Category Archives: Time Travel

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!

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

The Mallett Time Machine – Time Travel to the Past May Become Possible!

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Thanks to particle accelerators, like the Large Hadron Collider (LCH) 175 meters (574 ft) beneath the Franco-Swiss border near Geneva, Switzerland, physicist have been able to routinely demonstrate forward time travel (i.e., time dilation) using subatomic particles. In a sense, you can think of the Large Hadron Collider as a time machine. It is capable of sending subatomic particles to the future. Unfortunately, we do not have a similar machine that can send subatomic particles to the past. However, Dr. Ronald Mallett is attempting to change that.

Dr. Ronald Mallett is an American theoretical physicist and the author of Time Traveler: A Scientist’s Personal Mission to Make Time Travel a Reality (2007). Dr. Mallett is a full professor at the University of Connecticut, where he has taught physics since 1975.

Dr. Mallett is attempting to twist spacetime using a ring laser (i.e., a laser that rotates in a circle) by passing it through a through a photonic crystal (i.e., a crystal that only allows photons of a specific wavelength to pass through it). The concept behind spacetime twisting by light (STL) is that by twisting space via the laser, closed timelike curves will result (i.e., time will also be twisted). In this way, Dr. Mallett hopes to observe a violation of causality when a neutron is passed through the twisted spacetime. Dr. Mallett also believes he will be able to send communication by sending subatomic particles that have spin up and spin down. Note, the spin of a subatomic particle is part of the particle’s quantum description. As a simple example, we can consider spin up equal to 1 and spin down equal to 0. Using this technique, Dr. Mallett can send a binary code, similar to the binary codes used in computing.

Few scientists openly discuss their work on time machines. They fear ridicule. In this regard, Dr. Mallett is a pioneer. When Dr. Mallett was ten years old, his father died at age thirty-three from a heart attack. Dr. Mallett has shared that his initial drive to invent a time machine was to go back in time and visit with his father. Unfortunately, the science of time travel only allows a person to go back in time to the point when the time machine is first turned on. Dr. Mallett acknowledges this, but continues his quest.

Dr. Mallett’s concept of twisting space is close to the concept of creating a wormhole, as discussed in my last post. Dr. Mallett is using laser light as means of creating the mouth of the wormhole. In a publication (R. L. Mallett, “The Gravitational Field of a Circulating Light Beam,” Foundations of Physics 33, 1307–2003), Dr. Mallett argued that with sufficient energies, the circulating light beam might produce closed timelike lines (i.e., time travel to the past).

Is Dr. Mallett’s theoretical foundation solid? According to physicists Dr. Olum and Dr. Everett, it is fatally flawed. In a paper published in 2005 (Ken D. Olum and Allen Everett, 2005, “Can a Circulating Light Beam Produce a Time Machine?”, Foundations of Physics Letters 18 (4): 379–385), they argue three points:

  1. Dr. Mallett’s analysis contains unusual spacetime (i.e., mathematical) issues, even when the power to the machine is off.
  2. The energy required to twist spacetime would need to be much greater than lasers available to today’s science.
  3. They note a theorem proven by Stephen Hawking (chronology protection conjecture—1992), namely, it is impossible to create closed timelike curves in a finite region without using negative energy.

Although Dr. Mallett did not address their criticism in a formal publication, he did argue in his book, Time Traveler, that he was forced to simplify the analysis due to difficulties in modeling the photonic crystal. This, however, is far from a complete response.

Who is right? In the physical sciences, we are judged by the weakest link in our theories. If I use this criterion, I would say the argument favors Dr. Mallett, since the chronology protection conjecture, which we will discuss in the next chapter, has come under serious criticism, and it is not clear that it presents a valid challenge. Nonetheless, Dr. Olum and Dr. Everett are highly regarded physicists. Therefore, at this point, it is hard to know who is right, and right about what. Perhaps the mathematical analysis is flawed, and the approach published by Dr. Mallett requires more energy than is available via today’s technology. However, we are witnessing a significant event in science. A respected physicist, Dr. Mallett, is openly publishing his work on building a backward time travel machine. Other respected physicists, Dr. Olum and Dr. Everett, are entering into a scientific debate regarding Dr. Mallett’s theoretical basis. From my point of view, this is how it should be in science. The debate is healthy. As a theoretical physicist, I know that the debate will end only when either:

  1. The Mallett time machine works, or
  2. The Mallett time machine enters the rubbish pile of scientific failures, along with astronomer Ptolemy’s Earth-centered model of the solar system and the flat Earth theories.

