Posts tagged theoretical physics

A Quantum Paradox

The idea that quantum mechanics applies to everything in the universe, even to us humans, can lead to some strange conclusions. Consider this variant of the iconic Schrödinger cat thought experiment that Nobel laureate Eugene P. Wigner came up with in 1961 and David Deutsch of the University of Oxford elaborated on in 1986.

Suppose that a very able experimental physicist, Alice, puts her friend Bob inside a room with a cat, a radioactive atom and cat poison that gets released if the atom decays. The point of having a human there is that we can communicate with him. (Getting answers from cats is not that easy.) As far as Alice is concerned, the atom enters into a state of being both decayed and not decayed, so that the cat is both dead and alive. Bob, however, can directly observe the cat and sees it as one or the other. Alice slips a piece of paper under the door asking Bob whether the cat is in a definite state. He answers, “yes.”

Note that Alice does not ask whether the cat is dead or alive because for her that would force the outcome or, as physicists say, “collapse” the state. She is content observing that her friend sees the cat either alive or dead and does not ask which it is.

Because Alice avoided collapsing the state, quantum theory holds that slipping the paper under the door was a reversible act. She can undo all the steps she took. If the cat was dead, it would now be alive, the poison would be in the bottle, the particle would not have decayed and Bob would have no memory of ever seeing a dead cat.

And yet one trace remains: the piece of paper. Alice can undo the observation in a way that does not also undo the writing on the paper. The paper remains as proof that Bob had observed the cat as definitely alive or dead.

That leads to a startling conclusion. Alice was able to reverse the observation
because, as far as she was concerned, she avoided collapsing the state; to her, Bob was in just as indeterminate a state as the cat. But the friend inside the room thought the state did collapse. That person did see a definite outcome; the paper is proof of it. In this way, the experiment demonstrates two seemingly contradictory principles. Alice thinks that quantum mechanics applies to macroscopic objects: not just cats but also Bobs can be in quantum limbo. Bob thinks that cats are only either dead or alive.

Doing such an experiment with an entire human being would be daunting, but physicists can accomplish much the same with simpler systems. Anton Zeilinger and his colleagues at the University of Vienna take a photon and bounce it off a large mirror. If the photon is reflected, the mirror recoils, but if the photon is transmitted, the mirror stays still. The photon plays the role of the decaying atom; it can exist simultaneously in more than one state. The mirror, made up of billions of atoms, acts as the cat and as Bob. Whether it recoils or not is analogous to whether the cat lives or dies and is seen to live or die by Bob. The process can be reversed by reflecting the photon back at the mirror. On smaller scales, teams led by Rainer Blatt of the University of Innsbruck and by David J. Wineland of the National Institute of Standards and Technology in Boulder, Colo., have reversed the measurement of vibrating ions in an ion trap.

In developing this devious thought experiment, Wigner and Deutsch followed
in the footsteps of Erwin Schrödinger, Albert Einstein and other theorists who
argued that physicists have yet to grasp quantum mechanics in any deep way. For decades most physicists scarcely cared because the foundational issues had no effect on practical applications of the theory. But now that we can perform these experiments for real, the task of understanding quantum mechanics has become all the more urgent. —V.V.

(Source: physics.utoronto.ca)

     You are hovering some planet in a galaxy far far away, uncertain whether it is made of matter or antimatter and hence whether or not it will be safe to land. The planet is inhabited by friendly aliens with whom you have made radio contact. They are very intelligent and understand you, and being advanced, know all about matter and antimatter.

     Naturally, they insist that they are made of matter; after all, it would be surprising if anyone chose to define their own stuff as ‘anti.’ How can we decide if their dictionary and ours coincide? What questions will unambiguously reveal whether they are made of the same stuff as us, or are anti-aliens?

     If matter and antimatter were always perfectly symmetrically counterpoised, there would be no way to settle the issue, other than gambling with a close approach of firing a tiny unmanned probe and seeing what  happens when it hits the atmosphere or anti-atmosphere. However, we know that there is an asymmetry, small but measurable, and that is what the electrically neutral variety of K mesons can reveal. They do so when they decay, producing a pion that is either positively or negatively charged accompanied by an electron or positron respectively. If matter and antimatter were perfect opposites, these two decays would also be precisely matched, the chance of each being the same. In reality, they are slightly different.

     The neutral K and anti-K are welded together in nature in such a way that they sometimes die quickly, but at other times live longer. The two possibilities are quite distinct and are known as the short- and long-lived versions. Each of these shows an asymmetry between matter and antimatter, but it is the long-lived one where the effect is biggest, they decay that leads to a positron being slightly more likely to happen than giving an electron: out of every two-thousand examples, on the average, 1,003 will give a positron and 997 give an electron. Now at last we have something to discuss with the alien.

