If you want to meet someone, you have to tell them both a place and a time. In a city like New York, that could be a cross street. The Broadway Plaza Hotel is on 29th and Broadway. You can also tell them to meet you on a floor, say, the 6th, at a time, 7 pm.
In general, to locate a place in space, you need three numbers, one for each dimension, right/left, back/forth, up/down. On Earth, a latitude, longitude, and an altitude fixes the location of a moving aircraft or orbiting spacecraft. To locate a place in space and time, you need a fourth coordinate, a when.
If you take all the points in space and all the whens and put them together, you get a four dimensional spacetime.
Einstein was the first to show that time behaves a lot like space. When you go faster, your time appears to move more slowly to those watching you accelerate. Your length also appears to get shorter in the direction you are moving. Time stretches. Space compresses.
Gravity bends space and time as well. You can, in fact, change the rate at which you move through time relative to other things just by changing how far you are from the Earth.
In the 1970s, they put a super accurate clock on an airplane and flew it around the world. It showed that more time had definitely passed on the airplane than on Earth. That is, time moves more slowly closer to Earth because the gravitational field bends time more.
Recently, a vastly more accurate clock has been able to show that time passes differently, because of Earth gravity, with just a millimeter difference in altitude.
All of this suggests that time bends like space, but it doesn’t explain why we can’t move around in time the way we can in space. It also doesn’t explain why we experience time moving forward at a fixed rate or why we can remember the past but not the future. How silly would it be if we could only remember things on our left but not on our right?
Time is not like space.
And if you look at those four coordinates, they are missing an important piece of information. I can tell you to meet me at a particular time and place, sure, but to make that happens I also have to match up “now” with that time and place for it to occur. That is, I have not only an awareness of places in time and space but also an awareness of when “now” is.
That might seem like a trivial observation but it is critical to understand why time doesn’t make sense as a dimension like space.
If I have a chess board where both down and across board act like space dimensions, and I want to put two pieces in adjacent squares, I can just put them there. But if say the across board is like space and the down board is like time, then it is like the board a series of rows on a moving conveyor belt and the pieces can move only left or right while it moves them forward. In order to meet, they have to be in adjacent squares at a particular moment.
Yet, if that’s true then there is a problem because I have inserted a third dimension into the two dimensional chess board, the dimension of time. So now I have across board, down board, and time.
Or think of it this way, imagine I take a series of snapshots of the moving board at different times, at each time “now” is at a different location. Now I stack all those snapshots on top of one another. This gives me a cube with dimensions of space, across board, time, down board, and “now” in the vertical dimension.
While it is perfectly correlated with position in time, now must be an additional dimension, the one we actually experience as time moving, while the down board time is the one we refer to when we give a time for some event.
The thing is, I don’t have to tell anybody, when I ask them to meet me at a particular time, to make sure that they also meet me when that time is now. That is taken care of for us by the movement of now through time, so while it seems like a dimension it also isn’t one we have any control over.
There are four ways to interpret this observation:
Assume that the dimension of time is an illusion and only the now dimension exists.
Assume that only the time dimension exists and the now dimension is an illusion.
Assume that now and time are somehow the same dimension but different aspects of it.
Treat both as real separate dimensions.
Presentism
The first assumption is called the Presentist point of view. Until Einstein’s theory of relativity, Presentism made perfect sense, and Isaac Newton enshrined it in his theories. We can’t move forwards and backwards in time, after all. And we only appear to exist now, so now is all we need to define time. The past is gone and the future is unmade.
Relativity, however, showed that different observers could not agree on when now is.
To understand why, we need to define what now means.
If two things happen now, then they are happening at the same time. So now includes all events that are occurring simultaneously.
This works fine in everyday life, but, when it comes to objects moving very fast or near massive bodies, simultaneous events are in the eye of the beholder.
