Do the Past & Future Exist? (Script) (Patreon)

Is all that exists just whatever exists right now? Is the past erased and the future a void yet to be filled? Well the answer lies in between the past and the future - in the elusive, ever-moving eye-blink that we call the present.

Today we’re starting a deep dive into the nature of time - and down that rabbit hole we’ll encounter the physical origin of time, the question of determinism, and even what this tells us about consciousness and free will. Big questions that’ll take us to the edge of what physics can answer, and perhaps beyond. But today we’re definitely doing physics, because it already tell us a lot about the reality of the past, future and present.

We think of the world that exists as the world of the now. The past is gone except in our memories, and the future is a blank slate ready to be written into the past. But interpreted some ways, physics suggests that the future and the past exist eternally, while the present is an illusion. To show you why this really might be the case, we need to actually visit the past. As with many ideas in physics, this one originates with Isaac Newton. Newton was the first to come up with a set of laws that allow us to predict the motion of all objects in the universe.

Newton’s revelations led to the notion of the deterministic universe - the idea that, by knowing the current position and velocity of every particle in the universe, as well as the forces that act between those particles, you could calculate all future and all past states of the universe.

Now quantum mechanics might argue otherwise, depending on which quantum mechanics you prefer, but whether or not the universe IS deterministic we’ll come back to in an upcoming episode. But Newton’s picture of the universe included another assumption that we know for sure is wrong. It included the idea of an absolute, clearly definable “now”. Newton assumed that all particles, all observers, all points in space were ruled by a single, constantly ticking clock. This universal clock meant it was possible to define a notion of “now” that everyone would agree on, and so everyone would also agree on what was past and what was future.

Here’s a nice way to represent Newton’s picture of space and time. Let’s cut out one dimension of space - space then becomes a 2-D slice at a given instant in time. Now let’s stack successive instants so that time becomes the 3rd dimension. Every slice is the same universe at a particular instant, and it evolves from one slice to the next according to the laws of physics. We only experience a single slice at a time, and we usually think of the universe in that slice as the one that currently exists.

But we can also imagine a perspective from outside both space AND time in which this entire structure just exists. From that perspective, time doesn’t have a particular direction, and there’s no preferred “present”. Time is just a dimension like space, and we only observe a flow of time if we play the slices in sequence like a flip-book. We sometimes use the term “block universe” or “block time” to refer to this view of all space and time just existing in this 4-dimensional chunk.

The flip-book of the block universe has to be played in the right direction to see emergent phenomena like the sequence of cause and effect, evolving patterns of structure and information, entropy, and even our conscious experience, which itself emerges in the forward evolution of patterns of information in our brains. Our awareness of the universe rides this forward-moving wave of the present. To us, none of the rest of the block universe exists, because our existence emerges from the forward evolution of a razor-thin slice. Although by the way - that perception of existing in a single instant in time is definitely an illusion - our awareness - in fact our conscious existence - is sort of smeared out over a couple of hundred milliseconds. But that’s a topic for another, well, time.

Another way I like to think about it is that 4-D spacetime is a vinyl record, and our subjective experience is the music coded in the grooves. Viewed from outside, the whole thing exists, but the music only emerges if played in the right direction. Nothing “plays” the block universe - it just is, and temporal phenomena like us are just embedded in it, when you look at it in the right way.

There’s a name for this idea that all of time, future and past, sort of just exists as this atemporal structure. It’s “eternalism”. The opposite viewpoint would be presentism, which posits that only the current instant has a meaningful existence - the past is erased, the future still blank. And the middle viewpoint would be that of the growing block - that’s the idea that this shock-front of the present creates the block universe as it proceeds forward, weaving the past out of nothing. Believe it or not, we can actually science these very philosophical ideas, with just a little help from the smartest guy in the block universe.

The representation of the block universe that I showed you is very Newtonian. We had a single time axis that everyone agreed on, and everyone agreed what slice of the block was currently the present, and which sides  were the future or the past. Newton says there’s only one way to slice the universe. Einstein says otherwise. In his special theory of relativity, Einstein showed that there’s no absolute way to define whether two events happen simultaneously - the present, past, and future are relative concepts.

Let’s pick this apart. How do we even say what “now” means for other parts of the universe? Nothing, not even information, can travel faster than light. That means any distant event is already in the past by the time we see it. We represent the region of the block universe that we can possibly perceived with a light-cone - signals from things inside that cone have had time to reach us. The boundary of the cone holds the paths of light-speed signals. Whatever events live on that cone represents the universe that we see around us, carried to us by light.

Any event above that cone is unseeable in the present - we’d have to wait until our lightcone moves upwards to encompass it. Which is the same as saying we have to wait until its light has had time to reach us.

Our entire time-slice of our present is never observable in the present. To map it, we need to wait. From your point of view, you are motionless. After all, motion is relative so we can define your motion as stillness. That means as time passes you move straight up - fixed in space, only moving through time. As that happens, signals begin to reach you from the time-slice you’re trying to map. Successive shells of light from more distant parts of that time-slice arrive one after the other, expanding your view.

OK, so the present seems definable - if only in the future.

But if they’re moving relative to you things get weird. To you, they seem to make a mistake. They are racing towards one set of incoming signals and away from the other. At the same time, their entire perception of the spacetime grid is warped due to their motion.

We talked about this warping via the Lorentz transformation in a lot of detail previously. The upshot is that the moving observer builds an entirely different map of that time slice - they define the present differently. In fact, it’s possible for another observer to be in your slice of the present, but for you not to be in theirs.

So that’s the effect of Einstein’s special relativity - space and time tilt into each other, so that different observers will slice up block time at different angles depending on their velocities.

