Even In A Quantum Universe, Space And Time Might Be Continuous, Not Discrete
“In General Relativity, matter and energy tell space how to curve, while curved space tells matter and energy how to move. But in General Relativity, space and time are continuous and non-quantized. All the other forces are known to be quantum in nature, and require a quantum description to match reality. We assume and suspect that gravitation is fundamentally quantum, too, but we aren’t sure. Furthermore, if gravity is ultimately quantum, we don’t know whether space and time remain continuous, or whether they become fundamentally discrete.
Quantum doesn’t necessarily mean that every property breaks down into an indivisible chunk. In conventional quantum field theory, spacetime is the stage upon which the various quanta act out the play of the Universe. At the core of it all should be a quantum theory of gravity. Until we can determine whether space and time are discrete, continuous, or unavoidably blurred, we cannot know our Universe’s nature at a fundamental level.”
If you could look at the Universe down to the smallest possible scales, fundamentally, what would you find? Would you discover that space and time really could be broken up into tiny, indivisible entities where the was a length scale and a timescale that could be divided no further? Would you discover that space and time were quantum in nature, but were instead a continuous fabric? Or would you discover something else, like that space and time weren’t quantum or that there was a fundamental “blurring” that prevented you from seeing below a specific scale?
Quantum, surprisingly to many, doesn’t necessarily mean it can be broken up into indivisible chunks. Space and time might not be discrete even if they’re quantum. Time to learn the difference.
LIGO’s Lasers Can See Gravitational Waves, Even Though The Waves Stretch The Light Itself
“But this is where the puzzle comes in: if space itself is what’s expanding or compressing, then shouldn’t the light moving through the detectors be expanding or compressing too? And if that’s the case, shouldn’t the light travel the same number of wavelengths through the detector as it would have if the gravitational wave had never existed?
This seems like a real problem. Light is a wave, and what defines any individual photon is its frequency, which in turn defines both its wavelength (in a vacuum) and its energy. Light redshifts or blueshifts as the space it’s occupying stretches (for red) or contracts (for blue), but once the wave has finished passing through, the light returns to the same wavelength it was back when space was restored to its original state.
It seems as though light should produce the same interference pattern, regardless of gravitational waves.”
Have you ever thought about how gravitational wave detectors work? By passing light down two mutually perpendicular arms, reflecting them back and reconstructing an interference pattern, we can detect a passing wave by how it changes the arm-lengths of the light. But the light itself also gets compressed and expanded, and shouldn’t those effects cancel out?
Clearly, LIGO, Virgo and KAGRA all work, as many detected events bear out. But have you ever thought about how? Come get the answer today!
This Is Why Time Has To Be A Dimension
“But even two different objects with the same exact three-dimensional spatial coordinates might not overlap. The reason is easy to understand if you start thinking about the chair you’re sitting in right now. It can definitely have its location accurately described by those three spatial coordinates familiar to us: x, y, and z. This chair, however, is occupied by you right now, at this exact moment in time, as opposed to yesterday, an hour ago, next week, or ten years from now.
In order to completely describe an event in spacetime, you need to know more than just where it occurs, but also when it occurs. In addition to x, y, and z, you also need a time coordinate: t. Although this might seem obvious, it didn’t play a large role in physics until the development of Einstein’s relativity, when physicists started thinking about the issue of simultaneity.”
When you describe where you are in the Universe, you typically think of the coordinates you’d need to give to describe your location. This includes an x, y, and z-direction: the three spatial coordinates corresponding to where we live in our three spatial dimensions. But this doesn’t fully tell you everything you’d need to know, because your location is defined not only by your spatial location but when you’re located there: you need a time coordinate, too. If we take a deep look into the relationship between space and time, first put forth by Einstein over a century ago, we’d find that it isn’t even enough to put in an additional coordinate. Time is more than a separate value; it’s every bit as much a dimension as any of the three spatial dimensions.
If you’ve ever wondered why we say that time is the fourth dimension, come read this. It couldn’t be any other way.
