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Answers to questions, Nov 24
1. Two black holes
Question:
What would be the implications of two black holes orbiting each other?
Short answers:
As they circle one another, they would lose energy in the form of gravitons (gravitational waves). Eventually, the two black holes would spiral into each other and cause an explosion that would make another black hole (Ka HJ) .
If two black holes orbited each other, the more massive one would eventually pull the less massive one in and they would become one, while emitting gravitational waves (Kim P) .
The more massive black hole would pull the smaller one into it. Gravitational waves would be emitted from their spiraling (Brooke W) .
... Eventually, the two black holes would collide, producing a huge explosion and form a bigger black hole (more than the two combined?). Would the energy of this explosion go anywhere or would the gravity suck it all back? (Chris B and John K)
Comments:
(a) Gravitational waves are emitted in the explosion. They are not "all sucked back". Basically, the two black holes rush into each other at tremendous speed, and the enrgy of this motion can be radiated away. Still, nothing escapes from the inside of the black holes, if one ignores quantum processes (Hawking radiation). For stellar-size black holes, the Hawking radiation is extremely weak and can be ignored.
(b) The two black holes form one black hole that has an area of the event horizon that is "more than the two combined". However, its mass is "less than the two combined". To understand this, recall that the mass of a non-rotating black hole is proportional to its radius, according to r = 2Gm/c2. If two non-rotating black holes of mass m collide, they can form a non rotating black hole of mass less than 2m, and radius R that is less than 2r, but still such that R2 is greater than 2r2. Most likely, however, the resulting black hole has a lot of spin (is rapidly rotating).
(c) Several people wrote that the black hole that is formed is ''denser'' than the two colliding black holes. If we define density as mass divided by the volume inside the event horizon, the contrary is true: a small-mass black hole has such a small event horizon that it is denser, on the average, than a large-mass black hole. However, the stuff inside a black hole keeps collapsing towards the singularity at the core of the black hole, and eventually the density near the singularity becomes higher in a more massive black hole.
2. Event horizon
Question:
Can you give a layman's explanation of an event horizon?
Answer (Kim P):
An event horizon is the boundary around a black hole that light can't escape from. It is the farthest reaches of the gravitational pull of the black hole that are inescapable to light and all other forms of energy and matter.
Answer (Brooke W):
The event horizon is the outer boundary of the black hole. Objects enter the black hole here, but can never escape. It is the boundary at which light is trying to escape the black hole unsuccessfully. Nothing travels faster than light, therefore nothing can escape. The event horizon is the boundary of the no-escape region, farthest away from the singularity.
Answer (Chad D):
Event horizon: a sphere-like boundary past which nothing can escape a black hole, not even light. From what I understand, exactly at the event horizon any light emitted perfectly perpendicular to it (and in the direction away from the black hole) is actually caught and suspended there.
Answer (Ka HJ):
It is the ''surface'' of a black hole, at some distance from the center, where nothing can escape. Even light tries to escape but can't, so it is sucked in.
Answer (Chris B):
The distance from a black hole at which not even light can escape from the black hole. Closer than that, everything is sucked back. Farther than that, light may be able to escape....
Answer (Rena M)
Stars start falling in on themselves when they run out of the fuel that keeps them hot (hot gases try to expand). Gravitational forces within them (b/c of their incredible mass) start drawing all the particles together. A star with enough mass would completely collapse. The gravitational field of the star would start affecting the paths of light rays emitted by it. The more condensed the star becomes, the more light rays are bent inwards as well by gravity. At a certain point, the gravity is so strong that light can no longer escape. This is the event horizon, the point at which in space time light rays no longer come from the star and so basically the star-black hole is in a kind of different universe than ours. We can't learn of any events that happen beyond the event horizon because there is no light to report them to us.
Comment:
The event horizon is not a ''point in space time'', but rather a collection of points that, at any time, form a surface around the black hole.
Question:
If a person went into a black hole, could they communicate with people on the outside? How? Could they maintain a cable of extreme length that transmitted information?
