Category Archives: Time Travel

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The Top Five Unsolved Mysteries of Science

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There are numerous unsolved mysteries in science. In this post, I will delineate the top five that I consider the most profound.

  1. What caused the Big Bang? Cosmologist are in strong consensus that the Big Bang resulted in the evolution of the Universe, but there is no scientific consensus as to what caused the Big Bang. There are several theories, including one that I put forward in my book, Unraveling the Universe’s Mysteries. However, none of the current theories, including the one that I forward in my book, have garnered consensus in the scientific community. The origin of the Big Bang is arguably the greatest scientific mystery of all time, and it remains an area of considerable research.
  2. How did life start on Earth? There are two fundamental theories regarding the origin of life on Earth. The first theory, panspermia, holds that life exists throughout the Universe and is distributed by meteoroids, asteroids and planetoids. This theory is compelling, but it still leaves us with another profound question, “How did life originate in the Universe?” There are no widely accepted theories to address that question. The second theory, regarding how life started on Earth, is termed biopoesis. It holds that life forms from inorganic matter through natural processes. This theory is also compelling, but no experimental process has resulted in life forming from inorganic matter. By simple logic, one or even both of these theories is correct. Obviously, in the early Universe, life had to form from inorganic matter. It is also possible that life also started on Earth via the same process. It is also possible that once life formed in the Universe, it was spread by meteoroids, asteroids and planetoids.
  3. What is the nature of time? Some scientists, myself included, argue time is real. This stance suggests that time travel would also be possible. In my book, How to Time Travel, I devote considerable attention to the various philosophies of time and to experiments that suggest time is real. I also delineate experiments that prove time travel to the future is real, as well as experiments that prove reverse causality is real (i.e., literally, the effect precedes the cause). I also delineate experiments that prove that something in the future can alter the past. Some philosophers and scientists argue that time is a mental construct. It is not real. That humans invented time to measure change. If that is true, time travel would not be possible, except in your mind. However, scientific experiments, such as time dilation and reverse causality suggest otherwise. What do you think?
  4. What is the fundamental theory of physics? Modern physics rests on two pillars, The first pillar is Einstein’s theories of relativity. The second pillar is quantum mechanics. Although Einstein’s theories explain phenomena on the macro-scale (i.e., the typical scale we observe in our every day life), it fails to explain phenomena on the quantum level (i.e., the level of atoms and subatomic particles). To explain phenomena on the quantum level we must turn to quantum mechanics. This would be acceptable, except Einstein’s theories of relativity are incompatible with quantum mechanics. They do not come together to adequately explain gravity. Physicists have long sought the “theory of everything.” Some physicists, like world renown cosmologist Stephen Hawking, suggest that M-theory (i.e., the most comprehensive string theory) fits the bill. However, there is no consensus or proof that M-theory is even valid. Until the next Einstein comes along and solves the problem, we don’t have a fundamental theory (i.e., a single unifying theory) of physics.
  5. Does life exist on other planets or is the Earth unique? Almost every scientist agrees that given the vastness of the Universe and the numerous Earth-like planets that have been discovered, there must be life somewhere else in the Universe. Indeed, many believe, myself included, that advanced aliens, similar or more advanced than ourselves, must also exist. However, there has been no definitive publication that proves life exists elsewhere in the Universe. I will refrain from getting into UFOs, government conspiracies and similar material. I don’t refute such theories, but as a scientist I must base my conclusions on definitive evidence. To date, we have no definitive evidence (i.e., widely accepted by the scientific community) regarding life on other planets. However, mathematically, I think life on other planets is a certainty. What do you think?
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Reverse Causality – The Future Can Change the Past

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Most people find reverse causality intriguing, but impossible. Yet, it has a strong basis in science. In my book, How to Time Travel, I discuss a number of reverse causality examples. Here are some from the book.

Twisting the Arrow of Time

The flow of time, sometimes referred to as the “arrow of time,” is a source of debate, especially among physicists. Most physicists argue that time can only move in one direction based on “causality” (i.e., the relationship between cause and effect). The causality argument goes something like this: every event in the future is the result of some cause, another event, in the past. This appears to make perfect sense, and it squares with our everyday experience. However, experiments within the last several years appear to argue reverse causality is possible. Reverse causality means the future can and does influence the past. For example, in reverse causality, the outcome of an experiment is determined by something that occurs after the experiment is done. The future is somehow able to reach into the past and affect it. Are you skeptical? Skepticism is healthy, especially in science. Let us discuss this reverse causality experiment.

