# What is Space and time: Definition and 139 Discussions

In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive differently where and when events occur.
Until the 20th century, it was assumed that the three-dimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was independent of one-dimensional 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. However, in 1905, Albert Einstein based a work on special relativity on two postulates:

The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference)
The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.The logical consequence of taking these postulates together is the inseparable joining together of the four dimensions—hitherto assumed as independent—of space and time. Many counterintuitive consequences emerge: in addition to being independent of the motion of the light source, the speed of light is constant regardless of the frame of reference in which it is measured; the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true.
Einstein framed his theory in terms of kinematics (the study of moving bodies). His theory was an advance over Lorentz's 1904 theory of electromagnetic phenomena and Poincaré's electrodynamic theory. Although these theories included equations identical to those that Einstein introduced (i.e., the Lorentz transformation), they were essentially ad hoc models proposed to explain the results of various experiments—including the famous Michelson–Morley interferometer experiment—that were extremely difficult to fit into existing paradigms.
In 1908, Hermann Minkowski—once one of the math professors of a young Einstein in Zürich—presented a geometric interpretation of special relativity that fused time and the three spatial dimensions of space into a single four-dimensional continuum now known as Minkowski space. A key feature of this interpretation is the formal definition of the spacetime interval. Although measurements of distance and time between events differ for measurements made in different reference frames, the spacetime interval is independent of the inertial frame of reference in which they are recorded.Minkowski's geometric interpretation of relativity was to prove vital to Einstein's development of his 1915 general theory of relativity, wherein he showed how mass and energy curve flat spacetime into a pseudo-Riemannian manifold.

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1. ### I About quantum interference: space and time

Hello to all, Questions that I hope are not completely devoid of physical meaning. Firstly, about space. Let be a Hilbert space, in which we can by definition establish the existence of complete and orthonormal vector bases; and a Psi vector (state) that we write as a linear combination (a...
2. ### I What is the fabric of space-time made up of?

Also why can't we apply laws of gravity to the quantum world? Can gravitational forces affect the subatomic particles such as protons and neutrons.
3. ### B Is there a lining to our Universe?

I was wondering if someone could answer my question. I know I may be dumb but I don't really understand why our universe has a limited capacity for mass. If I remember correctly black holes occur when concentration of mass causes a hole in spacetime. I don't really understand I guess how space...
4. ### I Topological phase transitions for the whole Universe...?

Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "*The Universe in a Helium Droplet*" where he explained his ideas. You can find a brief discussion here...
5. ### I Consequences of the absence of global symmetries...?

I found some interesting discussions in this site (e.g: https://www.physicsforums.com/threads/smolin-lessons-from-einsteins-discovery.849464/; https://www.physicsforums.com/threads/relatismo-to-the-max.83885/) which are related to Lee Smolin's ideas that laws are not immutable and can therefore...
6. ### I Models where all symmetries would be approximate?

I found this interesting discussion here in Physics Forums (https://www.physicsforums.com/threads/are-all-symmetries-in-physics-just-approximations.1005038/) where the topic of all symmetries being approximate is discussed Is there any model (for instance, some type of spacetime metric or...
7. ### I Non-homogeneous and anisotropic metric and laws of physics...?

In this popular science article [1], they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to place in the entire universe. And according to this paper [2] anisotropy in spacetime could...
8. ### B Exploring Space-Time: Is Einstein's Theory Overfitted?

I understand people collapse multi dimensional functions to make simpler visualisations, eg if you have a 500 dimension objective function in machine learning you can collapse it to 2D or 3D to get a visual idea of the objective-space.is this why Einstein did it as well? to make simpler...
9. ### B Mass of Space (B but approaching I)

Since space is expanding, and that expansion moves things, then those moved things must be moved by something. If moved by space itself, then space must be providing the energy. And if gravity is a function of space and time interacting, then what is the energy or mass of a volume of space...
10. ### A Small and large extra dimension(s) of the physical space

Trying to make sense of small and large extra dimension(s) of phyiscal space in a simple intuitive example. Consider a two dimensional manifold like R2 and we are trying to add a small and a large extra dimension. Do we mean by smale extra dimension in this case something like (0,1)×R (the...
11. ### I Uncertainty Principle in QFT & Early Universe Conditions

I have a question related to the uncertainty principle in QFT and if it is related to the early universe conditions. Do we still have four-vector momentum and position uncertainty relation in relativistic quantum theory? I have been following the argument related to the early universe and the...
12. ### B Is A Universe w/o Mass Possible? Exploring Time & Matter

Is it possible to have a universe with only space and time but no mass ? I ask this question because a friend told me that time is an illusion. In fact, time does not exist. Because of the existence of matter, time can be felt through the movement of matter. If matter does not exist, time does...
13. ### B Does the CMB give us absolute space and time in practice?

