Some of the questions below have been slightly altered, either for the sake of clarity or in order to combine related questions. Since many people asked similar questions, we have not identified each source; any list of names would be incomplete. In some cases, several related questions have been grouped together and answered simultaneously. Rich Talcott, Associate Editor at ASTRONOMY magazine, who authored the Program Essays in this Web companion piece to the PBS series, has furnished several answers for us. Other explanations come from Dr. Michio Kaku, Henry Semat Profesor of Theoretical Physics at the City University of New York, who also contributed two essays to Unsolved Mysteries.


The current universe is an ever-expanding one. But, if we shrink or rewind the expanding sphere, should we not expect to find out the approximate location where the Big Bang begins?

Into what does the universe expand? Is there empty space that is being "filled"?

Is it possible to speculate or to speak intelligently about what might have existed or happened before the Big Bang?

If everything originated from a single point, why isn't the universe like a balloon, with everything on the outer boundary and nothing in the center?

Is space something in which the universe existed prior to the Big Bang and subsequently expanded in? Or, was space a product of the bang and if so, "where" was the universe prior to the bang?

If the universe is not bounded by the beginning and end of time, wouldn't universal expansion in four dimensions follow? That is, if the No-Boundary Universe can be represented by a sphere, shouldn't the sphere expand in all directions?

Is it possible that an infinite number of other universes exists beyond the range of distance that can be observed from Earth?

I've heard of references to "multi-universes" when discussing theories of the universe. How is "another universe" defined, i.e. where does "our" universe end and "another" begin? In addition, if we are to define occupied space as "the universe," how can there be "another universe"?

Assuming the Big Crunch theory is right, is there any validity to the idea that once the crunch has occurred that the present universe has become the starting point for a succeeding singularity, which will give rise to another Big Bang, and the entire will repeat itself endlessly?

How can theory predict precise details about the Big Bang such as knowing that by the end of the first second of time, the building blocks of matter had formed, and after three minutes, helium and deuterium had formed?

If I remember correctly, it is said that galaxies are moving away from each other at different speeds. Why not at the same speed?

If everything originated from a single point, why are galaxies colliding instead of speeding away from that point at equal speed?


Imagine a child blowing up a balloon. Imagine that there are dots painted on the balloon. Notice that all the dots are moving away from each other. The farther any two dots are, the faster they are moving apart.

Now imagine there is an ant living on the balloon. To the ant, the balloon is infinite in two dimensions. The ant, walking on the balloon, could go an infinite distance around the balloon and never reach "the end of the balloon." To an ant, the "universe" would be a two-dimensional, expanding surface, such that the farther the dots are, the faster they move.

If you were to ask the ant, "What is the universe expanding into?," the ant would reply that the question has no meaning. The ant can only move on the surface of the balloon, yet the expansion of the balloon lies in the third dimension, in hyperspace, which is beyond the understanding of the ant. All that the ant understands is that the space between dots is expanding. But it cannot understand "into what is it expanding," since that requires knowledge of the third dimension, or hyperspace, which is beyond the ant's comprehension.

Also, if you ask the ant, "From where did the balloon expand?," the question would have no meaning. The expansion point lies at the center of the balloon, which is off the "universe" of the ant. Thus, the balloon's Big Bang also lies in hyperspace, beyond the understanding of the ant.

To us, however, all these answers are trivial. We live in hyperspace (the third dimension) so we can see that the balloon is finite and is expanding in the third dimension, and that the balloon's Big Bang lies in the center, also in the third dimension.

Likewise, there may be other balloons floating in hyperspace. The ant, which has difficulty understanding its own balloon, would have an even greater problem understanding the fact that there might be other balloons, with other ants on them.

Similarly, we are like the ant, except that our universe appears to be infinite in three dimensions. We can go an infinite distance in any direction, and never reach the "end of the universe."

Likewise, the space between our galaxies is expanding, such that the farther a galaxy is, the faster it is going (this is Hubble's Law). (However, there are also random motions, so galaxies can sometimes collide. For example, our own galaxy may one day be gobbled up by the Andromeda Galaxy.)

But the question, "Into what is the universe expanding?" makes no sense to our three-dimensional brains. The location of the Big Bang is in hyperspace.

(If we try to retrace the early history of the universe, we still cannot locate the Big Bang. If we go back 15 billion or so years, the universe might have been as big as, say, a bowling ball. The entire universe, with all its space and matter, was only that big. But nowhere on the bowling ball was the Big Bang.)