In 2009, physicist John Howell of the University of Rochester and his colleagues devised an experiment that involved passing a laser beam through a prism. The experiment also involved a mirror that moved in extremely small increments via its attachment to a motor. When the laser beam was turned on, part of the beam passed through the prism, and part of the beam bounced off the mirror. After the beam was reflected by the mirror, the Howell team used “weak measurements” (i.e., measurement where the measured system is weakly affected by the measurement device) to measure the angle of deflection. With these measurements, the team was able to determine how much the mirror had moved. This part of the experiment is normal, and in no way suggests reverse causality. However, the Howell team took it to the next level, and this changed history, literally. Here is what they did. They set up two gates to make the reflected mirror measurements. After passing the beam through the first gate, the experimenters always made a measurement. After passing it through the second gate, the experimenters measured the beam only a portion of the time. If they chose not to make the measurement at the second gate, the amplitude of the deflected angle initially measured at the first gate was extremely small. If they chose to make the measurement at the second gate, the deflected angle initially measured at the first gate was amplified by a factor of 100. Somehow, the future measurement influenced the amplitude of the initial measurement. Your first instinct may be to consider this an experimental fluke, but it is not. Physicists Onur Hosten and Paul Kwiat, University of Illinois at Urbana-Champaign, using a beam of polarized light, repeated the experiment. Their results indicated an even larger amplification factor, in the order of 10,000.

The Double-Slit Experiment

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.

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, which is well accepted in the scientific community. This is termed the dual nature of light. This portion of the double-slit experiment simply exhibits the wave nature of light.

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:

a. The individual photon light acted as a wave and went through both slits, interfering with itself to cause an interference pattern.
b. 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:

a. 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.
b. 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.

5. 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.

Summary

The above experimental results raise questions about the “arrow of time.” It appears that under certain circumstances, the arrow of time can point in either direction, and time can flow in either direction, forward or backward. This is a scientific result. It may be hard to believe, but the above experiments have been repeated. In the case of the double-slit experiment, it has been repeated numerous times. No one has been able to provide a widely accepted explanation. Reverse causality is a true mystery of science.

 

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Is Time Travel Possible?

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Few topics in science capture the imagination like time travel. Science fiction, like H. G. Wells’ classic novel, The Time Machine, published in 1895, and science fact, like time dilation, continues to fuel interest in time travel. Let us start with the most important question: Is time travel possible?

Of course, time travel is possible. We are already doing it. At this point, I know my answer may come across a bit flippant. However, my answer has a kernel of truth. We are traveling in time. We continually travel from the present to the future. This is what philosophers refer to as the arrow of time. In our everyday experience, it moves in one direction, from the present to the future. I think, though, on a more serious note, what people want to know is can we travel back in time—or to a future date in time.

In theory, it is possible. Indeed, numerous solutions to Einstein’s special and general relativity equations predict time travel is possible. In general, no law of physics prohibits time travel. We will begin by considering two methods science proposes to travel in time .

Method 1: Time Travel to the Future – Faster-than-light (FTL)

Using faster than light or near the speed of light, time travel appears to offer methodologies grounded in science fact. Consider two examples:

1) Assume you build a spaceship capable of traveling near the speed of light. With such a spaceship, you literally can travel into the future. This may sound like science fiction, but it is widely accepted as scientific fact. Particle accelerators confirm it. We discussed it when we discussed time dilation and the twin paradox. All you need is the spaceship, and an enormous amount of energy to accelerate it near the speed of light. However, this is an enormous problem. From Einstein’s special theory of relativity, we know that as we begin to accelerate a mass close to the speed of light, it becomes more massive, and approaches infinity. Thus, to accelerate it close to the speed of light, we need an energy source that approaches infinity. Perhaps we would have to learn how to harness the energy of a star, or routinely create matter-antimatter annihilations to create energy. Today’s science is nowhere near that level of sophistication.

