# What is 4d: Definition and 127 Discussions

4D film or 4-D film is a marketing term for an entertainment presentation system combining a 3D film with physical effects that occur in the theatre in synchronization with the film. Effects simulated in a 4D film may include rain, mist, bubbles, fog or smoke, wind, temperature changes, strobe lights, scent, vibration and motion. Seats in 4D venues may vibrate or move a few centimeters during the presentations. Other common chair effects include air jets, water sprays, and leg and back ticklers. Auditorium effects may include smoke, rain, lightning, bubbles, and smell.
Because physical effects can be expensive to install, 4D films are most often presented in custom-built theatres at special venues such as theme parks, amusement parks and zoos. However, some movie theatres have the ability to present 4D versions of wide-release 3D films. The films Journey to the Center of the Earth (2008) and Avatar (2009) are among the films that have received a 4D treatment in certain theatres. There are also mobile 4D theaters, which are mounted inside vehicles such as enclosed trailers, buses and trucks.
4D films are distinct from four-dimensional space. Notable historical formats for providing different aspects of a "fourth dimension" to films include Sensurround, and Smell-O-Vision. As of June 2015, about 530 screens worldwide have installed some 4D technology.

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1. ### How would your body change if you were in a 4D space?

I don't believe this question will interest anyone else, but remains worthy of notice because the solution is a formula that depends on the 36th root of a number. You unexpectedly find yourself in a space with four Euclidean dimensions. Fortunately your molecules have been rearrange into a four...
2. ### Surf 4D: Ride the Wave, Make a 360°

In four spatial dimensions it is possible to have a breaking ocean wave that is horizontally circular. Maybe you could get all the way around it. It is possible to get the board to do a 360 whilst facing forward the whole time. That is, the board planes as it normally does while this is going...
3. ### A 4D HyperEarth: 1000x More Space for Real Estate

Let’s dubiously assume that these 4D atoms would have the same radius as 3D atoms. Let’s further dubiously assume that a 4D HyperEarth has the same number of atoms as 3D Earth. Then the diameter of HyperEarth would be less than a kilometer. Four dimensional Earth is much more compact. Wouldn’t...
4. ### I 4d integration/differentiation notation and the total derivative

This is probably a stupid question but, ## \frac{d\partial_p}{d\partial_c}=\delta^p_c ## For the notation of a 4D integral it is ##d^4x=dx^{\nu}##, so if I consider a total derivative: ##\int\limits^{x_f}_{x_i} \partial_{\mu} (\phi) d^4 x = \phi \mid^{x_f}_{x_i} ## why is there no...
5. ### Equator of a Planet with Four Spatial Dimensions is a 4D Torus

A 4D planet has no axis of rotation. Nothing special about planets, all freely rotating 4D bodies have two perpendicular planes of rigid rotation. (Clifford proved this in the 19th century.) Now there is nothing stopping us from thinking of planes of rotation here in 3D Universe. It's the...
6. ### I Assumptions about the convex hull of a closed path in a 4D space

Hello everyone, I am struggling to get insight into a certain set in 4D space. Given is a closed path in 4D-space with constant Euclidean norm $$\vec{\gamma} (\theta):[0,2\pi]\to\mathbb{R}^4, \ \ \vec{\gamma}(0)=\vec{\gamma}(2\pi), \ \ ||\vec{\gamma}(\theta)||_2 = \mathrm{const.}$$ I am looking...
7. ### B The 3D Universe: A Speck in the 4D Universe

Assume a TV screen or a book page represents 2D information viewable within our 3D world. From our 3D world, we see the entirety of the 2d world...all of it. Now imagine if somehow we lived within the 2D TV or book page. From our 2D world, we would be incapable of viewing, interacting or...
8. ### I 4D d'Alembert Green's function for linearised metric

Q. Calculate the linearised metric of a spherically symmetric body ##\epsilon M## at the origin. The energy momentum tensor is ##T_{ab} = \epsilon M \delta(\mathbf{r}) u_a u_b##. In the harmonic (de Donder) gauge ##\square \bar{h}_{ab} = -16\pi G \epsilon^{-1} T_{ab}## (proved in previous...
9. ### I What are the significant angles between intersecting planes in 4D space?