     First, identify the K. It is no use giving its name, since the alien will certainly call it something else, but we can identify it by something we will agree about: its mass. It weighs in at slightly more than half the mass of a proton or antiproton and there are no other particles than can be confused with it. So tell the alien that we are interested in a particle whose mass is slightly more than half that of the massive particle that exists in the ‘nucleus’ at the center of the alien’s simplest atom, the proton in the hydrogen atom (or antiproton in an atom of antihydrogen.) That identifies the K.

     In addition to the neutral K, with no electric charge, there are also a K-plus and K-minus with positive or negative charge. So we much make sure that the alien and we are talking about the electrically neutral version. We must say that the property that holds the atom together is what we call ‘charge’ and that we are interested in the K that has no charge. The alien will be aware that this neutral K has two forms: one with a short life and one with a relatively long one. It is the latter that we will focus on.

     Now we come to the critical bit. In our world of matter, when the long-lived K decays into a pion and an electron or positron, it is the positron mode that is the most likely. So we ask the alien: ‘Is the lightweight particle that is produced most often in these decays the same as you find in your atoms, or is it the opposite?’ If the alien answers that it is the same, it is a positron, the alien is made of antimatter and we should look but not touch. If the alien replies that it is the opposite, an electron, then we are all made of matter and it is safe to land.

Antimatter, Frank Close

25 July 2011 by Marcus Chown

IT HAS been called the Goldilocks paradox. If the strong nuclear force which glues atomic nuclei together were only a few per cent stronger than it is, stars like the sun would exhaust their hydrogen fuel in less than a second. Our sun would have exploded long ago and there would be no life on Earth. If the weak nuclear force were a few per cent weaker, the heavy elements that make up most of our world wouldn’t be here, and neither would you.

If gravity were a little weaker than it is, it would never have been able to crush the core of the sun sufficiently to ignite the nuclear reactions that create sunlight; a little stronger and, again, the sun would have burned all of its fuel billions of years ago. Once again, we could never have arisen.

Such instances of the fine-tuning of the laws of physics seem to abound. Many of the essential parameters of nature - the strengths of fundamental forces and the masses of fundamental particles - seem fixed at values that are “just right” for life to emerge. A whisker either way and we would not be here. It is as if the universe was made for us.

What are we to make of this? One possibility is that the universe was fine-tuned by a supreme being - God. Although many people like this explanation, scientists see no evidence that a supernatural entity is orchestrating the cosmos. The known laws of physics can explain the existence of the universe that we observe. To paraphrase astronomer Pierre-Simon Laplace when asked by Napoleon why his book Mécanique Céleste did not mention the creator: we have no need of that hypothesis.

Another possibility is that it simply couldn’t be any other way. We find ourselves in a universe ruled by laws compatible with life because, well, how could we not?

This could seem to imply that our existence is an incredible slice of luck - of all the universes that could have existed, we got one capable of supporting intelligent life. But most physicists don’t see it that way.

The most likely explanation for fine-tuning is possibly even more mind-expanding: that our universe is merely one of a vast ensemble of universes, each with different laws of physics. We find ourselves in one with laws suitable for life because, again, how could it be any other way?

The multiverse idea is not without theoretical backing. String theory, our best attempt yet at a theory of everything, predicts at least 10⁵⁰⁰ universes, each with different laws of physics. To put that number into perspective, there are an estimated 10²⁵ grains of sand in the Sahara desert.

Fine-tuned fallacy

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Oddly enough, dark energy — for all the surprise around its discovery — is not an entirely new concept in physics. There is historical background for this idea, and it comes from the preeminent astronomer of the 20^th century, Albert Einstein.

In 1917, Einstein was applying his new theory of general relativity to the structure of space and time. General relativity says that mass affects the shape of space and the flow of time. Gravity results because space is warped by mass. The greater the mass, the greater the warp.

But Einstein, like all scientists at that time, did not know that the universe was expanding. He found that his equations didn’t quite work for a static universe, so he threw in a hypothetical repulsive force that would fix the problem by balancing things out, an extra part that he called the “cosmological constant.”

Then, in the 1920s, astronomer Edwin Hubble, using a type of star called a Cepheid variable as a “standard candle” to measure distances to other galaxies, discovered that the universe was expanding. The idea of the expanding universe revolutionized astronomy. If the universe was expanding, it must at one time have been smaller. That concept led to the Big Bang theory, that the universe began as a tiny point that suddenly and swiftly expanded to create everything we know today.

Once Einstein knew the universe was expanding, he discarded the cosmological constant as an unnecessary fudge factor. He later called it the “biggest blunder of his life,” according to his fellow physicist George Gamow.

Today astronomers refer to one theory of dark energy as Einstein’s cosmological constant. The theory says that dark energy has been steady and constant throughout time and will remain that way.

A second theory, called quintessence, says that dark energy is a new force and will eventually fade away just as it arose.

If the cosmological constant is correct, Einstein will once again have been proven right — about something even he thought was a mistake.

