Physics Feed

September 28, 2008

Quantum Suicide and The Large Hadron Collider

Stern-Gerlach Experiment
Stern-Gerlach experiment. Source: Wikipedia Commons. Licensed under GNU Free Documentation License version 1.2.

Quantum mechanics is a theory that describes the behavior of objects at the atomic scale. The effects of quantum mechanics are typically observable only at this small scale, and not at larger ones, except in unusual or contrived situations.

Electron Spin

Electrons have a property called spin that may be measured in relation to an arbitrary axis. The name is somewhat misleading. It's not quite the same concept as a ball rotating around an axis but there are some useful similarities. Since an electron has an electric charge, its spin causes it to interact with a magnetic field, deflecting the electron's path in a manner similar to the way a charged sphere's course would be altered. An electron can have its spin measured by passing it through a magnetic field. If electrons were truly spinning spheres, a beam of electrons would spread out smoothly when passed through a shaped magnetic field since each rotating sphere would take on an arbitrary spin alignment.

However, what is actually observed is amazing and counter-intuitive. The 1922 Stern-Gerlach experiment showed that spin is quantized and only two values are observed - denoted up and down.

Standard Interpretation

In the standard Copenhagen interpretation of Quantum Mechanics, the electron does not have a definite spin until a measurement is made, and the quantum wave function collapses to a definite value. Schrödinger's Cat is a famous thought experiment which was originally conceived by Austrian physicist Erwin Schrödinger as a critique of the Copenhagen interpretation. In a variation of this thought experiment, one imagines that a cat is placed in a box with a flask of poison and a device that can measure electron spin.

If a single electron that is passed through the device is measured with spin up, the flask of poison is released and the cat expires. If the spin is down, the cat survives. There is a 50 percent chance of either outcome. If the box is sealed so that it is impossible to determine the state of the experiment from outside, the cat will exist in a superposition of states to the outside world with equal probability of it being alive and dead. It's not that the cat actually exists in one state or another according to the Copenhagen interpretation. The cat has become entangled in the quantum wave function describing the contents of the box and truly exists in a superposition of both states.

Quantum Suicide; Many Worlds
iStockphoto / Sirin Buse.

Quantum Suicide

However, in the Many-Worlds interpretation of Quantum Mechanics, two different worlds exist - one in which the cat remains alive, and another in which the cat has perished.

A thought experiment called Quantum Suicide has been crafted as a hypothetical test of the Many-Worlds interpretation. In this experiment, an observer takes the place of the cat and the experiment is performed many times. In some worlds, the observer perishes, but his conscious experience continues in the worlds in which he survives. He will never observe his own death. The observer perishes in half of the worlds, but it does not appear that way from his point of view. After repeating the experiment as many times as necessary to satisfy his curiosity, the observer concludes that the Many-Worlds interpretation is correct.

With the Large Hadron Collider shut down for two months due to a malfunction, some have suggested with tongue-in-cheek that the Quantum Suicide experiment is being conducted in real time with our own world. In some parallel universes, the LHC creates stable black holes which destroy the Earth. We only remain conscious to observe this in universes where that doesn't happen. In those universes, events happen that prevent the LHC from creating those kinds of black holes.

While the LHC's troubles are more likely explained by mundane problems, the idea behind the Quantum Suicide thought experiment is still an intriguing one.


September 22, 2008

Searching for the Higgs Boson

Higgs Boson Production
The Higgs Boson may be produced through the decay of two gluons. Source: Wikipedia Commons. Licensed under GNU Free Documentation License, Version 1.2.

The Large Hadron Collider's Search

The Higgs boson is the only particle left that has not yet been observed by experimental research in the Standard Model of particle physics which lists some 40 species of elementary particles. One of the goals of CERN's Large Hadron Collider, situated beneath the border between France and Switzerland, is to search for this particle when it reaches full operation.

The Higgs boson is a component of the proposed Higgs field. Even in completely empty space, the Higgs field has a value that is non-zero. It is theorized that this non-zero value gives mass to other elementary particles that do in fact have mass.

How Does Mass Arise?

