# Tag Info

## Hot answers tagged time

21

Theoretically, it is possible, at least if by 'original state' you mean 'macroscopically identical' - if you want the microscopic state to be identical, you encounter a problem, that it is impossible to precisely measure the microscopic state, especially after it was altered, so the 'original state' is unknown. However, practically, we don't have the ...

13

Let us first consider what exactly happens when an egg breaks. Chemical bonds are broken in the egg shell (mainly calcium carbonate) and the energy is converted to heat and sound. The interior of egg once exposed evaporation takes place and some chemical reactions might degrade the yolk. If we are only concerned about the exterior of the egg going back ...

11

I assume a chicken egg. A hen can create a new egg that is macroscpically identical to the old one. Elementary particles are indistinguishable so even the fact you're holding the remains of the old egg doesn't matter. Hens manage to create eggs because they absorb low entropy food and excrete high entropy feces (this way they also manage to keep their ...

6

There's no such thing as "frequency of Cs-133 atom" to be defined. Atoms don't have any natural freqencies. But the light produced when atoms change their state does. You can define the frequency of the light produced by the transition between two energy levels of Cs-133 to be 9,192,631,770 Hz, and that will lead to an equivalent definition of a second. ...

6

Others have written excellent answers, but I just wanted to make an analogy using a jigsaw. Imagine if you had a 10,000 piece jigsaw, made from a picture of an egg. If that jigsaw was jumbled up, then it is extremely unlikely that any amount of continued jumbling would return it to its completed state. However, by adding external energy (in the form ...

4

You are trying to apply your everyday intuition to photons. But your everyday intuition of how the world works has been developed from years of dealing with macroscopic objects such as tables, chairs, cups etc. It is therefore a very unreliable guide when it comes to predicting the behaviour of objects such as photons. So we have to rely on mathematical ...

4

No. You may choose a representation of the Hilbert space that is time-dependent, but the Hilbert space itself is a vector space that contains all possible states of the system, and it does not depend on time (that is, on the evolution of the state of the system within this space).

3

i am wondering if saying that we “move through spacetime at the speed of light” is something genuinely derived or just a matter of definition I would say that the truth is somewhere in the middle. The four-velocity is defined as $$\frac{dx^{\mu}(\tau)}{d\tau}$$ We can look at this expression from the perspective of Newtonian physics. The top is a change in ...

3

The other answers to your question should provide you with sufficient insight, summarized as follows: The energy in the system is so partitioned and dissipated following the egg's breakage that the probability of energetic fluctuations perfectly reversing every mechanistic step is practically zero, even though classical dynamics are time-reversible. To ...

3

As a trivial exercise, break the egg, scoop up the contents, use glue to stick the fragments of eggshell together, and insert the contents. Barring any chemical changes to the contents due to exposure to the atmosphere, you've unbroken the egg, having increased your own entropy considerably. Now you may argue that the glue joints aren't the same as the ...

3

In GR the equations are time symmetric. So if we calculate a geodesic for an object falling in a gravitational field the object could be moving in either direction. We can do this even for the simplest of black holes i.e. the uncharged, non-rotating Schwarzschild black hole. With time flowing in the usual direction objects fall into the black hole, but the ...

3

Time dilation occurs in both Special relativity, where a stationary observer and a moving observer would measure two different time intervals between the same event, and General relativity, where an observer in a strong gravitational field measures a different time interval between two events than an observer in a weaker gravitational field. In the Special ...

2

In relativity, there is no rest frame of photon in ordinary sense, so the whole question of photons experience makes no sense. No matter what you will do, the photons frame is unreachable and you are basically left only with experience of how the photon acts in other frames. In those frames, the time is well defined and all the processes - like photons ...

2

For fundamental questions about time you must refer to the fundamental notion of proper time instead of coordinate time. One essential difference between general relativity and Newton's system of space and time is the fact that instead of one absolute time concept there are two time concepts - coordinate time and proper time. Both time concepts are linked ...

