Before I begin, it should explained why understanding the nature of time is seemingly important. Unlike the strong and weak nuclear forces, electromagnetism and to a certain extent, gravity, we have extremely limited control over time. To understand time would potentially allow us – or whomever controls the knowledge or technology – to gain one more advantage over nature. If the nature of time could be understood, it is assumed we could control or at least manipulate it. Based upon our current understanding of physics and cosmology, I believe I’ve come to understand the nature of time. Equally important is that in understanding how time works, this understanding destroys the extravagant notion of time travel* altogether.
[*Time travel into the past, that is. Time travel into the future is known to be logically possible, but this concept of time travel is not regarded as ‘sexy’ because it does not allow any control or interaction with the past, much less the ‘now.’]
Why would we think time travel is possible in the first place? Human beings have been the stewards of impossible ideas for a long time and time travel is no exception. It is an even more bizarre thing to contemplate time travel when you take some of the most popular theories of time into consideration. For example, in the B-Theory of Time aka Tenseless Time or Eternalism, it is hypothesized that the past, present and future exist all at once and this would certainly not allow time travel. The B-Theory of Time is most likely a false hypothesis given Einstein’s Theory of Special Relativity as it relates to the relativity of simultaneity and time dilation due to the effect of gravity (it is well confirmed that time runs differently, say, near massive objects as opposed to further away due to gravity). A similar criticism can be leveled against what is called Block Time, in which it is hypothesized that the past and present are real but the future is not; that the future amounts to an ever shifting ‘now’ that somehow leaves a trail of breadcrumbs we could potentially go back to and pick up. There’s less of a problem with imagining that only right now exists given anyone’s perspective from any point in three dimensional space, though upon reflection, what now is there? For as soon as it is now, now is in the past. This last point brings us to whether or not there is a flow to time.
Does time flow? That is, does time flow like a river or it does it manifest as a series of infinitesimally small pieces, like a quantum-sized roll of film? While some scientists believe that the latter is the case and have scaled packets of time down to what is called Plank Time (trust me, a super-duper small measurement) there is no explanation as to how one frame of time ‘becomes’ or seemingly ‘flows into’ another. It also seems odd that any scientist would insist on dividing time into packets since this would make time appear to be unlike anything else in physics; the strong and weak nuclear forces, electromagnetism and gravity are not even matter are cleanly divided into packets. (Of course, just because the others are not a divisible ‘something’ doesn’t necessarily rule this out of time’s nature.) Where would solid matter even go during the intervals in which time changes in this view? If on the other hand time is part of a fabric that comes from the stretching of space, this would appear to make more sense as there would be a connection between space and time that makes it plausible that any interaction with gravity would have differing effects. These differing effects are what ultimately causes time to flow in the only direction we have ever known – forward. Thus, whether or not time flows is rather irrelevant; it’s the seemingly forward direction of time that matters*. And, the reason why time only goes in one direction is because it is an emergent property of the expansion of space and not a dimension unto itself.
[*On the macroscopic level of organization in which we interact with the world. Time appears to have little meaning in the subatomic world, though overall this has little impact on time as we experience it.]
In trying to understand time, people seem to overthink or want to complicate the answer as to its nature because time is so hard to pin down, metaphorically speaking. Anything so elusive must be difficult to understand, is what conventional wisdom seems to want to say. But the question and answer are not intractable. What is time? It is the perception or measurement of space and matter in relationship to each other. Time is observed as changes in the differences between these two aspects of nature. And these things change in relationship to each other – all the time – because space itself is always expanding.
We know that on both a cosmic and local scale, space itself is expanding and doing so in every direction (thus there is no ‘center’ to the universe). It is also expanding at a speed per distance in every direction that is faster than the ability of light to transverse already interstellar distances, which is about 70km/sec per megaparsec and accelerating (we’ll see what this has to do with anything soon). Because of this, most galaxies are getting further and further away from each other (and not themselves necessarily ‘moving’ away from each other). Keep in mind for a moment that the expansion of space at the subatomic and local scale is overcome – that is, not taking place or doing so imperceptibly – due to the strong and weak nuclear forces, electromagnetism, and even gravity whereas massive objects like a planet, solar system, or galaxy is concerned. In deep space, space itself is expanding (more rapidly than at the subatomic and local scale) and as it expands, the change of objects in relation to each other changes. These changes are perceived as the passage of time. For example, in the smaller confines of space, such as within the area of a galaxy – which is small, cosmically speaking – a galaxy’s position changes in relationship to everything else in the universe and because the matter therein changes in relation to space – which is not static in any direction – this is what we construe as time. Even if space were not expanding and instead were collapsing, we would still see the arrow of time go forward because of the different arrangement of matter in relationship to space. (Or in another scenario, we would be unable to tell if time were flowing backwards or forward since we’d be part of the changes taking place. And now you’ve just realized that, heck, that means time could be flowing backwards right now and we wouldn’t know it.)
