space & astrophysics

In a philosophical sense, we’re taught to doubt and question everything. Even the reality of ourselves and our own experiences are up for debate, as we have to make certain assumptions about how trustworthy our sensors — and our own senses, for that matter — actually are in order to arrive at any satisfactory conclusions. Sure, certain things might appear real, but isn’t it possible that those appearances are deceiving, and that quantities or concepts that we take for granted might be nothing more than very convincing illusions?

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From a physical, scientific perspective, however, these sorts of questions take on a different meaning. We’ve learned lots of surprising and counterintuitive lessons from our investigations of time. Time is relative, not absolute. Time always marches forwards, not backwards, but we still lack an explanation for the arrow of time. Thermodynamically, the Universe has an arrow of time, which “flows” in the same direction as increasing entropy. And when we investigate the Universe on a fundamental level, it turns out that time may not be fundamental at all.

But existence itself? It’s very, very difficult to take that property away from time and to still wind up with a Universe consistent with what we observe. Here’s why.

space & astrophysics

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role. However, whether humans or any other life forms are present or not, time still is required for exist in the Universe.

(Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn)

When it comes to the question of existence, physics is very simple and straightforward about what it considers to be a satisfactory answer.

  • Can you measure it?
  • Can you quantify it?
  • Can you define it in a mathematically self-consistent way?
  • It is, itself, an observable quantity, and do other observables depend on it in an inextricable way?

If your answers to these questions are all in the affirmative, there’s no way out of it: you’ve got yourself a quantity that exists.

The reason why is simple: as far as reality goes, “what is real” are those things that themselves are measurable, observable, quantifiable, and not pathological. In layperson’s terms, pathological is what happens when you pose a reasonable question to the Universe and you get inconsistent nonsense back. There are plenty of questions that do yield pathological behavior, and in those instances, the pathologies indicate to us that we have further work to do. “What happens at a black hole’s central singularity?” “What happens to quantum fluctuations on length scales smaller than the Planck length?” “What happens when a mass travels through the spacetime that is distorted by the presence of that mass itself?” These are all questions that, at present, are as pathological as dividing by zero.

space & astrophysics

Even for a complicated entity like a massive, rotating black hole (a Kerr black hole), once you cross the (outer) event horizon, regardless of what type of matter or radiation you’re composed of, you’ll fall towards the central singularity and add to the black hole’s mass. What happens at the central singularity is not well-described by current physics, however, as its behavior is pathological.

(Credit: Andrew Hamilton/JILA/University of Colorado)

You might think, then, that perhaps time itself is pathological. Sure, we can measure it, quantify it, and even observe both its passing and the consequences of its passing. But shouldn’t it matter that your measurements of “how much time has passed” between the start and end of an event depends entirely on where you are and how you’re moving when you’re making those observations?

For example, if you’re on a moving train and you shoot a light wave from one end of the train to the other, you’ll get a value for how long it takes the light to reach the far end of the train. If you’re on a platform, however, watching the person on the train shoot the light from one end to the other, you’ll get a different answer.

For the person on the moving train, they’ll measure that a certain amount of time must elapse for the light to travel down to the far end of the train. But for the person on the ground, they’ll not only get a different, longer answer, but they’ll conclude that the person (and, for that matter, everything) on the train is actually aging more slowly than they are. To the stationary observer, an object in motion ages more slowly than an object at rest.

space & astrophysics

This illustration of a light-clock shows how, when you are at rest (left), a photon travels up-and-down between two mirrors at the speed of light. When you are boosted (moving to the right), the photon also moves at the speed of light, but takes longer to oscillate between the bottom and the top mirror. As a result, time is dilated for objects in relative motion compared to stationary ones.

(Credit: John D. Norton/University of Pittsburgh)

Is this paradoxical? Is it pathological?