For example, suppose Bob is on a spaceship traveling at a significant fraction of the speed of light relative to Alice. Bob sets up a light gun in the center of the ship that fires pulses of light at detectors at either end of the ship. Bob sets up the gun so that it fires pulses in both directions using a splitter from a single source so that the light emerges from the center towards the two ends at exactly the same time.
For Bob the light pulses hit the detectors at each end of the ship simultaneously. Yet that is not what Alice sees. Both Alice and Bob see the light pulses traveling at the speed of light, in accordance with relativity. Because of the ship’s motion, however, in Alice’s frame the aft detector rushes up to meet the pulse of light traveling opposite to the direction of the ship’s motion while the forward detector rushes away from the pulse. From her perspective, the aft pulse hits the aft detector before the other one hits the forward detector.
This means that Bob and Alice cannot agree on when now is, i.e., they cannot agree on which collection of events occur now and which are in the past.
This is a variant of what is sometimes called the train paradox. In that version, lightning strikes each end of a moving train. Thus, the simultaneous events happens in the “now” of the observer who is not on the moving train but at different times for an observer who is on the train.
It gets even more complex in general relativity when you bring gravity into the mix. People near different gravitating bodies can’t agree on when now is either. Near a black hole, time can move so slowly that an hour is a year to those further away. Time is not a constant universal drumbeat as Newton supposed.
Relativity makes Presentism hard to accept in its original form. A few original Presentists soldier on, particularly in the Bohmian mechanics interpretation of quantum theory. They assume a universal now exists in a quantum realm we can’t measure.
The majority of Presentists just do away with the universal aspect of the Present and suggest that the universe is Presentist but each observer has its own local now. This requires that the universe decompose into slices, like a loaf of bread, called a foliation. Italian physicist and best selling author of The Order of Time, Carlo Rovelli, has published a number academic papers on this idea.
Eternalism
The second approach is called the Eternalist or Block time approach. Einstein favored this interpretation to the point where it became a major part of his philosophy of life. In one version of this interpretation, now is an illusion. We are minds experiencing a fixed four dimensional reality, like a person scanning a painting with their eyes. Our attention is wandering over the breadth of time and taking in what there is to see, hear, touch, taste, and smell.
Our experience of time flowing in an orderly linear direction is an illusion created by our brains. Your mind cannot know anything but what the brain tells it, and the brain convinces the mind that time is moving, but, like a movie, it isn’t. It’s just a sequence of snapshots presented to the mind, with memories to convince you that you existed just a moment before.
This interpretation is sometimes called the B-theory of time and dates back to J.M.E. McTaggart’s 1908 book The Unreality of Time. It is necessarily dualist because it assumes an immaterial mind and attributes the experience of time to it. The physical brain creates the illusion of the flow of time. Yet the mind must somehow be able to experience different moments and no two moments at the same time.
Kurt Vonnegut incorporated the B-theory into his classic 1960s World War Two novel, Slaughterhouse Five. If anything, the novel shows how absurd the idea is.
Eternalism is deeply problematic. It not only assumes a mind disembodied from the body but one able to move about through time like a ghost, perhaps through supernatural means. This seems to create more problems than it solves and doesn’t have the status of a scientific theory.
Oddly enough there are many scientists who are both Eternalists and do not believe in non-physical causes, which puts them in a philosophical bind that would be remedied with a second time dimension as we will see.
Half-Block Time
The third option is to assume that time is composed of the past but now is the edge of that dimension, and it is growing like the top of a plant towards the Sun. The half block time suffers from the same problems as Presentism and so needs a multi-time interpretation to make sense with relativity. Because of the similarity, I won’t address it further.
Eternalism with another time dimension
The fourth approach is Eternalism with not one but two temporal dimensions. The dimension of relativity is that of events and history. The dimension of now means the present moment moving through history.
Because our position in the now and the current moment in history are perfectly correlated, we lose (at least locally) what physicists call a “degree of freedom”, which is why the universe appears to be four and not five dimensional. We are not allowed to move in history without moving in now by the same amount. And the only way we can distinguish between the two dimensions is through relativity.
