Tag Archives: time travel paradoxes

Diagram of a double slit experiment setup showing a monochromatic light source, double slit, and interference pattern on a screen.

The Classic Double Slit Experiment Is a 100 Year Old Time Travel Paradox that Continues to Baffles Modern Science

Categories, Time Travel, , , ,

First, let’s define a time travel paradox. It is an occurrence that apparently violates some aspect of causality typically associated with time travel. Many science students are introduced to the oddities of the double slit experiment in an advanced high school science class or in an entry level college science class. The double slit experiment is a paradox that has to do with the the future changing the present or the past. The effect has been known for well over a hundred years. It continues to this day to baffle science.

There are numerous versions of the double-slit experiment. In its classic version, a coherent light source, for example a laser, illuminates a thin plate containing two open parallel slits. The light passing through the slits causes a series of light and dark bands on a screen behind the thin plate. The brightest bands are at the center, and the bands become dimmer the farther they are from the center. See the figure below:

3-17-2014 3-08-41 AM Double Slit Fig 1 small

The series of light and dark bands on the screen would not occur if light were only a particle. If light consisted of only particles, we would expect to see only two slits of light on the screen, and the two slits of light would replicate the slits in the thin plate. Instead, we see a series of light and dark patterns, with the brightest band of light in the center, and tapering to the dimmest bands of light at either side of the center. This is an interference pattern and suggests that light exhibits the properties of a wave. We know from other experiments—for example, the photoelectric effect (see glossary), which I discussed in my first book, Unraveling the Universe’s Mysteries—that light also exhibits the properties of a particle. Thus, light exhibits both particle- and wavelike properties. This is termed the dual nature of light. This portion of the double-slit experiment simply exhibits the wave nature of light. Perhaps a number of readers have seen this experiment firsthand in a high school science class.

The above double-slit experiment demonstrates only one element of the paradoxical nature of light, the wave properties. The next part of the double-slit experiment continues to puzzle scientists. There are five aspects to the next part.

  1. Both individual photons of light and individual atoms have been projected at the slits one at a time. This means that one photon or one atom is projected, like a bullet from a gun, toward the slits. Surely, our judgment would suggest that we would only get two slits of light or atoms at the screen behind the slits. However, we still get an interference pattern, a series of light and dark lines, similar to the interference pattern described above. Two inferences are possible:
    1. The individual photon light acted as a wave and went through both slits, interfering with itself to cause an interference pattern.
    2. Atoms also exhibit a wave-particle duality, similar to light, and act similarly to the behavior of an individual photon light described (in part a) above.
  2. Scientists have placed detectors in close proximity to the screen to observe what is happening, and they find something even stranger occurs. The interference pattern disappears, and only two slits of light or atoms appear on the screen. What causes this? Quantum physicists argue that as soon as we attempt to observe the wavefunction of the photon or atom, it collapses. Please note, in quantum mechanics, the wavefunction describes the propagation of the wave associated with any particle or group of particles. When the wavefunction collapses, the photon acts only as a particle.
  3. If the detector (in number 2 immediately above) stays in place but is turned off (i.e., no observation or recording of data occurs), the interference pattern returns and is observed on the screen. We have no way of explaining how the photons or atoms know the detector is off, but somehow they know. This is part of the puzzling aspect of the double-slit experiment. This also appears to support the arguments of quantum physicists, namely, that observing the wavefunction will cause it to collapse.
  4. The quantum eraser experiment—Quantum physicists argue the double-slit experiment demonstrates another unusual property of quantum mechanics, namely, an effect termed the quantum eraser experiment. Essentially, it has two parts:
    1. Detectors record the path of a photon regarding which slit it goes through. As described above, the act of measuring “which path” destroys the interference pattern.
    2. If the “which path” information is erased, the interference pattern returns. It does not matter in which order the “which path” information is erased. It can be erased before or after the detection of the photons.

This appears to support the wavefunction collapse theory, namely, observing the photon causes its wavefunction to collapse and assume a single value.