2) Assume you can move information (like a signal) faster than light. Theoretically, if we could send a signal from point A to point B faster than the speed of light, it would represent a form of time travel. However, a significant paradox occurs. Here is an example.
An observer A in an inertial frame A sends a signal to an observer B in an inertial frame B. When B receives the signal, B replies and sends a signal back to A faster than the speed of light. Observer A receives the reply before sending the first signal.

In 1907, Albert Einstein described this paradox in a thought experiment to demonstrate that faster-than-light communications can violate causality (the effect occurs before the cause). Albert Einstein and Arnold Sommerfeld in 1910 described a thought experiment using a faster-than-light telegraph to send a signal back in time. In 1910, no faster-than-light signal communication device existed. It still does not exist, but the possibility of its development is increasing. From quantum physics, it appears that certain quantum effects “transmit” instantaneously and, therefore, appear to transmit faster than the speed of light in empty space. One example of this is the quantum states of two “entangled” particles (particles that have physically interacted, and later separated). In quantum physics, the quantum state is the set of mathematical variables that fully describes the physical aspects of a particle at the atomic level. When two particles interact with each other, they appear to form an invisible bond between them. When this happens, they become “entangled.” If we take one of the particles, and separate it from the other, they remain entangled (invisibly connected). If we change the atomic state of one of the entangled particles, the other particle instantaneously changes its state to maintain quantum-state harmony with the other entangled particle. Significant experimental evidence indicates that separated entangled particles can instantaneously transmit information to each other over distances that suggest the information exchange exceeds the speed of light. Initially, scientists criticized the theory of particle entanglement. After its experimental verification, science recognizes entanglement as a valid, fundamental feature of quantum mechanics. Today the focus of the research has changed to utilize its properties as a resource for communication and computation.

Method 2: Time Travel to the Past – Using Wormholes

Scientists have proposed using “wormholes” as a time machine. A wormhole is a theoretical entity in which space-time curvature connects two distant locations (or times). Although we do not have any concrete evidence that wormholes exist, we can infer their existence from Einstein’s general theory of relativity. However, we need more than a wormhole. We need a traversable wormhole. A traversable wormhole is exactly what the name implies. We can move through or send information through it.

If you would like to visualize what a wormhole does, imagine having a piece of paper whose two-dimensional surface represents four-dimensional space-time. Imagine folding the paper so that two points on the surface are connected. I understand that this is a highly simplified representation. In reality, we cannot visualize an actual wormhole. It might even exist in more than four dimensions.

How do we create a traversable wormhole? No one knows, but most scientists believe it would require enormous negative energy. A number of scientists believe the creation of negative energy is possible, based on the study of virtual particles and the Casimir effect.

Assuming we learn how to create a traversable wormhole, how would we use it to travel in time? The traversable wormhole theoretically connects two points in space-time, which implies we could use it to travel in time, as well as space. However, according to the theory of general relativity, it would not be possible to go back in time prior to the creation of the traversable wormhole. This is how physicists like Stephen Hawking explain why we do not see visitors from the future. The reason: the traversable wormhole does not exist yet.

Hard as it may be to believe, most of the scientific community acknowledges that time travel is theoretically possible. If fact, time dilation of subatomic particles provides experimental evidence that time travel to the future is possible, at least for subatomic particle accelerated close to the speed of light. Real science is sometimes stranger than fiction. What do you believe?

 

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Is Time Travel to the Future Possible?

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Since the future doesn’t exist, how would it be possible to travel into the future? This question has been debated by both philosophers and scientists. However, time travel to the future is the only experimental evidence we have of time travel. To understand this, we will need to understand Einstein’s theories of special and general relativity.

The science of time travel was launch in 1905,  when Einstein published his special theory of relativity in the prestigious Annalen der Physik (i.e., Annals of Physics), one of the oldest scientific journals (established in 1790). The paper that Einstein submitted regarding his special theory of relativity was titled “On the Electrodynamics of Moving Bodies.” By scientific standards, it was unconventional. It contained little in the way of mathematical formulations or scientific references. Instead, it was written in a conversational style using thought experiments. If you examine the historical context, Einstein had few colleagues in the scientific establishment to bounce ideas off. In fact, Einstein essentially cofounded, along with mathematician Conrad Habicht and close friend Maurice Solovine, a small discussion group, the Olympia Academy, which met on a routine basis at Solovine’s flat to discuss science and philosophy. It is also interesting to note that Einstein’s position as a patent examiner related to questions about transmission of electric signals and electrical-mechanical synchronization of time. Most historians credit Einstein’s early work as a patent examiner with laying the foundation for his thought experiments on the nature of light and the integration of space and time (i.e., spacetime).

