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What Is Space-Time?

Long exposure star trail image taken at Hehuan Mountain, Taiwan.
Space-time is the conceptual model that best explains how the universe works.
(Image: © Shutterstock)

The fabric of space-time is a conceptual model combining the three dimensions of space with the fourth dimension of time. According to the best of current physical theories, space-time explains the unusual relativistic effects that arise from traveling near the speed of light as well as the motion of massive objects in the universe. 

Who discovered space-time? 

The famous physicist Albert Einstein helped develop the idea of space-time as part of his theory of relativity. Prior to his pioneering work, scientists had two separate theories to explain physical phenomena: Isaac Newton's laws of physics described the motion of massive objects, while James Clerk Maxwell's electromagnetic models explained the properties of light, according to NASA.

Related: Newton's Laws of Motion

But experiments conducted at the end of the 19th century suggested that there was something special about light. Measurements showed that light always traveled at the same speed, no matter what. And in 1898, the French physicist and mathematician Henri Poincaré speculated that the velocity of light might be an unsurpassable limit. Around that same time, other researchers were considering the possibility that objects changed in size and mass, depending on their speed.

Einstein pulled all of these ideas together in his 1905 theory of special relativity, which postulated that the speed of light was a constant. For this to be true, space and time had to be combined into a single framework that conspired to keep light's speed the same for all observers. 

Einstein's theory of special relativity posited that the speed of light was constant because light always travels at the same speed.  (Image credit: Shutterstock)

A person in a superfast rocket will measure time to be moving slower and the lengths of objects to be shorter compared with a person traveling at a much slower speed. That's because space and time are relative — they depend on an observer's speed. But the speed of light is more fundamental than either. 

The conclusion that space-time is a single fabric wasn't one that Einstein reached by himself. That idea came from German mathematician Hermann Minkowski, who said in a 1908 colloquium, "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."

The space-time he described is still known as Minkowski space-time and serves as the backdrop of calculations in both relativity and quantum-field theory. The latter describes the dynamics of subatomic particles as fields, according to astrophysicist and science writer Ethan Siegel.

How space-time works 

Nowadays, when people talk about space-time, they often describe it as resembling a sheet of rubber. This, too, comes from Einstein, who realized as he developed his theory of general relativity that the force of gravity was due to curves in the fabric of space-time. 

Massive objects — like the Earth, sun or you — create distortions in space-time that cause it to bend. These curves, in turn, constrict the ways in which everything in the universe moves, because objects have to follow paths along this warped curvature. Motion due to gravity is actually motion along the twists and turns of space-time. 

A NASA mission called Gravity Probe B (GP-B) measured the shape of the space-time vortex around the Earth in 2011 and found that it closely accords with Einstein's predictions.

Related: Ripples in Space-Time Could Reveal the Shape of Wormholes

But much of this remains difficult for most people to wrap their heads around. Although we can discuss space-time as being similar to a sheet of rubber, the analogy eventually breaks down. A rubber sheet is two dimensional, while space-time is four dimensional. It's not just warps in space that the sheet represents, but also warps in time. The complex equations used to account for all of this are tricky for even physicists to work with. 

"Einstein made a beautiful machine, but he didn't exactly leave us a user's manual," wrote astrophysicist Paul Sutter for Live Science's sister site, Space.com. "Just to drive home the point, general relativity is so complex that when someone discovers a solution to the equations, they get the solution named after them and become semi-legendary in their own right."

The simplest way to understand the fabric of space-time is to imagine a curved sheet of rubber that directs how everything in the universe moves. But the analogy isn't entirely accurate because space-time has four dimensions, while a sheet of rubber only has two.  (Image credit: Shutterstock)

What scientists still don't know

Despite its intricacy, relativity remains the best way to account for the physical phenomena we know about. Yet scientists know that their models are incomplete because relativity is still not fully reconciled with quantum mechanics, which explains the properties of subatomic particles with extreme precision but does not incorporate the force of gravity. 

Quantum mechanics rests on the fact that the tiny bits making up the universe are discrete, or quantized. So photons, the particles that make up light, are like little chunks of light that come in distinct packets.