Today, cosmologists are grappling with the question, "What happened before the Big Bang?" Einstein's equations break down at that point, so we need a theory which combines the quantum theory and general relativity (the unified field theory). So far, the only candidate for such a fabled theory is the 10-dimensional superstring theory.

To quantize the universe, let us first consider an electron. We know from chemistry class that the electron can exist simultaneously in infinitely many orbitals surrounding the nucleus. (Because of the Uncertainty Principle, we can never know for sure precisely which state the electron is in, until an observation is made.) These are "parallel electrons." This strange fact about electrons has been verified thousands of times in the laboratory.

Now if we quantize the universe, we must treat it like an electron. Because of Uncertainty, this means that the universe must also exist simultaneously in an infinite number of states. These are parallel universes.

Imagine boiling water (a quantum mechanical phenomenon). Out of nothing, bubbles form and then quickly expand. Similarly, the leading theory among cosmologists today is the multiverse theory, which states that quantum universes are constantly being created out of Nothing. Many of them are probably short-lived; they have a Big Bang, but then rapidly have a Big Crunch and disappear back into Nothing.

(This does not violate the conservation of matter and energy; the matter of the universe has positive energy, but the gravitational field has negative energy, such that the total energy for a closed universe is zero, so it takes zero energy to create a closed universe.)

This means that Big Bangs are probably happening all the time, with entire bubble/universes springing out of the vacuum. However, life probably does not exist in most of these universes. Protons need to be stable for several billion years in order to create DNA (or at least some other auto-catalytic, self-replicating form of stable matter). So many of these other universes in the multiverse are probably lifeless, consisting of, say, a sea of electrons, neutrinos, and photons.

Our universe is probably one of the few in which the expansion is so great that the universe lives for many billions of years, enough for stable matter to form.

This may ultimately explain the Anthropic Principle: the puzzle that the physical constants of our universe seem "fine-tuned" to allow for intelligent life to form. Some have speculated that it was no accident that the physical constants of the universe are precisely those which allow for life to form. If the constants were a bit different, then deuterium and the higher elements would never have formed, and hence DNA could not exist.

However, this multiverse idea argues against that. It says that there are indeed an infinite number of dead universes, and our universe just happens accidentally to be one in which the constants of the universe accidentally came out consistent with life, so we are here to debate the question in the first place.

Can the multiverse theory be tested? Its critics say no, since a real test involves re-creating the Big Bang, which is impossible.

However, a new generation of satellites will soon make precise measurements of the microwave background radiation. Very small perturbations in the smooth "echo of the Big Bang" may possibly prove some version of the "inflationary universe theory," which in turn is nicely explained by the multiverse idea.

Similarly, many physicists (myself included) believe that we will one day solve the superstring theory, in which case we will be able to make precise statements about what happened before the Big Bang.

So until then, the multiverse theory is just that: a theory. However, it is a theory which has generated much excitement and a rash of papers in theoretical physics journals. I, for one, believe that we will one day prove the theory.  — M.K.


If black holes devour their neighboring matter, won't this lead to ever-larger black holes which will eventually swallow ALL matter, with a final merging of all the black holes into the final singularity, with repetition of Big Bang etc.?

Since black holes "swallow" all matter near them, doesn't it follow that at some point in the distant future they will consume all matter in the universe?

If a black hole is a "gateway" to another universe, wouldn't there be foreign (other universe) black holes poking into OUR universe, and if so, how should they be recognizable as such?


About a dozen black holes have now been seen in outer space, such as M-87 and NGC-4258. They are huge galactic black holes, with swirling masses of gas traveling at about a million miles per hour, and are tens of millions of light years away. They have been seen by the Hubble Space Telescope and the Very Large Array Radio Telescope. As expected, the black holes eat up enormous quantities of stellar material and gas.

However, they will not eat up the universe. This is because all of them are widely separated from each other, so the empty vacuum of space prevents them from swallowing up all matter.

Also, black holes are expected to slowly radiate over time. This is because, by the quantum theory, particles should be able to "tunnel" through the huge gravity fields, which are powerful enough to create anti-matter/matter out of the vacuum. This is called Hawking radiation. It means that black holes, over billions of years, will eventually evaporate.