Einstein’s special theory of relativity gave us numerous new important insights into reality, among them the famous mass equivalence formula (E = mc2) and the concept and formula for time dilation. Time dilation lays the foundation for forward time travel, so let’s understand it in more depth.

According to special relativity’s time dilation, as a clock moves close to the speed of light, time slows down relative to a clock at rest. The implication is that if you were able to travel in a spaceship that was capable of approaching the speed of light, a one-year round trip journey as measured by you on a clock within the spaceship would be equivalent to approximately ten or more years of Earth time, depending on your exact velocity. In effect, when you return to Earth, you will have traveled to Earth’s future. This is not science fiction. As I mentioned above, time dilation has been experimentally verified using particle accelerators. It is widely considered a science fact.

What scientific experimental evidence do we have that time dilation is real. Here are several experiments that validate time dilation caused when particles move close to the speed of light.

Velocity time dilation experimental evidence:

Rossi and Hall (1941) compared the population of cosmic-ray-produced muons at the top of a six-thousand-foot-high mountain to muons observed at sea level. A muon is a subatomic particle with a negative charge and about two hundred times more massive than an electron. Muons occur naturally when cosmic rays (energetic-charged subatomic particles, like protons, originating in outer space) interact with the atmosphere. Muons, at rest, disintegrate in about 2 x 10-6 seconds. The mountain chosen by Rossi and Hall was high. The muons should have mostly disintegrated before they reached the ground. Therefore, extremely few muons should have been detected at ground level, versus the top of the mountain. However, their experimental results indicated the muon sample at the base experienced only a moderate reduction. The muons were decaying approximately ten times slower than if they were at rest. They made use of Einstein’s time dilation effect to explain this discrepancy. They attributed the muon’s high speed, with its associated high kinetic energy, to be dilating time.

In 1963, Frisch and Smith once again confirmed the Rossi and Hall experiment, proving beyond doubt that extremely high kinetic energy prolongs a particle’s life.

With the advent of particle accelerators that are capable of moving particles at near light speed, the confirmation of time dilation has become routine. A particle accelerator is a scientific apparatus for accelerating subatomic particles to high velocities by using electric or electromagnetic fields. In 1977, J. Bailey and CERN (European Organization for Nuclear Research) colleagues accelerated muons to within 0.9994% of the speed of light and found their lifetime had been extended by 29.3 times their corresponding rest mass lifetime. (Reference: Bailey, J., et al., Nature 268, 301 [1977] on muon lifetimes and time dilation.) This experiment confirmed the “twin paradox,” whereby a twin makes a journey into space in a near-speed-of-light spaceship and returns home to find he has aged less than his identical twin who stayed on Earth. This means that clocks sent away at near the speed of light and returned near the speed of light to their initial position demonstrate retardation (record less time) with respect to a resting clock.

Time dilation can also occur as a result of gravity. Our understanding of this comes from Einstein’s theory of general relativity. What is the difference between the special and general theory of relativity? Einstein used the term “special” when describing his special theory of relativity because it only applied to inertial frames of reference, which are frames of reference moving at a constant velocity or at rest. It also did not incorporate the effects of gravity. Shortly after the publication of special relativity, Einstein began work to consider how he could integrate gravity and noninertial frames into the theory of relativity. The problem turned out to be monumental, even for Einstein. Starting in 1907, his initial thought experiment considered an observer in free fall. On the surface, this does not sound like it would be a difficult problem for Einstein, given his previous accomplishments. However, it required eight years of work, incorporating numerous false starts, before Einstein was ready to reveal his general theory of relativity.

In November 1915, Einstein presented his general theory of relativity to the Prussian Academy of Science in Berlin. The equations Einstein presented, now known as Einstein’s field equations, describe how matter influences the geometry of space and time. In effect, Einstein’s field equations predicted that matter or energy would cause spacetime to curve. This means that matter or energy has the ability to affect, even distort, space and time. One important aspect prediction of general relativity was that gravitational fields could cause time dilation. Here are some important experiments that prove this aspect of general relativity is correct.