Some theorists have speculated that perhaps space-time itself also comes in these quantized chunks, helping to bridge relativity and quantum mechanics. Researchers at the European Space Agency have proposed the Gamma-ray Astronomy International Laboratory for Quantum Exploration of Space-Time (GrailQuest) mission, which would fly around our planet and make ultra-accurate measurements of distant, powerful explosions called gamma-ray bursts that could reveal the up-close nature of space-time. 

Such a mission wouldn't launch for at least a decade and a half but, if it did, it would perhaps help solve some of the biggest mysteries remaining in physics. 

Additional resources: 

  • SJBauer
    A well written summary for the definition of space-time. I like the history and the introduction of how space-time works, but the focus on a reconciled quantum mechanical model to explain gravity seems like a math problem. A more logical approach might be to consider the 'Big Bang' theory from a pre-existing fabric of space-time without any real matter, as a proposed one dimensional determinant, its inception starts with the unfolding perspective of this dimensional determinant for space-time fabric towards existence. The sequence is somewhat understood from an expansion from our one dimensional space-time into a two dimensional space-time fabric, and then into a three dimensional space-time fabric, and so on. The expectation is that ordinary matter creation took place within a pre-existing dark energy medium of space-time. Indeed, the existence of matter would be an intrusion upon this pre-existing universal medium of space-time which maintains a zero sum difference that is the balance of our cosmological continuum.
    With this understanding, any perturbation in this medium would engender a warping of this pre-ordered dark energy template of space-time fabric in the evolutionary perspective of its dimensional unfolding. Wherein the creation of matter as a whole induces a complementary displacement, or warping, in the dark energy medium of the space-time fabric, its promulgation is interdependent on its insistence and persistence. For within this warping, there is yet another pertubation in the whole matter created; a dual relationship of newly created positive density matter in an envelopment of negative density matter. The complementary displacement insulates the newly created positive density matter in an envelopment of negative density matter. This envelope of negative density matter, known as dark matter, then infiltrates the spaces in matter, providing it with the ability to interact, bond, and evolve. Indeed it would require much more dark matter to fill the spaces among ordinary matter down to its smallest constituent parts.
    So if dark matter is what engenders a force of gravity for ordinary matter to bond, then the accretion and accumulation of ordinary matter is just the resultant consequence of this force. And if the black holes are nothing but dark matter, then it would also follow that dark matter can be accumulated, separate of ordinary matter. It would therefore also follow that the gravitational force is more representative of negative density mass than positive density mass.
    Upon this hypothesis then, one can expect that there is a require transition to separate ordinary matter from its complementary dark matter. It starts first with the disintegration of matter, as a whole, as it interacts with the event horizon of the black hole. As the positive density mass is 'squeezed' upon its own gravitational acceleration toward the black hole, liken to the spaghettification effect, its matter changes to allow for its disintegration via transmutation and the massive release of photons due to alpha decay and beta decay. This is the effect wherein positive density mass is collected within the event horizon, into a plasma, increasing its photon density. This 'squeezing' effect is like extracting out the dark matter from the whole matter, allowing for the ordinary matter to be reduced to its smallest constituent components. The dark matter is then absorbed into the black hole, and the remnants of ordinary matter are discarded and radiated out at high velocity back into the cosmos; to start, once again, to reintegrated into the universe via bonding and evolving.
    If you're interested in exploring this concept more, please review the alternative theories presented in the book, 'The Evolutioning of Creation: Volume 2', or even the ramifications of these concepts in the sci-fi fantasy adventure, 'Shadow-Forge Revelations'. The theoretical presentation brings forth a variety of alternative perspectives on the aspects of existence that form our reality.
    Reply
  • jimdodds
    Interesting stuff, but what if spacetime is itself a web, that has naturally occurring fluctuations of density and where spacetime is already denser is where matter accumulates?
    Reply
  • sward
    jimdodds said:
    Interesting stuff, but what if spacetime is itself a web, that has naturally occurring fluctuations of density and where spacetime is already denser is where matter accumulates?

    Mind boggling!
    Reply
  • Manuel Peniche Osorio
    About SJB comment, if formation of space-time was a consequence of the so called BB event - as many scientists consider- , then the "pre-existing fabric" in which it happened could not have a spatial nor a temporal nature. It would rather be a "negative net" - to say so- wth no space-time existance where the appearance of an initial zero D singularity (a point with incipient spatial existance) would eventually have arisen (due perhaps to those "density fluctuations" of the primordial grid suggested by Jimdodds).

