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117 votes
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 ...
Bob D's user avatar
  • 72.8k
101 votes
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 ...
Dale's user avatar
  • 102k
100 votes
Accepted

Is light actually faster than what our present measurements tell us?

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 $\...
John Rennie's user avatar
93 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 (...
Chris's user avatar
  • 17.2k
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 ...
Floris's user avatar
  • 119k
84 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 ...
C. Kent's user avatar
  • 860
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 ...
knzhou's user avatar
  • 103k
73 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 $15$ orders of magnitude away. (Cosmic rays with a center-...
coconut's user avatar
  • 4,693
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 ...
rob's user avatar
  • 90.9k
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 ...
Brick's user avatar
  • 4,909
67 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} \...
rob's user avatar
  • 90.9k
65 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 ...
DanielSank's user avatar
  • 24.6k
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 ...
probably_someone's user avatar
59 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 ...
Luboš Motl's user avatar
57 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 ...
knzhou's user avatar
  • 103k
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 ...
dmckee --- ex-moderator kitten's user avatar
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 ...
Christopher Creutzig's user avatar
49 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 ...
Pritt Balagopal's user avatar
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 ...
AccidentalTaylorExpansion's user avatar
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 ...
SG8's user avatar
  • 4,580
48 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 ...
ZeroTheHero's user avatar
  • 45.7k
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 ...
g s's user avatar
  • 13.9k
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 ...
Chris's user avatar
  • 17.2k
46 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 ...
joseph h's user avatar
  • 29.8k
44 votes

What does the notation $8.9875517923(14)$ mean?

It's the uncertainty in the last two digits: $$8.9875517923(14) = \color{blue}{8.987\,551\,79}\color{red}{23} \pm \color{blue}{0.000\,000\,00}\color{red}{14}. $$
SuperCiocia's user avatar
  • 24.8k
43 votes

What's the significance of the nickel crystal in the Davisson and Germer experiment?

They were looking at the scattering of electrons from samples of nickel out of general interest, and because their nickel target was polycrystalline the electrons came off pretty much in all ...
RogerJBarlow's user avatar
  • 10.1k
43 votes
Accepted

Why do my experimental data for Hooke’s law not match the expected data?

The coils of the spring are touching one another and the spring is initially under self-compression so it takes a finite force to move all the coils away from one another and for the spring to behave ...
Farcher's user avatar
  • 97.1k
42 votes
Accepted

Why are LIGO's beam tubes so wide?

The LIGO beam is 200 W as generated at the input mode cleaner; the beam is then recycled multiple times in the arms, increasing the power density significantly. This requires large optics with near ...
Peter Diehr's user avatar
  • 7,185
42 votes

What's the problem with my Young slits experiment?

The angle between maxima in the double-slit pattern is $$ \theta \approx \frac\lambda d $$ for wavelength $\lambda$ and slit separation $d$. I wild-guess that the slits in your photograph are about 5 ...
rob's user avatar
  • 90.9k
42 votes

How exactly do you avoid fooling yourself?

My favorite story (which I learned about recently) is about Frank Dunnington and his measurements of electron properties in about 1930. He was measuring the ratio $e/m_e$. Experiments took quite a ...
lesnik's user avatar
  • 4,202

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