Gravitational time dilation experimental evidence:

In 1959, Pound and Rebka measured a slight redshift in the frequency of light emitted close to the Earth’s surface (where Earth’s gravitational field is higher), versus the frequency of light emitted at a distance farther from the Earth’s surface. The results they measured were within 10% of those predicted by the gravitational time dilation of general relativity.

In 1964, Pound and Snider performed a similar experiment, and their measurements were within 1% predicted by general relativity.

In 1980, the team of Vessot, Levine, Mattison, Blomberg, Hoffman, Nystrom, Farrel, Decher, Eby, Baugher, Watts, Teuber, and Wills published “Test of Relativistic Gravitation with a Space-Borne Hydrogen Maser,” and increased the accuracy of measurement to about 0.01%. In 2010, Chou, Hume, Rosenband, and Wineland published “Optical Clocks and Relativity.” This experiment confirmed gravitational time dilation at a height difference of one meter using optical atomic clocks, which are considered the most accurate types of clocks.

The above discussion provides some insight into time dilation, or what some term time travel to the future. However, is it conclusive? Not to my mind! Although we have numerous experiments that demonstrate time dilation (i.e., forward time travel) involving subatomic particles is real, we have been unable to demonstrate significant human time dilation. By the word “significant,” I mean that it would be noticeable to the humans and other observers involved. To date, some humans, such as astronauts and cosmonauts, have experienced forward time travel (i.e., time dilation) in the order of approximately 1/50th of a second, which is not noticeable to our human senses. If it were in the order of seconds or minutes, then it would be noticeable. Scientifically speaking, there is no documented significant evidence of human time travel to the future.

To answer the subject question of this post, time travel to the future appears to have a valid scientific and experimental foundation. However, to date the experimental evidence does not include significant (noticeable)  human time travel to the future, which leaves the question still unanswered. My own view is that when we develop space craft capable of speeds approaching the speed of light with humans on board, time dilation (time travel to the future) will be conclusively proven.

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Is Time Travel to the Past Possible?

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For time travel to the past to be possible would require that the past have a physical reality, namely that it continue to exist. If it did not continue to exist, it would suggest time travel to the past is impossible.

Time travel to the past has it theoretical foundation in Einstein’s special relativity. in the way of background, in November 1915, Einstein presented his general theory of relativity to the Prussian Academy of Science in Berlin. The equations Einstein presented, now known as Einstein’s field equations, describe how matter influences the geometry of space and time. In effect, Einstein’s field equations predicted that matter or energy would cause spacetime to curve. This means that matter or energy has the ability to affect, even distort, space and time.

Many of the predictions of general relativity have been scientifically verified. Two of the most important predictions for our study of time travel are (1) gravitational time dilation and (2) closed timelike curves.

Gravitational time dilation predicts that a clock in a strong gravitational field will run slower than a clock in a weak gravitational field. Therefore, a clock on the surface of Jupiter, a massive gas planet three hundred times larger than the Earth, resulting in a significantly stronger gravitational field, will run much slower than a clock on the surface of the Earth. This phenomenon was first verified on Earth, with clocks at different altitudes from the Earth’s surface. Using atomic clocks, time dilation effects are detectable when the clocks differ in altitude by as little as one meter.

Gravitational time dilation also occurs in accelerating frames of reference (i.e., noninertial frames of reference). According to Einstein’s general theory of relativity, an accelerated frame of reference produces an “inertial force,” also termed a “pseudo force,” that results in the same effect as a gravitational force in an inertial frame of reference. The equivalence of the inertial force in a noninertial frame of reference (i.e., an accelerating frame of reference) to a gravitational force in an inertial frame of reference (i.e., a frame of reference moving at a constant velocity) is termed the equivalence principle. The equivalence principle refers to the equivalence of “inertial mass” and “gravitational mass.” Therefore, a blindfolded person in a rapidly ascending elevator would experience a force equivalent to an increase in gravity, as if standing on a planet more massive than Earth. The blindfolded person would not be able to determine if the force experienced is inertial or gravitational. This effect also holds true for time dilation. Time moves slower in a highly accelerated frame of reference in much the same way it would as if it were in a strong gravitational field. It is important to note, a frame of reference can accelerate in two fundamental ways. It can accelerate along a straight line, or it can accelerate by rotating.