Answer:
There is no communication possible from points inside the ''event horizon'' to points outside. Signals in a cable could travel, at best, at the speed of light, and could not come out of the black hole. Even if it were possible to maintain a cable (which it is not), it would not help.
3. Center of the universe
Question:
If we measure the microwave background to be the same intensity in every direction, wouldn't that suggest that we could be the center of the universe?
Answer (Evan R)
No, the intensity would be the same everywhere you moved...therefore, you would be the center of the universe, not the Earth.
Answer (Ka HJ)
The universe is expanding like the surface of a balloon, making points grow further away from each other in every direction. There is no identified ''center'' of the universe.
Answer (Glen M)
There is no true center of the universe ... the universe is expanding at an equal rate from all points in the universe, so no single point can be the center.
Answer (Chris B):
Yes, but the universe is expanding from every single point at the same time. In other words, there is no real center of the universe.
Answer (John K)
The intensity is the same in every part of the universe. Therefore, when you measure it anywhere it is constant from every direction there too. So: no, that does not suggest that we are the center of the universe.:
Answer (Matt D)
Trying to determine the center of the universe is a pointless expedition. In an infinite system, every point could be equally described as the center, but that is not the same as a Ptolemaic or even Copernican view.
Answer (Kim P):
We wouldn't necessarily be the center of the universe because we are only able to measure background radiation to a certain distance. If the radiation does change in frequency as it radiates further from the origin of the big bang, we wouldn't necessarily be able to detect that with our technology. We could only see that the radiation is the same around us
Comment:
See Comment on question 8.
4.
How is it possible to make space-time more or less warped, and how do we know it is warped to certain degrees?
Answer (Brooke W):
Space-time is warped because of the bodies of mass in it, like the sun. We know this because the light from stars that passes close to the sun is bent around the sun. The gravitational pull of the sun causes this to happen, and it was proved by observations during an eclipse. Space -time is more or less warped because of the different large and small masses in it.
Answer (Ka HJ):
Space time is strongly warped by huge masses, like the sun, or black holes. This ''warping'' can be seen as ''gravity'', since in flat space-time objects want to travel in straight lines.
Answer (Kim P):
It is possible to strongly warp space-time by a large mass being formed , or ''placed'' in the universe. It's like putting a ball in the middle of a stretched-out sheet: it warps the sheet. We know this because we can see that light bends around planets (and the sun in an eclipse).
5. Energy in food
Question:
Why is heat a disordered form of energy while food is not?
Answer (Kim P)
Heat is a form of energy. Food is matter that is digested by our body into simpler, less ordered molecules, and then used as a source of energy to fuel our body systems. .
Answer (Brooke W):
Heat is a disordered form of energy and food is not because the molecules are more tightly packed and ordered in food. As food matter breaks down in the body, becoming less ordered, energy is given off as heat, resulting in a move to higher entropy. Therefore, heat signals less order.
Answer (Chris B):
A lump of bread stays a lump of bread until it is either eaten or decomposes. It doesn't change shape by itself. You could heat an ordered lump of bread to the point that it is no longer ordered, at which point it would vaporize.
Answer (Ka HJ):
Food becomes a source of heat when it is eaten and transformed by the body into disordered states to create energy/heat. Heat is simply the energy of particles moving around in random directions. .... You could heat an ordered lump of bread to the point that it is no longer ordered, at which point it would vaporize.
Answer (Dagim W):
If we were able to turn body heat back into food, we would not need farms.
Comment:
Actually, ''heat is given off'' because the food is ''burned'' in your body. Anaerobic bacteria get their energy by breaking down their food, we must breathe too. Oxygen is breathed in, travels in the blood stream to the tissues, combines with carbon in the digested food to form carbon dioxide, and also combines with hydrogen in the food to form water. Then the carbon dioxide and the water vapor are breathed out. In a way, Hawking's example is not well chosen. There was some frustration with this question, resulting in the following.