2

As I put in my comments, "now" is more of a philosophical concept, especially when you want to think of it as special. Given that "now" is as tricky of a word as you think it is, science typically strives to identify models in which "now" is not special -- any "now" is as good as any other "now." If it can make the model independent of any specific "now," ...

2

I don't consider this a physics question, although it is an interesting one. But what of my 'now', now? And your 'now' as you read this: 'now'? What's happening at these moments (S-T points) that is not happening there when the 'now' has gone elsewhere? Given no universal time, there is presumably no universal 'now'. All 'now's are relative. But even so, ...

2

Consider a harmonic potential with a constant $K$ which explicitly depends on time: $$U(t,x) = \frac{K(t)}{2} x^2\:.$$ Next define $$A(t) := -\frac{\hbar^2}{2m} \frac{d^2}{dx^2} + U(t,x)\:.$$ Here the derivative you pointed out gives a contribution. Another possibility is $m=m(t)$, giving rise to a temporal dependence in the kinetical energy operator which ...

2

It seems to me that you have not realised that stars live so long in general. For example, it seems likely that the Sun (which has a current age of 4.57 billion years) will not consume all its core hydrogen for another 5 billion years and will only reach the tip of the red giant branch (as a hydrogen shell-burning star) when it is 12.2 billion years old (e.g....

2

Your answer is correct but a better way to represent it would be as follows- $$y=x^n$$ Differentiating both sides we get $$dy=nx^{n-1}\,dx$$ Dividing one with the other \begin{align} \frac {dy}y&=n\frac {x^{n-1}}{x^n}dx\\ \frac {dy}y&=n\frac {dx}{x}\\ \end{align} Hence, the power is multiplied with the error when dealing with exponents. However ...

2

To make a broken egg in reality (so not in a video) return to the non-broken state you have to reverse all the momenta of all the particles that are part of the broken egg, and you have to include all the surrounding particles which are affected by the breaking as well. Including the motions of all particles that constitute you if you are looking at the egg ...

2

There is no such thing as a stationary point in space, so there is no universal time.

2

That's funny, I had the exact same thought while I was first taught special relativity. This can definitely work, though you would require enormous amounts of energy. An example would be a satellite in orbit around Earth. Given high enough speed, the people inside the satellite could undergo significant time dilation(everyone who is moving undergoes time-...

1

Yes, time exists without gravity! In the Minkowski spacetime of Special Relativity there is time but no gravity. Did you learn about Lorentz transformations and kinematic time dilation? They have nothing to do with gravity. Quantum field theories like QED and the whole Standard Model are typically formulated in flat spacetime and thus with no gravity. For ...

1

Well gravity according to Einstein's relativity theory is nothing but bending of 3D space and time called space time and this bending is caused by matter for reasons we don't know for sure. Well by no gravity you mean there's no matter in that part of space-time to bend it! So even though there's no gravity over there but space- time exists . Hence the ...

1

I think you should rather think: if the Hilbert space changed, in what could it change to? Finite dimensional (complex) Hilbert spaces are characterized by a single number, its dimension. All Hilbert spaces with the same dimension are isomorphic to each other. The same essentially happens in infinite dimension (for separable Hilbert spaces). Think now in ...

1

Nope. I think u are mistaken the age of the Universe with the particle horizon which settles the limit of the observable Universe. The $\Lambda$CDM model gives us a value of $d=45 \text{ }\mathrm{by}$. With time, as the Universe expansion is faster than light for certain distince (Hubble radius), the observable Universe will change, galaxies which are in ...

1

The lifetimes of stars are basically set by the combination of how much fuel they have for nuclear fusion (roughly proportional to their mass) and how fast they consume that fuel (e.g., by fusing hydrogen into helium). The fuel consumption rate -- which determines the star's luminosity or absolute brightness L -- is also set by their mass, but in an ...

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