This material is based on my new book, How to Time Travel.

A digital tunnel formed by cascading blue binary code creating a futuristic data flow effect.

Traversable Wormholes – Time Travel to the Past May Be Possible!

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Traversable wormholes may enable time travel to the past. This post is based on material from my new book, How to Time Travel.

Let us begin our discussion by understanding the scientific meaning of a “wormhole.” There are valid solutions to Einstein’s equations of general relativity that suggest it is possible to have a “shortcut” through spacetime. To picture this, consider a piece of paper with a dot at opposite corners. In Euclidean geometry, normally taught in high school, we learn that the shortest distance between the two points is a straight line. However, valid solutions to Einstein’s general relativity equations suggest that the two points on the paper are connectable by an even shorter path, a wormhole. To visualize this, simply fold the opposite corners of the paper with the dots, such that the dots touch. You have created a representation of a wormhole. You have manipulated the space between the dots by folding the paper to allow them to touch.

Unfortunately, there is no scientific evidence that wormholes exist in reality. However, the strong theoretical foundation suggesting wormholes (i.e., valid solutions to Einstein’s equations of general relativity) makes their potential existence impossible to ignore.

The first type of wormhole solution to Einstein’s equations of general relativity was the Schwarzschild wormhole, developed by German physicist Karl Schwarzschild (1873–1916). Unfortunately, although the Schwarzschild mathematical solution was valid, it resulted in an unstable black hole. The unstable nature of the Schwarzschild wormhole suggested it would collapse on itself. It also suggested that the wormhole would only allow passage in one direction. This brought to light an important new concept. Faced with the unstable nature of Schwarzschild wormholes, American theoretical physicist Kip Thorne and his graduate student Mike Morris demonstrated a general relativity “traversable wormhole” in a 1988 paper. In this mathematical context, a traversable wormhole would be both stable and allow information, objects, and even humans to pass through in either direction and remain stable (i.e., would not collapse on itself). As is often the case in science, one discovery leads to another. Numerous other wormhole solutions to the equations of general relativity began to surface, including one in 1989 by mathematician Matt Visser that did not require negative energy to stabilize it.

As discussed above, traversable wormholes may require negative energy to sustain them. Several prominent physicists, including Kip Thorne and British theoretical physicist/cosmologist Stephen Hawking, believe the Casimir effect proves negative energy densities are possible in nature. Currently, physicists are using the Casimir effect in an effort to create negative energy. Obviously, if successful, the amounts of negative energy will likely be small. Because of the amount of negative energy that may result, I suspect the first wormholes developed will be at the quantum level (i.e., the level of atoms and subatomic particles).

We have merely scratched the surface regarding the science of wormholes, but we did accomplish one important objective. We have described how a traversable wormhole would allow spacetime travel via shortcuts in spacetime. This means we could connect two points in time or two points in space via a traversable wormhole. However, there is a hitch regarding time travel to the past. According to the theory of relativity, we cannot go back to a time before the wormhole existed. This means that if we discover how to make a traversable wormhole today, a year from now we can go back to today.

You may wonder why a wormhole constructed today would not allow us to go back to yesterday. To understand this conundrum, we need to understand just how a wormhole works as a time machine. Here is one scenario. Imagine you are able to accelerate one end of a wormhole to a significant fraction of the speed of light. Perhaps you could use a high-energy ring laser (i.e., a laser than rotates in a circle). As you twist the space, you create the “mouth” of the wormhole, something like a tunnel. After you enter the mouth of the wormhole, you are now somewhere in the wormhole’s “throat.” A “tunnel” is a good analogy to what is occurring. Now imagine you are able to take the other entrance of the tunnel, which is at rest and called the “fixed end,” and bring it back close to the origin. Time dilation causes the mouth to age less than the fixed end. A clock at the mouth of the wormhole, where spacetime accelerates near the speed of light, will move slower than a clock at the fixed end.

Given the above understanding of how a wormhole acts as a time machine, let us address why it is only possible to go back to the time of the wormhole’s construction. Imagine you have two synchronized clocks. If you place one clock at the mouth, and you place the other clock at the fixed end, they will initially read exactly the same time, for example, the year 2013. However, the clock at the mouth, influenced by the twisted space, is going to experience time dilation, and therefore move slower than the clock at the fixed end. Let us consider the case where the clock at mouth of the wormhole moves, based on the rate of twisting spacetime, one thousand times slower than the clock at the fixed end. In one hundred years, the clock at the fixed end, which experiences no time dilation, will read 2113. The clock at the mouth will still read 2013; only one tenth of one year will have passed due to time dilation at the mouth of the wormhole. From the fixed end, where no time dilation is occurring (i.e., the clock reads 2113), you can walk back to the mouth of the wormhole, where the clock still reads 2013. You will have walked one hundred years into the past. Notice, though, you cannot go back beyond the time of the traversable wormhole’s construction.