Another noob relativity / cosmology question (although at least this time won't turn out to be a coding bug, as no code is involved...) AIUI, according to relativity, there is no privileged reference frame, and any inertial reference frame is as "correct" as any other. But... In practice, in...
14. ### I Infinite Frequencies & Extraterrestrial Signals: What Are We Missing?

And if there are an infinite amount of frequencies, doesn't that mean that an extraterrestrial civilization could be reaching out without us being able to receive their signals. And even if we did receive their signals, how would we understand their form of communication? What if they...
15. ### I Are there types of spacetime where no symmetries are valid?

We derive the most basic laws of physics from several fundamental symmetries (those from Noether's theorems, gauge symmetries, Lorentz symmetry...). But are there any types of spacetime where no symmetries, no matter how fundamental, would be valid? Any special metric, geometry or shape?
16. ### B Time Dilation: How Does Speed Affect Aging?

Hello everyone, I wanted to know how speed can dilate time. For example, if there is a star 100 light years away from Earth and I started traveling at light speed, how long would it take for me to get there? I understand that the time differs from the point of reference (ie time from Earth's...
17. ### I Cosmological principle about space and time

AFAIK there is no cosmological principle formulated about space and time. If it would be formulated, it would more or less state that spacetime is an interconnected whole, and has no gaps, edges or boundaries. It doesn't need to state wether spacetime is finite or infinite, that is an open...
18. ### Space and time -- Is the term "light year" really correct?

When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
19. ### B How Do Points M and O' Move Relative to Point O in Space and Time?

I consider three material points O, O', M, in uniform rectilinear motion in a common direction, so that in relation to the point O, the points O' and M move in the same direction with the constant velocities v and u (u>v>0). Assuming that at the initial moment (t0=0), the points O, O', M were in...
20. A

### Exploring the Final Frontier: Join Our Team at Robert Neal Higgins Incorporated!

Hi Folks, Found a saved bookmark from an old science class somewhere. Should have been here working with all of you all these years, but worlds apart do much better with an exchange of art. So who would like to participate: interstellar travel; extra-dimensional movement; and temporal...
21. ### I A quick question regarding the application of Aleph numbers to reality

Hi everyone, I have a quick question about Aleph numbers. Are they even possible? By containing infinity to a finite set, isn’t that essentially disproving the infinity in the first place? Can they be used in an actual scenario’s, or are they just purely hypothetical? Can they be used to...
22. ### I Why Do Objects Fall Along Spacetime Ripples?

When people try to explain how gravity works, the following example is constantly used . However, I don’t understand how this explains HOW gravity works. By using this example, gravity itself is used as a bias to explain how gravity works. How can explain gravity by saying “things fall along...
23. ### B Space & Time: How Are They Related?

All help is welcome!
24. ### B Did energy begin to exist? Can energy exist without space and time?

Did energy begin to exist at the Big Bang? Can energy exist without space and time? Or don't we know? When I've tried to research this I get a mix of different answers. I have virtually no understanding of science or physics in general FYI.
25. ### I What does it mean to say space and time are fundamental in physics?

What feature of the equations of classical physics show that space and time are fundamental and how would the equations differ if space and time were not fundamental but emergent? I heard all of this from a previous talk and I would appreciate any help and any further reading recommendations.
26. ### B How constrained are the initial conditions of the universe?

Do we know? Do we have any idea? There seems to be 2 schools of thought. 1. The initial conditions can be almost any value with universes inflating with different laws of physics. This is the multiverse and string theory 10^500 false vacua view. 2. The initial conditions are more restrained...
27. ### A Effects of gravity throughout space-time?

Can the effects of gravity possibly extend through time? Is there a natural law that forbids this? Could this be a possible explanation for dark matter?
28. ### A Do Holographic Screens eliminate holographic dualities?

Do Holographic Screens eliminate the need of finding holographic dualities? There are various models in physics based on the famous holographic principle (https://en.wikipedia.org/wiki/Holographic_principle) This does not always work since in these models we must find a correlation between two...
29. ### I Is Space-Time Continuous or Quantized?

A free photon can have any wavelength and energy; no discreteness there. Just because something is quantized, or fundamentally quantum in nature, doesn’t mean everything about it must be discrete. The idea that space (or space and time, since they’re inextricably linked by Einstein’s theories...
30. ### A Is there any theory that can be modeled in any type of space?

Is there any theory in physics that can be modeled in any type of space (Hilbert space, Euclidean, Non-Euclidean...etc)? And if yes, could that theory also contain/be compatible with all types of (physical) symmetries?
31. ### I Black Holes: Time Bidirectional?

I just watched a youtube video saying that somewhere in a black hole, time is bidirectional (can go to the future and can go to the passed), but space is one-directional. Any introduction material on this subject? Thanks in advance!
32. ### B Could singularities be just rips in the space-time fabric?

