Hot answers tagged

5

There are (at least) two types of water clock: constant flow per unit time, and constant drop in height per unit time. If you want constant flow, you need a mechanism to keep the pressure constant - this was the subject of this question If you want constant change of height with time, you need to change the area as a function of height above the orifice. ...


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 ...


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} = ...


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 ...


3

This is a question that cannot be answered without understanding what time is. And I think the best we can say is that time is a quantity we use to compare events. But the compared feature is the ordering. Bearing that in mind, we cannot say much about its reality, even less about its direction. In other words, despite the fact that you can order events, ...


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 ...


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 ...


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 = ...


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 ...


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 ...


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. ...


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 ...


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 ...


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 ...


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 ...


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

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, ...


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...



Only top voted, non community-wiki answers of a minimum length are eligible