Suppose I have two intersecting planes in a four dimensional space. It seems to me that there are two angles between these planes. If the two planes intersect in a line then one of those angles is zero. If the two angles are non-zero then the planes intersect in a point. If one plane is the...
10. ### B Why Does Mass Pull on Other Stuff?

Why doesn't it repel things... or just pass through and leave distance unchanged?
11. ### Python 4D array with elements only on one side of the diagonal

Given the parameters ##n## and ##m##, I'd like to initialize a ##n \times n ## 4D array where each entry is an ##m \times m ## matrix, and where each column of the matrix is an array of type numpy.linspace(0,1,m). Furthermore, I'd only like to have entries on the diagonal and above it, or only...
12. ### I Electrodynamics of Dielectrics: 4D to 3D

Hi, so the four-dimensional generalization of $$\vec{B}=\mu\vec{H}$$ is $$F_{\lambda \mu}u_{\nu} + F_{\mu \nu}u_{\lambda} + F_{\nu \lambda}u_{\mu} = \mu (H_{\lambda \mu}u_{\nu} + H_{\mu \nu}u_{\lambda} + H_{\nu \lambda}u_{\mu})$$ From these four-tensors and four-vector I should be able to...
13. ### I Intersection of a 4D line and a 3D polyhedron in 4D

Is the intersection of a 4D line segment and a 3D polyhedron in 4D a point in 4D, if they at all intersect? Intuitively, it looks like so. But I am not sure about it and how to prove it.
14. ### I Explaining Time Crystals: Matter, Mass & 4D

Can someone simply explain to me what are the time crystals? What are they built from (matter, do they have mass)? I cannot find a clear explanation of them. I just know that ordinary crystals are 3d, time crystals - 4d.
15. ### I Exploring 4D Wave Propagation in 3D Solids

I am uncertain if this belongs in the differential geometry thread because I don't know what area of mathematics my question belongs into begin with, but of the math threads on physics forums, this one seems like the most likely to be relevant. I recently watched a video by PBS infinite series...
16. ### Python Invert a matrix from a 4D array : equivalence or difference with indexes

I have a 4D array of dimension ##100\text{x}100\text{x}3\text{x}3##. I am working with `Python Numpy. This 4D array is used since I want to manipulate 2D array of dimensions ##100\text{x}100## for the following equation (it allows to compute the ##(i,j)## element ##F_{ij}## of Fisher matrix) ...
17. ### I Question about why QFT in 4D is undefined

I'm a beginner in QFT and I'm always hearing people say that QFT is undefined in 3+1 dimensions or that the Hamiltonian cannot be rigorously defined, what do these statements mean exactly? In particular I'd like to know what makes the 3+1D case that much worse than non-relativistic QFTs like the...
18. ### A Where is the actual space inside a tesseract?

Hey, i can't see actual space in tesseract. Like in 3 dimensional cube, you have squares as faces and between them there is a space. But in tesseract where you have 8 cubes as faces, this leaves 0 space for actual space inside, i can see only 8 faces and nothing between them. I first thought it...
19. ### B Is the Universe a 3D Cut of a 4D object?

I have just simple Knowledge, but recently a question came To my mind. An Entity Living in 2d Space cannot Directly Observe a sphere, but rather a Circle Changing its Size when a sphere is traversing trough the observable 2d plane. Similar, a 4d sphere would Appear To us as 3d sphere Changing...
20. ### Can 4D Objects Be Understood Using a 2D Analogy?

That 2D analogy part is fantastic!Just trying to share great videos.
21. ### B Is our 3D universe a shadow of a 4D Universe?

Is it possible that our 3D universe is just a shadow in 3D space of 4D or higher dimensional reality? And we are limited in our perception of it by our 3-dimensionality. In other words like Flatlanders would just see s square of a cube sitting on the plane or passing through the plane...
22. ### A Physical meaning of "exotic smoothness" in (and only in) 4D

I see that this has been discussed before, but the old threads are closed. As Carl Brans and others note, it seems too big a coincidence to ignore. Why is exotic smoothness "good" (in the sense that it permits richer physics or something like that)? Exotic Smoothness and Physics,arXiv "there...
23. ### 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...
24. ### I String theory and Lorentz invariance - 10D vs. 4D....

