Time travel basics

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Time travel is a fascinating topic that has been explored in science fiction and pop culture for many years. But is it really possible? How does it work? This article will explore the different types of time travel that exist and answer all your questions about this fascinating topic.

What is time travel?​

Time travel is the movement of a person or objects from one point in time to another, like moving a person or object from one location in space to another. Time travel is generally described within the context of hypothetical equipment known as a time machine, a concept first popularized by H. G. Wells’ 1895 novel The Time Machine. Time travel is a popular topic in philosophy and fiction, especially science fiction.

It is unknown whether physical time travel to the past is conceivable. Forward time travel, outside of the conventional notion of time perception, is a widely seen and well-understood phenomenon within special and general relativity frameworks. However, with current technology, making one body advance or delay another by more than a few milliseconds is not possible. In terms of backward time travel, general relativity solutions, such as a rotating black hole, are possible. Traveling to an arbitrary space-time location has a minimal theoretical basis. It is usually associated with quantum mechanics or wormholes.

How does time travel work? Is it really possible to travel through time?​

Many theories propose that suitable geometries of space-time or specific types of motion in space may allow time travel into the past and future if these geometries or motions are possible.

In scientific articles, physicists talk about closed time-like curves, which are world lines that make closed loops in space-time and let things go back to their own past. It’s possible that some solutions to general relativity equations that describe space-times with closed time-like curves, like the Gödel space-time, are not valid.

Many scientists feel that backward time travel is highly implausible. Any theory that allows for time travel would pose potential causality issues. The “grandfather paradox” is a classic example of a causality problem: what if one could travel back in time and kill one’s own grandfather before one’s father was conceived? The Novikov self-consistency principle or a version of the many-worlds interpretation with interacting worlds may help avoid these kinds of temporal paradoxes.

Time travel and general relativity​

Some general relativity space-time geometries make it possible to travel faster than light back to the past. These geometries include cosmic strings, wormholes that can be walked through, and Alcubierre drives. These geometries allow faster-than-light travel. The theory of general relativity suggests that it is possible to go back in time in some very unusual situations. Semiclassical gravity claims that these loopholes may be closed when quantum effects are added to general relativity. Hawking came up with the chronology protection conjecture based on these semiclassical arguments. It says that the fundamental laws of nature prevent time travel. Still, physicists can’t make a definitive decision on the issue until they have a theory of quantum gravity to combine quantum mechanics and general relativity into a single theory.

Relativistic geometries

There are field equations in the theory of general relativity that determine the metric, or distance function, of space-time. These equations are called “field equations.” In these equations, exact solutions include closed time-like curves, which are world lines that cross each other. At some point in the causal future of the world line, it is also in the causal past, which can be called time travel. Solution: Kurt Gödel came up with one called the Gödel metric. It requires the universe to have physical characteristics that it doesn’t have, like rotation and no Hubble expansion, which it doesn’t seem to have at all. Whether general relativity bans closed time-like curves in real-world situations is still being studied.


Wormholes are hypothetical warped space-times permitted by Einstein’s field equations of general relativity. A time machine based on a wormhole that can be crossed would work like this:

One end of the wormhole is sped up to a significant fraction of the speed of light, maybe with help from an advanced propulsion system. Then, the wormhole is returned to where it came from.

Another way to move a wormhole entry is to move it to an object with higher gravity than the other one. Then move it back to a place near the other one.

These methods make the moved end of the wormhole age less, or become “younger” than the stationary end as seen by an outsider. However, time connects differently through the wormhole than outside it, so that synchronized clocks at both ends of the wormhole will always stay in sync, even if the two ends move around. This means that if someone were to enter the “younger” end, they would exit the “older” end when it was the same age as the “younger” end, going back in time from the outside. One of the main limitations of a time machine like this is that it can only go back in time as far as the machine was made. In other words, it is more like a path through time than a machine that moves through time. It would not allow the technology itself to be moved back in time, so it would not work.