(Source: hubblesite.org)

When the word first got out that the expansion of the universe was accelerating, many astronomers questioned the results. They felt that the observations must be wrong, or the interpretation must be flawed. The whole concept was so difficult to believe because it requires significant changes in our understanding of the way the universe works.

Say you step outside and throw a baseball up into the air. The gravity of Earth begins immediately to act on the baseball, slowing it down even as it rises into the air. The upward speed of the baseball slows until it stops at its peak, then gravity’s pull causes it to drop down at an ever-increasing speed. What you can’t see is that the baseball also has a tiny gravitational pull that acts upon Earth. Gravity always acts to pull matter together.

Now consider a spaceship. If launched with enough speed, a spaceship will escape Earth’s gravity to the extent that it will not fall back to the planet. However, it hasn’t escaped the pull of Earth entirely. Though it travels away, the spaceship will be continuously slowed — just not to the point where it stops.

Competing Models

These same concepts apply to the expansion of space. That expansion was launched in the Big Bang, and ever since then, each bit of matter in the universe has been attracted to every other bit by the force of gravity. This should have been slowing down the expansion.

Before the discovery of dark energy, scientists had two models of how the universe’s expansion would work. In one scenario, there would be enough matter in the universe to slow the expansion to the point where, like the baseball, it would come to a halt and start to retract, everything crashing back together in a “Big Crunch.”

In the other scenario, there would be too little matter to stop the expansion and everything would drift on forever, always slowing and slowing but never stopping — like the spaceship. The galaxies would drift apart from each other until they were out of view. The universe would continue growing larger as countless generations of stars faded and died out. It would end in a vast, dark, and cold state: a “Big Chill,” if you will.

Does the Matter Matter?

By the early 1990s, astronomers had calculated how much mass was in the universe, and decided on the Big Chill as the most likely end of the universe. But then dark energy showed up in our observations.

According to the Big Chill, the universe should be expanding more slowly today than it did in the past, because gravity has had time to work on slowing the universe down over all these billions of years. But astronomers found that the universe is moving faster today than it was a billion years ago, meaning something must be working to speed it up.

This result seems crazy because gravity always pulls and slows — it never pushes. Yet some force appears to be pushing the universe apart. Astronomers, concluding that we just don’t know what this force is, have attributed it to a mysterious dark energy.

The Big Rip

With dark energy, the fate of the universe might go well beyond the Big Chill. In the strangest and most speculative scenario, as the universe expands ever faster, all of gravity’s work will be undone. Clusters of galaxies will disband and separate. Then galaxies themselves will be torn apart. The solar system, stars, planets, and even molecules and atoms could be shredded by the ever-faster expansion. The universe that was born in a violent expansion could end with an even more violent expansion called the Big Rip.

So out of the three scenarios for the fate of the universe — re-collapse to a Big Crunch, expand ever more slowly to a Big Chill, or expand ever faster to a Big Rip — we have managed to narrow the possibilities down somewhat.

Evidence has ruled out the Big Crunch. The Big Chill is probably the least that will happen. Whether or not the universe goes all the way to a Big Rip depends on what dark energy really is, and whether it will stay constant forever or fade away as suddenly as it appears to have arisen. And that we do not yet know.

No matter which scenario is right, the universe still has at least a few tens of billions of years left — which leaves us plenty of time to look for the answers.

(Source: hubblesite.org)

So what is dark energy? Well, the simple answer is that we don’t know. It seems to contradict many of our understandings about the way the universe works.

We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.

Einstein’s famous equation, E = mc², teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy.

Something from Nothing

But energy is supposed to have a source — either matter or radiation. The notion here is that space, even when devoid of all matter and radiation, has a residual energy. That “energy of space,” when considered on a cosmic scale, leads to a force that increases the expansion of the universe.

Perhaps dark energy results from weird behavior on scales smaller than atoms. The physics of the very small, called quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.

It could be that dark energy creates a new, fundamental force in the universe, something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.

Or perhaps the answer lies within another long-standing unsolved problem, how to reconcile the physics of the large with the physics of the very small. Einstein’s theory of gravity, called general relativity, can explain everything from the movements of planets to the physics of black holes, but it simply doesn’t seem to apply on the scale of the particles that make up atoms. To predict how particles will behave, we need the theory of quantum mechanics. Quantum mechanics explains the way particles function, but it simply doesn’t apply on any scale larger than an atom. The elusive solution for combining the two theories might yield a natural explanation for dark energy.

Stranger and Stranger

We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity’s force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.

It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It’s as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we’ve caught sight of the waves, we want to know what this huge, strange, powerful entity really is.

The strangeness of dark energy is thrilling.

It shows scientists that there is a gap in our knowledge that needs to be filled, beckoning the way toward an unexplored realm of physics. We have before us the evidence that the cosmos may be configured vastly differently than we imagine. Dark energy both signals that we still have a great deal to learn, and shows us that we stand poised for another great leap in our understanding of the universe.

(Source: hubblesite.org)