Not at all. Noting, about time, that “it’s relative” doesn’t mean it’s pathological. For our question about light traveling from one end of a moving train to the other, it’s possible that the train can come to rest, and the “on the ground” and the “on the train” observer can meet up again. Both of their measurements will be different, individually, but they’ll be consistently different with one another. When you perform the calculations for how much time passes for one observer relative to the other, each observer will be able to correctly predict not only what their own watches and clocks say, but the other observer’s as well. All it takes is a knowledge of special relativity.

Yes, you get different answers to the question of “how much time has passed?” or “when did this event occur?” or even “which event happened first?” depending on where you are and how you’re moving, but no one is more “right” or “wrong” than anyone else. Instead, we just need to transform our idea of time — according to the laws of relativity — to match what someone at either a different location or moving at a different relative speed would conclude.

space & astrophysics

A ball in mid-bounce has its past and future trajectories determined by the laws of physics, but time will only flow into the future for us.

(Credit: MichaelMaggs Edit by Richard Bartz/Wikimedia Commons)

So, then, the notion that “time is relative” isn’t sufficient to claim that time doesn’t exist. But could it be the case that, perhaps, we only perceive time to exist, and that it isn’t, in fact, actually real?

We can consider this from a particular perspective: looking at the notions of symmetries in physics. After all, the laws of physics, at least as we know them, are time-symmetric. If you watch a ball falling under the influence of gravity, you have no idea whether:

  • you’re watching time running forward as gravity pulls the ball down from a dropped position high above where you’re looking now,
  • or whether you’re watching time running backwards as a ball, having been thrown upwards from a lower position, is rising higher and higher as the force of gravitation resists its motion.

In fact, almost all of the laws of physics — including motion, gravitation, electromagnetism, and even the strong nuclear force — are completely time-reversible. They are the same forwards and backwards in time, and you cannot discern, simply by watching a physical system unfold, which one is occurring.

space & astrophysics

Heavy, unstable elements will radioactively decay, typically by emitting either an alpha particle (a helium nucleus) or by undergoing beta decay, as shown here, where a neutron converts into a proton, electron, and anti-electron neutrino. Both of these types of decays change the element’s atomic number, yielding a new element different from the original, and result in a lower mass for the products than for the reactants.

(Credit: Inductiveload/Wikimedia Commons)

But there are two ways we can identify a physical way that progressing forward in time differs from progressing backwards in time. The first is by looking at reactions that proceed via the weak nuclear force, such as radioactive decays.

Let’s imagine that you have a heavy atomic nucleus, full of protons and neutrons. If there are a large number of neutrons in that nucleus for the given number of protons that are present, there’s a chance that the nucleus will undergo a specific type of radioactive decay: beta decay. Beta decay is what happens when one of the neutrons in the nucleus decays into a proton, an electron, and an anti-electron neutrino, and it even happen for free (unbound) neutrons that aren’t part of any larger atomic nucleus.

It will often happen that a neutron decays into a proton, an electron, and an anti-electron neutrino. But it never happens that a proton, an electron, and an anti-electron neutrino spontaneously react together to form a neutron. In fact, in a variety of ways, the weak interaction is the poster-child for time-asymmetric reactions in physics.

space & astrophysics

When ice is put into liquid water, the ice will melt and the water will lower its temperature. The end state, of fully melted ice, takes us not only closer to thermal equilibrium than the initial state, but less energy can be extracted from it than from the initial state.

(Credit: Victor Blacus/Wikimedia Commons)

The second way, however, is even more familiar to most of us. Every time you:

  • scramble an egg,
  • cook a scrambled egg,
  • drop a full glass of water onto the ground and watch it shatter,
  • or simply open the door between a hot room and a cold one,

you are creating a situation where there will be a thermodynamic arrow of time.

You may have heard of the concept of entropy before, which is often incorrectly defined as a “measure of disorder” of your system. But really, what’s going on is this: any physical system you can take will have some level, inside of it, of energy gradients. If you have an unscrambled egg, there’s an energy gradient between the albumen (the white part) and the yolk; the barrier around the yolk is what keeps things from mixing evenly. In an uncooked egg, there’s chemical potential energy that will be released — and new bonds will form — if you cook the egg. There’s potential energy in the structure of the glass, and shattering it will release it.