Answer (Evan R):
Food, such as pizza or Hoos Hungry, are ordered forms of energy only when you call and have it delivered.
6.
What does it mean for time to ''loop back on itself?'' Is it realistic to think that someday we may be able to time travel? On the other hand, if our concept of time depends upon our position in space-time, then don't we time travel to a certain degree anyway?
7. Matter and antimatter
Question:
How do we know that we are made of matter and not anti-matter?
Answer (Ka HJ)
Matter and anti-matter are just names. It is all relative. We could be made of antimatter, and not matter.
Answer (Glen M)
Matter and anti-matter are just words we use. We could reverse them and there would be no consequences on our existence. There could be some alien civilization at the far reaches of the universe that is made up of what we call ''antimatter'' ... These aliens would probably describe our civilization as being made out of anti-matter while theirs is made of matter.
Answer (Cat S):
Basically, it's all a ''matter'' of point of view. Because we see most of the ''stuff'' around us in one state, we have named this ''matter'', while assuming that its anti-particle is anti-matter - just the opposite. It's merely a question of defining a norm, in this case matter, and then defining its opposite, in this case anti-matter, in terms of this norm. We could just as well be living in a world composed almost entirely of anti-matter, only it would look the same to us because of our starting point of view.
Answer (Chris B) : We don't. It's just easier to stick THAT particular label on our universe. To them, the inhabitants of an anti-universe are made of matter, not anti-matter. Remember, it's all relative.
Answer (Jon B):
Matter has been defined as the stuff we are made of while antimatter has been defined as the material that annihilates matter. It is just as correct to label what we are made out of antimatter as the stuff that annihilates what we are made of.
Answer (Ginny W):
The reason we know that we are made of matter and not antimatter is the same reason that we know that the sky is blue and not green - because we define it to be that way. The terms ''matter'' and ''antimatter'' are arbitrary human designations for what are essentially the states of ''us'' and ''not us.'' Matter is very simply whatever we (meaning all the ''stuff'' in the universe) are, and antimatter is the opposite of what we are. Blue is blue because we say it is - there is no distinct state of ''blue'', there is only a state of absorbing certain wavelengths of light and reflecting others which corresponds to that which we have decided to call blue. The case is the same for matter and antimatter
8. Center of expansion
Question: a) If the universe is everything, and it is expanding, then what is it expanding into? b)Where is the earth in relation to the universe? Will our position change as the universe expands?
Answer (Chris B):
a): Whatever is beyond our universe is not made of the same stuff we're made of, so there's no way of even conceptualizing it. Maybe it's god, maybe it's a void, maybe it's chicken soup. Nobody knows, and there's no way of knowing.
b): The earth is inside the universe, and will stay inside the universe. But the rest of the universe, as it expands, will continue expanding away from the earth, and away from Neptune, and away from the farthest galaxy we can see. I think the position of the earth depends on where one is looking from.
Answer (Ka HJ):
a) The universe is expanding, but it is also infinite. If our universe is like a bubble, then anything outside of that bubble would not affect us, so we would not know.
b) We have not yet explored the whole universe, so we would not know. All that is known is that the earth is inside the universe and as the universe expands, it will grow away in distance from everything else.
Answer (Lindsay B):
We do not know what the universe is expanding into because it does not exist. If it did exist we could measure it , but if it were measurable it would be a part of the universe and not outside it.
Comment:
There are two opposite schools of thought:
1) We have seen it all or nearly all. The universe is finite, closed, and will recollapse. It is expanding onto itself, like the surface of a balloon. It creates its own space-time as it expands. There is nothing outside it. It has no center.
2) We have seen nothing yet. What we can see, is a puny part of the whole universe. The universe is infinite and parts of it will expand forever, others will recollapse, new ''baby universes'' will be born. Our neighborhood will expand indefinitely and it is not like a sphere: it has negative curvature everywhere, a geometry so bizarre that it cannot be shown graphically. The boundaries of our local region are well beyond the point where we can see, and things look the same in all directions. That does not mean that we are in the center of our local region. Beyond the boundaries are other neighborhoods, about which we know nothing (yet).