Originally, it was thought that anyone falling into a black hole would die horribly, compressed down to a tiny point. Now, physicists are not so sure. In 1963, mathematician Roy Kerr discovered a new solution of Einstein's equations in which the black hole is spinning and hence collapses into a spinning ring, rather than a dot. Miraculously, anyone falling though the ring would not experience infinite gravity, but might actually fall right through (like in Alice's Looking Glass) and live. The frame of the Looking Glass corresponds to the black hole (wormhole).

Since then, hundreds of other solutions to Einstein's equations have been discovered which, in principle, may allow one to fall through the wormhole and enter a parallel universe (or a distant point in the same universe).

There is much debate about this solution. Some think that it might be unstable, such that the wormhole might close as you enter it. Others think that quantum radiation effects might become infinite as you enter the wormhole. Others say that "white holes" should exist somewhere, which correspond to the other side of a wormhole. (White holes, which should spew enormous amounts of matter into space, have never been seen.)

My personal point of view is that all calculations showing the wormhole to be unstable are suspect. This is because these calculations use a simple (and therefore incorrect) version of Einstein's theory coupled with the quantum theory.

A real calculation of the stability of a wormhole will have to wait until we have a full-blown quantum theory of gravity (such as superstring theory, which is still too difficult to solve this question). Until then, it's an open question whether we can journey through the Looking Glass.  — M.K.


Part 4 of STEPHEN HAWKING'S UNIVERSE was puzzling. Everyone seems to be completely convinced of the existence of "dark matter" because of someone's measurement of the speed of individual stars in our galaxy. Yet all attempts to find or mathematically confirm its existence proved futile. Now the search is on for a particle that is not known to exist but is present in sufficient quantity to confirm the measurements.

As Karl Popper said, "We hate the very idea that we may be mistaken, so we cling dogmatically to our conjectures." Is it possible that all these measurements of speed (rate of expanding universe, velocity of stars in our galaxy, etc.) are based on false assumptions and, thus, leading the scientists astray?


It is always possible for a theory to be wrong. However, this means that we must come up with a better one, rather than just complaining!

For example, there are literally hundreds of data points which fit the Big Bang theory precisely, so we have confidence in the theory (e.g. nucleosynthesis of elements, red shifts of galaxies, microwave background radiation).

It's a free country, so anyone can voice their aesthetic displeasure at the Big Bang theory, but not everyone can come up with a rival theory which explains these hundreds of data points! It's easy to criticize: it's much, much harder to come up with a better theory. Similarly there are experimental reasons for believing that dark matter exists, although the experiments are not as convincing as the Big Bang theory. Not only is dark matter needed to explain why galaxies do not fly apart, but dark matter can be indirectly measured (e.g. by analyzing how it distorts light and other things in its path). New generations of satellites will be able to calculate the amount of dark matter with much greater accuracy.

However, anyone who disagrees with the dark matter theory should come up with a rival theory which can explain the hundreds of data points describing galactic rotations.  — M.K.


If the most distant galaxies we see today appear as young galaxies about 1 billion years old and the entire universe is probably about 15 billion years old, where would they be located in the universe today when you consider that 14 billion years has passed prior to our observations?


Surprisingly, there is no definitive answer to this question—the answer depends on the kind of universe we live in. If light left an object 1 billion years after the Big Bang and is now arriving at Earth 14 billion years later, all you can say with certainty is that the light traveled a distance of 14 billion light-years.

If the universe contained no matter—which would make for a pretty dull universe—you can calculate for this example that the light left the young galaxy when it was 0.93 light-year from the Milky Way. But the real universe is more complicated than that. Because the universe contains matter, gravity has been continually slowing its rate of expansion. Cosmologists have not yet been able to measure how great this effect is, or for that matter how much the universe is curved or whether a cosmological constant exists. These unknowns mean that the two galaxies could have been either closer or farther away than in the no-matter universe.  — R.T.


The residual energy of the Big Bang is supposed to be detectable with a simple radio receiver. If this is true, at what frequency can it be detected?


The cosmic background radiation has a temperature of about 3 kelvins, or 3° Celsius above absolute zero. At this temperature, the radiation’s peak intensity occurs at a wavelength of about 1 millimeter, in the radio region of the electromagnetic spectrum, and can be picked up with a sensitive receiver.  — R.T.


Why was there slightly more matter than antimatter in the early universe and not equal amounts of each?


