# Tag Info

1

I'd like to add something to these answers. In the classical mechanics, we cannot distinguish a moving body from the body at rest, if we look at it at any particular instant. So, we have to add some hidden information to the picture, that is instantaneous velocity. But that's what physics only knew in the 19th century. In 20th century physics, there have ...

0

The answer is more simple than you think. Time is that, which is measured by (technologically suitable) clocks. Physical theories will simply tell you how clocks behave under certain conditions. This is purely descriptive. There is not a single physical theory out there, that gives a microscopic description of time, although the similarity of time with ...

1

Okay, I am going to try and give this a shot, but this is most probably not going to be a decisive answer. Let us operate with the term event time and duration and consider only special relativity (SR). The conclusions of general relativity should be the same for reasonable space-times. (e.g. without closed time-like curves etc.) We expect event time to ...

1

Duration is certainly a more physical concept than time. Duration is something you may measure between timelike separated events while time is always something you compute by adding up duration measurements + an arbitrary constant to fix the origin. Duration is experimental and relational while time (e.g. GPS time) is an abstract a posteriori ...

3

At a "frozen" instant of time, the arrow may not be moving - but this is a tautology, since movement is something that requires time. However, even in that frozen instant the arrow does have a velocity (instantaneous velocity, if you will). Imagine that time is a series of hug number of discrete frames (or instead imagine that it is continuous, and that we ...

1

Take a landscape. It can be modeled by a function f(x,y,z). If all the derivatives, df/dx, df/dy, df/dz are zero, the landscape is flat to infinity and nothing interesting exists in the landscape. If one of the derivatives is different than zero, then we perceive a shape, and generally a landscape has a shape. As an example, suppose that we have a cone ...

2

What makes heat move from hot to cold? Entropy. How can you calculate entropy microscopically? Start counting states! What makes the universe change irreversibly from yesterday to tomorrow? Start counting states!

0

Planes of simultaneity in special relativity don't really mean much of anything. The real physical structure of spacetime is in the light cones. The takeaway from "relativity of simultaneity" is not that there are "different time orderings for different observers", but rather that there is no meaningful time ordering for spacelike separated events. They ...

1

Even though the forces started at different times, is there any displacement of the metal box in any of the situations? Or is there any movement at all but is the net displacement zero? Sure. If you think of each force as causing an acceleration, the first one begins an acceleration in one direction, the second an acceleration in the other (or a ...

8

What you're asking about is the existence of surfaces of simultaneity. In SR, surfaces of simultaneity can be defined by measurement procedures such as Einstein synchronization, and they turn out to depend on one's frame of reference. In GR it gets a lot tougher to do this. We don't even have global frames of reference. It turns out that what you need in ...

9

It sounds like you're interested in when a spacetime admits a Cauchy surface. The answer is that every spacetime that is globally hyperbolic has this property. This was proved by Geroch in 1970 (article here, see Section 5). This includes most of the textbook relativistic spacetimes --- Schwarzschild, Kerr, FLRW, and many others. But there are some ...

1

"How do we know that clocks slow down relative to each other?" Experimentally. This has been observed many times in the lab. The same answer is true for ANYTHING in physics and science in general. We only know that it is true, because we have experimental evidence for it.

0

No, since by the principle of relativity: A body in constant velocity motion cannot determine whether it is in motion in a certain direction or whether everything else is in motion in the other direction. No physical experiment can determine this hence for all purposes a body in motion will simply claim that it is at rest while the other body is in motion. ...

2

Yes, the two are intimately related. One way, as in QMechanic's answer, is via Wick rotations, but in general there is a lot more freedom once you allow integration contours to go over into the complex plane. In my area, strong field physics, the use of complex time to understand tunnelling problems is everyday bread and butter for many people, and it is the ...

6

Yes, quantum tunnelling in the double well potential can be solved in a Wick-rotated Euclidean formulation $$S_E[x]~=~\int \! dt_E \left[ \frac{1}{2}\left(\frac{dx}{dt_E}\right)^2 - (-V) \right],$$ see e.g. Ref 1. Here $t_E=it_M$ denotes Euclidean time. The Euclidean action is in turn interpreted as the usual kinetic minus potential term with a potential ...

1

In quantum mechanics velocity is not an easy concept. Here particle motion is replaced by a wave. Momentum, is easier to define in quantum mechanics. A complex wave $\exp(ikx)$ describes a particle with momentum $p=\hbar k$ where $\hbar$ is Plancks constant divided with $2\pi$. It is fundamental in quantum mechanics that momentum cannot strictly be defined ...

