# Tag Info

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If the universe is 4-dimensional, with 3 dimensions of space and 1 of time, then the present 3d universe can be a 3d surface. Normal to the surface is the time direction. As time goes by, the shell grows (in the expansion phase) into the time direction. If the universe were just closed, it would be like a spherical shell growing bigger with time as the ...

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BECAUSE WE MEASURE WITH ATOMS (or atomic properties) a change in the measured value can be either a true change of the observed quantity or a change in the properties of the ruler (the atoms). I'll revert the paradigm the space expands to this one: the atoms are shrinking THEN I have simple answers : If the universe is expanding, what is it ...

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Your cousin is right. The Universe is a 4D sphere W = r + Ix + Jy + Kz = [f, V] The universe is defined by energy W = -vh/2pir + cP where -vh/2pir = -vp = -mv^2, a real number potential energy. Newton found this in his theory of Gravity W=-mGM/r = -vh/2pir = -vp = - mv^2. Newton's energy is a real number or scalar 1 dimensional energy. Newton ...

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1. If the universe is expanding, what is it expanding into? The universe isn't expanding into anything. Space-time isn't curving into a higher-dimensional space. So what do we mean by "curved" and "expanding", words usually having a meaning only for objects in space? The answer is it is just an analogy. Mathematicians have found properties of space an ant ...

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This paper on the Arxiv claims to have measured an anisotropy at the $2\sigma$ level. I'm not familiar enough with the subject to pass judgement, though I get the impression that the general view is interesting but unproven at the moment. If nothing else the paper gives a convenient literature review. Various anisotropies have been suggested, such as a ...

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Yes. This is the idea behind some versions of Eternal Inflation. In that model, most of an infinite universe is assumed to be in a higher-energy vacuum state, in which pockets chaotically inflate. Some of those pockets also decay, and the combination produces Big Bangs which make universes like ours embedded in a large surrounding non-decayed, non-inflated ...

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You ask: Is it that space is not expanding within the smaller structures or is space expanding through these structures? where I've highlighted what I think is the key issue. The phrase space is expanding is a convenient metaphor to describe the expansion of the universe, but it is only a metaphor and taking it too literally can lead to confusion. It ...

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why not relative to the Earth? Scientists do express things relative to the Earth, where that makes sense. I couldn't imagine trying to forecast the weather or model the global circulation of the Earth's atmosphere from the perspective of a non-rotating frame with it's origin at the solar system barycenter. Astronomers, at least those dealing with ...

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The sun is much more massive than the planets, so even though the earth and the sun exert equal forces on each other, the sun barely moves. It's like when you jump off a small thing and it moves a great deal but you jump off a big heavy thing and it doesn't move much at all. So the earth and the sun orbit their common center of mass, but the sun barely ...

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When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre. For example in ...

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A reference frame at rest with the Sun is, with a good approximation, an inertial system (much better than one at rest with our planet or other bodies in the Solar system, essentially in view of the hugely larger mass of the Sun). Physics in inertial reference frames has the simplest form. For instance the motion of planets around the Sun is described along ...

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It's all about the context in which you want to analyze particular issue. If you are studying the solar system, the most suitable, would be to consider the sun as the center of the system. If you are studying the Milky Way, the sun is not a good reference point, you should take the center of the galaxy. Similarly, to locate the stars from an observer on ...

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Too long to be a comment, this is an extension to Chris's answer. Suppose a macroscopic object, a thermometer, for example, was placed in that hot intracluster medium (ICM) Chris mentioned in his answer. Even though that thermometer is surrounded by this hot gas, the thermometer will not get hot. It will instead cool to a tiny bit above the cosmic microwave ...

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To avoid more complex definitions of temperature (which do not require matter), you could say instead that "an object in interstellar space would be in thermal equilibrium with its environment when it is at a temperature near $3K$." The matter nearby is too diffuse to affect the temperature much. Instead, it is thermal equilibrium mostly due to radiation. ...