We can be thankful for the slight excess of matter—without it, we wouldn’t exist. If particles and antiparticles were created in exactly equal numbers, they would have annihilated one another, leaving a universe consisting only of radiation.

Physicists think that in the first fraction of a second after the Big Bang, equal numbers of particles and antiparticles existed. According to some of the grand unified theories that attempt to unite the strong nuclear force with the weak force and electromagnetism, that changed as the universe cooled down. These theories predict that certain massive particles, which formed soon after the Big Bang, decayed in such a way that they created slightly more particles than antiparticles. Although it is still a hypothesis, this scenario does offer a way to give us our universe of matter.  — R.T.


Because looking out in space is looking back in time, how can we map the present universe? For example, a galaxy 1 billion light-years away appears as it did a billion years ago, not as it does today.


That’s true, as it is for all observations in astronomy and cosmology. We see the Moon as it was a little more than a second ago, the Sun as it was about eight minutes ago, and the stars in our galaxy as they were anywhere from several years to tens of thousands of years ago. But most of these objects change so slowly that astronomers have a good idea of the “current” state of our galaxy.

The same holds true for the nearby universe, out to a distance of a couple of billion light-years. For example, we see the enormous Virgo Cluster of galaxies as it was about 50 million years ago. But, except for the occasional collision, galaxies in the cluster change very little in that time span. So cosmologists have a good inventory of the present, nearby universe, and with the use of Hubble’s Law can map the universe pretty well out to a couple of billion light-years. Beyond that, however, other factors come into play.  Until cosmologists get a firm handle on how rapidly the expansion of the universe is slowing down and on how galaxies evolve over many billions of years, they won’t be able to map the entire universe with great accuracy.  — R.T.


If it turns out that the universe is closed and at some point falls back on itself, what can we expect from time? Will it actually reverse itself and the cosmos run backwards like a movie projector?


If the universe is closed, it will eventually stop expanding and begin to contract. As seen by some supernatural, outside observer, the universe would appear to be a movie running in reverse. The galaxies would come together again, slowly at first but with increasing speed as time goes on. Time itself, however, would keep operating as it does and not in reverse. That implies that the observed universe would continue to evolve—stars would continue growing old and dying, and the light from the most distant galaxies would still appear redshifted because we would be seeing them as they were at a time before the contraction started.  — R.T.


I once heard you could visualize the expanding universe as being similar to a balloon being inflated. Would this mean that all, or most, of the galaxies exist on the surface of this imaginary balloon? If so, what occupies the space between the center of the balloon and its surface? And when we look at a distant object, are we looking through the interior of the balloon to the surface on the other side?


An inflating balloon makes a good analogy for thinking about the expanding universe. The thing to keep in mind is that this is strictly a two-dimensional analogy to our three-dimensional universe.

Imagine a species of antlike creatures that exist in just two dimensions (they have length and width but no height). Then further suppose that these creatures live in two-dimensional galaxies that dot the surface of a balloon, which makes up their entire two-dimensional universe. To the ants, the surface of the balloon appears flat because they have no concept of a third dimension. As the balloon inflates, an ant astronomer would see the other galaxies moving away, with the more distant galaxies moving faster because there’s more rubber between them to stretch. Only sophisticated measurements could tell the ant astronomers that their universe was curved, but they wouldn’t be able to visualize the third dimension it curved into. Likewise, humans can’t visualize the fourth dimension that our three-dimensional universe curves into.

In the balloon analogy, every galaxy (indeed the entire universe) lies on the balloon’s surface. That means that the space between the center of the balloon and the surface simply doesn’t belong to the universe, because it doesn’t lie on surface. And when you view a distant object, you are looking at it across the balloon’s surface and not through the interior of the balloon.  — R.T.


How can the Universe be both infinite and expanding? An infinite universe would, by definition, be everywhere, and would have no other place into which it could expand.  Can you give me an answer that will help me grasp this concept? Thank you.

I have read that the universe has no center, but also that the shape of the universe is a hypersphere, which—if I remember my topology correctly—has a center. So what is meant by a center-less universe?

Also, in structures througout the universe, some residual angular momentum seems to cause a disk shape preference over a spherical shape. Is there any reason to believe that the universe itself has angular momentum and thus exhibits a disk shape?