According to current theories about how wormholes work, a substance with negative energy, sometimes called “exotic matter” is needed to make a wormhole that can be crossed. The wormhole space-time requires an allocation of energy that doesn’t satisfy several energy laws, including the null energy condition and the weak, strong, and dominant energy rules. Many physicists think that the negative energy required by quantum physics may be possible because of the Casimir effect. However, it is known that quantum processes can cause minor, quantifiable deviations from the null energy rule. Even though early estimates said that much negative energy would be needed, later calculations showed that the amount of negative energy can be made as small as possible.

Suppose you have two wormholes with different clocks at their mouths. In that case, you can’t bring them together without having a quantum field and gravitational effects that could cause the wormhole to collapse or push the two mouths away. This is what Matt Visser said in 1993. As a result, the two mouths couldn’t be close enough to break causality. Although Visser noted in 1997 that a “Roman ring” of wormholes could still be used as a time machine, this is more likely to result from a flaw in classical-quantum gravity theory than proof that causality can be broken.

What are the implications of time travel? What would happen if we traveled back in time?​

It’s an inspiring idea, but there are many questions about the implications of time travel. Since at least the time of ancient Greece, philosophers have debated the nature of time; for example, Parmenides proposed that time is an illusion. Centuries later, Isaac Newton defended the concept of absolute time. However, his colleague Gottfried Wilhelm Leibniz contended that time is just a relationship between events and cannot be expressed independently. The latter method eventually gave rise to relativistic space-time.

Many philosophers have claimed that relativity entails eternalism, the belief that the past and future exist in a genuine sense, rather than only as changes that have occurred or will occur in the present. Dean Rickles, a philosopher of science, disagrees with these qualifications but observes that “the consensus among philosophers appears to be that special and general relativity are incompatible with presentism.” Some philosophers believe that time is a dimension equal to spatial dimensions, that future events are “already there” in the same way that distinct places exist. There is no objective flow of time; this is a controversial viewpoint.

Presentism is a philosophy that says that the future and the past only exist as changes to the present that have happened or will happen. They don’t have a separate existence of their own. It’s not possible to go back and forth in time from this point of view because there is no future or past to go to. Keller and Nelson say that even if past and future objects don’t exist, there can still be certain truths about past and future events, so a truth about a time traveler returning to the present could explain why the time traveler actually showed up in the present. However, some authors disagree.

In classical space-time, presentism holds that only the present exists; this is incompatible with special relativity, as demonstrated by the following example:

Alice and Bob are both watching event O at the same time. When it comes to Alice, some event E happens simultaneously as O. But when it comes to Bob, event E is in the past or in the future. Classical presentism doesn’t agree with Alice and Bob’s thoughts about the present. It’s called “here-now presentism,” and it tries to make sense of this by only acknowledging the time and space of a single point. This doesn’t work because objects coming and going from the “here-now” are both real and unreal, and there isn’t a privileged “here-now.” It’s called “relativized presentism” because it recognizes that there are infinite frames of reference, each with a different set of simultaneous events. This means that there can’t be a single “real” present, so either all events in time are real, or each frame of reference exists in its own reality. It looks like there are no more options for presentism in special relativity. Gödel and others think presentism may be true for some forms of general relativity. The idea of absolute time and space is believed to be incompatible with general relativity because there is no universal truth about the absolute position of events that happen at different times. There is no way to figure out which point in space at one time is at the universal “same place” at another time. The principle of diffeomorphism invariance says that all coordinate systems are on the same level.

Is time travel dangerous, and could it have negative consequences?​

Fictional stories about time travel often explore the paradoxes that occur because of multiple futures or because one cannot change history. In some stories, the hero knows what happens in the future. He travels back in time only to change events, creating some variation of The Grandfather Paradox.

