What is Space?
We can see in our detectors. We measure long distances in space in "light-years," representing the distaance From the perspective of an Earthling, outer space is a zone that occurs about 100 kilometers (60 miles) above the planet, where there is no appreciable air to breathe or to scatter light. In that area, blue gives way to black because oxygen molecules are not in enough abundance to make the sky blue.
Further, space is a vacuum, meaning that sound cannot carry because molecules are not close enough together to transmit sound between them. That's not to say that space is empty, however. Gas, dust and other bits of matter float around "emptier" areas of the universe, while more crowded regions can host planets, stars and galaxies.
No one knows exactly how big space is. The difficulty arises because of what it takes for light to travel in a year (roughly 5.8 trillion miles, or 9.3 trillion kilometers).
What is a Space time?
In physics, spacetime is any mathematical model that combines space and time into a single interwoven continuum. The spacetime of our universe is "usually" interpreted from a Euclidean space perspective, which regards space as consisting of three dimensions, and time as consisting of one dimension, the "fourth dimension". By combining space and time into a single manifold called Minkowski space, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.
Explanation
In non-relativistic classical mechanics, the use of Euclidean space instead of spacetime is appropriate, because time is treated as universal with a constant rate of passage that is independent of the state of motion of an observer. In relativistic contexts, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends on the object's velocity relative to the observer and also on the strength of gravitational fields, which can slow the passage of time for an object as seen by an observer outside the field.
In cosmology, the concept of spacetime combines space and time to a single abstract universe. Mathematically it is a manifold consisting of "events" which are described by some type of coordinate system. Typically three spatial dimensions (length, width, height), and one temporal dimension (time) are required. Dimensions are independent components of a coordinate grid needed to locate a point in a certain defined "space". For example, on the globe the latitude and longitude are two independent coordinates which together uniquely determine a location. In spacetime, a coordinate grid that spans the 3+1 dimensions locates events (rather than just points in space), i.e., time is added as another dimension to the coordinate grid. This way the coordinates specify where and when events occur. However, the unified nature of spacetime and the freedom of coordinate choice it allows imply that to express the temporal coordinate in one coordinate system requires both temporal and spatial coordinates in another coordinate system. Unlike in normal spatial coordinates, there are still restrictions for how measurements can be made spatially and temporally (see Spacetime intervals). These restrictions correspond roughly to a particular mathematical model which differs from Euclidean space in its manifest symmetry.
Until the beginning of the 20th century, time was believed to be independent of motion, progressing at a fixed rate in all reference frames; however, following its prediction by special relativity, later experiments confirmed that time slows at higher speeds of the reference frame relative to another reference frame. Such slowing, called time dilation, is explained in special relativity theory. Many experiments have confirmed time dilation, such as the relativistic decay of muons from cosmic ray showers and the slowing of atomic clocks aboard a Space Shuttle relative to synchronized Earth-bound inertial clocks. The duration of time can therefore vary according to events and reference frames.
When dimensions are understood as mere components of the grid system, rather than physical attributes of space, it is easier to understand the alternate dimensional views as being simply the result of coordinate transformations.
The term spacetime has taken on a generalized meaning beyond treating spacetime events with the normal 3+1 dimensions. It is really the combination of space and time. Other proposed spacetime theories include additional dimensions—normally spatial but there exist some speculative theories that include additional temporal dimensions and even some that include dimensions that are neither temporal nor spatial (e.g., superspace). How many dimensions are needed to describe the universe is still an open question. Speculative theories such as string theory predict 10 or 26 dimensions (with M-theory predicting 11 dimensions: 10 spatial and 1 temporal), but the existence of more than four dimensions would only appear to make a difference at the subatomic level.
What is Time travel?
Time travel is the concept of movement (often by a human) between different points in time in a manner analogous to moving between different points in space, typically using a hypothetical device known as a time machine. Time travel is a recognized concept in philosophy and fiction, but travel to an arbitrary point in time has a very limited support in theoretical physics, usually only in conjunction with quantum mechanics or Einstein–Rosen bridges. Sometimes the above narrow meaning of time travel is used, sometimes a broader meaning. For example, travel into the future (not the past) via time dilation is a well-proven phenomenon in physics (relativity) and is routinely experienced by astronauts, but only by several milliseconds, as they can verify by checking a precise watch against a clock that remained on Earth. Time dilation by years into the future could be done by taking a round trip during which you move at speeds approaching that of light, but this is not currently technologically feasible for manned vehicles.