But how can one particle give rise to mass in another particle? This would seem at first glance to involve circular reasoning. The Exploratorium gives a great analogy here:

Imagine you're at a Hollywood party. The crowd is rather thick, and evenly distributed around the room, chatting. When the big star arrives, the people nearest the door gather around her. As she moves through the party, she attracts the people closest to her, and those she moves away from return to their other conversations. By gathering a fawning cluster of people around her, she's gained momentum, an indication of mass. She's harder to slow down than she would be without the crowd. Once she's stopped, it's harder to get her going again.

One reason that the Higgs boson has not yet been observed is because of the predicted large amount of energy necessary to create it. Generally, physicists believe that the Higgs boson will have a mass between 114 and 1,000 GeV / c2. The LHC will be able to operate at up to 7,000 GeV  / c2 on two beams.


September 19, 2008

Puzzling Discrepancies in Space Probe Trajectories

Gravity Waves
© / Karl Dolenc

The Pioneer 10 and 11 space probes were launched in 1972 and 1973 respectively with missions to survey Jupiter and the outer solar system. At the end of their successful missions, both probes had trajectories which left them on hyperbolic courses to exit the solar system forever.

After their primary missions were completed, NASA continued to monitor the probes until they were no longer able to transmit signals. The last time Pioneer 11 was heard from was in November 1995, and Pioneer 10's signal has not been detected since January 2003.

Unexplained Acceleration

Close examination of data regarding the paths of the spacecraft has shown that there is a very small acceleration towards the sun that cannot be accounted for after every known force is taken into account. A large number of possible effects have generally been ruled out including fuel leakage, the solar wind, and navigational errors.

The Pioneer probes are not the only probes that have experienced unexplained changes in acceleration. A number of more recent missions have also experienced small changes in velocity as they passed close to the Earth for gravitational-assist maneuvers:

Possible Causes

The cause of the effect is still an open question and their is not enough data to resolve the question conclusively. A number of potential causes have been suggested, including:

A mission to specifically to study the effect has been proposed, but has not been approved. Scientists will be especially interested in the third flyby of Earth by the Rosetta mission which will occur on November 13, 2009.


September 01, 2008

LHC Nearing Full Operation, May Produce Black Holes

CMS Higgs Event
Source: CMS Media/CERN

The Large Hadron Collider (LHC) at CERN continues on target to ramp up to full operation after a second and final test of the beam synchronization systems was completed on Friday, August 22, 2008.

Scientists are excited about the possibility that the collider will produce unstable, short-lived black holes or even dark matter. Physicists Steven Giddings and Michelangelo Mangano have ruled out the potential for dangerous, stable black holes to be created in a paper entitled Astrophysical implications of hypothetical stable TeV-scale black holes published in the journal Physical Review D on August 15, 2008.

The first attempt to circulate a beam of particles is set for September 10, 2008 and will be webcast live.

July 22, 2008

Speculations on Gödel's Incompleteness Theorems, the Halting Problem, and The Simulation Argument

Fermi Paradox
© / David Marchal

Kurt Gödel

Kurt Gödel was a mathematician whose 1931 seminal work was the proof that all formal mathematical systems of sufficient complexity are necessarily incomplete. In other words, there are mathematical statements within these systems that are true, but which can never be proven within the system itself. Gödel proved this by showing that statements can be created which state that they can never be proven within the formal system. While these statements are in fact true, they can't be proven so - if they could, by definition they would not be true!

An analogy is the sentence "This sentence is false". This sentence cannot be a true statement, because if it were, we would have to believe what it states - that it is false. Similarly, it cannot be a false statement, because if it were, it would be true.

Nick Bostrom

Nick Bostrom is the Director of the Future of Humanity Institute at Oxford who has authored a Simulation Argument. Essentially, it states that:

Unless one of the following statements is true,

  • The human species goes extinct before reaching a posthuman stage.
  • Humans never become capable of running (or desire to run) computer simulations of their history.

then we are most likely living in a simulation now.

Turing Machines and the Halting Problem

The halting problem is a question in computability theory which asks if an algorithm can be found that decides whether a program (a Turning machine) will finish, or run forever, once given a description of such a program and a finite amount of input. Alan Turing proved in 1936 that a general algorithm to solve the halting problem for all possible program-input pairs cannot exist. The ideas within Gödel's incompleteness theorems are quite similar to those presented by the halting problem.


Suppose that the universe that we live in is in fact a simulation, and it is being simulated by the equivalent of a Turing Machine. What are the ramifications of the halting problem and Gödel's incompleteness theorems in this regard? The "Scientific and technological approaches" section of the Simulated Reality entry in Wikipedia has some interesting speculations on software glitches, Easter Eggs, limitations on processing power, and the Heisenberg uncertainty principle.