    The subsecuent "unfolding" of the initial 0D singularity into a 1D (linear) reality and then to a 2D ( plane) one and a 3D (volumetric) universe would imply the nested presence of the time factor in each step to produce a new following dimension, "time" being allways the next non-spatial dimension.

    In our familiar 3D spatial reality, time is then the 4th dimension - as it is usually considered- along which "reality" unfolds trough.
    Reply
  • Tensologist
    sward said:
    Mind boggling!
    Yup a mind boggler. Considering also in 1952 Einstein wrote, “Physical objects are not in space, but these objects are spatially extended. In this way the concept ‘empty space’ loses its meaning.”
    So physical objects being 4D spatially-extended, as @jimdodds put it "fluctuations of density," reflects the fact that what the CERN LHC high-energy physics experiments actually measure is 4D electromagnetic energy density pressure concentrations. Just as AI pattern recognition famously aided in the discovery of the Higgs boson by recognizing collision pattern energy densities.

    I suggest the 'Quantum Crisis' CERN LHC experimentalist Maria Spiropulu reported on in 2014 (see at 20:32) arises from the fact the standard model of physics (SM) models the collision pattern energy density concentrations as zero-dimensional (0D) mathematical point particles -- which provide no information content towards replicating the 4D spacetime universe. The Quantum Crisis then being the observed 4D physical properties are conjectured to be hidden dimensional unknown material string or membrane mechanism properties -- which remain undetected at LHC energy levels.

    What Is Space-Time? Well, we can only measure 4D spacetime relative to energy density (mass density) concentrations...which concentrations are themselves 4D spatially-extended energy density and mass density integrations. So a new 4D spatially-extended nonstandard model of energy density and mass density integrations is going to be required to resolve the Quantum Crisis, since as Alessandro Fedrizzi and Massimiliano Proietti found Objective Reality Doesn't Exist, Quantum Experiments Shows.
    Just as Paul Sutter pointed out in Where Are All the 'Sparticles' That Could Explain What's Wrong with the Universe?: "Or, more depressingly, they don't exist. And that would mean that these creatures — along with their supersymmetric partners — are really just ghosts dreamt up by feverish physicists, and what we actually need is a whole new framework for solving some of the outstanding problems of modern physics."
    Reply
  • Tak
    SJBauer said:
    A well written summary for the definition of space-time. I like the history and the introduction of how space-time works, but the focus on a reconciled quantum mechanical model to explain gravity seems like a math problem. A more logical approach might be to consider the 'Big Bang' theory from a pre-existing fabric of space-time without any real matter, as a proposed one dimensional determinant, its inception starts with the unfolding perspective of this dimensional determinant for space-time fabric towards existence. The sequence is somewhat understood from an expansion from our one dimensional space-time into a two dimensional space-time fabric, and then into a three dimensional space-time fabric, and so on. The expectation is that ordinary matter creation took place within a pre-existing dark energy medium of space-time. Indeed, the existence of matter would be an intrusion upon this pre-existing universal medium of space-time which maintains a zero sum difference that is the balance of our cosmological continuum.
    With this understanding, any perturbation in this medium would engender a warping of this pre-ordered dark energy template of space-time fabric in the evolutionary perspective of its dimensional unfolding. Wherein the creation of matter as a whole induces a complementary displacement, or warping, in the dark energy medium of the space-time fabric, its promulgation is interdependent on its insistence and persistence. For within this warping, there is yet another pertubation in the whole matter created; a dual relationship of newly created positive density matter in an envelopment of negative density matter. The complementary displacement insulates the newly created positive density matter in an envelopment of negative density matter. This envelope of negative density matter, known as dark matter, then infiltrates the spaces in matter, providing it with the ability to interact, bond, and evolve. Indeed it would require much more dark matter to fill the spaces among ordinary matter down to its smallest constituent parts.
    So if dark matter is what engenders a force of gravity for ordinary matter to bond, then the accretion and accumulation of ordinary matter is just the resultant consequence of this force. And if the black holes are nothing but dark matter, then it would also follow that dark matter can be accumulated, separate of ordinary matter. It would therefore also follow that the gravitational force is more representative of negative density mass than positive density mass.
    Upon this hypothesis then, one can expect that there is a require transition to separate ordinary matter from its complementary dark matter. It starts first with the disintegration of matter, as a whole, as it interacts with the event horizon of the black hole. As the positive density mass is 'squeezed' upon its own gravitational acceleration toward the black hole, liken to the spaghettification effect, its matter changes to allow for its disintegration via transmutation and the massive release of photons due to alpha decay and beta decay. This is the effect wherein positive density mass is collected within the event horizon, into a plasma, increasing its photon density. This 'squeezing' effect is like extracting out the dark matter from the whole matter, allowing for the ordinary matter to be reduced to its smallest constituent components. The dark matter is then absorbed into the black hole, and the remnants of ordinary matter are discarded and radiated out at high velocity back into the cosmos; to start, once again, to reintegrated into the universe via bonding and evolving.
    If you're interested in exploring this concept more, please review the alternative theories presented in the book, 'The Evolutioning of Creation: Volume 2', or even the ramifications of these concepts in the sci-fi fantasy adventure, 'Shadow-Forge Revelations'. The theoretical presentation brings forth a variety of alternative perspectives on the aspects of existence that form our reality.
    Sorry, but dark matter has nothing to do with black holes or dark energy. Dark matter is called "dark" because it does not seem to feel any force except gravity. So, it does not by definition interact with electromagnetic energy, meaning that it can't clump up or slow down, which would be necessary to bond in any way with ordinary matter or to form a black hole (which is collapsed ordinary matter). Instead, it just orbits in diffuse clouds around massive objects like galaxies or star clusters withour ever settling down. And dark energy is a force which has nothing to do with either black holes or dark matter, since it seems to pervade the universe uniformly, causing it to expand uniformly at all points. Since it is very weak, it is only measured over very large distances. This is all very well established physics.
    Reply
  • Tak
    "A person in a superfast rocket will measure time to be moving slower and the lengths of objects to be shorter compared with a person traveling at a much slower speed. "