1

CPT does exchange particles with their antiparticles, so if there were a time direction associated with particles then it might make sense to say that, by CPT, the antiparticles would have to have the opposite time direction. But there's no time direction associated with particles. It doesn't even make sense to say that something is "going forward in time"; ...

0

Yes, CPT is compatible with the idea that matter and antimatter are created at equal amounts. CPT is a basic theorem of quantum field theories, QFT:s. QED i a QFT and thus obeys CPT. The original QED is a theory of electrons, positrons and photons. In QCD, when a positron is produced an electron is produced at the same time (pair production). Likewise, when ...

2

The most common explanation for the "matter-antimatter asymmetry of the Universe" is $\rm CP$ violation in interactions involving leptons. This scenario is usually called leptogenesis because it generates a net excess of leptons compared to anti-leptons. This $\rm CP$ violation is currently unconfirmed by experiment (though there is also not yet any evidence ...

-1

1st Law of Motion Every object in a state of consistent motion tends to remain in that state of motion unless an external force applied to it. 2nd Law of Motion It is pertaining to the relationship between an object’s mass, its acceleration, and the applied force. In this law, the direction of the force vector is the same as the direction of the ...

1

The crucial point here is, that a reference frame where photons are at rest simply cannot be defined in special relativity: There is no Lorentz transformation which transforms you from a given inertial system into a reference frame or a space-time where some photon is at rest. This, however, means that concepts such as travelled distance and proper time do ...

0

Photons are not considered as observers which would be able to observe their proper time. But if you would do so, their hypothetical proper time would be zero. You obtain this result by multiplying the time observed by any real observer with reciprocal gamma (which is 0 for v=c), see http://en.wikipedia.org/wiki/Time_dilation. However, you may not forget ...

0

You provided already the most important answer, saying “Time is just …” You are right: time is very much over-estimated. Time is just and nothing more than aging of matter (proper time of matter), every mass having its private time which may be observed by anybody else. You will hear many other things about time, but without any proof, without physical ...

1

Philosophically, both time and distance are illusions. Distance is actually more disturbing than time. So first, let's define what "time" is. It is the number of transitions of an atomic state (see atomic clock wiki). Distance, a meter, is defined to be the length a photon (light) travels in $\frac{1}{299,792,458}$ of a second (source) which ...

-1

we start somewhere, that start must exist to determine our end point, and to define a new start is not easy. so speed is a relative term and can be measured by comparison. Therefore $$S = D / U$$ Here U is new unit.

1

You could use the Length contraction equation of Special Relativity to determine speed. If you know the dimensions of an object at rest relative to you, you would know its velocity relative to you by finding which dimensions are length contracted and by how much when you measure the body in motion relative to you. But, full disclosure, I think time exists.

1

We know that to position objects in spacetime requires four coordinates e.g. $(t, x, y, z)$. So time certainly exists. The point you're addressing is about the flow of time. Incidentally this point is discussed in some detail in the question Is there a proof of existence of time?. Any object traces out a worldline that is a curve in spacetime, and we can ...

0

Note: I am not allowed to write comments yet, so I have to post this as an answer. Besides of John Rennie's formal objections to the concept of expanding time (at "Does time expand with space"), I still like the explanation of the quatum physical state of superposition that is provided with your question: The idea that i.e. the interference pattern which ...

0

The time-energy uncertainty relation (and other time-"observable" uncertainty relations that can be constructed) is (considered) not to have same meaning as canonical uncertainty relations. Meaning uncertainty relations costructed from canonical dynamical variables/observables (in the Hamiltonian sense), like position and momentum, since time parameter is ...

0

I don't have the reputation to comment, so I'll comment about David Hammen's (accepted) answer here. His conclusion is correct, but mentions "Those formulae do imply a singularity for the clock that is closest to you" in reference to the forumale he thought you were referring to. He also mentions "In between, you'll get a nice continuous change from faster ...

0

I want to check that I get it right. [...] [...] we would be associating time with objects and not with the points of space itself. In physics, the word "time" is used in various related but quite different meanings (which I describe in some detail below). Therefore, if you care to "get it right" (which is of course commendable) you should avoid ...

2

First off, there are a couple of reasons why we don't have complete symmetry between space and time: We have 3 spatial dimensions and only one timelike one. The particles in the standard model have timelike or lightlike world-lines; as far as we know, there are no tachyons. For these reasons, it only makes sense to talk about laws of physics that take ...

0

The answer seems to me to go to Einstein's first postulate of special relativity: The laws of physics (as we know them) are the same in all reference frames. If you define a reference frame as an observer's location in space-time moving at a given speed, then two different observers should measure the same change in entropy no matter where they are in space. ...

