# Tag Info

Accepted

### Why doesn't increasing the temperature of something like wood or paper set them on fire?

Before answering your question, it is important to understand how ignition of a solid material occurs. For fuels that contain hydrogen and carbon like paper, Ignition is a gas phase phenomenon . It is ...
• 63.1k
Accepted

### General relativity (and other theories) when proven wrong

My question would be, what happens in the scientific community if one experiment proves it wrong We have already seen what happens in this circumstance by looking at what happened to Newtonian ...
• 81.6k
If we take air, then the refractive index at one atmosphere is around $1.0003$. So if we measure the speed of light in air we get a speed a factor of about $1.0003$ too slow i.e. a fractional error $\... • 343k 91 votes Accepted ### Why do we need large particle accelerators? There are many competing limits on the maximum energy an accelerator like the LHC (i.e. a synchrotron, a type of circular accelerator) can reach. The main two are energy loss due to bremsstrahlung (... • 16.9k 85 votes Accepted ### Why is the vibration in my wire acting so oddly? Your wire is not quite round (almost no wire is), and consequently it has a different vibration frequency along its principal axes1. You are exciting a mixture of the two modes of oscillation by ... • 118k 85 votes ### Does old light contain clues to its age? Light does not "experience" time, the concept "age" does not apply to light in a meaningful way (with respect to human experience). [As background; recall clocks slow for objects as they near the ... • 870 78 votes Accepted ### Why is a leading digit not counted as a significant figure if it is a 1? Significant figures are a shorthand to express how precisely you know a number. For example, if a number has two significant figures, then you know its value to roughly$1\%$. I say roughly, because ... • 98.1k 77 votes Accepted ### Cause for spikes in Trinity nuclear bomb test The answer is in wikipedia The photograph on the right shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball ... • 229k 76 votes ### Why is the detection of gravitational waves so significant? Chris' answer provides an excellent explanation as to why gravitational waves are useful to detect in general. Here's my take (as someone who works in the theory of black holes) on what is ... • 2,183 72 votes Accepted ### 2002 research: speed of light slowing down? That result has been controversial since the beginning. A comparable survey looking at a different part of the sky saw no effect, but the original authors and some new collaborators combined data ... • 80.3k 72 votes Accepted ### How far away are we from probing Planck scale physics directly? The highest energy reached so far by an accelerator is$13\,\text{TeV}$in the LHC. The Planck scale is$\sim 10^{16}\,\text{TeV}$, so we are$15orders of magnitude away. (Cosmic rays with a center-... • 4,563 71 votes ### Is light actually faster than what our present measurements tell us? The answer by John Rennie is good so far as the impacts of the imperfect vacuum go, so I won't repeat that here. As regards the last part of your question about whether this should be accounted when ... • 4,807 68 votes Accepted ### Physicists adding 3 decimals to the fine structure constant is a big accomplishment. Why? The fine structure constant tells us the strength of the electromagnetic interaction. There are some misleading statements in the news story. The big one is how to read the result, \begin{align} \... • 80.3k 66 votes Accepted ### Are random errors necessarily Gaussian? Are random errors necessarily gaussian? Errors are very often Gaussian, but not always. Here are some physical systems where random fluctuations (or "errors" if you're in a context with the thing ... • 23.7k 65 votes ### How exactly do you avoid fooling yourself? There are lots of different strategies that are employed by the scientific community to counteract the kind of behavior Feynman talks about, including: Blind analyses: In many experiments, it is ... 60 votes Accepted ### Why does LIGO do blind data injections but not the LHC? After they told me about their impressive "LHC Olympics" in which physicists (often hardcore theorists) were reverse engineering a particle physics model from the raw (but fake) LHC data, I proposed ... • 176k 59 votes ### Can water falling from a tap follow a spiral path? You are right that, without a force acting on it, water falling from a tap could not follow a spiralled path. The tap, however, creates an illusion - the water appears to be spiralling, but it isn't - ... • 14.9k 56 votes Accepted ### Is there any evidence that dark matter interacts with ordinary matter non-gravitationally? There are some standing anomalies that could be explained by non-gravitational dark matter interactions. For example, Fermi-LAT is an indirect detection experiment (i.e. an experiment that looks for ... • 98.1k 55 votes ### Why is the detection of gravitational waves so significant? In additions to what Chris White lists, I'd like to point to the fact that, except for a few meteorites and some dust collected on the plates of satellites and rocks from Mars (and cosmic rays and a ... • 10.5k 54 votes Accepted ### Why do physicists assume that dark matter is weakly interacting? The short answer is that they don't assume that. But among all the proposals that remain for what dark matter might be, weakly interacting stuff is the easiest to detect,1 so that is what is getting ... 50 votes ### Understanding which string breaks when one pulls on a hanging block from below While I haven't seen the video, the description matches an old science trick using inertia: if you want the top string to snap, pull slowly. To snap the bottom string, pull suddenly - the inertia of ... 50 votes ### How can I determine the rpm of a wheel that's spinning really fast? There's a very interesting way to find the angular velocity of a wheel that's spinning so fast that you can't measure using a stopclock. We'll be using a strobe light (a light that flashes on and off ... • 3,177 49 votes Accepted ### Is there any physical evidence for motion? According to classical physics: no. It is impossible to tell how fast something is moving from a snapshot. According to special relativity: yes. If we choose a frame of reference where one of the ... 49 votes ### Can a mathematical proof replace experimentation? No. Physics remains an experimental science and so it is not possible to replace experiment by a proof. Descartes tried this when he proposed his theory of propagation of light - very elegant - but ... • 42.3k 49 votes Accepted ### Did LIGO measurements prove that the speed of gravity equals the speed of light? Yes. In principle, the speed of gravitational waves can be measured using the data of LIGO. In fact, using a Bayesian approach, the first measurement of the speed of gravitational waves using time ... • 4,480 48 votes ### Does gravitation really exist at the particle level? For the interaction of one small (atom scale) mass and one large mass, measurements of the Earth's atmosphere that anyone could do with a homemade barometer and a nearby mountain constitute direct ... • 11.2k 47 votes Accepted ### How does one store a muon? As the paper says, the muons are deflected with a magnet into a storage ring, where they are stored for a fraction of a second. The muons are still moving with nearly the speed of light, which extends ... • 16.9k 45 votes ### How seriously can we take the success of the Standard Model when it has so many input parameters? It is inaccurate to think that all of the standard model of particle physics was determined through experiment. This is far from true. Most of the time, the theoretical predictions of particle physics ... • 27.2k 44 votes ### What does the notation8.9875517923(14)\$ mean?
It's the uncertainty in the last two digits: $$8.9875517923(14) = \color{blue}{8.987\,551\,79}\color{red}{23} \pm \color{blue}{0.000\,000\,00}\color{red}{14}.$$