    Um, this is just plain wrong (or maybe just badly expressed). Both of those people would measure their own flow of time and lengths of objects around them in what would be to each them to be completely normal ways. However, when observing each other, each observer would measure the other's time to be passing at a slower rate, and lengths on each other's ships to be compressed in the direction of motion, relative to themselves. The greater the relative motion, the greater the observed effect.
    Reply
  • NewPhysics
    The Space-Time was described in the Po Theory potheory.com

    The Po Theory is a new theory of physics presenting fundamental issues, among others construction of space-time and describes the properties of space-time, e.g. its decay (expansion) resulting in the generation of time. In The Po Theory there are many models describing the structure of matter, e.g. particle generation model, particle mass (energy) generation model, preon structure model, etc. All these models are supported by mathematical equations describing the properties of particles such as particle radius, its mass, characteristic time associated with the particle, life time, range of interaction. The Po Theory gives, for example, a formula for calculating the radius of an electron, its mass or electric charge.

    Po theory describes the known and yet unknown sets of elementary and fundamental particles, objects that are black holes - collapsars, including the Universe and Bi Universe collapsars. Describes interactions - forces that govern these particles and objects. It shows the correlations between the parameters of these particles and physical constants, symmetries between interactions, and above all determines the basic particle - the Po collapsar. The particle which is the smallest collapsar and the basic particle with the highest mass among the elementary particles, which is part of both planckon and fundamental and elementary particles, which is the basic "building block" of matter.