This post is based on material from my new book, How to Time Travel. Click How to Time Travel to browse the book free on Amazon.

A digital blue background with glowing horizontal lines and light flares creating a futuristic effect.

Faster-Than-Light Time Travel to the Past May be Possible

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Based on Einstein’s special theory of relativity, if we are able to move information or matter from one point to another faster than the speed of light, there would be some inertial frame of reference (i.e., a frame of reference moving at a constant velocity) in which the signal or object is moving backward in time. Let us understand why this is the case.

Consider sending a signal from one location to another. The first event is sending the signal. The second event is receiving the signal. As long as the signal travels at or below the speed of light, according to the “relativity of simultaneity,” the first event will always precede the second event in all inertial frames of reference. Although this squares with our everyday observation of reality, that cause precedes effect, you may have a question. What is the relativity of simultaneity?

The relativity of simultaneity is a concept introduced by Einstein in the special theory of relativity. The simultaneity of an event is not an absolute to all observers, but depends on the observer’s frame of reference. For example, if one observer is midway on a train car, and a second observer is at rest on the platform at the train station, they will see the simultaneity of an event differently. As the two observers pass, assume the observer in the train takes a picture using a flashbulb. From the viewpoint of the observer within the train, the light reaches both the front and rear of the train car at the same time. However, the observer on the platform sees a different situation. From the observer on the platform’s viewpoint, first the flashbulb goes off, and then the light reaches the back of the train car, since it was moving toward the fixed observer on the platform. Lastly, the observer sees the light reach the front of the train car, since it was moving away from the observer. The effect is more pronounced as the speed of the train approaches the speed of light.

Based on the relativity of simultaneity, if a signal propagates faster than the speed of light, there would always be some frames of reference where the signal arrives before it was sent. To illustrate this, let us go back to the above example and assume the train is traveling close to the speed of light. The observer is now closer to the end of the train car when the flashbulb flashes. Let us also assume the light exceeds the speed of light in a vacuum. For example, we could assume the interior of the train car contains a negative energy vacuum, which some in the scientific community believe would allow light to travel faster than it would in a normal positive vacuum. Given these two inertial frames of reference, the train moving close to the speed of light, and the observer situated closer to the rear of the train car when the flashbulb goes off, it would appear that the light reached the end of the train car prior to the light from the flashbulb reaching the observer on the platform. Why is this? (You might want to draw this out on a piece of paper to visualize the light paths.) The light inside the train instantaneously reaches the back of the train car, and then travels a short distance in the inertial frame of the observer, who records the event. This is witnessed ahead of the light reaching the observer from the source, since now the observer is farther away from the source. Therefore, the observer first witnesses the light reach the back of the train, and then observes the light from the source (i.e., flashbulb goes off). From the viewpoint of the observer at the station, the effect preceded the cause. If the light within the train did not travel faster than the speed of light in a vacuum, the effect of reverse causality would be lost.

From inside the train car, nothing changes for the observer seated midway in the car. The faster-than-light signals reach the front and back at the same time. In summary, the observer on the platform witnesses reverse causality. The light signal reaches the back of the train car before the light from the flashbulb reaches the observer on the platform. This thought experiment, illustrating reverse causality, suggests the observer on the platform witnesses an event taking place in the past (i.e., light reaching the end of the train car), since the flashbulb light at the source will reach the observer on the platform later (i.e., the future).

Does anything travel faster than the speed of light in the real world? Maybe! Some quantum physicists believe the phenomena of quantum entanglement (i.e., two particles that have interacted to the point that the physical state of one particle is dependent on the other) exhibits effects that travel faster than the speed of light. However, this is controversial, and more data is required to make an irrefutable case that this is true.

This post was based on material taken from my new book, How to Time Travel. It is available from Amazon in a paperback or Kindle editions. Click How to Time Travel to browse the book free.

A silhouette of a person with a clock face behind them, symbolizing the concept of time and human existence.