3

Yes, really. But that doesn't mean that we're floating around loose without a foundation. In Galilean relativity all observer could agree on a number of things about a interaction of process. Things like How long it took (the same for all the bits) How far each part of the system traveled in that time What the mass of bit ... In Einsteinian relativity ...

4

You're mixing up two things in this question: 1) How we label individual points (Einstein calls them "events") in space-time 2) What results someone would get when looking at their clock when they pass through spacetime points. The first thing is almost completely arbitrary, especially in full general relativity. The second thing is an unambiguous result ...

0

Due to the work of Julian Barbour and others, time is defined (in a closed system) by keeping track of all the changes (of particles and so on). In this respect we would say that in a classical system (macroscopic) that time would be continuous since the motions of such objects are essentially continuous and the way that you parameterize the changes would ...

3

If you measure the absorption of microwaves by a gas of caesium atoms you'll get a spectrum looking something like this: NB not a real spectrum - I drew this as an illustration When the microwave frequency is 9,192,631,770 Hz the microwave photons have exactly the right energy to flip the outer electron spin, so at this frequency they are more strongly ...

3

It does not mean that it switches the hyperfine state that often. I means that, if you hit the atom with a photon that (by $E = \hbar\omega$) corresponds to exactly that frequency, then the energy of that photon will precisely be the energy to lift the electron from the lower hyperfine state to the upper hyperfine state. If hit, it will then, as with all ...

1

When "year" is used as a unit for things unrelated to Earth's orbit, such as distances in light years or the age of the universe in years, it is the Julian year of exactly 365.25·86400 SI seconds.

30

Which year? The sidereal year? The tropical year? The anomalistic year? The calendar year (and whose calendar)? The sidereal year is the average amount of time it takes the Earth to make one complete orbit about the Sun with respect to the fixed stars. The tropical year is the amount of average amount of time between successive spring equinoxes. The ...

0

I am assuming we are talking about the one dimensional case in which we can "move through" clocks. Assuming you synchronise the clocks in your own frame, those further away from you will show an older time than those closer because it takes longer for light to reach you from them. On top of that, all clocks will be equally time dilated since they are all ...

1

A year is defined by the time earth takes to circle the sun once: http://en.wikipedia.org/wiki/Year This does however not correspond to an even number of days. The SI unit of time is the second and $1\,\mathrm{day} = 24\cdot 3600\,\mathrm s$

42

I assume you used the formulae $f_o = fs\sqrt{\frac{1+v/c}{1-v/c}}$ for the clocks ahead of you and $f_o = fs\sqrt{\frac{1-v/c}{1+v/c}}$ for the clocks behind you. Those formulae do imply a singularity for the clock that is closest to you. Which equation to use? The answer is neither. Those expressions assume the travel is along the line of sight to the ...

2

If you consider a straight-on trajectory, there really will be a discontinuity. That is the same as with the audible Doppler effect. There is a smooth drop in frequency of a fire truck's siren passing you on the street. The reason is, that there is a certain distance between you and the truck at the closest point. If that were not the case, i.e. the siren ...

3

The signal from the clock moving towards you is the Doppler shifted version of the value you "know" it to be - that is, first slow it down by gamma (clock moving relative to your frame of reference), then speed it up by Doppler shift. Ditto, with sign reversed, for clocks behind you. Now the clock moving at right angles shows what you expect and there is no ...

1

I do not understand all the nitty details, but the gist goes as follows: You have a constant probe beam, which is cast on a sample and analysed later. Suppose the sample has a very predictable behaviour, with an event every $24\mu \text{s}$. Then the analysis of the beam will show exactly that. This is the blue line in Fig. 4. Now, suppose you want to ...

0

In general it depends on the history of the acceleration. For the case of constant acceleration your question can be answered quite simply, and I've described the calculation in the Q/A How long would it take me to travel to a distant star? or see John Baez's article on the relativistic rocket for a list of useful related equations. The derivation I used is ...

0

So my question is, is it like future of an isolated system is already determined but is just not perfectly predictable by an observer because of limitations in observability? In quantum mechanics the outcome of measurements is in general not determined when given the exact physical state of the system in general. But apart from the uncertainty arising ...

-4

The universe cannot be predicted from a single data point about a moment in time because inertia does not exist in any one moment but is critical to how a system would develop.

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A deterministic universe need not be predictable. And even a deterministic universe not hampered by any limits to observability need not be predictable. As an example take a toy universe consisting of an infinite chain of $0$'s and $1$'s. This 1D cellular universe evolves according to cellular automata rule-110: the state of a cell becomes $1$, unless the ...

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