Time is perhaps the greatest mystery of all and is deeply wrapped up in our conscious experience of things. Since antiquity, time has naturally attracted the interest of philosophers and scientists determined to understand and explain its true nature. At the heart of the question is whether time is an actual reality of the physical world or simply an artificial construct of the human mind.

The pre-Socratic Greek philosopher Parmenides, for example, saw time as merely an illusion, while his contemporary Heraclitus believed the flow of time to be real and the very essence of reality itself. For Newton time was absolute and moved at a consistent pace everywhere throughout the universe, while with Einstein time became more flexible and relative in scope. Nevertheless, the concept of time continues to be one of nature’s greatest mysteries and no one has been able to fully explain what it is really.

## 4-Dimensional Space-Time

In the 17th Century, René Descartes (1596-1650) envisioned there to be three dimensions of space, with a totally separate dimension of time that together specify an object’s position in physical space. Simply stated, three dimensions are used to specify an object’s location/movement in space (forward-backward, left-right and up-down), while a fourth dimension locates its position in time. This approach was adopted by Isaac Newton (1643-1727), the founder of classical mechanics, and classical physicists who saw time as an absolute, universal quantity completely independent from space. As such, early scientists used time to delineate a universal system of coordinates through 3-dimensional space.

In 1905, Albert Einstein then published his Special Theory of Relativity which was instrumental in introducing the concept of four-dimensional space-time. According to Einstein, time, space, and motion all act upon each other, and as an object’s speed increases, its time slows down in order to preserve the cosmic speed limit of light. This new concept merged space and time into space-time and helped introduced a new framework for the whole of physics.

## We Are 3-Dimensional Creatures

Being three-dimensional creatures (possessing length, width, and height), humans are unable to see the fourth dimension as our physical world is constructed within these three physical dimensions. We might feel or intuit time’s presence, but we can never actually detect it with our three-dimensional senses because it extends beyond our universe. Humans only perceive the fourth dimension of time as memories lodged at variable intervals, the result of which is our apparent perception of time moving forward in a straight line.

Nevertheless, time exists as a dimension and objects cross it in a similar way as they do the others, although three-dimensional humans are only able to move in one direction forward through time. If we could see an object’s fourth-dimensional space-time (or world-line) it would resemble a spaghetti-like line stretching from the past to the future showing the spatial location of the object at every instant in time.

## Space and Time are Inseparable

Space And Time are simultaneous phenomena (like mass and energy), and together form the fabric of the universe known as space-time. A demonstration of four-dimensional space-time’s inseparability is the fact that, as astronomers often remind us, we cannot look into space without looking back into time. We see the Moon as it was 1.2 seconds ago and the Sun as it was 8 minutes ago.

Also, in accordance with Einstein’s general theory of relativity, a massive object in space distorts the fabric of both the space and time around it. In other words, gravity is actually the result of mass stretching its surrounding space-time. For example, our Sun’s mass bends the space around it so that the Earth moves in a straight line but also circles within the Sun’s curvature in space. The Sun’s effect on time is to slow it down, so time runs slower for those objects close to the massive stellar object.

Interestingly, gravity also has an infinite range such that no matter how far apart two masses are in space they will always experience some gravitational pull toward each other. Theoretical physicists have tried to explain this phenomenon in terms of gravitons, S-Theory, and M-Theory, but even today a successful quantum theory of gravity has yet to be found.

## How Time **Changes at Relativistic Speeds**

A property of light is that it always travels at the same constant speed in a vacuum of 186,000 miles a second (700 million mph) and you can’t go any faster. Let us now take a look at the effect special relativity and traveling at high speeds have on the concept of time. According to the mathematical formula:

**Speed = Distance ÷ Time**

As we now know, the Speed of light (c) is fixed/absolute and represents the inviolable cosmic speed limit. As you travel at relativistic speeds or those speeds in which the relativistic effect becomes significant, then the distance and time values in the equation become flexible and are forced to change relative to one another. What actually happens is that time and distance are ‘relative’ to one another, and as you travel close to the speed of light, distances become shortened while time is lengthened. This is explained in Einstein’s theory of special relativity.