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February 13, 2008

Boltzmann Brain Paradox

Digital Brain
© Sebastian Kaulitzk

Random Fluctuation Created Universe

Ludwig Eduard Boltzmann was an Austrian physicist who made important contributions to the area of statistical thermodynamics. He lived in the last half of the 19th century and proposed that the low-entropy (high order) universe that we live in is the result of a random fluctuation in a larger, higher entropy (lower order) metaverse.

Quantum Fluctuations

Although Boltzmann's proposal was made in advance of quantum mechanics, his idea is similar to modern day theories that the universe arose from a quantum vacuum fluctuation. Quantum mechanics predicts that particles can spontaneously arise from the vacuum if they are short-lived. Even in a perfect vacuum, pairs of particles and anti-particles are constantly being created and destroyed. This is possible because the total energy of the particle anti-particle pairs is zero.

In fact, the total energy of the universe appears to be zero [Stephen Hawking, A Brief History of Time, chapter 8]. Particles have positive energy, and the negative energy represented by the gravitational field of the entire universe appears to be exactly enough to cancel out the positive energy of the particles.


This idea leads to the Boltzmann Brain Paradox. In a metaverse that is larger than ours, random fluctuations of the size to create a universe such as our own will happen. Due to the size and number of particles in such a universe, these fluctuations will be exceedingly rare. The anthropic principal - the fact universes will only be observed when they are hospitable to observers - makes the amount of time between such fluctuations meaningless. These fluctuations could be happening every quadrillion years, or once every googolplex number of years. Fluctuations of a much smaller magnitude that simply create one fully formed brain for a brief amount of time should be happening with enormously higher frequency than universe-creating fluctuations. Such brains would be the smallest possible creations that would give rise to a sentient observer and are called Boltzmann Brains. The fact that such brains do not appear to exist is called the Boltzmann Brain Paradox.

There are a number of ways out of this paradox. One of the base assumptions could be false. Perhaps there is no metaverse or such quantum fluctuations do not happen on large scales.

Or, it possible that the concept of the Boltzmann Brain is true and you are the only sentient observer in the universe right now, complete with false memories of a life which did not exist. False inputs to your brain only make it appear that there are other observers with you. If true, it's possible that you will cease to exist in just a ...

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January 31, 2008

CERN's LHC particle accelerator to begin operating in May, 2008

Update September 1, 2008: The LHC continues to ramp up to full production and the first attempt to circulate a beam of particles on September 10, 2008 will be broadcast live. See the full update here.

Update August 7, 2008: The LHC is scheduled to begin operation on September 10, 2008 and ramp up to full production in early 2009.


CERN, the European Organization for Nuclear Research, is nearing completion of the world's largest and most complex scientific instrument. The final element in the Large Hadron Collider (LHC) was lowered in place on January 22, 2008. The LHC is a particle accelerator which will be used to accelerate tiny particles to extraordinarily high velocities to observe their resulting transformation in collisions with one another. The facility is scheduled to begin operation in May, 2008. There have been a number of news articles in the past few years that have speculated on remotely possible catastrophic risks posed by the LHC.

One speculation is that the LHC will create miniature black holes. If this happened, standard physics theory expects that these black holes would evaporate very quickly due to Hawking Radiation. However, some have suggested a remote possibility that these miniature black holes would not evaporate, but would grow to consume the Earth within a short period of time.

Another highly unlikely possibility is that strangelets could be produced. Strangelets are hypothetical objects that are comprised of roughly equal numbers of up, down, and strange quarks. The concern is that these objects would begin to change other matter that they come in contact with into strange matter, resulting in a runaway process that converts the entire Earth to strange matter.

The consensus within the physics community is that the operation of the LHC is safe. Studies have been conducted which have analyzed the risks and concluded that these catastrophic scenarios are not credible. The official position of CERN is that experiments in particle accelerators are completely safe and the risks have already been adequately analyzed and dismissed in previous studies. A small minority, however, disagree. James Blodgett noted that Dr. Walter L. Wagner is so concerned about the existential risks of experiments to be conducted at the LHC that he is proceeding with an effort to initiate legal action against CERN to force it to conduct additional safety studies.

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