What is Flat: Definition and 495 Discussions

So as of 2013 physicist are 99.6% sure that the universe is flat, all evidence points to a flat universe, but we live on a spherical 3D planet... I'm having extreme issues comprehending the definition of flat in regards to the universe. Does it mean flat as in flat like the surface of the...

In his book on GR, Wald writes "the notion of the mass and spin of a field requires the presence of a flat background metric...which one has in the linear approximation but not in the full theory..." Why do we need a flat background metric to define a field's mass and spin?

Homework Statement Hello, I posted a similar question in the physics section but no one was able to help, I am first going to include a link to the older problem where I was attempting to find the ,(Finding the local flat space of the Poincare half disk metric), and explain what is different...

Greetings everyone. Does anyone know of any flat pressure switches? I am trying to build a keyboard sewn into a shirt and have not found any suitable sensors. There are force sensors (like this one https://www.sparkfun.com/products/9375) but it would become too expensive for a full keyboard and...

ΩHomework Statement Given the metric ds^2 = \frac{dx^2 + dy^2}{y^2} find a set of coordinates which yield local flat space. i.e. ( g_{\mu\nu} = \delta_{\mu\nu} + second order terms). My text outlined a process to go through to find the flat space coordinates, but the actual execution is...

I keep on reading that cosmologists contemplate two possibilities: either the universe is closed, unbounded and therefore finite, or else it is open (possibly flat) and infinite. I hear that a flat, finite universe "introduces many problems" and is discarded. My question would be "what...

Hi there. I have a dump question for you guys. I really wonder about curvature of spacetime. I read that due to Omega_tot=1 the Universe is assumed to be flat. But on the other hand something like the curvature of the universe is mentioned... I also thought that the energy stress tensor...

... I was thinking of a thought experiment concerning different systems of gravity-like acceleration. I've noticed that using a stringline in construction is close, but not very accurate because the string always sags to some degree. I have sometimes fabricated a water level by attaching...

Hello All, I have a fixture that is used for lifting an engine. See attached image for the part in question. You can see in the 'front' view the hole where a pin passes through the part and the lifting lug of the engine. You can also see the loading. We want to machine the part down such...

It seems as though the contemporary consensus among cosmologists is that the universe is basically flat and Euclidean: http://en.wikipedia.org/wiki/Shape_of_the_Universe However, Einsteins relativity equations describing events in space-time appear to be hyperbolic...

Hello everybody! This is my first post! I was wondering about the fact that we have measure our universe to be flat very accurately. This means that it is also infinite and if it is infinite now it was infinite from the beginning! This means that at bb the time collapses but not the space...

Homework Statement Hi all, I have a question about a standard flat key being specified to prevent failure. I'm just quickly giving a problem description because once I understand what's going on I can easily run calculations but I'm missing something fundamental. Essentially the problem I...

Homework Statement A flat plate lies in the region: 0<x<35, 0<y<inf The temperature is steady (not changing with time), and the boundary conditions are: T = { x if 0<x<35; y=0 70-x if 35<x<70; y=0 0...

A train applies steel wheels to a steel track. Therefore, without any distortion of the wheel or track, the contact patch would be extremely small. Now, if you slice a chord from the wheel, creating a flat spot contacting the track, the result is a larger contact patch. With a larger...

Heat Transfer Qts Engine oil at 60 oC flows over flat plate whose temperature is at 20 oC with a velocity of 2 m s-1. The length of the plate is 5m. What is the film temperature and the rate of heat transfer per unit width of the entire plate? Additional information (at the film temperature)...

We very well know how to calculate curvature in gravitation. But this time i just need a physical explanation to this question on my mind: "In order to describe n dimensional space with constant curvature why do we need to go to n+1 dimensional flat space? Why don't we use just n-dimensional...

How does sheet resistance change when comparing a flat sheet to a folded sheet? Is this change predictable? Would the change also depend on the number of folds? Is the capacitance of the fold an important factor in the change?

I'm not sure if this belongs here or in Cosmology - possibly either would do, I think. I was reading the thread about open/closed/flat universes that's currently ongoing in Cosmology, and a related question occurred to me. According to post #7 you can have geometrically flat universes that...

Meaning of a "Flat," "Open," and "Closed" Universe Hello all, I'm reading up on cosmology and the potential shapes of the universe based on its density in relation to the critical density. When one says that a universe is "flat," what precisely does this mean? Does this mean that the...

New to cosmology and somewhat confused. I get all the ideas indvidually, that inflation is the exponential growth in the beginning of the universe, than that gravity and matter cancel each other out to make the zero energy universe, and that a flat universe is the only one that is zero energy...

Hi all! This isn't actually a homework question, but thought this subforum is the place for this anyway. It's been some ten years since I last touched the topic of mechanics and I guess this is a real simple question, but what can you do when I don't get it! Any help is appreciated! How do...

In Minkowski spactime (Flat), if the coordinate system makes a rotation e.g. around y-axis (centred) , for the metric ds^2, how to make the tertad (flat spacetime) as the coordinate system rotats?

The CMB data suggests that the Universe is flat with in 0.4%. CMB data also shows that the expansion of the Universe is accelerating (from the sum of angles of triangle formed by distant hot spots- Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess). The pushing out dark energy is about 74% of...

I know most salts' have increased solubility in 100g of water with an increase in temperature, a few have an inverse relationship, but why does NaCl flatline regardless of temperature? Like is there a mechanism that explains this phenomenon? Thanks in advance.

Here's a question from my textbook: "Why do you not see a spectrum of colors when light passes through a flat pane of glass?" However I think that a spectrum of colors will be formed when light passes through a flat pane of glass. The colors will all be parallel to one another, unlike a prism...