Who is right? Take your pick. Astronomical evidence clearly shows that our neighborhood will not recollapse. Many theorists refuse to believe this evidence and still side with (1), maybe only in secret, because
(a) the astronomers have been wrong before
(b) it would be so neat if things were made according to (1).
Hawking used to side with (1), and at one point confidently predicted ''the end of physics'' by the year 2000 or so. The younger cosmologists are mostly in camp (2).
Some of the answers to question 3 are definitely within viewpoint (1), while the answers given to question (8) are in line with viewpoint (2).
9. Action and reaction
Question:
''For every action there is an equal but opposite reaction''. What is the opposite reaction to the formation of a black hole? of the Big Bang?
Answer (Evan R):
A White Donut will be formed. A Small Tap.
Comment:
More serious answers to this question were also off the mark, because the question is misleading. Newton's third law says that if you push on any object, the object pushes back on you with an ''equal reaction''. In an explosion, the fragments of the explosion, and the hot gases, push on each other as they fly apart. The third law does not say that an implosion is the '`opposite reaction'' of an explosion.
In the formation of a black hole, the various pieces of the collapsing star (or whatever) pull gravitationally on each other, in an equal and opposite way. In the Big Bang, one cannot point to forces between the particles that caused them to fly apart. Rather, space and time were being created at an explosive rate. However, the general principle that underlies Newton's third law must still apply, we strongly believe. This is the principle of conservation of momentum, which is as basic as conservation of energy. When object A pushes, or pulls, on object B, it causes the momentum of B to change. At the same time, B causes the momentum of A to change by an equal but opposite amount, so that the total momentum of A+B is unchanged. The total momentum of all the stuff in the universe has not changed at all from the first instant of the Big Bang (assuming that what we see is the whole universe).
10.
Question:
Why is it that disorder increases with time? How does anyone go about figuring this out?
Answer (Evan R):
Put a lot of people in a room and lock them in there for a few months without food. See the disorder that develops; who gets sacrificed for food, etc.
Answer (Jon B):
Time has been defined using disorder, so to change the direction disorder goes in, you would change the (thermodynamic) arrow of time.
Answer (Ka HJ):
Since disorder is more probable (more possibilities of a disordered state than an ordered state), disorder increases with time. Entropy is a measure of disorder. If entropy decreased with time, then we would see broken teapots gathering themselves from the floor and jumping back up on the cupboard.
Answer (Kim P):
.... One finds this out by noticing the beginning and end products of all systems. In the beginning things are more in order, at the end there is energy lost and/or a visible amount of disorder. Nothing can be ordered without a loss of energy, which increases the disorder (Comment : what she means is that the system that is becoming more ordered must lose energy to its surroundings, which become more disordered as a result)
Answer (John K):
The world tends towards entropy, so the more time the more entropy or disorder. People figure this out by observation.
Answer (Glen M)
This is the second law of thermodynamics... All systems proceed towards a maximum number of discernible arrangements, or maximum entropy. The entropy of some system can decrease, by being dumped onto the rest of the world, but any isolated system or the universe as a whole necessarily proceeds in a direction towards maximum entropy.
Answer (Ginny W):
The second law of thermodynamics is, I think, an extremely confusing concept. It talks about something called entropy - alternatively described by various people as disorder, heat, and chaos - which increases invariably. ... When entropy increases, as it does overall all the time, we don't particularly notice it because it is, for us, simply the natural process of the universe. We wouldn't exist if it weren't. Entropy increasing is manifested in the signs of natural processes all around us: when a house is left abandoned, it gradually deteriorates, until it becomes covered with vines, or caves in completely, or otherwise simply disorders. This is the way of the universe. We wouldn't expect a hundred year old house to look exactly how it looked the day it was built if it hadn't been being cared for - we expect the process of natural decay, of entropy. Another manifestation of entropy is Hawking's teacup falling off the table. When things are allowed to be acted upon purely by nature - that is, when we stop artificially holding the teacup in the air by keeping it on the table, it naturally falls to the ground and disorders, breaking into pieces. We may decrease entropy locally by gluing it back together, but we have increased it globally by exerting effort, and releasing heat, which, as the most disordered state of energy/matter in the universe, is considered to be pure disorder. We know, instinctively, that disorder increases because it is a fact of our existence, and we would not exist without it.