Hi all, Clarification question: I've read that string theory is manifestly Lorentz invariant - however, I'm confused about this being true in 4D spacetime or in the full 10D setting of the theory (well, one version anyway). At some point I'd also read in a paper that 4D Lorentz invariance...
25. ### B Can there be many Worldlines in 4D spacetime?

Can many worldlines occur in 4D spacetime where observers in each worldline think they're the "real" worldine? Is there a mechanism preventing many worldlines from occurring in 4D spacetime. Einstein said this in his book Relativity: Look at this scenario. Event A occurs outside of the light...
26. ### A Flat s-t 4d killing vectors via solving killing equation

So I know what these are 4 translation : ##\frac{\partial}{\partial_ x^{u}} = \partial_{x^u}## 3 boost: ##z\partial_y - y \partial_z## and similar for ##x,z## and ## y,x## 3 rotation: ##t\partial_x + x\partial_t ## and similar for ##y , z## however I want to do it by solving Killing equation...
27. ### I Can a 3D object displace a 4D object?

I was just thinking about this randomly today. I feel like the answer is that it "does" and it "doesn't." A math teacher I had said that the 4th dimension is perpendicular to everything and anything in the 3rd, so does that mean that it can't be displaced, or is the answer more complicated than...
28. ### I Can string theory reproduce Hawking radiation in non-extremel black holes in 4D?

can string theory reproduce hawking radiation in non-extremel black holes in 4D? i.e physically realistic black holes. do they exactly match hawking's calculations? what is the interpretation of hawking radiation in string theory?
29. ### I Why isn't spacetime considered as true 4D?

...and what is the difference between true 4D and the minkowski space? To me, it would be much easier to see universe as a 4D and us humans just experiencing the dimension of time differently. In my mind i pictured the universe as a complete 4D structure which we humans experience in one...
30. ### B From a 4D perspective would 3D infinity exist?

From the perspective of a 4D observer would 3D infinity appear to exist? Why/why not?
31. ### I Papers on 4D ball through 3D space

Could anyone please recommend any papers that describe the path of a 4D ball through 3D space?
32. ### Calculating Covariant Riemann Tensor with Diag Metric gab

Using Ray D'Inverno's Introducing Einstein's Relativity. Ex 6.31 Pg 90. I am trying to calculate the purely covariant Riemann Tensor, Rabcd, for the metric gab=diag(ev,-eλ,-r2,-r2sin2θ) where v=v(t,r) and λ=λ(t,r). I have calculated the Christoffel Symbols and I am now attempting the...
33. ### MHB Creating a 4D System with a Specific Solution: How Can It Be Done?

I know how to solve systems, but the question asks to work backwards. "Create a 4-d system that has a solution (-2,5,-6,1)." How would i do this?
34. ### Would NMSM or vMSM + 4D QG like LQG or AsG be TOE?

thus far the LHC has not seen any SUSY in run 2, with possible exception of the 750 scarlar boson which may disappear with further data. If LHC run 2 provides no BSM type physics, would either The New Minimal Standard Model Hooman Davoudiasl, Ryuichiro Kitano, Tianjun Li, Hitoshi Murayama...
35. ### Are Some Atomic Orbitals From a 4D Wave Function?

"To visualize the standing waves (or orbitals) of electrons bound to a positively charged nucleus in three dimensions, we will need a four-dimensional plot of the wave function vs. x, y, and z." http://www.grandinetti.org/electron-orbital-shapes"The wavefunctions in the N=2 family are vectors in...
36. ### Exploring 4D Worlds: A Visual Journey Through Extra Dimensions

I've been wondering what spaces with an extra physical dimension would look like. That is, instead of our 3+1D space it would be a 4+1D space. In general, it can't be done. There is too much information for a human brain to handle. But some special cases can work. The main one I thought of...
37. ### Torus used to model 4D rotation

A torus can be used to model rotations of a sphere in 4 dimensions. Such rotations have two planes of rotation at right angles to one another. So one rotation plane corresponds to rotation around the major axis of the torus, and the other rotation plane to rotation around the minor axis...
38. ### Exploring Einstein's Corner Term in General Relativity

Question outline: In the case of 5d Kaluza (Klein) GR with NO charge and NO gauge field we expect 5d to reduce to 4d GR exactly. So this should be a very simple useful sanity check.s the side and corner terms of Einstein, Ricci and Energy tensors are zero, then R would be the same in 4d or 5d...
39. ### How can I construct the 4D real representation of SU(2)?