  1. If the detector replaces the screen and only views the atoms or photons after they have passed through the slits, once again, the interference pattern vanishes and we get only two slits of light or atoms. How can we explain this? In 1978, American theoretical physicist John Wheeler (1911–2008) proposed that observing the photon or atom after it passes through the slit would ultimately determine if the photon or atom acts like a wave or particle. If you attempt to observe the photon or atom, or in any way collect data regarding either one’s behavior, the interference pattern vanishes, and you only get two slits of photons or atoms. In 1984, Carroll Alley, Oleg Jakubowicz, and William Wickes proved this experimentally at the University of Maryland. This is the “delayed-choice experiment.” Somehow, in measuring the future state of the photon, the results were able to influence their behavior at the slits. In effect, we are twisting the arrow of time, causing the future to influence the past. Numerous additional experiments confirm this result.

Let us pause here and be perfectly clear. Measuring the future state of the photon after it has gone through the slits causes the interference pattern to vanish. Somehow, a measurement in the future is able to reach back into the past and cause the photons to behave differently. In this case, the measurement of the photon causes its wave nature to vanish (i.e., collapse) even after it has gone through the slit. The photon now acts like a particle, not a wave. This paradox is clear evidence that a future action can reach back and change the past.

To date, no quantum mechanical or other explanation has gained widespread acceptance in the scientific community. We are dealing with a time travel paradox that illustrates reverse causality (i.e., effect precedes cause), where the effect of measuring a photon affects its past behavior. This simple high-school-level experiment continues to baffle modern science. Although quantum physicists explain it as wavefunction collapse, the explanation tends not to satisfy many in the scientific community. Irrefutably, the delayed-choice experiments suggest the arrow of time is reversible and the future can influence the past.

Source: How to Time Travel (2013), Louis A. Del Monte

Multiple overlapping clock faces with various times, creating a surreal and abstract time concept in blue tones.

Do Time Travel Paradoxes Negate the Possibility of Time Travel?

Categories, Time Travel, , ,

Do time travel paradoxes spell doom to time travel? The short answer is no. Many in the scientific community do not think time travel paradoxes present an insurmountable barrier to time travel. Many physicists have suggested solutions to time travel paradoxes. In fact, discussing them all would result in a book. I will discuss the major ones. For the sake of convenience, I have divided them into four categories:

  1. Multiverse hypothesis—The multiverse hypothesis argues that time travel paradoxes are real, but they lead to alternate realities. The most famous theory in this category is American physicist Hugh Everett’s many-worlds interpretation (MWI) of quantum mechanics. According to Everett (1930–1982), certain observations in reality are not predictable absolutely by quantum mechanics. Instead, there is a range of possible observations associated with physical phenomena, and each is associated with a different probability. Everett’s interpretation is that each possible observation corresponds to a different universe, hence the name “many-worlds.”  Let us consider a simple example. If you toss a coin in the air, it can come down heads or tails. The probability of getting heads is equal to the probability of getting tails. If you toss the coin, and it comes down heads, then there is another you, in another universe, who observes tails. This sounds like science fiction. However, according to a poll published in The Physics of Immortality (1994), 58% of scientists believe the many-world interpretation of quantum mechanics is true, 13% are on the fence (undecided), 11% have no opinion, and 18% do not believe it. Among the believers are Nobel laureates Murray Gell-Mann and Richard Feynman, and world-famous physicist/cosmologist Stephen Hawking. In our everyday reality, many of us would reject the many-world interpretation of quantum mechanics because we do not experience it directly. However, let me point out, we do not experience the individual atoms of a book when we hold it. Yet, we know from sophisticated experimental analysis that the book is a collection of atoms. Unfortunately, in the strange world of quantum mechanics, our intuition and experience rarely serve us. I leave it to you to formulate your own conclusions.
  2. Timeline-protection hypothesis—The timeline-protection hypothesis asserts that it is impossible to create a time travel paradox. For example, if you travel back in time and attempt to prevent your grandfather from meeting your grandmother, you fail every time. If you attempt to shoot yourself through a wormhole, the gun jams, or something else happens, which prevents you from changing the past. Several other paradox resolutions fit under this category. They are:
    • The Novikov self-consistency principle, suggested by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s, which asserts anything a time traveler does remains consistent with history. For example, if you travel to the past and attempt to keep your grandfather from meeting your grandmother, something interferes with any attempt you make, causing you to fail in the attempt. In other words, the time traveler is unable to change history.
    • The self-healing hypothesis theory, which states that whatever a time traveler does to alter the present by traveling to the past sets off another set of events to cause the present to remain unchanged. For example, if you attempt to prevent Abraham Lincoln’s assassination, you may succeed in preventing John Wilkes Booth from carrying out the assassination only to find someone else assassinated Lincoln. In essence, time heals itself.
  3. Timeline-corruption hypothesis—The timeline-corruption hypothesis suggests that time paradoxes are inevitable and unavoidable. Any time travel to the past creates minute effects that inevitably alter the timeline and cause the future to change. For example, if you inadvertently step on an ant in the past, it changes the future. Popular science fiction literature calls this the “butterfly effect,” namely, that the flutter of a butterfly’s wings in Africa can cause a hurricane in North America. Under this theory, anything you do will have a consequence. It may be small and benign. Alternatively, it may be large and disastrous. The destruction-resolution hypothesis fits in this category. It holds that anything a time traveler does resulting in a paradox destroys the timeline, and even the universe. Obviously, if the destruction-resolution hypothesis is true, any time travel would be disastrous. However, I doubt the validity of the destruction-resolution hypothesis, since we are able to perform time dilation (i.e., forward time travel) experiments with subatomic particles using particle accelerators.
  4. Choice timeline hypothesis—The choice timeline hypothesis holds that if you choose to travel in time, it is predestined, and history instantly changes. This implies you can time travel to the future and leave an item there that you will need sometime in the future. It will be there for you when the future becomes the present. For example, assume you are in New York City, and someone is about to assault you. You have no escape or means of protection. According to the choice timeline hypothesis, you can use your time machine to travel to the future. You hide a gun near the place where the assault is about to occur. When the assault occurs, you retrieve the hidden gun and scare off the attacker.

There are numerous other time-paradox resolution hypotheses. Most fall under one of the above categories, or are not as popular as the above. I left them out in the interest of clarity and brevity. The four categories above give us a reasonable framework to understand the major time-paradox resolution theories, and the current thinking regarding their impact on the timeline.

The majority of the scientific community does not think time paradoxes inhibit time travel. For example, Kip Thorne, an American theoretical physicist and professor of theoretical physics at the California Institute of Technology until 2009, argues that time paradoxes are imprecise thought experiments which can be resolved by numerous consistent solutions. The scientific consensus appears to be that time paradoxes may or may not occur, but they do not exclude the possibility of time travel. This position appears validated by the time dilation (i.e., forward time travel) experiments routinely performed using particle accelerators.

This post is based on my book, How to Time Travel (2013)

A black and white image of a clock face with a spiral effect distorting the numbers and hands.

Explore Two Famous Time Travel Paradoxes – The Grandfather Paradox & Twin Paradox

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Time Travel Paradoxes – The Grandfather Paradox & Twin Paradox

What is a time travel paradox? It is an occurrence that apparently violates some aspect of causality (i.e., cause precedes effect) typically associated with time travel. Although there are numerous time travel paradoxes, let us explore two famous ones: The grandfather paradox and the twin paradox.