So far science has been slow to catch on to this philosophical conclusion. Theories of time, in fact, tend to ignore the experience of time altogether — the fact that every point in time is now at some “point” and that this requires an additional dimension to explain is left out of all the equations.
Those that do look at it tend to focus on the Presentist multi-time approach. Yet it is unclear that we can foliate the universe entirely including all types of black holes. There is a type of foliation called “York time” that can take them into account. York time however would need to be universal throughout the entire universe, from the scale of the cosmos down to the Planck length, for Presentism to make sense.
For an Eternalist interpretation of time to make sense, you must have two time dimensions, full stop. There is no way to interpret motion through a block time without another dimension to explain the motion unless you appeal to the supernatural.
The benefit of two-time dimensional Eternalism is that you can admit geometries of spacetime that can’t be sliced. These include Closed Timelike Curves (CTCs), wormholes that go back into the past, which a foliation excludes. Since we do not observe anything traveling back in time, this isn’t a physical observation that we need to account for, but potentially at the smallest quantum scales CTCs would appear.
In addition, quantum physics naturally includes correlations between all points in spacetime simultaneously. Indeed, we can show that, in quantum field theory, relativity is violated with exponentially decreasing probability with distance. This can also lead to a strange effect called retrocausality, the future affecting the past, a phenomenon that has never been observed but is a prediction of standard quantum theory.
The prediction appears because all quantum theories are time symmetric. They do not distinguish between the future and the past.
A Presentist perspective completely excludes retrocausality because it is time asymmetric. Yet, CTCs that occur at the quantum foam level in a theory of quantum gravity could make short distance retrocausality a regular feature of our universe.
The two time dimensional Eternalist approach, meanwhile, is time symmetric, in agreement with quantum theory, because it has another dimension that is not symmetric which doesn’t appear in any equations (yet).
Retrocausality also makes philosophical sense with free will if you have to have two time dimensions but not if you have one. In the two time dimension case, you can have a history, including all of time, in our ordinary time dimension with two events, A and B, such that B occurs later than A but causes A. For example, suppose B is a time traveler going back in time and A is that time traveler killing his own grandfather.
Now, if we can observe that A was caused by a later event B, we have the free will to prevent B from happening, e.g., destroying the time machine before the time traveler can use it, creating a time paradox. Yet, because we are moving in a second time dimension, we are free to prevent B and remove the retrocausal link from history since we are allowed to “change the past” without paradox. This is because all of history is able to move and change in that second time dimension.
If quantum experiments were to determine retrocausality occurs, it would automatically exclude the Presentist point of view. And for all the reasons given above, such an observation would also exclude a single dimension of time whether we have free will or not.
Another benefit of the two time approach is that it offers an explanation of the quantum measurement problem. I have already written extensively on that topic (links below) so I won’t go into it here.
General relativity may create quantum physics
General relativity is the science of the very large, stars, galaxies, and the universe as a whole. Quantum physics…medium.com
A 5th dimension may explain the quantum interpretation paradox
Combining the best of many worlds and consistent histories, the 5th dimension may resolve a 100 year old debate.medium.com
Thus, a 5D universe might explain two mysteries of modern physics, time and quantum theory, with one idea. That certainly makes it worth study.
Rovelli, Carlo. “General relativistic statistical mechanics.” Physical Review D 87.8 (2013): 084055.
Roser, Philipp. “Gravitation and cosmology with York time.” arXiv preprint arXiv:1609.03942 (2016).
McTaggart, J. Ellis. “The unreality of time.” Mind (1908): 457–474.
Friederich, Simon and Peter W. Evans, “Retrocausality in Quantum Mechanics”, The Stanford Encyclopedia of Philosophy (Summer 2019 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/sum2019/entries/qm-retrocausality/>.
Leifer, Matthew S., and Matthew F. Pusey. “Is a time symmetric interpretation of quantum theory possible without retrocausality?.” Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 473.2202 (2017): 20160607.