An element of SU(2), such as for example the rotation around the x-axis generated by the first Pauli matrice can be written as U(x) = e^{ixT_1} = \left( \begin{array}{cc} \cos\frac{x}{2} & i\sin\frac{x}{2} \\ i\sin\frac{x}{2} & \cos\frac{x}{2} \\ \end{array} \right) = \left(...
40. ### "Simple" 4D Kerr Geodesics simulator, source code and docs

Looks like my main pet GR project is about to enter something akin to maintenance mode, since it now does all I currently need it to. It's nothing earth-shattering at first glance, but is very concise (e.g. ~100 lines of Python for the simulator script) and should be easier to understand than...
41. ### Do any and all 4D or 5D AdS satisfy AdS/CFT correspondence

does any and all 4-D 5-D AdS spaces always have a 3-d or 4-D (d-1) CFT corresponding ? or are there 4D and 5D AdS where there is NO corresponding CFT? i.e is it possible to posit a 4D (5D) space that is AdS that has no 3D (4D) CFT ? since our universe is 4D, should AdS in 4D be studied with...
42. ### Black holes are the shadows of 4D objects (idea)

this is the idea...
43. ### Exploring 4D Space: Mysteries of Hypercubes

I've read the descriptions and watched the videos over and over that describe these shapes, and my skepticism has simply not gone away. Can anybody explain what I might be missing? Here is my reasoning: Physics supercedes, or at the very least, is parallel to math. Math is not something that...
44. ### Can a 4D shape exist in our universe?

Can a 4 spacially dimensional shape exist in our 3 spacially dimensional universe? Could one exist and we just cannot perceive like a 2D animal could think it lives on a flat plane yet it actually lives on a sphere or donut shape? And could our universe be a 4 spacially dimensional shaped?
45. ### Wrapping a 3D space wihtin a 4D space

Let’s imagine a moving point bound to a 2D plane. If we wrap this plane on a sphere within a 3D space the point would now eventually end up at the same position while moving in a seemingly fixed direction (it is actually not fixed in the 3D space). I am now wondering: Can a 3D hyperplane be...
46. ### Origin of our universe 4D black hole

The brains at the Perimeter Institute recently published a paper describing how our 3 dimensional universe could possibly exist as the event horizon of a 4 dimensional black hole in a 4 dimensional universe as the event horizons of black holes have one less dimension than the black hole itself...
47. ### Exploring Dimensions: The 4D Schrodinger Equation Made Simple

This is a rather naive question concerning the dimension of the schrodinger equation. If the Schrodinger equation can be wrtiten in a three dimensional form using the laplacian operator can it be written in a 4d version. I understand that the schrodinger equation shows the development of the...
48. ### Mobius Strip & 4D: Exploring Relationship & Dimensions

What is the relationship with the Mobius strip (or loop) and the 4 dimension? Is Mobius strip a four dimensional object?
49. ### I Calculating the Riemann Tensor for a 4D Sphere

Dear All, I am trying to calculate the Riemann tensor for a 4D sphere. In D'inverno's book, I have this equation R^{a}_{bcd}=\partial_{c}\Gamma^{a}_{bd}-\partial_{d}\Gamma^{a}_{bc}+\Gamma^{e}_{bd}\Gamma^{a}_{ec}-\Gamma^{e}_{bc}\Gamma^{a}_{ed} But the exercise asks me to calculate R_{abcd}. Do...
50. ### Wieland's new action for 4d simplicial gravity can be significant

It is too new for me to know for sure but I think it probably is. The action has a new kind of momentum, consisting of tetrahedral volume flowing along an edge-network: a "one dimensional branched manifold" derived from a simplicial decomposition of the original 4d manifold. Wieland calls the...