Next, let us discuss closed timelike curves. What is a closed timelike curve? It is an exact solution to Einstein’s general relativity equations demonstrating a particle’s world line (i.e., the path the particle follows in four-dimensional spacetime) is “closed” (i.e., the particle returns to its starting point). Closed timelike curves theoretically suggest the possibility of backward time travel. The particle’s world line is describable by four coordinates at each point along the world line, and when it closes on itself, the four coordinates at the start equal the four coordinates at the end. The particle, conceptually, went back to its past (i.e., the starting point). You can think of this like a horse racetrack. As the horse runs around the track, the horse eventually crosses the finish line, the starting point. If we allow the horse racetrack to represent a world line, then when the horse crosses the finish line, the horse has returned to its past (i.e., the starting point). In the mathematics of general relativity, the starting four coordinates, including the fourth dimensional coordinate that includes a time component, equal the four coordinates at the finish line.

The first person to discover a solution to Einstein’s general relativity equations suggesting closed timelike curves (CTCs) was Austrian American logician, mathematician, and philosopher Kurt Gödel, in 1949. The solution was termed the Gödel metric. Since 1949, numerous other solutions containing CTCs have been found, such as the Tipler cylinder and traversable wormholes, both of which will be discussed in section 3. The numerous solutions to Einstein’s general relativity equations suggest that time travel to the past is theoretically possible. However, the entire scientific community is not in complete agreement on this last point.

The largest issue that physicists have with backward time travel is causality violations (cause and effect), where the effect precedes the cause. These violations of causality are termed “time travel paradoxes.” Some physicists suggest that time travel paradoxes inhibit backward time travel, while other physicists argue that time travel paradoxes can be reconciled, and backward time travel is possible. There is no scientific consensus regarding the reality or practicality of time travel to the past. Although, there are a number of experiments that suggest reverse causality is scientifically possible.

Let us consider a recent experiment that demonstrates reverse causality is not only possible, but a scientific fact. In 2009, physicist John Howell of the University of Rochester and his colleagues devised an experiment that involved passing a laser beam through a prism. The experiment also involved a mirror that moved in extremely small increments via its attachment to a motor. When the laser beam was turned on, part of the beam passed through the prism, and part of the beam bounced off the mirror. After the beam was reflected by the mirror, the Howell team used “weak measurements” (i.e., measurement where the measured system is weakly affected by the measurement device) to measure the angle of deflection. With these measurements, the team was able to determine how much the mirror had moved. This part of the experiment is normal, and in no way suggests reverse causality. However, the Howell team took it to the next level, and this changed history, literally. Here is what they did. They set up two gates to make the reflected mirror measurements. After passing the beam through the first gate, the experimenters always made a measurement. After passing it through the second gate, the experimenters measured the beam only a portion of the time. If they chose not to make the measurement at the second gate, the amplitude of the deflected angle initially measured at the first gate was extremely small. If they chose to make the measurement at the second gate, the deflected angle initially measured at the first gate was amplified by a factor of 100. Somehow, the future measurement influenced the amplitude of the initial measurement. Your first instinct may be to consider this an experimental fluke, but it is not. Physicists Onur Hosten and Paul Kwiat, University of Illinois at Urbana-Champaign, using a beam of polarized light, repeated the experiment. Their results indicated an even larger amplification factor, in the order of 10,000.

The above experiment strongly suggest that the future can influence the past. This implies, the past must continue exist and have a physical reality. If it no longer existed, how could the future influence the past. as the above experiments demonstrate.

This is an exciting time for science. Physical experiments suggest that the past may continue to physically exist. If that is true, then time travel to the past may be possible. The is an old saying in physics, “That which is not forbidden by physical law is compulsory.” The exact origin of the saying is not clearly known, but is often attributed to Murray Gell-Mann (born 15 September 1929), an American physicist who received the 1969 Nobel Prize in Physics for his work on the theory of elementary particles. To my mind, this saying suggests it is only a matter of time before we discover how to time travel to the past.