# Tag Info

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A perfect vacuum never exists as mentioned in multiple other comments. All "messenger particles" are fluctuations of their respective fields (e.g. the graviton a place in the gravitational field that has a non zero energy value). All fields are subject to quantum fluctuations, in essence, they rarely have no energy at one point but the fluctuations average ...

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Your intuition is good, but you're mixing up some quantum and classical phenomena. In classical (ie. non-quantum) physics, a vacuum is a region of space with no matter. You can have electromagnetic fields in a vacuum, so long as the charges creating the fields are in a different region. By the same token you can have gravitational fields in a vacuum, ...

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The graviton is the hypothetical gauge boson associated with the gravitational field. I say hypothetical because it is far from clear whether gravity can be described by a quantum field theory, so it isn't clear whether gravitons are a useful description. In any case, you should not take the notion of virtual particles like the graviton too seriously. have ...

-2

Yes, gravity does exist in a vacuum. A vacuum does not need to be completely devoid of matter, it just needs to have a lower pressure than the area around it. Consider the syringe above. If I was to put my finger over the end, and then pull on the plunger, an imperfect vacuum would be created. If there was a solid mass in the syringe cavity, it would ...

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You are simply confusing vacuum with "nothingness", which is a philosophical concept. You can check the definition at wiki Vacuum is space that is devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure.[1] ...

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In quantum mechanics, it's impossible to remove all the particles from a vacuum. A volume of space time that contains only photons and gravitons in thermal equilribium (or not) sounds like a perfectly good vacuum to me.

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If we assume you are a sphere in space, at the same distance from the sun as Earth, then we can calculate the heat absorbed - and we can calculate how hot you need to be so heat in = heat out (assuming uniform surface temperature, and radiative heat transfer only). For this, we need the Stefan-Boltzmann expression for total emission at a given temperature: ...

3

No, because Haag's theorem states that there is no map between the free and interacting Hilbert spaces such that the fields and their commutation relations on one space are unitarily mapped onto the fields and their commutation relations on the other space. That is, the space of states of the interacting theory is as a representation of the commutation ...

1

Starlight, as emitted by a star, comes in a wide range of colours. For instance see the picture below. Now this is a picture, and pictures can often be tricky with their representation of colour, so you'll have to take my word for it that Betelgeuse does look significantly redder to the naked eye than say Vega until you get a chance to go look yourself on ...

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The colour of stars as observed by an observer on Earth varies just like the colour of our own Sun, depending on where in the sky the source is relative to the observer. However, the light of stars is generally too faint to notice this as clearly with the naked eye, because we cannot perceive colour for weak light sources.

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No, Rayleigh scattering models the probability (and angle) of scattering as a function of wavelength and of the particle sizes. All wavelengths travel a long way but the path followed (scatter or nonscatter) varies. Since space is mostly "empty", there's little scattering. Beyond that, your understanding of stars is quite incomplete. THey do in fact have ...

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No, you don't have to re-introduce the luminiferous aether (or aether of any sort) in order to make space/time and General Relativity sensible. You do not have to postulate any sort of spatial filling or philosophical substratum in order to keep General Relativity logical and experimentally intact. It would be helpful for you to conceptualize space/time as ...

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What you're describing is theoretically possible, or at least there is no contradiction inherent in its conception any more than any other "pure" quantum state is: you simply need to prepare the pure quantum state which is the zero particle number observable eigenstate for all the quantum fields (electron, photon, ...). This of course corresponds to all the ...

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The answer to the main question is no. The reason it is no, is because your reasoning is flawed. In addition to the vacuum created in front of the propeller, there is the impulse applied to the propeller by the reaction to the air being pushed away from the propeller. Although the force due to the vacuum reaches a limit, the one due to the impulse does not. ...

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In theory, yes – it’s an effect called ‘cold welding’ by which the metallic bonds that hold atoms together in each object effectively ‘bridge the gap’ between them to create a single solid object. In practice, this rarely happens on Earth because most metals form a protective oxide layer where their surface is exposed to the atmosphere. Slight bumps and ...

