LIGO is designed to detect changes in length on the order of 10,000ths the scale of a proton. I know they are extremely well isolated from their surroundings, but how could we even approach isolation at such a scale? Wouldn't even the vibration of the molecules making up the system be picked up by the sensor?
85810
Since LIGO is dealing with readings at nanometers, events such as vehicles driving nearby, and constant (but extremely minor) tremors of the earth can cause movement with the mirrors at nanometers.  Such tiny movements are extremely difficult to measure, but can easily cause errors when using such a small scale.  How does LIGO remove these factors, and take precautions to prevent false readings due to noise?
59014
So I'm an IT engineer and i just watched a video about Quantum entanglement, that somehow when we split the entangled photons, any change happening to one of them will be reflected to the other one INSTANTLY, and therefore we can use this technology to transfer data from one point to the other instantly, basically even faster than the speed of light because no travel is required so we can even send data without the need of encryption!!   and  China has developed a technology like this too   so since i have 0 knowledge about quantum  physics and this blows my mind, can someone here explain this in a simple manner that how is this possible? and does this mean we can communicate with other planets INSTANTLY no matter how far they are ? doesn't this change everything?  also what is this technology called? where we separate the entangled photons and use them to communicate, basically can you guys also tell where should i start because i want to know everything about this technology, but I'm not sure where to start!
48333
Has there been any experiments, or are there any references, demonstrating gravity between atoms? If so, what are the key experiments/papers? Or if not, what is the smallest thing that has actually experimentally been shown to be affected by gravity?  I don't know of specific papers demonstrating gravity between larger objects, but I can vaguely remember learning about them in my classical physics class as an undergraduate. However, I have never heard of experiments demonstrating gravity at atomic or subatomic levels.  I don't have a physics background so it's not obvious to me, so just looking to see the actual research/evidence behind it, so I can start to try to imagine how gravity works at a quantum level.
27847
Say you have a vial of tritium and monitor their atomic decay with a geiger counter. How does an atom "know" when it's time to decay? It seems odd that all the tritium atoms are identical except with respect to their time of decay.
58544
The Hubble constant, which roughly gauges the extent to which space is being stretched, can be determined from astronomical measurements of galactic velocities (via redshifts) and positions (via standard candles) relative to us. Recently a value of 67.80 ± 0.77 (km/s)/Mpc was published. On the scale of 1 A.U. the value is small, but not infinitesimal by any means (I did the calculation a few months ago, and I think it came out to about 10 meters / year / A.U.). So, can you conceive of a measurement of the Hubble constant that does not rely on any extra-galactic observations?  I ask because, whatever the nature of the expansion described by the Hubble constant, it seems to be completely absent from sub-galactic scales. It is as though the energy of gravitational binding (planets), or for that matter electromagnetic binding (atoms) makes matter completely immune from the expansion of space. The basis for this claim is that if space were also pulling atoms apart, I would naively assume we should be able to measure this effect through modern spectroscopy. Given that we are told the majority of the universe is dark energy, responsible for accelerating the expansion, I wonder, how does this expansion manifest itself locally?   Any thoughts would be appreciated.
118653
First thing I've been wondering is how the gravitational field is emitted. Matter emits gravitational waves, and I guess that those waves travel at around the speed of light. If that's not the case, please direct me to something that explains that. For now I'll assume that this emits a waves that travels at the speed of light.  Now those waves are emitted constantly without the apparent need of energy. Just the mass being there is enough to emit those waves, and the mass itself isn't consumed by the emission of gravitational waves. Again that's my assumption, please tell me if I'm wrong.  Now if we would be able to modulate the mass of an object, we would be able to modulate the gravitational waves this object emits. Hence we would be able to transmit information through those waves, if of course we'd have a device that detects gravitational waves with enough sensitivity.  Those waves would be transmitted at no cost, because the waves are emitted by the mass. The only cost of this transmission would be the "mass modulator", which has yet to be invented and which would require energy. However the actual transmission doesn't require energy, and the gravitational waves are harmless, unlike the EM ones.  So here comes my question (as in the title): would it be possible somehow to use the gravitational waves to transmit information?
47210
A  posted on Youtube.  How does this phenomenon work? I know he is using frequency to propagate water in a sine/cosine wave, but how does it exactly work this way? Why do we see it as if its paused, or in slow motion, or even in reverse?  Can someone please explain this? I am intrigued by the video and want to know it's physics.
56071
Consider the following statement:     Hadron Epoch, from $10^{-6}$ seconds to $1$ second: The temperature of the   universe cools to about a trillion degrees, cool enough to allow   quarks to combine to form hadrons (like protons and neutrons).   What does it mean to say "from $10^{-6}$ seconds to $1$ second"?   How is time being measured?  One particle might feel just $10^{-20}\ \mathrm s$ having passed and another could feel $10^{-10}\ \mathrm s$ having passed.   Is saying "1 second after the big bang" a meaningful statement?