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Temperature in a gas is the average kinetic energy per particle. As an intrinsic property its value is entirely decoupled from how much stuff has the property. Whether there are 100 particles per cubic centimeter or only 1 particle per cubic meter, the temperature can be anything. The coldest parts of the ISM are about 3 K, and getting colder than this is ...

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Your 12-year-old cousin might be correct; it isn't yet known for sure. However, some existing experiments are pointing in the direction of your cousin being wrong. What you're calling a "4D sphere" and a "3D sphere", a mathematician would call a "3-sphere" or a "2-sphere", respectively, because mathematically an "$n$-sphere" means something that's ...

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The simple answer is that your cousin could be correct. If his theory is that: the scale of the sphere is far larger than the observable universe there's no way to detect the 4th (spatial) dimension then no experiment we can do could prove him wrong. But then there's no experiment that we can do that could prove him right either, so as theories go it ...

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There's an old theory called "tired light" where the momentum is lost due to waves hands some other reason, but as far as I'm aware this has been pretty much discounted these days. The background behind the current-best-theory is this: When you look at light from a star it's not a smooth spectrum, it has a series of dark lines in it, an "emission ...

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As far as I understand, it has something to do with Hubble's Law. Essentially, based on looking back at energy density of a distant star at one point, and then looking at it again, and determining that it has diminished, or something along those lines. I assume it's a far fancier version of looking at a light you just passed as you drive down the road. If ...

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The interconversion of matter and energy is described by quantum field theory. If you're interested the question What keeps mass from turning into energy? is on this subject. The particular quantum field theory that describes our universe is called the Standard Model, and there are three important symmetries that apply to the standard model - charge ...

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Creating only matter without antimatter would violate several conservation laws. Mainly the electric charge conservation and color charge conservation. If you create a lot of electrons without antielectrons (positrons), you create a lot of negative electric charge out of nothing. That is not allowed by the conservation laws. Similarly, you cannot create a ...

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The answer is yes. The de Broglie wavelengths of freely propagating particles (i.e. forget the influence of interactions and gravity perturbations, just consider the Universe as a whole) are redshifted by the expansion of the universe. Another way of saying this is that their peculiar momenta with respect to a co-moving local volume decrease as the inverse ...

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We are continually told that the Universe will eventually be a void and everything will have burned up, no stars, no nothing. In my school days we were told that energy can neither be created or destroyed. So, if the universe does become "nothing" what has happened to the energy?

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The argument is sound given a few oft-omitted (but not too unreasonable) assumptions. Here is one way it can be formulated. Consider a volume $V$. Suppose it has a (possibly infinite) set of possible configurations; call this set of states $S$. Suppose we are interested in a particular configuration, $c \in S$, to within a certain tolerance. Let \$C ...

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The argument rests on the assumed validity of Ergodic theory (see http://en.wikipedia.org/wiki/Ergodic_theory). Quoting it "A central concern of ergodic theory is the behavior of a dynamical system when it is allowed to run for a long time. The first result in this direction is the Poincaré recurrence theorem, which claims that almost all points in any ...

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The great simulator idea is piffle, see See http://www.daviddeutsch.org.uk/wp-content/ItFromQubit.pdf. A brief summary of the problem. A set S of computational gates is universal if by composing gates in the set S you can do any computation allowed by the laws of physics. Both the classical and quantum theories of computation say that there are many ...

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The article you quote is about metaphysics: The idea that we might be living in an artificial reality constructed by something higher than ourselves has been a recurring philosophical hypothesis for centuries. Plato's Allegory of the Cave, and cannot really be discussed in a physics framework because it is searching for a Creator. It never ceases to ...

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Well, i would say no. Why? Because an absolute center of mass would require a uniform covering (coordinate system) over the whole manifold, which, even if it exists, will probably not be on the manifold itself. An analogy would be the center of mass of a spherical surface/manifold. It would be exactly on the center of the sphere (i.e not on the sphere ...

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