The following table shows the extent time (one hour) slows down relative to what percentage of the speed of light an object is traveling. As you can see, you don’t need to travel at light speed for time dilation to occur, but you won’t notice the effects until you go extremely fast. Bear in mind, also, that the fastest man-made object ever built, NASA’s Parker Solar Probe, has only managed to achieve a top speed of 430,000 mph (692017.92 km/h or 0.06412% of light speed.

- 0 % of c: 60.00 mins
- 10 % of c: 59.52 mins
- 20 % of c: 58.70 mins
- 30 % of c: 57.20 mins
- 40 % of c: 55.00 mins
- 50 % of c: 52.10 mins
- 60 % of c: 48.10 mins
- 70 % of c: 42.85 mins
- 80 % of c: 36.00 mins
- 90 % of c: 26.18 mins
- 92 % of c: 23.52 mins
- 95 % of c: 18.71 mins
- 99 % of c: 8.53 mins
- 99.9 % of c: 2.78 mins
- 99.997 % of c: 1.17 mins
- 100 % of c: zero mins

Roughly speaking, a person traveling at 99% the speed of light would experience timed slowed by roughly a factor of 7. If they were to travel to a star 7 light years away at 99% speed of light, it would thus take them 1 year to reach their destination, but to an observer on Earth, it would have seemed like 7 years have passed. However, if that person attained 99.9999% of the speed of light, only 1 year would pass onboard for roughly every 70 years back on Earth. Meanwhile, a speed of 0.9999999 % of c would equate to 2,236 years of time elapsing, a speed of 0.9999999999 of c would see 70,710 years pass on Earth, rising at 0.999999999999999 of c to a staggering 22,369,621 years.

## Traveling to the Stars and Time Dilation

As the table above indicates, traveling to the stars at high percentages of the speed of light would allow travelers to cover vast distances but experience very little time. The astronomical distances between the stars subsequently become no obstacle at all to traverse and a trip to another star system would feel near instantaneous. The following examples of the time it would take space crews traveling at near-light-speed to reach various destinations will help illustrate this point:

**Alpha Centauri**: The near-light-speed craft traveling to Alpha Centauri, our nearest extrasolar sun located 4.3 light years away, would take just 4.3 thousandths of a second to complete the journey.**Milky Way**: A space crew would experience 3.2 seconds of time while crossing the 300,000 light years distance to the center of our galaxy.**Andromeda Galaxy**: Located 2.2 million light-years away, the journey, as far as the crew are concerned, would last 3.5 minutes.**Virgo Cluster**: Located 40 million light-years away, the crew would experience a one-and-a-half-hour journey.**Edge of Universe**: An estimated 17 billion light years away, the edge of our universe could be reached within 19 days of crew time.

## Speed of Light and Immortality

Furthermore, if one day people were actually able to attain light-speed travel, then any journey undertaken at said speed would subsequently result in passengers experiencing no time at all. A photon, which is the basic unit that makes up all light, for instance, experiences no time whatsoever between its emission and its absorption.

In other words, despite a photon crossing billions of light years of space, the proper time it experiences between any two points on its path is zero and is reduced to just one instant. This idea is encapsulated in an observation made by sci-fi author ray Cummings in his 1919 short story The Girl in the Golden Atom, who noted: “Time . . . is what keeps everything from happening at once”.

Scientists believe photons have zero mass and so cannot decay. Some theories, however, suggest photons might have a minute rest mass and so can eventually decay into lighter elementary particles. According to one study, a photon’s lifetime within its own rest frame amounts to just three years. Nevertheless, photons are still capable of surviving for an estimated billion billion years (10^{18}) because of the time dilatation they experience traveling at the speed of light. Considering the universe is an estimated 13.8 billion years old, it’s fair to say that a photon, for all intents and purposes, lives forever.