As for why it is a fact of our universe that entropy must increase with time, that's a matter for some slightly circular logic. Hawking points out in his chapter on the arrows of time that the reason we feel entropy increases with time is that we calculate the direction of time based on the direction that entropy is increasing. It is a bit like the question of why blue is blue - because we define it to be that way. We say that ''forward'' is the direction in which entropy increases, and entropy increases in the direction that we call forward. It doesn't seem natural to us for a cup that has been broken to pick itself hack up and put itself back together because it was together in the past, and is broken in the future. We define past and future, however, simply by that direction in which things flow, the direction of entropy. But why should entropy increase as the universe expands, whether or not we call it forward? Here we must bring in the weak anthropic principle. If entropy didn't increase as the universe expanded, if entropy were decreasing right now and would continue to do so, we would not be able to exist. We could not break down proteins to get energy to live, we could not build or weave, or even order neural impulses to create thought. We would simply not be. Therefore, in a universe that contains us, entropy must increase as the universe expands, or else a universe containing us would not exist.
Comment:
The reason why time keeps moving inexorably forward is not fully understood, really, and the second law of thermodynamics is indeed ''confusing'' to many people. However, there is a precise definition of entropy and of its relation to ''heat'' and ''disorder''. For the relation of entropy to heat and temperature, see the answer to question 21; most definitely, entropy is not the same as heat. The relation of entropy to disorder is more subtle, but one can say simply that entropy is a measure of disorder. It would take too long to give a technical explanation.
11. White holes
Does a white hole have a singularity at the center like black holes do? What's a white hole?
Answer:
If one takes a black hole and applies time reversal (runs time backwards), one obtains a solution of Einstein's equations, starting off with a singularity at its center, that could be called a white hole. However, in this solution entropy decreases with time, which is a problem. This means that only an extremely simple black hole can be run backwards without an extremely improbable fine-tuning of the initial conditions. The second law of thermodynamics is at work here, and it implies that white holes of any reasonable complexity do not exist. This has not stopped some people from describing our expanding universe as the interior of a gigantic white hole.
For a rotating black hole, one can mathematically continue the solution of the equations to the ''other side'' of the singularity. The properties of this ''other side'' are not really understood, but in some ways it can be described as a ``white hole'' side of the solution.
12. Lunar colonies.
Question:
How far in the future are colonies on the moon?
Answer (Ka HJ):
Unknown. Too much money to scarf up and not really necessary for the survival of our species, so probably never.
Answer (Chris B):
More than a week. I believe we have the technologies to colonize the moon on a very small scale, but the cost, time, purpose, and political garbage will take about six billion years, and by then, the sun will have died.
Answer (John K):
I think we could build one right now, but what would be the point?
Answer (Matt D)
I disagree with those who take a comical approach to this question. They seem to be as unenlightened as the stolid pre-NASA morons who rejected space exploration and the notion that man could ever reach the moon. Technological advances , political stabilities, economic revolutions, and new discoveries of physical principles could make moon colonization possible within any period of time, such as less than 100 years.
13. Meteorites.
Question:
If it was a meteorite that killed off the dinosaurs, how much energy (in terms of nuclear bombs) was released?
Short answer:
According to a recent article in Scientific American [November 98, page 71], the energy released was the equivalent of one hundred million (108) Megatons of TNT. The standard thermonuclear weapon (hydrogen bomb) in the US arsenal today is believed to be 1 Megaton, so it would be 100 millions of those. The Hiroshima bomb was only 12 kilotons of TNT, 80 times less than 1 Megaton. So the dinosaur-killing meteor released the energy of 8 billion Hiroshima bombs. These are very rough estimates, could easily be wrong by a factor of 10.
