# Tag Info

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We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not. For example radioactive decay: We know how fast various elements decay, and we can observe the results of radioactive decay in distant supernovae. The conclusion is that, for at least some elements, the rate of radioactive decay is the same ...

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It is very suspicious! It points to the fact that the speed of light isn't just some random speed that light happens to travel at, but is a fundamental property of the universe. In fact, any massless particle will move at the speed of light. This is a consequence of relativity. Energy, mass and momentum($p$) are related by $$E^2 = m^2c^4 +p^2c^2$$ for a ...

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The issue here is whether pairs of planets can become gravitationally bound to each other. In the two-body problem the trajectories or orbits are ellipses (bound orbits), parabolas and hyperbolas (unbound). For all practical purposes, an encounter looks like they start out at infinity with some finite speed, approach each other, and then maybe fly away or ...

30

This is more of a question for the Physics stack, but I'll give it a shot, since it's fairly basic. You need to understand something before we begin. The theoretical framework we have to gauge and answer this sort of thing is called General Relativity, which was proposed by Einstein in 1915. It describes things such as gravity, black holes, or just about ...

28

What you're describing is basically the "collapsed star" (Eng) or "frozen star" (Rus) interpretation of black holes that was common prior to the late mid-1960s. It was a mistake. Suppose you are distant and stationary relative to the black hole. You will observe infalling matter asymptotically approaching the horizon, growing ever fainter as it redshifts. ...

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Yes, you are absolutely right, from OUR VIEWPOINT it does. From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy." “Like a rock dropped from a rooftop, the star’s surface falls downward (shrinks inward) slowly at first, then more and more rapidly. Had Newton’s laws of gravity been correct, this acceleration of the implosion would ...