Is Time travel possible?
Time travel's been one of man's wildest fantasies for centuries. It's
long been a popular trend in movies and fiction, inspiring everything
from Charles Dickens' A Christmas Carol to H.G. Wells' The Time Machine to the Charlton Heston shrine that is The Planet of the Apes. And with the opening of Interstellar today—n0t to spoil anything—we're about to fantasize about it even more.
The most fantastic thing? It's probably possible.What's almost impossible
Let's start with the bad news. We probably can't travel back in time and watch the Egyptians build the pyramids. In the last century scientists came up with a number of theories that suggested it is indeed plausible to take a leap into the future; going back in time, unfortunately, is much more complicated. But it's not necessarily impossible.
One option for would be a wormhole, also known as an Einstein-Rosen bridge. Along with physicist Nathan Rosen, Einstein suggested the existence of wormholes in 1935, and although we've yet to discover one, many scientists have contributed their own theories about how wormholes might work. Stephen Hawking and Kip Thorne are probably the most well known. Thorne, a theoretical physicist at CalTech, even helped Christopher Nolan with the science behind Interstellar.
So let's just assume that wormholes do exist. In the late 1980s, Thorne said that a wormhole could be made into a time machine. According to Einstein's theory of general relativity, a wormhole could act like a bridge though space-time by connecting two distant points with a shortcut. Certain types of wormholes, it's theorized, could allow for time travel in either direction, if we could accelerate one mouth of the wormhole to near-light speed and then reverse it back to its original position. Meanwhile, the other mouth would remain stationary. The result would be that the moving mouth would age less slowly than the stationary mouth thanks to the effect of time dilation—more on this in a second.
But there are several major caveats of traveling back in time with this method. Chief among them is the simple fact that we'd need a method for creating wormholes, and once created, the wormhole would only allow us to travel as far back as the point in time when it was created. So we'll definitely never be spectators to Great Pyramids' construction.
The other really serious caveat is that we'd need a way to move one of the mouths of the wormhole nearly the speed of light. In their seminal 1988 paper on wormholes, Thorne and his colleagues assumed that "advanced beings [would] produce this motion by pulling on the right mouth gravitationally or electronically." We can't do that right now, however.
What we can do is travel into the future—but only by a little bit.
What's almost certainly possible
In recent years, we've seen some aspects of Einstein's fanciful theories proven true. The latest and perhaps most exciting theory is the aforementioned effect called time dilation. Though we've based technology on the theory for decades, an experiment finally proved this year that time dilation is absolutely a real phenomenon. It's also a phenomenon that could allow us to travel into the future.
Time dilation basically refers to the idea that time passes more slowly for a moving clock than it does for a stationary clock. The force of gravity also affects the difference in elapsed time. The greater the gravity and the greater the velocity, the greater the difference in time. Black holes, like the one depicted in Interstellar, for instance,would produce a massive amount of time dilation, due to their extreme gravitational pull.
Thanks to the space program, we've actually been dealing with this effect for many years. This is why the clocks on the International Space Station tick just a little bit more slowly than clocks on Earth do. Since the space station is moving so fast and is affected by less gravity, time moves more quickly. It's also why no clock on Earth is perfectly accurate, since the effect of time dilation means that time moves more slowly closer to the planet's surface. Okay, maybe one is almost perfect.
A better example of time dilation at work involves GPS satellites. The GPS chip in your smartphone works because there are 24 satellites circling the globe at all times that triangulate your location based on how long it takes time-stamped information to travel to and from the device.
However, scientists learned when building the system that the atomic clocks on GPS satellites do indeed run a little bit fast, since they're moving 9,000 miles per hour in orbit. To be specific, they lose 8 microseconds a day. That's hardly perceivable, but it's enough to throw off the location data. And so GPS technology makes adjustments to the clocks on board to account for the relativistic effects. The equation used is kind of complicated.
The implications of all this are huge. What if you took this to the extreme? If you jumped in a spaceship that flew super fast, time would pass more quickly for people on Earth. You could do a lap around the galaxy and return to Earth in the future. This is basically what happens in Planet of the Apes. In effect, Charlton Heston's character is a time traveler.