39645
If there was a source of a continuous gravitational wave at (say) 50hz, and amplitude of say a micrometer (a typical sound wave displacement, I think), and you were nearby (standing happily on a planet in an atmosphere), with your ear pointing to the source, would you hear it?  It seems to me that since the gravitational wave is reducing and increasing the distance between points in the atmosphere right at your eardrum, surely the density and pressure of the air there is likewise increasing and decreasing, so you might expect to hear it.   What I can't "intuit" is whether you would actually hear it due to the fact that you yourself are also being distorted.  My tentative conclusion is that you would hear it.   At any given time, there appears to be a pressure differential across your eardrum due to this distortion in space pressurising the materials - so ... deflection?  (note: I know that in the recent LIGO announcement they talked about "hearing" the waves, but this is something completely different: an electro-acoustic rendition of the waveform.  I'm asking about direct physical sensing.)
83329
Consider a free electron in space. Let us suppose we measure its position to be at point A with a high degree of accuracy at time 0. If I recall my QM correctly, as time passes the wave function spreads out, and there is a small but finite chance of finding it pretty much anywhere in the universe. Suppose it's measured one second later by a different observer more than one light second away and, although extremely unlikely, this observer discovers that electron. I.e. the electron appears to have traversed the intervening distance faster than light speed. What's going on here?  I can think of several, not necessarily contradictory, possibilities:   I'm misremembering how wave functions work, and in particular the wave function has zero (not just very small) amplitude beyond the light speed cone. Since we can't control this travel, no information is transmitted and therefore special relativity is preserved (similar to how non-local correlations from EPR type experiments don't transmit information) Although the difference between positions is greater than could have been traversed by the electron traveling at c, had we measured the momentum instead, we would have always found it to be less than $m_e c$ and it's really the instantaneous momentum that special relativity restricts; not the distance divided by time. My question is ill-posed, and somehow meaningless.   Would anyone care to explain how this issue is resolved?
50431
Why the log? Is it there to make the growth of the function slower?  As this is a common experimental observable, it doesn't seem reasonable to take the range from $[0,\infty)$ to $(-\infty,\infty)$ (For a particle emitted along the beam axis after collision $\theta = 0$ wouldn't be better to have a number that says how close it is to zero rather than one that says how large a number it is. I hope that makes the question clear.)
13986
A while back I read about the super Kamiokande detector detected a large neutrino flux and then several hours later a supernova was seen. Anyone know of this with sources? I don't recall the source at the moment.
40666
Quantum mechanics allows us to describe a particle as a wave, and also a collection of particles, which a car happens to be. What separates a typical wave from a classical particle is that the position is not well defined for a wave.  I think that it is hard to tell exactly how big an atom is, because the electron of the atom may be anywhere, but the probability of finding the electron gets very small as you look far away from the nuclei. I think in some sense the atom is as large as we want, depending on the definition.  When a car drives past me, is the position of the car separate from my position in a quantum mechanical way, or is it correct to say that "part" of the car hit "part" of me? I understand that no matter the answer the force from the hit is so small that it is negligible in every way, but that is not the core of the question.  In addition, if I made any wrong claims, please correct me, thanks in advance.
84802
I've heard that it is incredibly difficult to detect a graviton, but I don't quite understand why. With all of the knowledge I have at the present time it seems like it should be possible to create a graviton in a particle accelerator. This is how I figure: a graviton is massless, so the collision shouldn't need to have too high of an energy. Since gravity acts upon anything with energy, a graviton field should couple with all of the other fields, so it seems like gravitons could be created in collisions. A graviton created this way would probably not show up on traditional detectors, but the two units of spin associated with the particle would be missing from the observed decay products. What is wrong with this picture?It may come from a naive understanding of scattering, as I am no physicist (yet.)   p.s. I have the same misunderstanding regarding axions. They are predicted to be light particles, so why aren't they created in collisions?
10484
If I've got an instance of a fundamental particle, how can I separate out the measurements of these four quantities?  (I think) I understand the theory behind them, and why the particles in the standard model are predicted to have the values they do. However, the process of validation of these numbers eludes me. How have they been historically measured?
115234
Will I receive an interference pattern on D0 detector   if D1, D2, D3, D4 exist in the set up of the experiment, but they are thousands of light years far? Simply, to set up the experiment so that I detect all of the signal photons on D0 before I detect any of idler photons!
94504
It is my understanding of the double-slit experiment that when a measuring device is activated, to measure which of the two slits a particle is travelling through, this measuring is responsible for a different outcome than when the device is left off. When the device is left off particles leave an interference pattern, suggesting a single particle (or its wave) interferes with itself; when the device is activated no interference pattern is left, and particles are observed to travel through a single slit.  Now, does all this imply that this experiment is typically carried out in a vacuum? If not, how is it that interaction with other particles in the environment of the experiment's set-up are not already influencing the outcome, regardless of whether the measuring device is on or left off?  This question may very well be a very naive one, due to my very limited understanding of particle interaction, vacuums and/or physics in general. If so, I would appreciate a brief, layman's, overview of where my understanding of the different subjects at play here is likely to be lacking.