25

Not at all a dumb question. As you have heard, it is true that time is affected by gravity. The stronger the gravitational field, the slower time passes. If you're far from any gravitating matter, time passes "normally". But to answer your question, we must specify what is meant by "the black holes's time" (let's call the black hole $\mathrm{BH}_\mathrm{Sgr\... 22 The easiest explanation for why the maximum distance one can see is not simply the product of the speed of light with the age of the universe is because the universe is non-static. Different things (i.e. matter vs. dark energy) have different effects on the coordinates of the universe, and their influence can change with time. A good starting point in ... 22 Yes, you are right. We don't only see the Sun 8 minutes in the past, we actually see the past of everything in space. We even see our closest companion, the Moon, 1 second in the past. The further an object is from us the longer its light takes to reach us since the speed of light is finite and distance in space are really big. 18 (I will assume a Schwarzschild black hole for simplicity, but much of the following is morally the same for other black holes.) If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon. In Schwarzschild ... 18 The answer is yes time dilation does affect how much time an observer experiences since the big bang until the present (cosmological) time. However there is a certain set of special observers called comoving observers, these are the observers to which the Universe appears isotropic to. For example we can tell the Earth is moving at about 350 km/s relative ... 17 Yes there are. They are mainly based on what dominates the energy density of the universe at the time and they are known as epochs. Thus we have the inflationary epoch in the first tiny fraction ($\sim 10^{-32}$) of a second, when the energy density was dominated by an inflationary field. Then we are in the electroweak epoch, when the weak nuclear and ... 16 In one word, yes. Anything and everything we see, we see the way it was a certain time ago—about 1.3 seconds for the Moon, about 13,000 years for your hypothetical planet. Like @Richyt pointed out, there is no way for information to travel faster than the speed of light. Because any and all celestial objects gravitationally affect any and all others, orbits ... 15 I think the question is referring to situating a very large mirror in space facing earth. If we were to put it several light minutes away, then events occurring opposite the mirror could be reviewed de novo with more preparation upon the warning we received upon the first light of the event arriving at earth. For example, a supernova going off in M31 might ... 15$t$signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors. The basics are pretty natural to understand. Suppose something happens, an event, like an apple falling off of a tree. In order to tell someone else about it you need the three space coordinates$x, y, z$and the time coordinate$t$. Without all four, you ... 14 Would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? No, that's not possible. If we could do that, it'd mean that the matter traveled from there to here faster than its light got here, and matter can't travel faster through space than light does. ... 13 What is the difference between time and space-time? Space-time is time plus space. How does gravity affect the passage of time? The higher the gravity of a planet or star and the closer to that body the slower the time. What is the speed of light and how does it relate to time? The speed of light is 299,792.4580 km/s in vacuum, the speed at which ... 13 Well, first things first. It's not likely to have a planet orbiting near a black hole and in significant time dilation because the tidal effects would likely tear anything that close apart. Certainly a planet orbiting a stellar mass black hole would need to be quite far away so as to not be torn apart, so any time dilation would be pretty small. Around ... 12 In short: things can not move faster that light by theirselves, but they can move faster than light due to universal expansion. The more far away, the faster they go away. 12 The arms are$4\,\mathrm{km}\,\times\, 1.2\,\mathrm{m}$: From the LIGO webpage: The 1.2 m diameter beam tubes were created in 19-20 m-long segments, rolled into a tube with a continuous spiral weld. While a mathematically perfect cylinder will not collapse under pressure, any small imperfection in a real tube would allow it to buckle (a crushed vacuum ... 12 When we talk about the universe, we are really talking about one of two things: The observable universe, which is everything we can possibly see. The Universe, which is everything that has ever existed, currently exists, and will exist. The observable universe has its own center, usually the Earth. It is a spherical region of everything that we can see, ... 12 The perturbation to the metric of spacetime (known as the strain), caused (for example) by an oscillating mass quadrupole, obeys a wave equation of the form $$\nabla^2 h^{\mu \nu} = \frac{1}{c^2} \frac{\partial^2 h^{\mu \nu}}{\partial t^2}\, ,$$ where$h^{\mu \nu}$is a 4x4 tensor. The solutions to this equation are plane waves travelling with a speed$c$, ... 11 We need to think about just where the time dilation effect occurs. By then thinking about the observations from each point of view, that is the free falling object and the external observer, we can come to terms with just what is happening as opposed to what appears to be happening. The experience of time We must remember that an object moving at a certain ... 10 Velocity is a form of kinetic energy, while height within a gravity well is a form of potential energy. For an orbiting body, conservation of energy will keep the total energy constant. So as a planet moves away from the parent star, it loses velocity and gains potential energy. As it moves closer, it trades the potential energy back for velocity. The point ... 10 Yes, we always look into the past, when looking somewhere. There is for instance a mirror on the moon. When sending a laser beam to that mirror, we can detect the reflected light about 2.5 seconds later. This could be interpreted as looking 2.5 seconds into the past, when the laser has been fired. Details here. 10 As others have said, mathematically, a singularity is when there is an attempt to divide by zero. Take, for example a Schwarzschild black hole. This is a black hole that has no electric charge or angular momentum; tt is the simplest type of black hole. According to general relativity, gravity is the bending of spacetime. The curvature of space can be ... 10 The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime. You can't get any more than that from the rubber sheet. If you have that idea in your head already then you are ready to drop the image because: The sheet is 2d but spacetime is 4d The 2d sheet is embedded in 3d ... 9 I was just thinking about that and here is my layman's explanation. Imagine you're tracing two dots on a crumpled piece of paper, the dots are moving, but as they are moving, so is the paper getting ‘uncrumpled’, the actual distance between the dots will be more than the sum of distances they have travelled. 9 Reason 1: Let's look at the Friedmann equations without the cosmological constant. $$\frac{\dot{a}^2 }{a^2} = \frac{8 \pi G \rho}{3}-\frac{kc^2}{a^2}$$ The term on the LHS is just the Hubble constant squared$H^2$which can be measured the direct measurement of recession velocity of galaxies The density term can be said to be a combination of$\rho_{...

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What makes you think that it is "obviously not 2013 on Earth"? In actual calculations, astronomers use the Julian day, which is a decimal representation of time. A Julian year is exactly 365.25 days of 86,400 seconds each. Astronomical coordinates are usually written in the J2000 epoch, which allows us to compensate for Earth's axial precession. Our ...

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