  • The grandfather paradox—Science fiction writer René Barjavel, in his 1943 book, Le Voyageur Imprudent (Future Times Three), originally proposed the grandfather paradox. It goes something like this. A person goes back in time and meets his grandfather before his grandfather meets his grandmother. The person in some way interferes with his grandfather meeting his grandmother. Consequently, the grandfather and grandmother never meet. The question becomes, what happens to the person? In theory, the person will never be born.  Is this just some illogical premise, similar to asserting that a square circle exists? Most of the scientific community considers it a valid concern regarding causality violations due to time travel. Some physicists believe that it actually presents a barrier to time travel. However, numerous theories exist to resolve time travel paradoxes. We will discuss those theories in the next section, but first let us explore another famous paradox.
  • The twin paradox—The is one of the most famous time travel paradoxes. It goes something like this: On Earth live a pair of twins. They are almost the same age, differing only by the order in which they were born. One twin boards a spacecraft capable of traveling near the speed of light. In the spacecraft, the twin embarks on a one-year journey, measured by the clock within the spacecraft. During the one-year journey, the spacecraft travels at 99.94% the speed of light. When the spacecraft returns to Earth, the twin on the spacecraft has aged one year, but learns his twin has aged almost thirty years. Although the example is fictitious, the science is real. The twin paradox has been experimentally verified using highly accurate atomic clocks, one on a jet plane and the other at the airport. There have been many variations of the twin paradox. The scientific community considers it a valid effect of Einstein’s special theory of relativity regarding time dilation.

There is a laundry list of time travel paradoxes. I discuss many of them in my critically acclaimed best selling new book, How to Time Travel. The paradoxes above are sufficient to illustrate causality issues. It is important to note that the time travel paradoxes are not simply in the category of thought experiments. Numerous time travel paradoxes, like the twin paradox and the double-slit delayed-choice paradox (discussed in a previous post), are experimental facts. They are real. The important question is: Do time travel paradoxes form a barrier to time travel? We will address this question in an up coming post.

 

Diagram of a double-slit experiment setup with light source, thin opaque plate, double slits, and screen.

A Classic Time Travel Paradox – Double-Slit Experiment Demonstrates Reverse Causality!

Categories, Time Travel, Universe Mysteries, , , , ,

Almost the entire scientific community has held for hundreds of years that for every effect, there must have been a cause. Another way of saying this is cause precedes effect. For example, if you hit a nail with a hammer (the cause), you can drive it deeper into the wood (the effect). However, some recent experiments are challenging that belief. We are discovering that what you do after an experiment can influence what occurred at the beginning of the experiment. This would be the equivalent of the nail going deeper into the wood prior to it being hit by the hammer. This is termed reversed causality. Although, there are numerous new experiments that illustrate reverse causality, science has been struggling with a classical experiment called the “double-slit” that illustrates reverse causality for well over half a century.

There are numerous versions of the double-slit experiment. In its classic version, a coherent light source, for example a laser, illuminates a thin plate containing two open parallel slits. The light passing through the slits causes a series of light and dark bands on a screen behind the thin plate. The brightest bands are at the center, and the bands become dimmer the farther they are from the center. See image below to visually understand this.

The series of light and dark bands on the screen would not occur if light were only a particle. If light consisted of only particles, we would expect to see only two slits of light on the screen, and the two slits of light would replicate the slits in the thin plate. Instead, we see a series of light and dark patterns, with the brightest band of light in the center, and tapering to the dimmest bands of light at either side of the center. This is an interference pattern and suggests that light exhibits the properties of a wave. We know from other experiments—for example, the photoelectric effect (see glossary), which I discussed in my first book, Unraveling the Universe’s Mysteries—that light also exhibits the properties of a particle. Thus, light exhibits both particle- and wavelike properties. This is termed the dual nature of light. This portion of the double-slit experiment simply exhibits the wave nature of light. Perhaps a number of readers have seen this experiment firsthand in a high school science class.

The above double-slit experiment demonstrates only one element of the paradoxical nature of light, the wave properties. The next part of the double-slit experiment continues to puzzle scientists. There are five aspects to the next part.

