, event horizons and the black holes that have them aren’t truly black—quantum mechanical fluctuations at the event horizon allow black holes to emit particles and radiation. This “Hawking radiation,” discovered by Stephen Hawking about 25 years ago, means that black holes aren’t truly black. Nonetheless, objects that fall into an event horizon never leave intact.

I’m grateful for your work and the availability of your site. Though I think tha…

I’m grateful for your work and the availability of your site. Though I think that your ignorant condemnation of the work of other professionals about whose work you know absolutely nothing is contemptuous. Once again the arrogance of the established order and refusal to open-minded investigation. I would not have this opinion if you had used the careful, open-minded, systematic investigation that you espouse before you let your ego expose your ignorance. Carolyne Myss, with a verifiable accuracy rate of 93% percent, should not be called a quack. I wonder if, in your answers on this site, you could attain that rate of accuracy. I sincerely doubt it. In fact, with your over-blown ego, you could really benefit from her work. So stick to the information about lightning and CDs and stay away from that which you obviously are quite ignorant! — Unsigned

This comment, which responds to a previous posting on this site, points out one of the most important differences between physical science and pseudo-science: the fact that pseudo-science isn’t troubled by its lack of self-consistency.

Physical science, particularly physics itself, is completely self-consistent. By that I mean that the same set of physical rules applies to every possible situation in the universe and that this set of rules never leads to paradoxical results. Despite its complicated behavior, the universe is orderly and predictable. It’s precisely this order and predictability that is the basis for the whole field of physics.

In contrast, pseudo-science is eclectic—it draws from physics and magic as it sees fit. It uses the laws of physics when it finds those laws useful and it ignores the laws of physics when they conflict with its interests. But the laws of physics only make sense if they apply universally—if there were even one situation in which a law of physics didn’t apply, physics would lose its self-consistency and predictive power. That’s just what happens with pseudo-science when it begins to ignore the laws of physics on occasion. Moreover, the new rules that pseudo-science introduces to replace the ones it ignores make the trouble even worse. Overall, pseudo-science is inconsistent and can’t be counted on to predict anything.

Pseudo-science might argue that the laws of physics are correct as far as they go, but that they’re incomplete. No doubt the laws of physics are incomplete; physicists have frequently discovered improvements to the laws of physics that have allowed them to make even more accurate predictions of the universe’s behavior. But in the years since the discoveries of relativity and quantum physics, the pace of such discoveries has slowed and what remains to be understood is at a very deep and subtle level. It’s extraordinarily unlikely that the laws of physics as they’re currently understood are wrong at a level that would allow a person to bend a spoon with their thoughts alone or predict the order of a deck of cards without assistance. Just because I haven’t dropped a particular book doesn’t prevent me from predicting that it will fall when I let go of it. I understand the laws that govern its motion and I know that having it fly upward would violate those laws. Similarly, I don’t have to watch someone try to bend a spoon with their thoughts to know that it can’t be done legitimately. Again, I understand the laws that govern the spoon’s condition and I know that having it bend without an identifiable force acting on it would violate those laws. I also don’t have to watch someone try to predict cards to know that it, too, can’t be done legitimately. Without a clear physical mechanism for transporting information from the cards to the person, a mechanism that must involve forces or exchanges of particles, there is no way for the person to predict the cards.

Does gravity have a speed at which it acts upon another body?

Does gravity have a speed at which it acts upon another body? — CP, Billings, Montana

Yes, the speed of light. The gravitational interaction between two objects can be viewed as the exchange of particles called “gravitons,” just as the electromagnetic interaction between two objects can be viewed as the exchange of particles called “photons.” Gravitons and photons are both massless particles and therefore travel at a special speed: the “speed of light.” Since light is easier to work with than gravity, people discovered this special speed in the context of light first. If gravity had been easier to work with, they might have named it “the speed of gravity” instead. Sometime in the not too distant future, gravity-wave detectors such as the LIGO project will begin to observe gravity waves traveling through space from nearby cosmic events, particularly star collapses. These gravity waves will reach us at essentially the same time as light waves from those events since the gravity and light travel at the same speed.

How does one find out the speed of a quark? Is it 7000 times the speed of light?…

How does one find out the speed of a quark? Is it 7000 times the speed of light? — D

It seems that quarks are forever trapped inside the particles they comprise—no one has ever seen an isolated quark. But inside one of those particles, the quarks move at tremendous speeds. Their high speeds are a consequence of quantum mechanics and the uncertainty principle—whenever a particle (such as a quark) is confined to a small region of space (i.e. its location is relatively well defined), then its momentum must be extremely uncertain and its speed can be enormous. In fact, a substantial portion of the mass/energy of quark-based particles such as protons and neutrons comes from the kinetic energy of the fast-moving quarks inside them.

But despite these high speeds, the quarks never exceed the speed of light. As a massive particle such as a quark approaches the speed of light, its momentum and kinetic energy grow without bounds. For that reason, even if you gave all the energy in the world to a single quark, its speed would still remain just a hair less than the speed of light.