Is Time Travel Possible? Let’s take a look at the science.

Time Travel In Pop Culture

Time travel has become a frequent motif in popular culture, with numerous time-travel plots in films, television, and literature. However, it is a concept that goes back a long way: the Greek tragedy Oedipus Rex by Sophocles, which was written over 2,500 years ago, may be considered the first time-travel narrative.

While the concept of time travel has been around for a long time, it wasn’t until the late 19th century that the idea began to be taken seriously by scientists. In 1895, H.G. Wells published The Time Machine, which popularized the idea of time travel and introduced many of the themes and tropes that are now common in time-travel narratives.

But is it feasible to travel through time? Given the public’s appetite for the idea, this is a fair concern.

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The simplest answer is that time travel isn’t feasible because we’d be doing it already if it were. The second law of thermodynamics and relativity are two examples that might be cited. It may be possible, but it will require a tremendous amount of energy.

There’s also the issue of time-travel paradoxes; we might — theoretically – address these if free will is an illusion, if there are many universes or if the past can only be observed but not experienced. Perhaps time travel is impossible simply because time must flow in a straight line and we have no control over it, or perhaps time is an illusion, and time travel is irrelevant.

Laws of physics

We’d all want to believe that time travel is prohibited by relativity since Einstein’s theory of relativity — which explains the nature of time, space, and gravity — is our most profound understanding of time. However, one of his peers at the Institute for Advanced Study, Kurt Gödel, devised a cosmos in which time travel was not just feasible but necessary. It’s called a Gödel universe, and it’s based on a rotating black hole.

Related Project Pegasus, Did DARPA Invent Time Travel

In a Gödel universe, there would be closed time-like curves, which are paths that return to their starting point in spacetime. These curves are allowed by the equations of general relativity; they’re just not found in our universe. The key is that they require matter to rotate, which warps spacetime in a way that makes time travel possible.

Gödel’s universe doesn’t violate the laws of physics; it just adds an extra dimension to them. It’s possible that our universe is like Gödel’s, and time travel is something that happens all the time — we just can’t do it because we’re not on a closed time-like curve.

Another example of a universe that contains closed time-like curves is a “cosmic string.” A cosmic string is a one-dimensional topological defect that may have formed during the early universe. If a string has enough mass per unit length, it will cause spacetime to curve in a way that produces closed time-like curves.

So, it’s possible that time travel is prohibited by the laws of physics, but it’s also possible that the laws of physics allow for time travel. It just depends on the universe you live in.

Creating A Time Machine

We can create time machines, but the majority of these (in principle) successful proposals need negative energy, or negative mass, which we don’t believe exists in our universe. A tennis ball with negative mass will fall upwards if you drop it. Because this explanation merely explains why we can’t time travel in practice by including another concept — that of negative mass — which itself is unproven, it doesn’t seem very satisfying.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe.

Time travel also runs contrary to the second law of thermodynamics, which states that entropy or disorder must always increase. Time can only move forward; for example, you cannot unscramble an egg. More precisely, by jumping into the past we are moving from a high-entropy state (now) into a lower-entropy past.

The notion that the cosmos is expanding has been used as an explanation for why we don’t notice any changes in the sky. This argument, at best, lacks completeness. It may stop you from going into the past, but it says nothing about time travel into the future. In practice, it is just as difficult for me to go to next Monday as opposed to last Monday.

Resolving paradoxes

If we could go back and forth in time at will, there would certainly be paradoxes. The “grandfather paradox” is the most well-known: one might theoretically use a time machine to travel to the past and kill their grandfather before their father’s conception, therefore preventing their own birth. You can’t both exist and not exist; logically, the grandfather paradox should be impossible.

But some physicists believe that time travel into the past is not only possible but, in fact, inevitable. In their view, time travel paradoxes can be resolved by appealing to the many-worlds interpretation of quantum mechanics.

In the many-worlds interpretation, there is a separate universe for every possible outcome of every quantum event. So, if you travel back in time and kill your grandfather, he still exists in another universe. In the universe where you were born, however, your father was never conceived and you don’t exist.

The idea of parallel universes may seem far-fetched, but it is supported by some evidence. For example, the double-slit experiment shows that particles can take multiple paths through space and time. If you could somehow observe the particle, it would “collapse” into a single path. But in the many-worlds interpretation, the particle actually takes all possible paths; it’s just that we only see one of them.

The many-worlds interpretation is the most popular way to resolve time travel paradoxes, but it is not the only one. Some physicists believe that time travel is impossible because any time machine would necessarily be destroyed by its own temporal effects. This view was first proposed by Kerry Thornley and later expanded upon by Robert A. Heinlein in his science fiction novel “By His Bootstraps”.

Time is a river

We can conceive of time as flowing past every point in the cosmos, like a river around a rock. However, it is difficult to express the notion precisely. A flow is a measure of change: the flow of a river is the volume of water that passes a given length in one second. As a result, if time is considered to be a flow, it would be a physical quantity with units of measure (e.g. seconds) and dimensions (e.g. length).

The problem is that there is no agreed-upon definition of time flow. A straightforward way to define it would be the number of events that occur in a given interval divided by the duration of that interval.

Stephen Hawking advanced the argument that there must be a “chronology protection conjecture,” an unknown physical principle that prohibits time travel. According to Hawking, we can’t know what goes on inside a black hole because we don’t have access to anything coming out of it.

Researchers are pursuing a more fundamental idea, in which time and space “emerge” from something else. Quantum gravity is the name for this theory, but it hasn’t been developed yet. Some physicists believe that space and time are emergent properties of a more fundamental underlying reality.

Time travel is a popular topic in fiction, but it’s still an open question in physics. We don’t yet have a good way to think about it, let alone a way to build a time machine. But the search for a theory of quantum gravity may one day lead us to a better understanding of time, and perhaps even to the ability to travel through it.

Source: The Conversation