The result of sampling the property of a system

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78
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24answers
9k views

How can you weigh your own head in an accurate way?

I read some methods but they're not accurate. They use the Archimedes principle and they assume uniform body density which of course is far from true. Others are silly like this one: Take a knife ...
36
votes
13answers
9k views

Home experiments to derive the speed of light?

Are there any experiments I can do to derive the speed of light with only common household tools?
25
votes
5answers
3k views

What if the universe is rotating as a whole?

Suppose in the milliseconds after the big bang the cosmic egg had aquired some large angular momentum. As it expanded, keeping the momentum constant (not external forces) the rate of rotation would ...
18
votes
2answers
2k views

How does one measure the mass of a galaxy? And other such large quantities

Trained as a pure mathematician, I see claims about the mass of a galaxy and other such huge measurements that are arrived at experimentally, and I just have to scratch my head. I know this is a bit ...
17
votes
1answer
2k views

How did they measure the speed of light observing Jupiter's moons, centuries ago?

I am interested in the pratical method and I like to discover if it is cheap enough to be done as an experiment in a high school. Thank you.
14
votes
1answer
887 views

Is the Uncertainty Principle valid for information about the past?

My layman understanding of the Uncertainty Principle is that you can't determine the both the position and momentum of a particle at the same point in time, because measuring one variable changes the ...
14
votes
1answer
214 views

GHz rate single photon counting

The fluorescent lifetimes of molecules used in biological applications tend to be in the sub-ns to a few ns timescale (let's say 0.8-4). The most direct methods to measure lifetimes typically involve ...
13
votes
1answer
3k views

Home experiments to measure the RPM of a pedestal fan without special equipment?

Is it possible to determine to an approximate degree, the revolutions per minute of a fan, for example a pedesal fan pictured below, without using some electronic/mechanical measuring device? One ...
13
votes
4answers
738 views

Can we measure an electromagnetic field?

As far as I can check, the Aharonov-Bohm effect is not -- contrary to what is claimed in the historical paper -- a demonstration that the vector potential $A$ has an intrinsic existence in quantum ...
12
votes
2answers
130 views

Is there a fundamental limit to the temporal resolution of signals from space?

In Earth-based experiments, we can measure phenomenon very rapidly in an experiment given appropriate equipment. Clearly if something takes a long exposure to see (due to a weak signal), then the ...
10
votes
3answers
428 views

How can I weigh liquid in a sealed container?

How do you measure the mass and volume of alcohol if it is in a sealed container? Are there clever ways of achieving this? The weight of the container is not known. I can't open the container, I ...
10
votes
3answers
3k views

How to combine measurement error with statistic error

We have to measure a period of an oscillation. We are to take the time it takes for 50 oscillations multiple times. I know that I will have a $\Delta t = 0.1 \, \mathrm s$ because of my reaction ...
10
votes
2answers
600 views

Unambiguous distinguishing of quantum states by local measurement

Let's have two orthogonal n-particle quantum states: $|\psi \rangle$ and $|\phi \rangle$. In theory it is always possible to make an unambiguous measurement. However, things get complicated when one ...
10
votes
4answers
794 views

Energy Measurements in a Two Fermion Double Well System

This question is related but my question here is much more elementary than discussions of the Pauli principle across the universe. There has been a fair amount of discussion around at the moment on ...
10
votes
1answer
56 views

Why is the measured distance to the Triangulum Galaxy much more uncertain than to the Andromeda Galaxy?

Why is the measured distance uncertainty for M33 (835 ± 105 kiloparsec) six times bigger than the measured distance uncertainty for M31 (778 ± 17 kiloparsec)? They are approximately at the same ...
9
votes
10answers
299 views

Could one measure a stick to an arbitrary precision by having its length estimated by enough people?

I remember reading somewhere that the problem of exact time-keeping on ships could have been solved a lot earlier than it was if somebody would have had the idea of keeping time with a whole array of ...
9
votes
3answers
432 views

If time standard clocks and any memories about the time standard are destroyed, can we recover the time standard again?