  1. Both individual photons of light and individual atoms have been projected at the slits one at a time. This means that one photon or one atom is projected, like a bullet from a gun, toward the slits. Surely, our judgment would suggest that we would only get two slits of light or atoms at the screen behind the slits. However, we still get an interference pattern, a series of light and dark lines, similar to the interference pattern described above. Two inferences are possible:
    1. The individual photon light acted as a wave and went through both slits, interfering with itself to cause an interference pattern.
    2. Atoms also exhibit a wave-particle duality, similar to light, and act similarly to the behavior of an individual photon light described (in part a) above.
  2. Scientists have placed detectors in close proximity to the screen to observe what is happening, and they find something even stranger occurs. The interference pattern disappears, and only two slits of light or atoms appear on the screen. What causes this? Quantum physicists argue that as soon as we attempt to observe the wavefunction of the photon or atom, it collapses. Please note, in quantum mechanics, the wavefunction describes the propagation of the wave associated with any particle or group of particles. When the wavefunction collapses, the photon acts only as a particle.
  3. If the detector (in number 2 immediately above) stays in place but is turned off (i.e., no observation or recording of data occurs), the interference pattern returns and is observed on the screen. We have no way of explaining how the photons or atoms know the detector is off, but somehow they know. This is part of the puzzling aspect of the double-slit experiment. This also appears to support the arguments of quantum physicists, namely, that observing the wavefunction will cause it to collapse.
  4. The quantum eraser experiment—Quantum physicists argue the double-slit experiment demonstrates another unusual property of quantum mechanics, namely, an effect termed the quantum eraser experiment. Essentially, it has two parts:
    1. Detectors record the path of a photon regarding which slit it goes through. As described above, the act of measuring “which path” destroys the interference pattern.
    2. If the “which path” information is erased, the interference pattern returns. It does not matter in which order the “which path” information is erased. It can be erased before or after the detection of the photons.

This appears to support the wavefunction collapse theory, namely, observing the photon causes its wavefunction to collapse and assume a single value.

If the detector replaces the screen and only views the atoms or photons after they have passed through the slits, once again, the interference pattern vanishes and we get only two slits of light or atoms. How can we explain this? In 1978, American theoretical physicist John Wheeler (1911–2008) proposed that observing the photon or atom after it passes through the slit would ultimately determine if the photon or atom acts like a wave or particle. If you attempt to observe the photon or atom, or in any way collect data regarding either one’s behavior, the interference pattern vanishes, and you only get two slits of photons or atoms. In 1984, Carroll Alley, Oleg Jakubowicz, and William Wickes proved this experimentally at the University of Maryland. This is the “delayed-choice experiment.” Somehow, in measuring the future state of the photon, the results were able to influence their behavior at the slits. In effect, we are twisting the arrow of time, causing the future to influence the past. Numerous additional experiments confirm this result.

Let us pause here and be perfectly clear. Measuring the future state of the photon after it has gone through the slits causes the interference pattern to vanish. Somehow, a measurement in the future is able to reach back into the past and cause the photons to behave differently. In this case, the measurement of the photon causes its wave nature to vanish (i.e., collapse) even after it has gone through the slit. The photon now acts like a particle, not a wave. This paradox is clear evidence that a future action can reach back and change the past.

To date, no quantum mechanical or other explanation has gained widespread acceptance in the scientific community. We are dealing with a time travel paradox that illustrates reverse causality (i.e., effect precedes cause), where the effect of measuring a photon affects its past behavior. This simple high-school-level experiment continues to baffle modern science. Although quantum physicists explain it as wavefunction collapse, the explanation tends not to satisfy many in the scientific community. Irrefutably, the delayed-choice experiments suggest the arrow of time is reversible and the future can influence the past.

This post is based on material from my new book, How to Time Travel, available at Amazon in both paperback and Kindle editions.

Image: Figure 3, from How to Time Travel (2013)

A silhouette of a person with a blank face in front of a large clock, set against a swirling cosmic background.

How to Time Travel: Explore the Science, Paradoxes, and Evidence

Categories, Time Travel, , , , , , , ,

Here is the entire introduction from my new book, How to Time Travel. Enjoy!