But perhaps, of all the examples, considering a hot room and a cold room right next to each other is the smartest way to talk about entropy.

space & astrophysics

This illustration shows two sides to a room: a hot one and a cold one, with a demon between them capable of opening and closing a divider between them. If the divider is opened, the gases will mix.

(Credit: John D. Norton, Entropy, 2013)

If you have a large number of particles on the hot side of the room, they’ll all be in what we call thermal equilibrium with one another. As they bounce off one another and interact, no part of the hot side will either be heated up or cooled down; there is no energy gradient for heat to flow from one part of that room to the other. (The cold side has precisely the same properties, except that thermal equilibrium occurs at a much lower temperature.)

But now, what if you remove the divider that separates the hot side of the room from the cold side? What happens?

The answer is that the hot particles and the cold particles will mix, and produce an intermediate-temperature room where all the particles come to the same equilibrium temperature. Before equilibrium is reached, energy can be extracted from the system; afterwards, it cannot. When we talk about a state of maximum entropy, we’re talking about a state from which no further energy can be extracted; a maximum entropy system cannot perform work, as we say in physics.

space & astrophysics

A system set up in the initial conditions on the left, with hot and cold rooms separated, will have each room reach its own thermal equilibrium. If the divider separating the two rooms is opened, the gases in the rooms will mix, gaining entropy in the process.

(Credit: Htkym and Dhollm/Wikimedia Commons)

Work is physically real; entropy is physically real; thermodynamics is physically real. Time, as a measurable and observable and quantifiable quantity, is no different than any of those.

However, there are two important caveats to this discussion. While it’s true that time is real, it’s important to keep the following facts in mind.

  1. We do not know what causes our perceived arrow of time. We always observe time to be flowing forwards and not backwards; we recognize the passage of time, and are subject to the laws of physics moving forward in time, just as all physical objects and quantities are. But whether the entropy of your system remains constant, increases slowly, increases rapidly, or is even artificially decreased by inputting energy into it, the perceived arrow of time never ceases to flow nor reverses direction.
  2. While time is definitely real, it may or may not be fundamental. In our present way of looking at the Universe, we view something like entropy as a derived quantity and treat time as though it’s fundamental. However, mathematically, it is possible to treat entropy as though it’s a fundamental quantity, and then time behaves as though it can be an emergent quantity. We do not yet know enough about the Universe to comment much about the potential validity of this approach.

space & astrophysics

When a quantum particle approaches a barrier, it will most frequently interact with it. But there is a finite probability of not only reflecting off of the barrier, but tunneling through it. The actual evolution of the particle is only determined by measurement and observation, and the wavefunction interpretation only applies to the unmeasured system; once its trajectory has been determined, the “past” is entirely classical in its behavior.

(Credit: Yuvalr/Wikimedia Commons)

Despite the popular trend to question the nature of time, its physical “realness” is not in doubt. Time is an integral part of the Universe, and the boundary between events that have been observed or measured to have a definitive outcome and those whose outcome has not yet been decided is the best way we have to define, precisely, what we mean by the moment of “now.” As esteemed physicist Lee Smolin put it in an exclusive interview with him:

“in the Copenhagen version of quantum mechanics, there is a quantum world and there is a classical world, and a boundary between them: when things become definite. When things that are indefinite in the quantum world become definite. And what they’re trying to say is that is the fundamental thing that happens in nature, when things that are indefinite become definite. And that’s what “now” is. The moment now, the present moment, that all these people say is missing from science and missing from physics, that is the transition from indefinite to definite.”

Time may or may not be fundamental, and our perceived arrow of time may or may not (my hunch is “not”) be related to the thermodynamic arrow of time. But the fact that we can measure, observe, and quantify it should put any doubts of its non-existence to rest.

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