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Is it possible that when you mean an absence of air, you mean to say an absence of moving air? If so, then there is a difference to how fast clothes will dry. If there is no moving air, then the only considerable means of heating up the clothes and the moisture on the clothes is the thermal radiation from the sun. The radiation heats up the moisture and it ...

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I think when you say "no air" you mean "no wind" In modern greek too "air" can mean "wind" and and also the content of the atmosphere. So if you hang clothes in the same sun but with no wind to supply convection, the clothes will try slower than when a wind is blowing, due to convection. Convection replaces the saturated air close to the clothes with ...

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Disclaimer before I get started: A perfect vacuum is impossible. As I answer your question, I will take your use of the word "vacuum" to mean "a chamber with an air pressure arbitrarily close to 0 Pa." When I use the word "vacuum" in my response, I mean the same. Your clothes don't need the air in order to dry, and in fact, will dry more quickly. ...

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I don't understand the difference between the first and the second question, but the answer is "No, you don't need air for the clothes to dry". In fact, it will dry faster if in vacuum, because the water will start to boil in zero pressure, even if the temperature is not 100º C. In fact, at zero pressure, water cannot exist in liquid, but will evaporate if ...

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Your clothes would dry very quickly in a vacuum, assuming that the temperature is still one that one would find on earth. This is because the water would boil out of your clothes. On earth normally, boiling takes a lot of heat energy. This is because of the air pressure. In your scenario there is no air pressure. so the water will boil easily.

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No air means no vapor too. So without air your clothes will dry more easily, because the wetness will vaporize more easily.

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I think: Without air arround, you have some kind of a black body. Depending on the distance to the sun your body will reach a constant temperature. If this temperature is above the boiling point, you wet clothes will become dry. Because the gravitational attraction of your clothes is much smaller than the pressure of the water vapor. Thus the water molecules ...

1

Suppose you have a box filled with water in a uniform fashion. Now if you try to stretch the box in the $z$-direction, say, while keeping the other dimensions constant, what is the energy required? Well if the water distribution remains uniform, you can approximate this by the Hookean law $E_\text{elastic} = \frac{1}{2}k(z-z_0)^2$. Note that the constant $k$ ...

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In quantum field theory the vacuum is the vector with no particles. It is defined starting from the Fock space, that is the sum of spaces with any number $n\in \mathbb{N}$ of (identical particles). The space with zero particles is, roughly speaking, one-dimensional, and its basis vector is the vacuum. In quantum mechanics, one can use the Fock space ...

1

'Negative ' pressure is strictly a relative state; relative to what one may wish to define as zero pressure, and here on earth we chose to define that as one standard atmosphere of pressure which is about 760 mm Hg absolute pressure. If you are capable of removing all gas particles from a space, then you will achieve -760 mm Hg gauge pressure, but you cannot ...

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First, it is not "well known, a vacuum balloon using the materials we have at our disposal is not possible, because of the sheer force they have to resist from the air outside." In our patent application (Akhmeteli, Gavrilin, Layered Shell Vacuum Balloons, you can find it at USPTO site or at ...

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Not physically, but practically there are (currently) better alternatives. The limiting issue with propellers is similar to the limiting issue with helicopters: propellers work like wing sections in that they must accelerate flow to work; when you're near the speed of sound, this means you are going to cause shocks to form, and this issue is particularly ...

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Your question, the way it is put, allows one fast answer: No, in principle, this airplane can fall with higher velocity then 1M. However - what only you need is to accelerate the air molecules around so that you gain momentum (and speed). (1) In principle, it is not forbidden to invent such a propeller. But normally, with a classical design, you will have ...