    Po theory was presented in the book: "Po Theory. From the smallest particle to the BiUniverse", www.potheory.com or www.teoriapo.pl
    Polish version
    https://drive.google.com/file/d/1-2LF-htgDbanPKLP5qpFoUFDPnEBGtYK/view?usp=drivesdk
    English version
    https://drive.google.com/file/d/1qTNhhqrgtuK63P06Fa3IhktYQg3YdsIA/view?usp=drivesdk
    Reply
  • raymitchell
    jimdodds said:
    Interesting stuff, but what if spacetime is itself a web, that has naturally occurring fluctuations of density and where spacetime is already denser is where matter accumulates?
    These ideas are ignoring some plainly obvious facts of logic that blows these theories to smithereans. This article and scientists are really promoting speculation fairy tales that cannot have any logic to them if one does not ignore some obvious facts that contradict these "far out theories" only found in their imaginations.
    For example, the definition of "space" is the absence of anything, ie, nothing. Since space is nothing at all, then space cannot be bent, since one cannot bend nothing. There is nothing to bend.
    Also, what is time? It is not some substance that floats around the universe as these super silly and illogical ideas state or suggest. Time is also nothing because all time is is the idea that some event happens after another event. We can measure the amount of "time" that passed by using some constant stream of motion that can create clocks or stopwatches. The fact that something happened after another something does not make time some substance that can be bent. Time is only in our minds and also in the movement of things.
    So to say space (nothing) is joined with time (also nothing, only our thoughts of knowing which happened first and how long ago) can be bent should be put in Grimm's fairy tales.
    To say the gravity is caused by the bending of nothing and nothing fairy tale is an extra illogical fairy tales which tries to explain the unexplainable, gravity. (The only real explanation if our loving Creator made gravity function.)
    Also, the speed of light is not constant as scientists in 2015 showed in experiments. They slowed down light photons (search the web for it).
    The theory of relativity also has lots of holes in it just using logic. Although it has lots of truth to it, its relative speed and slowing of the ageing and movement of things for those who travel more at the speed of light is a self contradiction. So how does their body know they are traveling at the speed of light to slow everything down? It does not know at all. Besides all speed is relative, so if a rocket takes off from Earth to a star at the speed of light, they are going the speed of light relative to the Earth and the star. But what if there is a 2nd star behind the one they are going to but the 2nd star is moving the speed of light away from earth in the same direction as the rocket. So the speed of the rocket is ZERO mph, not the speed of light. So how does the body now know which speed to age at since it could pick the speed of light for star 1 , or it could pick Zero speed if the rocket decides to go to star 2. Which one does the body pick to age and move at? And how does the body and rocket know which speed to use?
    None of this article makes any sense at all.
    Reply
  • raymitchell
    These ideas are ignoring some plainly obvious facts of logic that blows these theories to smithereans. This article and scientists are really promoting speculation fairy tales that cannot have any logic to them if one does not ignore some obvious facts that contradict these "far out theories" only found in their imaginations.
    For example, the definition of "space" is the absence of anything, ie, nothing. Since space is nothing at all, then space cannot be bent, since one cannot bend nothing. There is nothing to bend.
    Also, what is time? It is not some substance that floats around the universe as these super silly and illogical ideas state or suggest. Time is also nothing because all time is is the idea that some event happens after another event. We can measure the amount of "time" that passed by using some constant stream of motion that can create clocks or stopwatches. The fact that something happened after another something does not make time some substance that can be bent. Time is only in our minds and also in the movement of things.
    So to say space (nothing) is joined with time (also nothing, only our thoughts of knowing which happened first and how long ago) can be bent should be put in Grimm's fairy tales.
    To say the gravity is caused by the bending of nothing and nothing fairy tale is an extra illogical fairy tales which tries to explain the unexplainable, gravity. (The only real explanation if our loving Creator made gravity function.)
    Also, the speed of light is not constant as scientists in 2015 showed in experiments. They slowed down light photons (search the web for it).
    The theory of relativity also has lots of holes in it just using logic. Although it has lots of truth to it, its relative speed and slowing of the ageing and movement of things for those who travel more at the speed of light is a self contradiction. So how does their body know they are traveling at the speed of light to slow everything down? It does not know at all. Besides all speed is relative, so if a rocket takes off from Earth to a star at the speed of light, they are going the speed of light relative to the Earth and the star. But what if there is a 2nd star behind the one they are going to but the 2nd star is moving the speed of light away from earth in the same direction as the rocket. So the speed of the rocket is ZERO mph, not the speed of light. So how does the body now know which speed to age at since it could pick the speed of light for star 1 , or it could pick Zero speed if the rocket decides to go to star 2. Which one does the body pick to age and move at? And how does the body and rocket know which speed to use?
    None of this article makes any sense at all.
    Reply