Introduction

Few subjects evoke more emotion than time travel, the concept of moving between different points in time in a manner analogous to moving between different points in space. Humankind’s fascination with time travel dates back thousands of years. Although there is no consensus recognizing which written work was the first to discuss time travel, many scholars argue that the Mahabharata, from Hindu mythology, is the first, dating between 700 BCE (Before the Common/Current/Christian Era) and 300 CE (Common/Current/Christian Era). The Mahabharata, which is one of the two major Sanskrit epics of ancient India, relates the story of King Revaita, who travels to heaven to meet the deity Brahma. When King Revaita returns to Earth, he is shocked to learn that many ages have passed. In today’s science, we would assert King Revaita experienced time dilation.

What is time dilation? It is a scientific fact that time moves slower for any mass accelerated near the speed of light. If that mass were a clock, for example, the hands of the clock would appear to be moving slower than a clock in the hand of an observer at rest. That phenomenon is termed time dilation. If King Revaita used a spaceship capable of speeds near the speed of light to visit Brahma, a roundtrip journey that would appear to King Revaita to take one year would result in a time passage of thirty years on Earth. This may seem like science fiction, but time dilation is a well-established, experimentally verified aspect of Einstein’s special theory of relativity; more about this later.

Arguably, the greatest single written work that laid the foundation to fire the imagination of today’s generation regarding time travel is H. G. Wells’s classic novel, The Time Machine, published in 1895. It has inspired numerous popular movies, television programs, novels, and short stories. Why are we humans so obsessed with time travel? It appears to be an innate longing. How many times have you wished that you could go back to a specific point in time and select a different action? We all do it. Consider the number of times you have replayed a specific situation in your mind. Psychologists tell us we replay an event in our minds when the outcome is not finished to our satisfaction. This has accounted for numerous nights of tossing and turning. Another common need is to seek answers to important questions from a firsthand perspective. Perhaps you would like to be a witness during the resurrection of Christ, or be a witness behind the grassy knoll during the Kennedy assassination. Perhaps you miss a loved one who has passed on, and you would like to go back in time to embrace that loved one again.

Some of us also dream about time travel to the future. What outcomes will result from our decisions? Imagine the prosperity and happiness that could be ours if we were able to travel to the future. We would be able to witness the outcome of any decision, return to the present, and guide our lives accordingly. Picking the right profession or choosing the right mate would be a certainty. We could ensure there would be no missteps in our life. A life of leisure and prosperity would be ours for the taking.

It is little wonder that many people ask this deceptively simple question: Is time travel possible? The majority of the scientific community, including myself, says a resounding yes. The theoretical foundation for time travel, based on the solutions to Einstein’s equations of relativity, is widely accepted by the scientific community. The next question, which is the most popular question, is how to time travel. Of all the questions in science, the keyword phrase “how to time travel” is close to the top of Internet search engine searches. According to Google, the largest search engine in the world, there are 2,240,000 worldwide monthly searches for the keyword phrase “how to time travel,” as of this writing. Unfortunately, it is the most difficult question to answer.

Obviously, interest in time travel is high, and what people want to know most is how to time travel. This high interest, combined with the intriguing real science behind time travel, is what inspired me to write this book.

At this point, I would like to set your expectations. We are going to embark on a marvelous journey. We will examine the real science of time travel, the theoretical foundation that has most of the scientific community united that time travel is possible. We will also examine the obstacles to time travel, and there are many. However, even in the face of all the obstacles, most of the scientific community agrees it is theoretically possible to time travel. The largest issue in time travel is not the theoretical science. It is the engineering. Highly trained theoretical physicists understand the theoretical science of time travel. However, taking the theory and building a time machine capable of human time travel has proved a formidable engineering task. It has not been done, but we are amazingly close. We have already built time machines capable of sending subatomic particles into the future. If you will pardon the pun, it is just a matter of time before we engineer our way through the time travel barrier and enable human time travel.

In setting your expectations, I promise you significant insight into the real science of time travel and an equally incredible insight into the obstacles to time travel. I cannot promise that with this knowledge you will be able to overcome the obstacles and engineer how to time travel. However, you may be the one person destined to harness the science, glean the engineering simplicity, and journey in time. There is only one way to find out, namely, read on.

To browse the book free on Amazon, click this link: http://amzn.to/1dWyEkp