Sorry, Black Holes Aren’t Actually Black
“If you have an astrophysical object that emits radiation, that immediately defies the definition of black: where something is a perfect absorber while itself emitting zero radiation. If you’re emitting anything, you aren’t black, after all.
So it goes for black holes. The most perfectly black object in all the Universe isn’t truly black. Rather, it emits a combination of all the radiation from all the objects that ever fell into it (which will asymptote to, but never reach, zero) along with the ultra-low-temperature but always-present Hawking radiation.
You might have thought that black holes truly are black, but they aren’t. Along with the ideas that black holes suck everything into them and black holes will someday consume the Universe, they’re the three biggest myths about black holes. Now that you know, you’ll never get fooled again!”
So, you thought you knew all there way to know about black holes? That if you get enough mass together in a small enough volume of space, you create an event horizon: a region from within which nothing can escape, not even light. So how is it, then, that black holes wind up emitting radiation, even long after the last particle of matter to fall into them has ceased?
There are two ways this occurs, and both are completely unavoidable. Black holes aren’t actually black, and this is how we know it.
Ask Ethan: Is Spacetime Really A Fabric?
“I’d like somebody to finally acknowledge and admit that showing balls on a bed sheet doesn’t cut it as a picture of reality.”
Okay, I admit it: visualizing General Relativity as balls on a bedsheet doesn’t make a whole lot of sense. For one, if this is what gravity is supposed to be, what pulls the balls “down” onto the bedsheet? For another, if space is three dimensional, why are we talking about a 2D “fabric” of space? And for another, why do these lines curve away from the mass, rather than towards it?
It’s true: this visualization of General Relativity is highly flawed. But, believe it or not, all visualizations of General Relativity inherently have similar flaws. The reason is that space itself is not an observable thing! In Einstein’s theory, General Relativity provides the link between the matter and energy in the Universe, which determines the geometric curvature of spacetime, and how the rest of the matter and energy in the Universe moves in response to that. In this Universe, we can only measure matter and energy, not space itself. We can visualize it how we like, but all visualizations are inherently flawed.
Come get the story of how to make as much sense as possible out of the Universe we actually have.
There Was No Big Bang Singularity
“Every time you see a diagram, an article, or a story talking about the “big bang singularity” or any sort of big bang/singularity existing before inflation, know that you’re dealing with an outdated method of thinking. The idea of a Big Bang singularity went out the window as soon as we realized we had a different state — that of cosmic inflation — preceding and setting up the early, hot-and-dense state of the Big Bang. There may have been a singularity at the very beginning of space and time, with inflation arising after that, but there’s no guarantee. In science, there are the things we can test, measure, predict, and confirm or refute, like an inflationary state giving rise to a hot Big Bang. Everything else? It’s nothing more than speculation.”
The Universe, as we observe it today, is expanding and cooling, with the overall density dropping as the volume of space increases. If we ran the clock backwards, however, instead of forwards, things would appear to contract, become denser, and grow hotter. If you go back farther and farther in time, you’d come to an epoch before there were stars and galaxies; before neutral atoms could stably form; before atomic nuclei could remain; etc. You’d go all the way back to hotter and denser states, eventually compressing all the matter and energy in the Universe into a single point: a singularity. This was the ultimate beginning of everything according to the original Big Bang: the birth of time and space.
But this picture is almost 40 years out of date, and known to be wrong. Why’s that? Come learn how we know that there was no Big Bang singularity.
Ask Ethan: Could The Energy Loss From Radiating Stars Explain Dark Energy?
“What happens to the gravity produced by the mass that is lost, when it’s converted by nuclear reactions in stars and goes out as light and neutrinos, or when mass accretes into a black hole, or when it’s converted into gravitational waves? […] In other words, are the gravitational waves and EM waves and neutrinos now a source of gravitation that exactly matches the prior mass that was converted, or not?”
