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182

Imagine you are an electron. You have decided you have lived long enough, and wish to decay. What are your options, here? Gell-Mann said that in particle physics, "whatever is not forbidden is mandatory," so if we can identify something you can decay to, you should do that. We'll go to your own rest frame--any decay you can do has to occur in all reference ...


34

The statement is true for decays, where lifetimes can be measured. It is not true for interactions though. A suicidal electron meeting a positron has a good probability to disappear, together with the positron, into two gamma rays, at low energies. Electron-positron annihilation It is intriguing that this is not true for neutrinos. If an electron ...


15

In relativity there is no standard-clock that tells you which time is "right". That's the point about relativity. There is no need for a absolute reference to compare with. Everything is just the way you observe it (that is, relative to you). Things may slightly differ from observer to observer but the qualitative behaviour stays the same just as classical ...


12

This is not exactly true. It is believed that net charge is conserved, but there is a weak process called electron capture, where an electron is captured by a nucleus, (usually from an inner "orbital" so there is a spectroscopic signature), a neutrino is emitted and a proton changes to a neutron. So therefore your textbook is wrong!


10

This is an area rife with potential misunderstanding, so we need to be absolutely clear what we mean. Suppose I take a ruler and a clock and I use rulers to mark out $x, y, z$ axes in space and the clock to note the positions of events in time. Assuming spacetime is flat, I now have a universal coordinate frame that everyone who is stationary relative to me ...


8

In the real world, you push the atoms in the back end of the object, which push the atoms in the next layer, etc. This means that the front end of the object won't start accelerating until $t = t_0 + \Delta x/c_s$, where $\Delta x$ is the length of the object and $c_s$ is the speed of sound in the material of the object. If you consider the object to be one ...


4

Yes, there's a very famous example: muons produced in the upper atmosphere can be detected on the surface of the Earth. Moving at nearly the speed of light, it takes them over 300 microseconds to get down to the Earth's surface, but the average muon decays after 2.2 microseconds. If it were not for time dilation, only a few in every $10^{60}$ muons (so, ...


3

The practice of dividing the degree used to measure angle into sixty minutes of arc, and that into sixty seconds of arc is over 2000 years old. The corresponding practice of dividing the hour used to measure time into sixty minutes, and that into sixty seconds, is over 1000 years old. Why sixty? That's over 5000 years old. The Sumerians and Babylonians used ...


3

The answer is "no", not as a straight 1-body -> 2-body decay. Proof: let $E_\gamma, E_1$, and $E_2$ denote the energies of the initial photon and the two daughters, and define $\vec{p}_\gamma$, $\vec{p}_1$, and $\vec{p}_2$ similarly for their momenta. By conservation of energy and momentum, we have $$ E_\gamma = E_1 + E_2, $$ $$ \vec{p}_\gamma = \vec{p}_1 ...


2

First off, traveling at constant velocity in flat spacetime is not the same as traveling g in a uniform circular motion. Quite the contrary, free falling towards the gravitational source is actually equivalent to moving with constant velocity in flat spacetime. This is so because the objects are following a geodesic path defined by the geodesic equation. I ...


2

Why a black hole? Try earth, this very experiment has been done with precision atomic clocks, on at sea level and one in the mountains! When comparing the times shown on the clocks later, the one that was on the mountain advanced faster (cf. experimental confirmation of gravitational time dilation)! This effect even has to be considered for the precision ...


2

is there any requirement to determine (and possibly correct for) the perturbation, or "shift", of any and all primary frequency standards, besides the described "shift due to ambient radiation"? Yes. These are called "systematic errors" and they are the order of business pretty much all day, every day, at the metrology labs that implement frequency ...


2

Your idea does not seem to work if you have two particles at different temperatures. Assume you "stop" one of them but not the other. Then does the time slows down for only one particle and not the other? or how would you explain that?


2

It's historical. The second was originally defined so that $60\cdot 60\cdot 24\,\rm s$ added up to a solar day. But that's a little hairy to measure, because the length of the day varies through the year. The sunrise-to-sunrise time varies from winter to summer. The noon-to-noon time interval, which would be operationally defined as the interval between ...


2

Let me clear up a few misconceptions. The edge of our observable universe would contain information from the beginning of the universe, since it is a particle horizon. However, the edge of the observable universe is not currently visible to us. What we can currently see only goes as far back as the recombination era, when electrons first joined with nuclei ...


2

In layman use, across the world (ignoring time zones), someone who is walking observes the same time as you. In physics, however, that person has his/her own time, relative to that person. In classical thermodynamics, time is the same as layperson's time. It is just described in a different way. Quantum mechanics doesn't say that time doesn't exist.


2

The oldest perspective is the philosophical one established by Aristote; modified by Mach and then Barbour Time is an aspect of change (ie motion) There is the mathematical perspective Time when mathematically conceived is just a parameter in the equation. This in fact has nothing to do with time - it's purely formal Physically there are a couple ...


1

In everyday (or casual) use "time" is often (mis-)taken to mean duration, or a coordinate assignment to indications or entire events ("coordinate time"). In contrast, the current, correct and careful meaning of "time" in Physics is based on Einstein's definition of "time" as "the position of the little hand of my watch"; i.e. generally as any one ...


1

Here are the three primary definitions of time in 3 prominent English language dictionaries: American Heritage Dictionary also thefreedictionary.com 1a. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future. 1b. An interval separating two points on this continuum; a ...


1

There is no reference object that transcends all inertial frames of reference. Everything in this universe has an inertial frame of reference, and none of them are privileged. If there were any object that existed independently of the relativistic effects of acceleration/gravity or of observer movement, then theoretically it could provide a reference to ...


1

No, the relationship between the period of a pendulum and $g$ is simple Newtonian mechanics and unrelated to special or general relativity. This is discussed in the answers to Time period related to acceleration due to gravity (though I hesitate to link this as that question was not well received). Time dilation was actually known before Einstein formulated ...


1

HyperLuminal asked: "Does that mean that electrons are infinitely stable?" Think about Dirac's model of an electron, which includes left and right handed contributions. Now add the (Nobel-worthy) Brout-Englert-Higgs idea, that the left-handed bit interacts with a condensate of weak hypercharge, while the right-handed bit does not. This suggests a ...


1

You're right that there would be a disagreement over who signed the treaty first, but it would not be between the diplomats on the train; it would be between the people on the train and the people not on the train. The setup initially is that the two diplomats are sitting in the dining car with the curtains drawn, for security. Let's say the light that the ...


1

In a comment Kyle points out the question Does strong magnetic field cause time dilation?, which is closely related to your question. However it isn't a duplicate because you are specifically asking whether experiments have been done, not whether the effect theoretically exists. Theoretically we would expect a magnetic fied to contribute to the curvature of ...


1

You are confusing time with the flow of time. Time is just a coordinate like the spatial coordinates, that is we label spacetime points with four coordinates $(t, x, y, z)$. Indeed, in relativity (both flavours) the time and spatial coordinates get mixed up so different observers will disagree about what is time and what is space. But the obvious thing ...



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