New answers tagged

1

When we talk about the expanding universe we normally mean the FLRW metric (or some minor perturbation to it) and in the FLRW metric one of the assumptions is that the distribution of matter is completely homogeneous. In this situation the gravitational potential of all observers is the same, and the velocities of all observers are exactly described by the ...


2

Prologue This is a work in progress - in particular the third section on the arrow of time could be improved. All suggestions for improvements are welcome. This a community wiki answer so anyone should feel free to edit it. However if you want to make large changes, e.g. completely rewrite a section, please post the revised text as a separate answer and I ...


4

In a way, gravitational waves are "waves of time". Or maybe it would be better to say "waves of time dilation". Take a look at this physicsworld report on today's LIGO announcement: "The LIGO facility does not, however, measure the change in path-length because the gravitational wave compresses or expands the light's wavelength too. Instead, what the device ...


0

There is no easy way to calculate this for liquids because the heat exchange will depend on whether there is any convection in the liquid or not. You can calculate the solution for the heat (conduction) equation for your geometry, but this may or may not give the right answer. The problem is a lot better defined for solids which can not convect. The ...


0

So, I'm not sure how much relativity you know, but usually we write the 'Proper time' for a particle moving through the x direction and time as $$ (c \Delta \tau)^2 = (c \Delta t)^2 - (\Delta x)^2 $$ Where proper time $\tau$ is just the time that the particle would measure in it's own internal frame. So if you are travelling fast on a rocket ship, someone ...


1

I have just watched one of Brian Greene's videos which gave this Blocktime impression as well, yet it is misleading. https://www.youtube.com/watch?v=VYZQxMowBsw Perhaps this may help you. Say we have a very long train that is 600,000 km long. Clocks are located at the opposite ends of the train, and there is also one clock located in the middle of the ...


2

This is how you do the calculation. The elapsed time on an observer's clock is called the proper time, $\tau$, and it is calculated by integrating the metric: $$ c^2d\tau^2 = \left(1-\frac{2GM}{c^2r}\right)c^2dt^2 - \frac{dr^2}{1-\frac{2GM}{c^2r}} - r^2d\theta^2 - r^2\sin^2\theta d\phi^2 $$ In this case we'll assume all motion is radial so $d\theta = ...


0

The fact that the clock near the earth would run continually slower - i.e. the difference between the two would grow the more time they are seperated - is enough to be equivalent to different rates of acceleration. It is not like it runs more slowly for at bit and then runs at the same rate as the other one, but slightly behind.


0

The answer to this relies on knowing about the extended solutions for the Schwarzschild solution, namely the Kruskal solution. In this solution, there is not just a black hole, but a pair of a black hole and a white hole. Particles move from the white hole in the distant past, and then eventually fall into the black hole. Therefore, the time-reversed ...


0

Well technically speaking, time (or better say passage of time) is identical in two frames of reference that are not accelerating with respect to one another (i.e. at rest or moving at constant speed). So yeah, if an ant and an elephant are put on a missile and accelerated into outer space and back, they would have aged identically with respect to one ...


3

The time taken to travel to the planet, as seen by the bystander at point B, is the distance from A to B, is simply the distance divided by your speed. The time experienced by you doing the travelling is the time seen by B divided by your relativistic factor. It's just like moving from A to B in non-relativistic physics. So the faster you go, the shorter the ...


0

My short answer comes from reading this page on the web: https://www.4physics.com/phy_demo/at_clock/at_clock.htm For more details, please go to that page. It is very clear. The Cesium atom itself is not oscillating. The oscillation being measured is the frequency of the photon that is emitted by the Cesium valence electron when it jumps from an excited ...


1

Pulling together what's been said in various comments: 1) General relativity admits models where spacetime is foliated by spacelike leaves, all of which are indexed by a global time coordinate. The simplest of these models is Minkowski space. All of your observations about models with comoving observers apply equally well to Minkowski space, so if you ...


2

Suppose two observers, Alice and Bob, are moving relative to each other since the beginning of the universe. While they do it, they construct the chronologies of all the events of the universe, as they record them in their frame of reference. They will construct different chronologies. However, and this is key, each can reconstruct the other's chronology. ...