First off, this is just an assumption. My knowledge of the field is extremely limited and I beg you to come and correct my mistakes, so I can learn. So, I guess we all know how that space-time fabric is bended by gravity. When a star dies, all of the atoms are brought extremely close...
33. ### B How did Einstein come up with the thought of a space fabric?

I've always wondered how we came to come up with such an idea. Was he one day sitting around and thinking, then made a random assumption and go "ah hah!". Or did his idea come up through his calculations on the nature of how gravity should cause interaction? Is their a literal fabric of space...
34. ### I Space and time dependence of entangled particles

It seems that the entanglement of two particles does not change with time and can cross long distanced as long an neither particle decoheres with the environment. This makes me wonder if the wave function for that entanglement can have any time or space dependence? I only did a brief search for...
35. ### Does light move, or does space and time move past light?

Could it be possible for light to not move at all but remain still while space and time moves past it? The light would just exists as the continuum of space time moves past light.
36. ### I Space & Time Axes Coinciding: Consequences Explored

Let's consider two inertial frame S and S'. S' moves with speed v w.r.t S along x-axis towards the right. Now we can draw the two co-ordinates system. The t' axes will make an angle arctan(v/c) with t axes rotated towards x-axis and similarly the x' axis will be tilted towards the t axes...
37. ### I Theories without Fundamental Space and Time

Can string theory be made without time equations? According to Carlo Rovelli in his latest book "The Order of Time" https://www.amazon.com/dp/073521610X/?tag=pfamazon01-20 : "The equations of loop quantum gravity on which I work are a modern version of the theory of Wheeler and DeWitt. There...
38. ### I When should I use a plus or minus sign in space and time translations?

Hello! I am a bit confused about the sign in space and time translation operators acting on a state. I found it with both plus and minus sign and I am not sure which one to use when. The equations I am talking about are: $$U(t)=e^{\pm iHt/\hbar}$$ and $$T(x)=e^{\pm ixp/\hbar}$$. Is it a plus or...
39. ### B Motion vs Time: Does Time Affect Motion?

Why Space/TIME? I feel Motion affects Time, not the contrary. Thanks for claritication (no Math please).
40. ### B Do photons interact with spacetime?

The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?
41. ### Ek vs engine power in cars and space

Help me please:) (1) An idealized engine with constant torque curve means constant acceleration. So for example if there is no air resistance and no frictions and car is on fixed gear and at 3000Rpm it produces 50Hp so at 6000Rpm will produce 100Hp. So he doubles the speed , doubles power and...
42. ### I Earth Between 2 Suns: Time & Space Questions

if the Earth were set between 2 Sun (our sun) in the distance which the time passes the exact same as it is right now, does the distance between us and each sun would be the same as it is right now?
43. ### B Did the Big Bang create space and time?

As I understand it Georges Lemaître, upon learning about Hubble's discovery that space is expanding, came up with the big bang theory. He thought that if space is getting bigger, then it must have been smaller at one point, and if you go back far enough you get an extremely dense singularity...
44. ### Exploring the 3 Dimensions of Space and Time

Why do we see the dimensions of our universe as 3 dimensions of space and time instead of space, time, and matter? (Or another variation)
45. ### I How can General Relativity explain the Moon drifting apart from Earth

According to various sources, the Moon is separating from Earth 4 cm every year. I’ve searched for the explanation and I’ve found the following: The friction the seas and oceans from the Earth make with it’s soil causes the Earth’s rotation to slow down. This causes the Moon to speed up...
46. ### I Does a metric exist for this surface?

I have a surface defined by the quadratic relation:$$0=\phi^2t^4-x^2-y^2-z^2$$Where ##\phi## is a constant with units of ##km## ##s^{-2}##, ##t## is units of ##s## (time) and x, y and z are units of ##km## (space). The surface looks like this: Since the formula depends on the absolute value of...
47. ### Does black hole stop light or time?

I have been told the speed of light is constant and does not waiver. This would make sense as light has no mass I am aware of. Pictures taken during a solar eclipse seem to prove gravity bends light. Could it be however that the light is not bent, only the time/space light is traveling through...
48. ### I Is there an analog to Einstein's field equations for 2D?

I am not familiar with tensors and I would like to know if it's possible to understand GR without using them. I imagine we use them to describe four-dimentional space-time, because a regular vector or matrix wouldn't be enough. Is there an analog of Einstein's equations for a 2D space (plane)...
49. G

### B How is a Vaccum mathmatically described?

Anyone know what books and astrophysicist has this knowledge? Thank you, Greg
50. S

### B Exploring the Relationship Between Earth's Mass and Time: Debunking the Myth

Is it true that the time increase (24 hrs becomes 25 or 26 or..)when the mass of Earth is increase? ??