The grandfather paradox, also known as the argument of self-infanticide, is a common reason people don’t like the idea of going back in time. A person who could travel back in time would have problems and contradictions if they changed anything. If the past is changed, there would be a contradiction. The paradox is often shown as a person going back in time and killing their own grandfather, which stops their father or mother from being alive, and thus their own existence. Philosophers say that these paradoxes show that time travel is not possible. In contrast, others say that while backward time travel is theoretically possible, it is impossible to change the past in any way, like the Novikov self-consistency principle in physics.

What scientific theories suggest time travel is possible?​

In special and general relativity, there is a lot of evidence for time dilation and gravitational time dilation. This is shown by things like atmospheric muon decay, which is well-documented and easy to reproduce. According to relativity theory, the speed of light is the same for everyone in every frame of reference. It always stays the same. Time dilation is caused by the fact that the speed of light isn’t always the same. A person can use time dilation to make it seem like a small amount of time has passed while years pass outside the dilation. This is time travel into the future and can be done by moving at relativistic speeds or taking advantage of gravity’s effects.

When two identical clocks move in the same direction without accelerating, one clock perceives the other to be ticking slower. Because of the relativity of simultaneity, this is possible. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The twin paradox describes this as follows: one twin remains on Earth, while the other is accelerated to relativistic speed as they travel into space, turn around, and return to Earth; the traveling twin ages less than the twin who remained on Earth due to time dilation experienced during their acceleration.

General relativity says that acceleration and gravity effects are the same. It shows that time dilation happens in gravity wells, with a deeper clock in the well ticking slower than the clock closer to the surface. This effect calibrates the clocks on the Global Positioning System satellites. It could lead to significant differences in aging rates for people at different distances from a large gravity well like a black hole.

A time machine based on this theory may have a spherical shell with a diameter of five meters and the mass of Jupiter. A person in its center will move forward in time four times slower than distant onlookers. Squeezing the mass of a massive planet into such a compact structure is not likely to be technologically feasible in the foreseeable future. After a few hundred days of space travel, existing technologies can only cause a human traveler to age less than their Earthly friends by a few milliseconds.

How could time travel change humanity and our understanding of the universe?​

If genuine time travel is ever possible, it will be the most revolutionary discovery in all of human history.

Imagine traveling back in time to witness any event, past or present. To meet your parents before they conceived you. To see the events of the Bible. To be able to answer the question “Where did we come from?” and “why are we here?” The potential benefits to humanity are enormous. The discovery of time travel would affect history and science and be a true revolution from the influences of evil and propaganda.

Should we be worried about time travelers from the future coming to our present-day?​

Many people believe that the lack of time travelers from the future proves that such technology will never be produced, implying that it is impossible. This is akin to the Fermi conundrum, which is concerned with the lack of proof of extraterrestrial life. The fact that there are no alien visitors does not mean they don’t exist. Just like the fact that there are no time travelers doesn’t mean time travel is physically impossible.

It’s possible that time travel is physically possible but is never developed or used carefully. Carl Sagan once speculated that time travelers could be present but conceal their presence or are not recognized as time travelers. According to some interpretations of general relativity, time travel may be possible only in a region of space-time that is twisted in a specific way. So time travelers would be unable to travel back to earlier areas of space-time before this region existed. According to Stephen Hawking, this would explain why the globe hasn’t already been invaded by “future tourists.”

Several experiments have been conducted to lure future humans who may invent time travel technology to return and demonstrate it to people of the present. Permanent “advertisements” of a meeting time and place for future time travelers were highly publicized at events such as Perth’s Destination Day or MIT’s Time Traveler Convention. In 1982, a group known as the Krononauts organized a similar event in Baltimore, Maryland, greeting guests from the future. These tests could produce a positive outcome confirming the reality of time travel. Still, they have so far failed. No time travelers are known to have attended either event. Some variants of the many-worlds interpretation can be applied to


Theories of time travel are consistent with the theory of relativity and quantum mechanics. However, it is challenging to design experiments that do not already have the outcome expected by the experimenter built into their design. So far, time travel has not been determined to be impossible. Still, it also has not been discovered despite strenuous efforts by explorers.
Time travel basics

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