Long answer:
The energy delivered by a meteoroid depends of course on its mass and its speed. Most meteoroids are similar to stones or rocks found on earth, and are 2 to 4 times heavier than water. A stony meteoroid that is 20 meter (65 feet) across and enters the lower atmosphere at 20 km/sec (45 thousand miles per hour) delivers roughly the same amount of energy as a standard thermonuclear weapon, or one MT (MT = Megaton of TNT = 4.18*1015 joules). This meteoroid would have a mass of about 20 kilotons, and thus it would be 500 times more powerful than TNT, pound per pound.
The energy is proportional to the square of the velocity and to the mass, which is proportional to the cube of the diameter.
It is believed that a stony meteoroid will break up explosively in the air and will not create a crater, unless it is bigger than 100 meters across if it is falling straight down, and bigger yet if it is falling at an angle. The breakup is accompanied by a release of the kinetic energy in a tremendous blast, which may create more devastation than a ground impact. This is what happened in the Tunguska explosion.
A more rare type of meteoroid, made mostly of iron, is much more likely to hit the ground and create a crater (or several craters if it breaks into large chunks). Meteorites, properly speaking, are the chunks that fall on the ground. About 5% of meteorites are classified as iron meteorites.
14. High g.
Question:
How much acceleration/gravity can a human withstand?
Answer (Cat S):
The human body can tolerate about 9 G's (approximately 90 m/s2) for brief periods of time. Source: http://www.physics.isu.edu/ keeter/phys211dir/quiz8/pg2ap1.htm
Answer (Glen M):
Fighter jet pilots deal with this problem on a daily basis. Pulling negative G's is especially dangerous, and quickly leads to a ``read-out'' at as few as - 3 G's. Positive G's are dealt with by special pilot suits and diaphragm techniques that allow pilots to endure accelerations of up to 10 G's for short periods of time.
Answer (Chris B):
... Military pilots who fly supersonic jets wear g-suits that somehow keep your blood flowing around your body instead of straight to the back, when going faster than the speed of sound.
Answer (Evan R):
Humans can withstand tons of acceleration, until such a force brings them into contact with an object, such as pavement.
Comment:
It is not speed itself that matters, it's the acceleration. If you fly straight at any constant speed, you do not need a g-suit. However, a change of direction is also an acceleration, and it is a stronger acceleration at higher speed.
More precisely, what counts is the difference between your body's acceleration and the local gravitational acceleration.. You can free-fall safely in a huge g-field (as long as it is fairly uniform, so that the tidal force will not get you). When you stop a free fall, you are accelerating upwards.
15. Bomb the sun
Question:
If a nuclear bomb was sent to the sun, would it have devastating consequences on earth?
Answer (Evan R)
No, unless the sun were to return the favor.
Answer (Chris B):
We would be short one nuclear bomb.
Answer (Ka HJ)
Little or no consequence. The sun has nuclear reactions going on all the time, which produce a lot more heat than our puny bomb.
Answer (Glen M)
The sun is a constant process of nuclear fusion many times more powerful than our nuclear bombs, so the explosion of a bomb on the sun would have little noticeable effect here on earth.
Answer (Cat S):
Because the sun is, in effect, a giant nuclear reactor, sending a skimpy, human-made bomb would have no effect here on earth. It would be a lot of wasted money, though, so perhaps there would be some economically detrimental effects.
Answer (John K)
No, we wouldn'ttt even know, because that is insignificant to the sun.
Answer (Chad D)
The sun is equivalent to millions of fusion bombs exploding every second - one bomb from earth won't make a difference.
Comment:
The sun's output, in watts, is 
A typical fusion bomb
delivers 1 Megaton of TNT = 
joules. Since one watt is
one joule per second, the sun is equivalent to 90 billion fusion bombs a
second. The power that reaches earth from the sun, in watts, is 
. This is a very
small fraction of the total output, but still equivalent to 43 fusion bombs
a second.]