Meanwhile, as space expands and matter spreads itself out (for the most part) on the cosmic scale, the Earth is moving through space around the Sun at 30km/sec, the Sun is moving around the center of the Milky Way at about 200-250km/sec, and the Milky Way is moving approximately 600km/sec toward the constellation Hydra. This, too, provides us with a positional change between objects that we can observe and measure. Notice if you will how often scientists assert that time and space cannot be separated; this is a bit confusing because it is an incomplete assertion: time and space and matter altogether cannot be separated if time is to be observed and measured. Time and space may exist on their own together, but to do so would be irrelevant to the matter of which we are comprised. You will likewise hear scientists say that time didn’t exist before the moment the universe came into existence. As space came into existence after quantum fluctuations supposedly caused the Big Bang (or Big Bounce depending on what theory you want to go with), matter formed as space expanded and cooled, providing the universe with time as we know it*.
[*We’re not exactly sure how the Big Bang or Bounce could happen as quantum fluctuation would suggest the necessity of the passage of time. But it appears that the so-called arrow-of-time means little if nothing at the subatomic scale or at least little or nothing to massless particles such as a photon. Empty space, teeming with energy – meaning it’s not really empty – doesn’t rely on a direction of time for changes. I do realize this seems counterintuitive for those of us who live at the macroscopic level since we perceive changes as going forward in time. Quantum physics doesn’t play by the same rules, apparently.]
What implications does this hold for time travel? It makes time travel, at least to the past, impossible. (How could you use ‘forward’ time to go back in time anyway?) Space and time and their relation to matter do not allow for a ‘map’ in which there are points in the past we can revisit on a whim. Due to the expansion of space and the movement of matter within space – which is a drive towards the equal distribution of matter and energy in the universe – the position of the matter within the universe is constantly changing; to visit the past would necessarily no longer be the past we knew, but a new past that included the input of (our) new matter into the initial position of matter within the universe at the time in question. Some might speculate that to do such a thing would result in a new timeline/alternate universe, but there is no evidence to suggest creating an alternate timeline or universe is possible. Hence, to “go back in time” would be paradoxical and paradoxes are by definition impossible.
As I alluded to in passing several times now, quantum physics doesn’t play by the same rules of time that we are used to. Massless particles such as a photon, light, do not have experiences in the traditional sense because they are not directly affected by the forces that dictate the behavior of particles with mass. A photon can be pulled into a blackhole because of the curvature of space due to extreme gravity, but gravity has no direct effect on a photon. The curvature of space is what appears to make a photon – something that experiences no time – to experience time, by changing the photon’s position because space around it expands and/or curves. Although a photon has no mass (and can therefore exert no gravitational force or, again, be directly affected by gravity) and is essentially ‘frozen’ in time, it is perceived to experience the passage of time due to space expanding through the photon’s fixed points. All of this is to once again say and add to that, that not only is time a derivative of the expansion of space, but is an emergent property of changes to the positions of mass and massless particles due to the expansion of space while gravity curves that space.
Time travel to the past is not possible unless we can reverse the expansion of space and un-curve space to put mass and massless particles back in their previously experienced positions. Even if we could do this, we wouldn’t know that we’ve done it because we would all be in the same state we originally experienced. Again, if time were actually running backwards right now and the so-called arrow-of-time is just a phrase for the direction time flows, we wouldn’t know it because it is normal to our experiences. In my conception of time, none of the legendary logician Kurt Godel’s (1906-1978) ‘light cones’ are allowed to ‘tip over’ due to extreme curvatures of space and allow information or you to travel to the past.
Given this conception of time, some questions remain (though all are not directly tied to the nature of time itself). If it is assumed by cosmologists that approximately 80-90% of the universe’s mass is missing, how does this missing mass affect time, if at all? (Perhaps it isn’t missing and something we haven’t thought of is overcoming gravity to drive the expansion of space, though, Dark Energy is the name of this clandestine force I suppose yet sounds too precise for something totally unknown). It would also be interesting to know how consciousness allows us to perceive the passage of time.
Naturally, this theory of time as I have formulated it is without a certain level of preciseness as I am not a physicist. My theory, though, makes far more sense and possesses more explanatory power than other theories such as the B-Theory of Time or Block Time, both of which are not confirmable.
Time only goes in one direction, expanding so to speak, and this doesn’t allow for time travel. But don’t shoot; I’m just the messenger. Go back in time and kill me before I figure all this out if you don’t believe me. (Or, more courteously, simply point out where my argument fails. Thanks.)