Think of Columbus back in 1492. Back then, in two dimensions, the earth certainly looked flat. But Columbus showed that, sailing in one direction, the earth was actually curved slightly in the third dimension. So, in two dimensions, the earth was infinite, but in three dimensions the earth was finite (i.e. a ball). So an object can be both finite and infinite at the same time (depending on the dimension one is talking about).

But where was the center of the earth? Clearly, the center of the earth was located off the surface of the earth. Thus, the center of the earth was not on the surface of the earth at all, but was located in its interior (i.e., in hyperspace). By pointing a finger north, south, east, or west, one cannot locate the center of the earth. You have to point “down,” in the direction of hyperspace.

Similarly, Einstein's theory said that, locally, our universe appears to be flat and infinite in 3 dimensions. But it’s actually curved slightly in four dimensions. If we “sail” in a starship in one direction, we might eventually wind up where we began. But where is the center of the universe? It is located in hyperspace, off the surface of the universe. Therefore the center of the universe does not exist in our three-dimensional universe at all.

Now assume that our universe is expanding. The center of expansion does not exist in our universe. You cannot point a finger north, east, south, west, up, or down, and point to the center of the expansion. The universe is expanding in hyperspace. What is the universe expanding into? The answer: It is expanding in the fourth dimension, hyperspace, which is not visible, and exists off the surface of our hyper-bubble. In other words, hyperspace does not exist in our universe at all.

(Observationally, our universe does not seem to rotate, it expands. However, there are cosmological models—proposed by Kurt Gödel—in which the universe does rotate, and in which time travel seems to be possible. Einstein was disturbed by these rotating solutions of Gödel, but, in his memoirs, Einstein confidently ruled out such time travel solutions by stating that the universe expands, but does not rotate.)

There is still debate among cosmologists whether our universe is a hyper-bubble or if it is truly infinite even in the fourth dimension. But the fact that our universe is expanding and that space is curved has been verified by a number of experiments, both on the earth and in space.   — M.K.


Paradox 1: If someone in our future went back in time to change an event or thing, the change would not be in our known time but in the time that was to come after the change. So if I have three brothers and I am sitting with one and the other goes back in time and kills the one I am sitting with, wouldn’t he only be dead in the time in which he was killed and still be sitting next to me in our time?

Paradox 2: If I go back in time and meet myself at an earlier age, say at 15 years old (I am now 67), and I happen to select ahead of travelling there what day and hour it would be, and what was taking place at that time in the past, and I go back there to meet myself and talk to myself knowing full well that at that time a 67-year-old version of myself never showed up . . . then how could I think that I could actually be there since that moment never existed in the past? Once I did arrive there, then it wouldn’t be the “past” at all because I would be there saying things to myself and being in an environment where a 67-year-old version of myself had never been. I think that time is forever locked and the past (or the future for that matter) can never be reached because going there would alter what really took place, and then it would no longer be the actual “past.”


Time travel is still highly speculative, but a growing number of theoretical physicists now believe that time travel is probably possible, although not practical—it would require truly advanced technology beyond our means.

Time travel paradoxes can be resolved if one combines Einstein’s theory with the quantum theory. Einstein thought that time could be viewed as a river that meanders its way across the heavens; time slows down, and speeds up with passing stars and galaxies. The new twist is that we now believe that the river of time can have whirlpools and even fork into two rivers.

If time has a whirlpool, it simply means that time repeats itself and one fulfills the past. Thus, if you meet yourself as a kid, then (thinking back), you recall that an elderly person came up to you years ago and claimed that he was your future self. Thus, there are no real paradoxes here. (Unless you gave yourself the secret of time travel; then one must wonder: Where did the secret of time travel really come from?)

But if time forks, then one can actually “change” the past, i.e., change the past of another quantum universe. If you shoot your parents before you are born, then the universe splits in half, and you have killed people living in another quantum universe, so your parents in your universe indeed did get married, and had a child which eventually became you. In other words, you really cannot change your own past. You do not suddenly disappear if you change the past.

Physicists call the river of time a “time line” or “world line.” The time line never stops or vanishes. It may bend into a circle, or may fork, but it cannot suddenly disappear. (Even when we die, the time line of our atoms and molecules keeps right on going.) Thus, even in time travel scenarios, we do not disappear when we change the past.

For a much more detailed explanation of these and other cosmological issues, I suggest you see some of my books, HYPERSPACE, and VISIONS.   — M.K.