Answer (Sam R)
Nothing. A thermonuclear weapon undergoes the same fusion process as hydrogen nuclei in the sun. A thermonuclear weapon is just like a small portion of the sun. Given that the sun is far greater than a bomb in mass, the energy released by a thermonuclear weapon would be but a pinpoint. The sun in fact has solar flares larger than the earth. Thus the sun would remain unfazed. The question asked what would happen to the earth, however. It should be noted that NASA has had particular difficulty convincing the public that putting a handful of plutonium in a satellite would be safe, let alone a nuclear weapon. The American public has a pronounced fear of a Challenger-esque explosion raining fallout on them. In the event that the launch vehicle exploded, depending on the trigger mechanism of the weapon, it is possible for a high altitude detonation sending fallout throughout the troposphere and the stratosphere.
Comments:
The sun is similar to a huge thermonuclear weapon, but different in more ways than just size. A major difference is that the sun starts off with regular hydrogen (H) and manages to create deuterium first (H+H - D) and some tritium and finally (mostly) Helium, whereas the standard weapon uses, basically, a deuterium-tritium fusion. An unwanted thermonuclear explosion during or after a launch can be made very unlikely. An explosion of the rocket's fuel would not cause an on-board nuclear bomb to go off. However, a thermonuclear weapon contains Uranium-235 and Plutonium, because it uses a fission explosion to ignite fusion. A chemical explosion of the rocket itself (and of the chemical trigger charge in the weapon), could scatter 50 kg (100 lbs) of U-235 and Plutonium in the upper atmosphere. Actually, there has been a major effort to produce ``clean'' thermonuclear weapons by reducing the amount of U and Pu in them; it may now be possible to get by with 10 kg (20 lbs) or less of U-235 and Pu.
16. Vacuum
What is ``empty space''? I know the book describes it, but it did not make sense to me.
Answer:
A region of space is ``empty'' if there are no ``real'' elementary particles in it. There can be ``virtual'' particles in empty space, and that causes confusion. A virtual particle is always part of a particle-antiparticle pair, and its energy is not equal to mc2 (if it is at rest; a more technical restriction applies if it's moving). In classical relativity theory the energy must be equal to mc2 and virtual particles are not allowed, but in the quantum relativistic theory the energy is uncertain (because of Heisenberg's principle) and virtual particles are allowed, although they do not last long. They keep flickering on and off, in what are known as vacuum fluctuations. As a result of quantum fluctuations, all space, including `empty'' space, is full of activity if one looks on a fine enough scale (on a large scale, the fluctuations average out).
17.Faster than light?
Question:
After reading the time travel chapter, I am still not clear - would it be possible to travel faster than the speed of light WITHOUT going into the past? Or are they necessarily the same thing - i.e., you aren't really travelling extremely fast, you are just getting there before the forward motion of time would allow.
Answer (Marcus E)
Hawking says that it is impossible to travel that fast, period. (Comment: stated more fully, it is impossible to move faster than the local speed of light, i.e., faster than light travels in a vacuum at the point where you are.)
18. Anthropic principles.
Question:
I am confused by the difference between the strong and weak anthropic principles. Why does Hawking object to the strong principle and not to the weak one?
Answer (Marcus E)
The anthropic principle, in general, is the idea that the universe is defined the way that it is because we exist. The weak principle states that, in a large universe, only certain localized areas would develop life capable of observing the universe itself. Obviously, we live in one such area.
The strong principle takes various forms. In one version, it states that there must be a part of the universe where intelligent life can develop, or the whole universe would be pointless. (Comment: If there is a purpose to the universe, one can argue, there must have been a purposeful creator. It is a short step from there to traditional Christian theology.)
It seems that the general question of the anthropic principle is just a bad idea, because it's asking scientists to argue for or against the existence of a god and a divine creation, as well as other incalculable uncertainties. (Comment: Hawking probably agrees with this, although he would not use the same words.)