“How to Time Travel” – Explore What’s New In Time Travel Science

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How to Time Travel (Published September 2013, Amazon) delineates the latest scientific theories and experiments regarding the science of time travel, proposed time machines, time travel paradoxes and time travel evidence. It also provide several new contributions to this perennially popular topic. These include the Existence Equation Conjecture, the Grandchild Paradox, the Preserve the World Line Rule, and the Time Uncertainty Interval.

Numerous books, experiments, and highly regarded scientific papers, like Einstein’s special and general theories of relativity, have established time travel as not only theoretically possible, but as a science fact. For example, high-energy particle accelerators routinely prove that time travel to the future is a science fact for subatomic particles accelerated close to the speed of light. Although, current scientific capability  does not enable significant human time travel to the future, or even time travel to the past for  subatomic particles, many in the scientific community estimate that human time travel to the future and past will be accomplished by the end of the 21st century.

In this post, I discuss the new additions that How to Time Travel makes to the field of time travel science.

Existence Equation Conjecture

In How to Time Travel I delineate my own theoretical research, the existence equation conjecture, which explains the role energy plays in time travel. Using the equation, I am able to explain time dilation experiments (i.e., time travel to the future) within 2% accuracy. As I asserted in the book, I derived the existence equation conjecture from Einstein’s special theory of relativity. It lays bare the fundamental basis for time travel. I consider it an important addition to the science of time travel, since it formulates time travel directly in terms of energy, and not secondary phenomena such as particle acceleration. Please keep in mind that in science, a conjecture is a scientific opinion.

Grandchild Paradox

A host of new experiments and even a classical experiment (i.e., the double slit experiment) prove that events in the future can influence the past. This may come across as counter intuitive, but the data from the experiments make it an inescapable conclusion. I discuss the experiments in chapter 1, “Twisting the arrow of time,” and in chapter 6, “Time travel paradoxes.” Here is a statement of the “grandchild paradox”: The grandchild paradox refers to any situation involving reverse causality (i.e., effect occurs before cause). Any situation, real or imagined, that reverses the arrow of time and allows the future to influence the past, may be considered a grandchild paradox. The arrow of time refers to the direction of time, typically proceeding from the present to the future. Twisting the arrow of time refers to reversing the flow of time. Until recently, most of the scientific community would have agreed that the arrow of time pointed in only one direction, from the present to the future. These new findings argue the arrow of time can also point from the future to the past.

Preserve the World Line Rule

According to the general theory of relativity, all reality travels in four-dimensional space, termed a world line. Numerous solutions to Einstein’s equations of general relativity delineate “close timelike curves” (the world line of an entity returns its starting point). If the world line of any entity returns to its starting point, the entity is said to have returned to its past, suggesting backward time travel is theoretically possible. However, to date, we have not been able to experimentally verify that this aspect of Einstein’s general theory of relativity is true. Some in the scientific community believe that in time, we will find a way to send subatomic particles, information and eventually humans back in time. When and if this becomes a reality, nations possessing this capability can literally rewrite history. Faced with this possibility, I think there is one commonsense rule regarding time travel that would assure greater safety for all involved. I term the rule “preserve the world line.” Why this one simple rule? Altering the world line (i.e., the path that all reality takes in four-dimensional spacetime) may lead to chaos. History would become meaningless. We have no idea what changes might result if the world line is disrupted, and the consequences could be serious, even disastrous.

Time Uncertainty Interval

Planck time is the smallest interval of time that science is able to define. The theoretical formulation of Planck time comes from dimensional analysis, which studies units of measurement, physical constants, and the relationship between units of measurement and physical constants. In simpler terms, one Planck interval is approximately equal to 10-44 seconds (i.e., 1 divided by 1 with 44 zeros after it). It is widely believed in the scientific community that we would not be able to measure a change smaller than a Planck interval. From this standpoint, we can assert that time is only reducible to an interval, not a dimensionless sliver, and that interval is the Planck interval. Since the smallest unit of time is only definable as the Planck interval, this suggests there is a fundamental limit to our ability to measure an event in absolute terms. This fundamental limit to measure an event in absolute terms is independent of the measurement technology. The error in measuring the start or end of any event will always be at least one Planck interval. This means the amount of uncertainty regarding the start or completion of an event is only knowable to one Planck interval. I term this uncertainty of measurement the “Time Uncertainty Interval.”

The above concepts are both new and original, based on my own theoretical research. I suggest you greet them with open mindedness and skepticism. They are now part of the scientific literature landscape, included in my new book How to Time Travel, and await rigorous peer review.

Click How to Time Travel to browse the book free on Amazon.com.