Assume the time standard clocks and any memories about the time standard are destroyed. Can we recover the time standard again exactly? Recovering the time standard again means we can determine the ...
9
votes
3answers
715 views

How hot is the water in the pot?

Question: How hot is the water in the pot? More precisely speaking, how can I get a temperature of the water as a function of time a priori? Background & My attempt: Recently I started spend ...
8
votes
4answers
406 views

Doesn't the use of a thermometer alter the temperature of the system?

If I place a mercury thermometer in hot water, heat energy will transfer from the water to the mercury inside the thermometer. Will this continue until thermal equilibrium is reached and thus the ...
8
votes
4answers
6k views

Weight on carpet vs. hard floor

Will the scale say I weigh more, less, or the same on a carpet as compared to a hard floor? You can assume the scale works via spring mechanism. Free body diagrams encouraged!
8
votes
5answers
585 views

How would I go about detecting monopoles?

A question needed for a "solid" sci-fi author: How to detect a strong magnetic monopole? (yes, I know no such thing is to be found on Earth). Think of basic construction details, principles of ...
8
votes
4answers
2k views

Measuring the spin of a single electron

Is it possible to measure the spin of a single electron? What papers have been published on answering this question? Would the measurement require a super sensitive SQUID, Superconductive Quantum ...
8
votes
0answers
144 views

Why is the Planck length the shortest measurable length? [duplicate]

I quote from the Wikipedia article on Planck length: According to the generalized uncertainty principle, the Planck length is in principle, within a factor of order unity, the shortest ...
7
votes
1answer
2k views

Why is carbon dating limit only 40,000 years?

For an example, when they tried to get the carbon dating for presence of Aboriginal people in Australia they get to the number 40,000. But it could be much earlier. Why is that 40,000 years limit for ...
7
votes
2answers
97 views

Is a “shift in the meaning” of Accuracy and Precision occurring?

Accuracy and precision are among the most fundamental concepts in experimental physics, and, I always believed, completely unambiguous. Recently I found that the Wikipedia article on Accuracy and ...
7
votes
1answer
491 views

Weinberg angle measurement methods

I was reading up on the history of $W/Z$ bosons today and I got a little puzzled. I always assumed that people measured $M_Z$ and $M_W$ and then derived the Weinberg angle. But it appears that they ...
7
votes
1answer
968 views

How to measure a solid-solid surface energy?

Many techniques exist to measure the surface energy between a liquid and a liquid or a liquid and a gas (see e.g. the wiki page). Methods to measure the surface energy between a solid and a fluid are ...
6
votes
4answers
2k views

Finding the volume of this irregular shape I have

I have an approximately basketball-sized non-hollow piece of aluminum sitting in my house that is of irregular shape. I need to find the volume of it for a very legitimate yet irrelevant reason. ...
6
votes
9answers
924 views

How to measure speed of a ceiling fan?

I thought for fun I might measure how my ceiling fan slows down when turned off, and maybe find out what forces act on it when it slows down. My problem is that for that I need to measure its ...
6
votes
2answers
1k views

Mass of a galaxy via luminosity

Is there a general formula for calculating the mass of a galaxy, or even a nebula from the luminosity? Or, is there a way of calculating the total mass of a galaxy from its energy output? Is there a ...
6
votes
1answer
163 views

Is it feasible to measure the energy of cosmic ray muons with a consumer Digital Single Lens Reflex camera?

I have read this article SIBBERNSEN, Kendra. Catching Cosmic Rays with a DSLR. Astronomy Education Review, 2010, 9: 010111. and it talks about estimating the muon cosmic ray flux by means of a DSLR ...
6
votes
3answers
881 views

Pauli exclusion principle and Entangled pairs

It is true for fermions in the same potential that the total wavefunction of two particles must be antisymmetric with respect to exchange of electrons. Which means the spin wavefunction is given by ...
6
votes
4answers
6k views

Rough, easy DIY method of measuring magnetic field strength

How to easily, using standard DIY equipment measure the strength of magnetic field generated by a permanent magnet? Narrowing down the "loose language" of the above: strength of magnetic field: ...
6
votes
2answers
274 views

What is known about the Magnetic North Pole's location before 1800?