2

A comoving observer and an observer that has been moving at $0.866c$ since Big Bang will disagree on their measured age of the Universe by a factor of 2. While both measurements are correct, we can say that the comoving observer measures a more "natural" age of the Universe. For instance, the comoving observer is the only observer who will measure the ...


1

Gravity is acceleration. Einstein's equivalence principle says that gravity (with the vector pointing toward the center of the mass) is equivalent to actual movement with acceleration pointed "outward". That's why we observe gravitational blueshift. Now, blueshift means that the frequency of the photon received is increased as compared to its frequency at ...


1

Philo's answer is spot on, and I'll basically be rephrasing it here into a form that makes more sense to me. Hopefully it will help some others as well. Rather than only dialing back the clock 1B years, let's go waaaay back and see what things look like: we go back 13.82B years and look out into space... And there's no space! The universe is very ...


2

We cannot see anything closer than 380,000 years after the big bang because that is when radiation and matter decoupled. The CMB is a picture of what the universe looked like at that point. All clumping of matter into stars, galaxies, etc has occurred since then. If we had looked 1 billion years ago, we would see the same except that the CMB temperature ...


3

In a static universe it would indeed be true that if you looked at an object, say, 10 billion light years away you would be looking at it as it had been 10 billion years ago. This isn't really an application of special relativity and is merely a consequence of a finite speed of light. Our universe, however, is expanding and so you can actually see across ...


1

Yes and no. Remember in special relativity whenever someone asks a question, they always are told to draw a spacetime diagram. The same thing happens in general relativity. If you want to see what is possible, consider drawing a Carter-Penrose diagram. For a black hole you can draw the event of a test particle crossing the event horizon. The past light cone ...


3

There is a smallest measurable time interval, known as Planck time, which is the time required for light to travel the smallest measurable length which is known as the Planck length, $$\ell_\mathrm{P} =\sqrt\frac{\hbar G}{c^3} \approx 1.616\;199 (97) \times 10^{-35}\ \mathrm{m} $$. So, the Planck time will be $ t_\mathrm{P} = ...


5

Planck time - the amount of time it would take a photon (or other particle travelling at the speed of light) to cross a planck length - the fastest known speed travelling over the shortest known distance. Time, as distance divided by speed, doesn't get much smaller than that. It is about 5.39 x 10-44 seconds Which can be expressed as ...


0

Note that when you travel by any mean, and even when you stay in a chair, you do travel towards the future, right ? At 99.99% of c you would arrive very fast in your frame, because for everybody else looking you, you were almost frozen during the 4.22 times. But it's not different to the first case above concerning the "pre-existence" of the future. PS: ...


0

I have done some number crunching and found an equation for this exact question. For the town I live in the equation is 730-198sin ((2Pit/365)+(Pi/2) For any given location the 730 is the average between the longest and shortest day in minutes. This number may vary just slightly. For the 198sin, the coefficient 198 is the difference between the longest and ...


2

Such a spacetime is called an ultrahyperbolic spacetime, so called because it produces ultrahyperbolic equations (equations with more than one negative eigenvalue). Those spacetimes are not overly nice to work with. They pretty trivially include closed timelike curves, since a closed curve in any plane of two time directions will be timelike. They permit ...


0

The derivative with respect to proper time is the derivative with respect to time of the instantaneously comoving inertial frame. This does not mean the particle is at rest. That's why I had to have the word instantaneously in there. As for four velocity, that's the unit tangent vector to the world line. It tells you what direction in spacetime something ...


1

Your approach is correct; your ability to read data from a graph is suspect (the divisions are 2 m/s each). The initial velocity is -12 m/s, and at time t=9 s it is up to 18 m/s That should change your answer...


0

You would receive the photo as long as the light of the photografer taking the photo reaches you (just few seconds of delay between the 2 ), so basically you would receive the same data again, so the photographer sending the photo is no more reliable than the subject's light itself. Receiving the photo makes no stronger guarantee of subject being still there ...


1

Time is just a scale we use to measure the rate of processes or to measure the interval between 2 events. Time is not a stand-alone entity and it does not exist alone independently. All the means (like clocks) we employ to measure Time use some standard physical changes as their fundamental measuring units of Time. So Time has no direction of its own, ...



Top 50 recent answers are included