Even your own sense of the present changes with your cvmotion. Start moving forward and your slice of “now” will skew. Ahead of you, things once in the future will become the present, and behind you the past becomes the present and what was once the present is now in the future. Walk in circles around the room and your entire now-slice tilts crazily like a ship deck in a storm. Nearby the effect is tiny, but the “present” at the edge of the observable universe veers back and forth by a couple of centuries every time you switch direction. Try it, but be careful - it can make you dizzy.

Let’s get back to the question: what does all this say about the reality of the past and the future. Imagine that the future is created as the wave of the present sweeps out the block universe. But where is that wave? Whose slice? At a given location in space, you can lay down all possible time-slices representing possible presents. The largest tilts are for things traveling at the speed of light, and these sweep out the future light-cone. In front of the future light cone is the region that is in the future for everyone, no matter what their speed. So is that the future that doesn’t exist until created by the evolving present?

Well that can’t be right. Consider the time-slice of our present; we can imagine other observers on that time slice that we surely must “exist”. After all, they could have travel to those points from our own past. But they have a similar future light cone to our, offset in space from ours. So now fill the present with observers and all of the parts of block universe that we might consider the future get filled out. They’ve filled by what would be the present or past of someone else who exists in our present.

Let’s take stock: It’s hard to accept that only the present exists, that only the current “slice” of the block universe has a meaningful reality - because it’s impossible to define what that slice actually is. it’s also hard to accept that only the past exists and the future doesn’t - at least in the sense of a growing block universe, again, because the “present” of someone else on your slice of present could be literally any point in your future.

So we’re left with two options: 1) the entire block universe has a meaningful existence; or 2) if you don’t want the future to exist then you need to deny the existence of the present beyond your own immediate experience, which can include events only from your past light cone. Nothing outside that section of the block universe can be ascribed a definite reality until you interact with it.

That second stance feels extreme - it feels like definition of solipsism - that your own subjective experience is the only thing we can be sure exists. But there’s a way to humble this notion - to make it more palatable the materialist - to those who like to believe in an actual universe independent of the self. That means most physicists, including this one.

To rescue materialism without demanding a perfectly defined personal future we need quantum mechanics. Quantum mechanics tells us that all this stuff outside our past lightcone - and even unobserved aspects of the world within that cone - exist in a state of indeterminacy until observed. Depending on how you interpret quantum mechanics, that could be evidence in favor of this solipsistic denial of external reality. But quantum can also save materialism and determinism if you like- but to do so you need all possible realities to exist simultaneously and persist into the future. We’ve talked about these ideas before - the Copenhagen and Many Worlds interpretations - but soon we’ll dive back in, to see what these idea imply about time, determinism, and what it would mean to live in a block multiverse space time.

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629 Correction Audio

For example, let’s consider powers of 2—2, 4, 8, 16, 32, 64, 128, 256, 512. If we divide these powers by 5 and look at the remainder (an operation called “mod”) we get 2, 4, 3, 1, 2, 4, 3, 1, 2, 4, 3, 1—it repeats every four numbers.

630 Correction Audio

Okay assuming it is real, does this mean we’ve found life? Let’s first discuss why the presence of a simple molecule might get people excited about the possibility of life. So phosphine is a tetrahedtron - pyramid or d-4 - shaped molecule with one phosphorus and 3 hydrogen atoms.

630: Overview Shoutout, Patreon, Comments

Hey everyone. A few things before we get to comments. First up, if you're looking for more great science content, then you should check out Overview, on PBS Digital Studios' science and nature channel Terra. Overview is hosted by Joe Hanson from It's Okay to Be Smart and combines mesmerizing drone footage with deep science storytelling to reveal all the things shaping our planet from the 10,000 foot view. So head over to Terra, and make sure to (politely) tell them Space Time sent you!

Next up, we just wanted to shout out again to everyone who helps us out on Patreon. As always, your contributions make a huge difference. But today an extra big shoutout goes to Scott Grey who’s contributing at the Big Bang level - Scott, thank you so much. We’ve used your contributions so far to bake a nice apple pie and send to welcome our Venusian neighbors to the solar system - or at least into our awareness of their existence in the solar system. We look forward to neighborly relations with the gentle Venusian gas-bag civilization. And, ultimately, to ally with them in war against the cosmic string entities in the heart of the Sun that we discussed recently. So, Scott, I guess thanks for contributing to the future ascendency of humanity.

Okay, Last week we talked about post-quantum cryptography - possible solutions to the impending cryptography-cracking powers of quantum computers that don’t themselves require quantum technology. We were delighted to find several crypto-geniuses in our audience who called us out on a few errors.

First up, Mina86 pointed out that our use of mixing colors as an analogy for one-way functions in RSA protocol is more accurately analogous to Diffie-Helman key exchange. I agree, and we should have been more careful here. Our point was to illustrate one-way functions themselves, but the way we said it could have been interpreted as illustrating RSA. Thanks for catching that.

Several of you noticed that we skipped a power of 2 in one of our graphics. We jumped form 256 to 1024. Oops. I think it comes from my habit of skipping generations of processor in my computer upgrades. I go for every second Moore’s law doubling, which leaves my machines basically paralyzed about half the time.

And the last catch - a few of you noticed I used the expression “factor a prime number” - which is dumb, because the factors of a prime number are just itself and 1. Which I guess you don’t need a quantum computer for. What I meant was factorizing the product of two primes INTO those primes.

Anyway, thanks for keeping us straight.

Finally, a few of you expressed your distaste for lattice-based encryption protocols. Really, there were doubts that any salad-based protocols would be meaty enough to defeat quantum decryption.