For the first time in the history of Ask Ethan, I have a question from a Nobel Prize-winning scientist! John Mather, whose work on the Cosmic Microwave Background co-won him a Nobel Prize with George Smoot, sent me a theory claiming that when matter gets converted into radiation, it can generate an anti-gravitational force that might be responsible for what we presently call dark energy. It’s an interesting idea, but there are some compelling reasons why this shouldn’t work. We know how matter and radiation and dark energy all behave in the Universe, and converting one into another should have very straightforward consequences. When we take a close look at what they did, we can even figure out how the theory’s proponents fooled themselves.
Radiating stars and merging black holes do change how the Universe evolves, but not in a way that can mimic dark energy! Come find out how on this week’s Ask Ethan.
Are Space And Time Quantized? Maybe Not, Says Science
“Incredibly, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment where a single photon would pass through a crystal, causing it to move by a slight amount. Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, it ought to be possible to detect whether the “steps” that the crystal moves in are discrete or continuous. With a low-enough energy photon, if space is quantized, the crystal would either move a single quantum step or not at all.”
When it comes to the Universe, everything that’s in it appears to be quantum. All the particles, radiation, and interactions we know of are quantized, and can be expressed in terms of discrete packets of energy. Not everything, however, goes in steps. Photons can take on any energy at all, not just a set of discrete values. Put an electron in a conducting band, and its position can take on a set of continuous (not discrete) values. And so then there’s the big question: what about space and time? Are they quantized? Are they discrete? Or might they be continuous, even if there’s a fundamental quantum theory of gravity.
Surprisingly, space and time don’t need to be discrete, but they might be! Here’s what the science has to say so far.
Ask Ethan: If Mass Curves Spacetime, How Does It Un-Curve Again?
“We are taught that mass warps spacetime, and the curvature of spacetime around mass explains gravity – so that an object in orbit around Earth, for example, is actually going in a straight line through curved spacetime. Ok, that makes sense, but when mass (like the Earth) moves through spacetime and bends it, why does spacetime not stay bent? What mechanism un-warps that area of spacetime as the mass moves on?”
You’ve very likely heard that according to Einstein, matter tells spacetime how to curve, and that curved spacetime tells matter how to move. This is true, but then why doesn’t spacetime remain curved when a mass that was once there is no longer present? Does something cause space to snap back to its prior, un-bent position? As it turns out, we need to think pretty hard about General Relativity to get this right in the first place at all. It isn’t just the locations and magnitudes of masses that determine how objects move through space, but a series of subtle effects that must all be added together to get it right. When we do, we find out that uncurving this space actually results in gravitational radiation: ripples in space that have been observed and confirmed.
The deciding results are actually decades old, and were indirect evidence for gravitational waves long before LIGO. Come get the answer today!
If The Universe Is 13.8 Billion Years Old, How Can We See 46 Billion Light Years Away?
“There are a few fundamental facts about the Universe — its origin, its history, and what it is today — that are awfully hard to wrap your head around. One of them is the Big Bang, or the idea that the Universe began a certain time ago: 13.8 billion years ago to be precise. That’s the first moment we can describe the Universe as we know it to be today: full of matter and radiation, and the ingredients that would eventually grow into stars, galaxies, planets and human beings. So how far away can we see? You might think, in a Universe limited by the speed of light, that would be 13.8 billion light years: the age of the Universe multiplied by the speed of light. But 13.8 billion light years is far too small to be the right answer. In actuality, we can see for 46 billion light years in all directions, for a total diameter of 92 billion light years.”
Sure, the Universe is expanding, but how is it possible to see objects that are 46 billion light years away? After all, with an age of 13.8 billion years since the Big Bang, and a Universe where the cosmic speed limit is the speed of light, how can we see light that’s more than three times the expected distance away? It’s one of the most frequent questions that cosmologists get, and yet the root of the question is better framed as “how does the expanding Universe work?” While we normally think about things happening in space that doesn’t change much, or as individual objects moving relative to one another in a static space, we don’t, conventionally, have a solid intuition for how the fabric of space itself expands. But thankfully, the scientists who study it do!
Come learn, in plain English, how we can see so far away in such a young Universe!