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Don't forget that the aeroplane will be moving forward, so it's not relying on a vacuum filling ahead of the propellor to supply the latter with air. Now I daresay there are good engineering reasons why propellors are not efficient and even impracticable for supersonic flight, but I don't think there is a fundamental physics theoretical reason ruling them ...

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A propeller can work at supersonic speeds because as it approaches those speeds it is catching up with the air molecules as it moves. So you don't have to "wait" for the molecules to move into the vacuum you create. In other words the thrust of a propeller does not go to zero just because the plane reaches a certain speed. But it is not enough to have ...

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It is a standard exercise in most quantum field theory books that the 2-point function does not vanish outside the light-cone of a particle. Less technically; the probability that some particle $r$ away from a source feels the effect of the source quicker than light to travel would $r$ is non-zero. That is a good definition of superluminal motion. See for ...

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In practice, no. In theory, also no. The Universe is filled with photons with an energy distribution corresponding to 2.73 K. Every cm$^3$ of space holds around 400-500 of them. That means that if you place your "stable body" in an ever-so-isolated box, the box itself will never come below 2.73 K, and neither will the body inside. It will asymptotically go ...

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It´s impossible to drop a body temperature to absolute 0K. You must notice that, at the same time the body is radiating energy from its own temperature, it is also receiving temperature from other sources (regardless the distance of the source) like distant stars.

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It's a mixture of $c_\infty = c_0 = c$ and "the question doesn't make sense". So, first, how it does not make sense: What's the "speed" of a quantum object? It has, in general, no well-defined position, so $v = \frac{\mathrm{d}x}{\mathrm{d}t}$ is rather ill-defined. Instead, we should probably look at the mass of the photon, since all massless objects ...

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Note: In the case of gauge symmetry, the degeneracy of the vacuum under gauge transformations leads to topologically inequivalent vacua characterized by the winding number of the gauge fields, in which case the lagrangian in the path integral has a term which indeed depends on which (theta) vacuum you choose. However here we will consider a vacuum degeneracy ...

0

You don't have to write the generating functional in the ground state. More generally $$\langle\mathcal{O}\rangle=\frac{\text{tr}(\rho \mathcal{O})}{\text{tr}\;\rho}$$ where the trace is over all states and $\rho$ is the density matrix. This can be written as a path integral in general for any $\rho$, so I don't see a particular formal difficulty in ...

1

You would have quite a problem to keep your water liquid. Normally, the water will evaporate when pumping. So you should go to low temperatures, but it freezes there. I thin you should thing about some other material to make bubbles in vacuum.

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See, the internal pressure of a bubble exceeds the external pressure by $\dfrac{4S}{R}$ ; $S =$ surface tension,$R =$ radius. So, additional force must be imparted so as to keep the bubble in equilibrium. So, if you want your bubble to exist in the chamber, there must be some mechanism to nullify the outward pressure of the bubble; otherwise it'll burst. ...

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It will help if you study this diagram of what a vacuum tube is If a cathode is heated, it is found that electrons from the cathode become increasingly active and as the temperature increases they can actually leave the cathode and enter the surrounding space. When an electron leaves the cathode it leaves behind a positive charge, equal but ...

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people in spaceships opening doors and closing them again with no suits on. Is it possible in "real life" No, it is not. Any sane engineer will build doors that open inward, or have latches that over-center when closed so it is simply impossible to open an airlock in a pressurized vessel. An aircraft, for example, has about 6-8 tons of pressure holding ...

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The biggest, immediate problem with "openning the door" of a spacecraft is not that you would die immediately from exposure to the vacuum of space: you don't - you have of the order of minutes to do something about it. The problem is the violent outrush of air. User rob offers this answer to the Physics SE question Do airlocks in space decompress violently ...

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Bad Things Happen The air, as it has no pressure or enough gravity to keep it in the ship, will attempt to expand. Air, in fact, attempts to expand to fill the container it is placed in. If there is no walls to the container, like on a planet, it will only be stopped by gravity. When the airlock is unsafely open or a hole is made in a spaceship, the air ...

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