Answer (Heather J):
Strong Anthropic Principle: ``The Universe must have those properties which allow life to develop within it at some stage in its history'' (page 21, The Anthropic Cosmological Principle). Variations on it and (arguable) consequences:
(a) There is only one universe, that was ``designed'' for our observation and survival
(b) Without observers the universe would not come into existence
(c) Many other different universes perforce exist in order for our universe to exist
(d) Intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out.
I find this subject a very fascinating combination of philosophy and physics.
Comment:
The strong anthropic principle, in some of its many flavors (see above), is often used to argue that the universe is especially designed to produce human beings. Once `design' and `purpose' are brought in, it is a short step (for some) to argue that there is a God with all the attributes of traditional Christian theology. If there is a purpose to the universe, one can argue, there must have been a purposeful creator. On the other hand, the weak principle can easily explain, for instance, why the earth is so well suited for humans: if it weren'tttnt, we would not be around. There is no need to postulate that God made the earth especially for humans. Obviously, this line of thought appeals to those who like to keep God out of ``natural philosophy'', or at least out of its daily business. Hawking is probably one of them.
The prevailing view among scientists was put succinctly, if half-jokingly, by Philip Morrison. He noted that, within the family of the Anthropic Principle (AP), we have the Weak AP (WAP) and the Strong AP (SAP), with variant (b) known as the Participatory AP (PAP) and variant (d) known as the Final AP (FAP). He then proposed that the next in line should be called the Completely Ridiculous Anthropic Principle (CRAP).
19. Time' arrows
What would happen if the thermodynamic and cosmological arrows of time were not pointing in the same direction? not exist in harmony (i.e.: how can they not point in the same direction)? What would the state of the universe be if they were in opposition to one another? If the universe began to contract, would the arrow of time change direction?
Answer (Michelle L)
If the universe began to contract, the cosmological arrow of time would change direction, but the thermodynamic arrow of time would stay in the same direction as it is now, but it would get progressively weaker. They would then be pointing in opposite directions. (Comment: Hawking used to teach that the thermodynamic arrow would also reverse in a contracting universe, but has recanted).
If the thermodynamic arrow reversed, people would be living backwards. You would see broken cups gather themselves off the floor and go on the table. (Comment: You would adjust to this by reckoning time backwards, i.e., you would adjust your psychological arrow to agree with the thermodynamic arrow. See also the answers to question 7.)
20. No boundary
Question
How can space-time be finite but have no boundary? Is that what Hawking means by his ''surface of the earth'' analogy?
Answer (Michelle L)
I picture space-time being finite but having no boundary by analogy with the surface of the Earth. Only, one must imagine that the surface of the Earth is like a balloon, so when one ``puts air into the balloon'' it expands (like the Universe), and when ``air is released'' it contracts.
21. Units of disorder
Question:
How does Hawking determine how many ''units'' of disorder 1000 calories of heat brings to the universe? What are the ``units'' of which he speaks?
Answer:
A unit of ``disorder'' is a unit of energy divided by a unit of temperature, for instance one calorie divided by one degree Kelvin, abbreviated as cal/K. (A degree Kelvin is the same as a degree centigrade, but the Kelvin scale starts from absolute zero). The amount of disorder in a system is called the entropy of the system and is denoted by the letter S. The additional entropy generated in a large body by the inflow of a relatively small amount of heat Q is given by S=Q/T, where T is the absolute temperature of the body that receives the heat. Since the universe is large indeed and is (mostly) at a temperature of 2.57 degrees above absolute zero, we find that 1000 calories bring to it an additional entropy S= (1000/2.57) cal/K = 389 cal/K.
22. Stars and reactors
Question:
Hawking says that ``[a star] is like a controlled hydrogen bomb''. What does he mean?
Answer:
In a way, a star is like a continued explosion of hydrogen bombs, and the analogy with a nuclear reactor is imperfect. The products of the explosions are ``confined'' (by gravitation) rather than ``controlled''. However, stars can and do blow up eventually (supernovae) and reactors can go out of control (Chernobyl).
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