Homework Statement What is the tension of a string between 2 blocks of equal mass that are moving due to a pull of 17 Newtons?http://postimage.org/image/pmoibsy4p/ A. 8.5 B. 0 C. 17 D. 25 Homework Equations F=ma The Attempt at a Solution 17

I was wondering if someone wouldn't mind offering me an explanation as to the differences between a flat spacetime versus a conformally flat spacetime (if there even is a difference).

hello I have a question: i am trying to understand how we find out that the curvature of the universe is zero using the angular size of the hot spots of the d microwave background radiation. http://scienceblogs.com/startswithab...tire-universe/ there is a picture in this blog showing the...

I've heard that a flat Universe is one where there is enough density in the Universe to prevent a big crunch or ever-accelerating expansion, and that the fate of such a Universe is that the expansion slows down to zero as time goes to infinity. I have also heard that the Universe is accelerating...

Homework Statement A flat, square surface with side length 3.00cm is in the xy-plane at z=0 . Calculate the magnitude of the flux through this surface produced by a magnetic field B=(0.150T)i+(0.350T)j-(0.500T)k What I'am doing is I know that the magnetic flux= BAcos(theta) So I...

Homework Statement Consider a flat plate moving at V = 105 ft/s in an atmosphere with P = 2100 psf and T = 75F, with a viscosity of vis = 3.7373e-7 slug/ft-s. The plate has a 2 foot chord and a 3.5 foot span. a. Find the drag on the plate for: i. A boundary layer that is laminar over the...

I understand that in order for a space to be conformally flat that the vanishing of the Weyl tensor is a necessary condition. I was wondering if the following arguments are also (if not the same) in arguing that Robertson-Walker spacetime is conformally flat. Starting with the...

As an understanding exercise in my general relativity course my professor recommended proving to ourselves that all Robertson-Walker models are conformally flat. However, I am unsure of how to approach such a proof. Thanks in advance for any help.

Hi everyone! I'm not very good at math as I'm in high school, and so probably did something wrong in this, but I did some calculations and am now wondering if a flat Universe requires Ω0 ≠ 1? I started with the Friedmann equation, If you divide by 3 and take the inverse of both sides, you...

Homework Statement Greetings, I'm new here :). Straight to my problem. I have a ramp. A ball accelerates down at varying angles, and then it reaches the flat part and travels only in the x-direction. I need to find the velocity of the ball as it moves on this flat part, as it is no longer...

Hi folks :smile: I am thinking that there are plenty of people here that have these and was hoping for some input. I bought a 46" Insignia (Best Buy's own brand), 60 HZ, 1080p, LCD television. I got it on sale at Best Buy for $399 and it had fairly decent reviews (average of 4.3 stars out...

We have two surfaces One is completely flat even on molecular scale (hypothetically) the other is a little uneven (but still much less than actual values, ill go with the hills and valleys view) The object used is completely flat for both case( a block)will the completely flat surface have more...

http://www.sciencedaily.com/releases/2012/08/120824093523.htm If I understand the technology correctly these optical antennas substitute surface plasmon effects for the phase delays of the light passing through glass. They also use standard semiconductor manufacturing techniques meaning mass...

Can somebody explain in layman's terms the connection between local flat space (tangent planes) on a manifold and the Riemann tensor.

if a linear potential is applied to a well, why is the wavelength for a given energy E no longer constant?

I understand that the Wilkinson Microwave Anisotropy Probe has made observations of the angular size of early fluctuations in the Cosmic microwave background. As the observed angular size is the same as the expected angular size it seems that light traveling to us from those early fluctuations...

Hi guys, first time poster here. I have been struggling with the following problem for a few days now and could use some guidance. I believe I have the first part correct but the second is causing me trouble (see attachment). Is what I'm doing correct and how would I go about finding the metric...

Homework Statement A flat non conducting sheet lies in the x - y plane. The only charges in the system are on this sheet. In the half space above the sheet, z > 0, the potential is \varphi = \varphi _{0}e^{-kz}coskx. Describe the charge distribution on the sheet. Homework Equations The Attempt...

Is the universe "flat" ? I am only casually interested in astronomy so its rather difficult for me to articulate my thoughts, but I will try my best. Consider me a layman and please accordingly use layman's terms if possible when you write your answer. When I see planetary systems, galaxies...

"How Flat is Our Universe Really?" http://arxiv.org/pdf/1207.3000v1.pdf Interesting paper, trying to get a model-independent grip on largescale spatial curvature using the available datasets. I've acquired respect for Bruce Bassett from his other papers over the years, so am inclined to at...

I was wondering, if I were to drive in a flat road , and then on a banked road(steep), in a circular motion, which one would be more difficult to manoeuvre over. I thought of generalising the two, but obviously it won't work because in the flat road, it is the frictional force that acts as the...

A 2-m x 3-m flat plate is suspended in a room, and is subjected to air flow parallel to its surfaces along its 3-m-long side. The free stream temperature and velocity of the air are 20oC and 7m/s. the total drag force acting on the plate is measured to be 0.86N. Determine the average heat...

What is a "flat" universe? Hi I read about the different shapes that are postulated for the shape of the universe. But what is meant by a "flat" universe? Is it the shape of a sheet, or blanket? If it is, why would the universe expand in suck a manner? It doesn't seem to make sense that it...

There are some "standard" coordinate systems in flat spacetime, such as Minkowski (inertial), Rindler (uniform acceleration), and Born (rotation). Is there a "standard" coordinate system in flat spacetime which has at least one null coordinate?

What must the Hubble constant be if the Universe is flat. At the Lamda CDM article on wiki it says 70.4 km/s mpc. I'm not sure if that's what it must be if the universe is flat or if that's what experiments have measured it to be, I'm 99% sure that it's the former but I want to be 100% sure...