Why, during the formation of a black hole, does the star have to be perfectly round? Has this been proven, or is it just a theory?


Actually, a star doesn’t have to be a perfect sphere to become a black hole (which is fortunate for those of us who like the idea of black holes, since few, if any, objects in the universe are PERFECT spheres). According to general relativity, a black hole that doesn’t rotate must be perfectly spherical. This led many theorists to think that a black hole could arise only from the collapse of a spherical star. However, Roger Penrose and John Wheeler were able to show that a non-rotating star of any shape could collapse to form a spherical black hole.

The same basic physics holds for a rotating black hole, which differs from its spherical cousin in that it bulges outward near its equator (the faster it rotates, the bigger the bulge). In the early 1970s, physicists Brandon Carter, Stephen Hawking, and David Robinson proved that a rotating object of any shape would collapse into this kind of black hole. So the case has been proven theoretically—a black hole can form from the collapse of any object, regardless of its shape.   — R.T.


What is hyperspace?


Hyperspace simply refers to any space that consists of more than three dimensions. The 10-dimensional space-time that many cosmologists use to describe the universe in its infancy is one example of a hyperspace.   — R.T.


According to Hubble’s Law, the galaxies are receding from one another. If that’s true, how can two galaxies collide with one another? (The Milky Way is supposed to collide with Andromeda Galaxy; and the Hubble Space Telescope recently took pictures of two galaxies colliding.)


Gravity is both strong and pervasive. Shortly after the birth of the universe, pockets of slightly higher density formed. The slightly greater gravitational pull of these regions brought in still more matter, creating the seeds for the later development of galaxy clusters. It’s these galaxy clusters that obey Hubble’s Law, not the individual galaxies within them.

Each galaxy group or cluster contains enough material to gravitationally hold on to its individual galaxies. (In the same way, the individual stars in our galaxy are bound to the galaxy as a whole.) These galaxies move about in basically random orbits, and occasionally they collide. When they do, any gas in the galaxies becomes highly compressed, setting off a burst of star formation that can be seen across much of the universe.   — R.T.


What would an antimatter universe be like?


Surprising as it may seem, a universe made entirely of antimatter would appear no different from our universe of matter. That’s because gravity and the other forces in nature act the same on antimatter as they do on matter—for instance, an antimatter Sun would keep an anti-Earth in the exact same orbit as we have in the solar system. And an atom of antimatter (consisting of positrons, or antielectrons, in orbit around antiprotons), would produce light in the same way as matter and with exactly the same properties because a photon of light is its own antiparticle.   — R.T.


How do we know that galaxies with higher velocities are farther away? Isn’t it possible that a fast-moving galaxy could be closer to us than a slower moving galaxy?


We can accurately measure the speed at which a galaxy moves away by observing how much the lines in its spectrum are shifted toward the red. Measuring the distance to a galaxy is more difficult, but astronomers have nevertheless found several methods for gauging how far away galaxies are. The first step is to look for individual stars in the galaxy and compare them with similar stars in our own galaxy. By measuring how much fainter the more distant star appears, astronomers can calculate how much farther away it must be.

In a similar way, the brightnesses of whole clusters of stars or large gas clouds in another galaxy can be compared to those in our own to approximate the galaxy’s distance. And once a galaxy’s distance is known, that galaxy can be compared to similar, fainter galaxies that must lie even farther away. By building up this “distance ladder,” astronomers have gained a good measure of the universe. And when they compare the distances to these galaxies with the galaxies’ speed away from us, they invariably find that the more distant one moves away from us faster.   — R.T.


If Earth is over four billion years old, how is it possible that most of the radioactive elements are still here when many of them have half-lives of only thousands of years?


At first this sounds highly improbable (if not impossible), but there’s actually a good reason why radioactive elements with short half-lives are still around: The longer-lived elements eventually decay into the shorter-lived ones. Uranium and radium (the element discovered by Marie and Pierre Curie) make a good example. The most common form of uranium (uranium-238) has a long half-life of 4.5 billion years, so approximately half of the uranium incorporated into the early Earth still exists. As the uranium slowly decays, one of its so-called “daughter” products is radium-226, with a half-life of just 1,620 years. So even though atoms of radium don’t hang around very long, uranium continually resupplies the element. The same holds true for other long-lived radioactive elements like thorium and potassium and their short-lived daughters.   — R.T.

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