I was recently startled to find that the Earth's North Magnetic Pole is moving at upwards of 40 km per year, with an additional ~80 km daily elliptical drift about its mean position due to variations ...
6
votes
2answers
170 views

Mean of a measurement on periodic data: what is the use of the inverse of correlation length?

Correlation and autocorrelation is something that in my Bachelor's programme in physics has been somewhat overlooked, so I'm in trouble understanding their use in this paper (The prisoner’s dilemma on ...
5
votes
5answers
417 views

Uncertainty principle with two photons

Imagine an experimental setup in which you have to measure the momentum and location of a particle. To measure it we know we will have to affect it, and the uncertainty principle would come into the ...
5
votes
3answers
315 views

experimental technique for measuring temperature of an ant

I am taking a course on thermodynamics. I have a question from my text(halliday & resnick,physics-1). They asked me to measure temperature of an ant or an insect or a small body,like a small ...
5
votes
1answer
2k views

How do astronomers measure the distance of the planets from its star?

I know they have accurate means of measuring the distance of the planets in the Solar System from the Sun. I'm skeptical how can they use the same tools or techniques for other systems so far away as ...
5
votes
3answers
384 views

How Is It Possible To Measure Extreme Temperatures? (>2M Deg)

Linked Article: Fusion: X-ray laser zaps solid to 2 million degrees The quest to create nuclear fusion may have come a step closer when scientists heated solid matter to two million degrees with ...
5
votes
1answer
195 views

Measuring acceleration of a bus using water between two sheets of glass

I was riding a bus one day and noticed that the double windows had some water between them. As the bus accelerated, the water collected to the sides, first forming a trapezoid and then a right ...
5
votes
3answers
275 views

Setting up a local-coordinate system in space-time using only a single clock and light beams

I have a question to ask about the operationalist view of space-time. I am a mathematician who happens to be interested in physics, so if anyone thinks that my question is a silly or vague one, please ...
5
votes
3answers
4k views

Lowest gravity on Earth's surface?

I am trying to determine which on Earth's surface has the lowest gravity. Googling is not finding anything concrete. My natural inclination would be to think of Mount Chimborazo in Ecuador, being on ...
5
votes
2answers
150 views

Can quantum measurement process be thought of as a sieve?

Consider an observable represented by the Hermitian operator $$A=\sum_{a'}a' |a'\rangle \langle a'|.$$ As I read on Sakurai's textbook, the process of measuring $A$ throws a system ...
5
votes
1answer
296 views

Practical method to weigh human limbs with common household items?

What methods could be used to determine (or estimate within a reasonable margin of error) the mass of a living human's limbs, short of cutting them off? And more interestingly, how can this be done ...
5
votes
1answer
253 views

Maximum probability of success for distinguishing between two pure states with one measurement

Suppose you have the states such that $\langle \psi_1| \psi_2 \rangle = \cos(\alpha)$ and you have one measurement to distinguish between the two. It is claimed that the probability of success at ...
5
votes
1answer
317 views

How do we measure $i[\hat\phi(x),\hat\phi(y)]$ in QFT?

What operational procedure is required to measure $i[\hat\phi(x),\hat\phi(y)]$ in an interacting (or non-interacting) QFT? [assume smearing by test-functions, or give an answer in Fourier space, for ...
5
votes
2answers
555 views

Averaging decibels

Wikipedia: The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. If I ...
5
votes
1answer
257 views

How do I measure the temperature of a tiny water droplet?

How do I accurately (+/- 0.1 degrees Celsius or better) measure the temperature of a small (5 to 50 microliter) water droplet without noticeably affecting its temperature? The mass of a thermistor or ...
4
votes
10answers
1k views

Is it possible for a physical object to have a irrational length?

Suppose I have a caliper that is infinitely precise. Also suppose that this caliper returns not a number, but rather whether the precise length is rational or irrational. If I were to use this ...
4
votes
2answers
344 views

What is the duration of a snap?

What is the duration of a snap (of the fingers)? When someone says, "it's ridiculously fast, it's like [snap]" - where [snap] is them snapping their fingers - they would normally mean it is ...