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70

Photons can't have a perspective. If we have a particle with mass, we can imagine taking a frame of reference in which that particle is at rest. We can then see things "from the particle's perspective". But there is no frame of reference in which a photon is at rest. Photons always move at the speed of light in every frame of reference. If I try to set ...


52

1. Is material on Earth's surface not in free fall around Earth's center? No. Material on the Earth's surface -- or inside it -- is not in orbit, and so is not in free fall. You can temporarily put yourself into an orbit (and thus into free fall) by jumping up into the air, or jumping off a higher surface. When you do this, you are briefly in a very ...


44

What is a spiral arm? The reason that the Sun, in principle (but see below), may cross spiral arms is that galactic spiral arms are not rigid entities consisting of some particular stars; rather they are "waves" with a temporary increase in density. An often-used analogy is the pile-up of cars behind a slow-moving truck: At all times, all cars are moving ...


35

The answer to this is surprising: We are. And many (if not all) other galaxies. And they move faster than light. See, the universe is expanding, at an accelerating rate. The fabric of spacetime itself stretches out, so that galaxies seem to move away from each other. The interesting thing is that relativity does not forbid these from moving away faster ...


33

Ole Rømer did not measure a change in the frequency of light. He measured an apparent change in the orbital period of Io, one of Jupiter's moons. The orbit of Io can be measured very accurately by observing when it enters or leaves the shadow of Jupiter. When the Earth is moving away from Jupiter, Rømer noted that the orbit of Io appeared to be very ...


27

By convention, astronomy uses the Julian Year for the computation of a light year: Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. Wikipedia gives the length as $31 557 600 s \times 299 792 458 m/s = 9 460 730 472 580 800 m$ (exactly) The ...


21

Yes, the speed of light in vacuum (or c) is 299,792,458 m/s and one light-year is the distance the light travels in one Julian year (365.25 days), which comes out as 9.4605284 × 1015 meters. Since c is the maximum speed at which all energy, matter, and information in the Universe can travel, it is the universal physical constant on which the light-year (ly) ...


18

When a galaxy recedes from us, the light we see from it is redshifted. For galaxies at cosmological distances, this redshift is fundamentally different from a Doppler shift; whereas the latter is due to a velocity difference between the emitter and the receiver, a cosmological redshift is due to photons traveling through an expanding space$^\dagger$. Hence, ...


17

The accuracy is extremely good thanks to LIGO and a binary neutron star merger. It's accurate to about 3 parts in 1 quadrillion! See the paper Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A The paper constrains "the difference between the speed of gravity and the speed of light to be between $−3×10^{-15}$ ...


16

There is also another mediator particle which moves at the speed of light other than the photon. This is the gluon, which is the exchange particle for the strong force. The odd thing about the gluon is that it's never seen by itself (that is, outside of collections of other gluons). Also, though neutrinos do in fact have mass, they are neutral particles. ...


14

There are plenty of rapidly moving objects in astrophysics. A good place where one can get moving relativistically is near an event horizon of a black hole. A simple Newtonian estimate illustrates the point. Black hole has all its mass $M$ hidden under an event horizon of the radius of order $r_{g}=\dfrac{2GM}{c^2}$. An object moving circularly in the ...


13

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect. Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the ...


13

In a car, you have a perception of speed because of (a) the "wind" passing by as you rush through the air which is not moving at the same speed as the vehicle, and (b) you perceive the stationary objects nearby as "moving" off into the distance behind. As the earth moves in its orbit, you don't notice any "wind" from the planet rushing through space, as the ...


12

The so called expansion of the universe is not as trivial as most people think. What is happening, in fact, is that the distance between two points in space (note that I'm not talking about objects with velocities, but just coordinates in space) increases with time in a manner proportional to a given factor (in this case, the Hubble constant - which is ...


12

As it turns out, the fastest spinning neutron star found yet is a pulsar 18000 light years away in the constelation of Sagittarius which scientist catalogued as PSR J1748-2446ad. Pulsars are neutron stars that rotate, are highly magnetic and emit a strong perpendicular beam of electromagnetic radiation. This pulsar's speed is such that: At its equator it ...


12

As Walter says, gravity doesn't bend light. Light travels along null geodesics, a particular type of straight path. Since (affine) geodesics don't change direction by definition, geometrically light trajectories are straight. Moreover, the speed of light in vacuum is $c$ in every inertial frame, regardless of whether or not spacetime is curved, although a ...


11

A deeper answer is "yes and no". In the frame of reference of the light itself the journey from Proxima to here is instantaneous. In our frame of reference it takes four years - this is all bound up in relativity and the nature of spacetime. But in the everyday sense we are indeed looking back in time at light from the stars.


10

Speed of light being finite is one of the fundamentals of our Universe. If it were infinite, this would have a major implication in causality. Besides, in non-quantum physics, light is just an electromagnetic wave. Eelectromagnetic field is described by Maxwell's equations, which predict that the speed $c$ of electromagnetic waves propagating through the ...


10

It is a co-incidence. The units are what you need to look at, in both the speed of light and the distance of the Sun the units of length are the same (m) (your ratio is $\approx 0.002 s^{-1}$ when you use metres as the unit for the distance to the Sun). The definition of the second is in a sense arbitrary and is a consequence of how we have chosen to sub-...


9

So where are these measurements of galaxies moving faster than light? They're redshift measurements. Check out the Wikipedia redshift article. It's good stuff. "we can actually observe galaxies that are moving away from us at >c" It's true. You might think it cannot be, but it can. Um, I think I missed the groundbreaking headline that said ...


8

From a (more) physics standpoint our acceleration on earth is basically zero from what we can feel. Just like the others that posted about cars traveling at a certain constant velocity, you won't feel a change. If a car is traveling at a constant velocity there is essentially no feelable force acting on your body. Therefore you you don't feel any effects of ...


8

I think the original answer has confused you somewhat - it is spacetime that is expanding faster than light - when viewed from sufficiently far away - we certainly are not moving away from the Sun at anything even remotely close to the speed of light (in fact the expansion of spacetime is not a motion at all in the true sense, but a change in the underlying ...


8

Comets don't cross Earth's orbit really. Orbits are one-dimensional objects and their chance of crossing in 3D space is 0. Henceforth, I consider a comet at distance 1AU from the Sun. What's the maximum speed of a returning comet at 1AU from the Sun? This can be easily worked out from the orbial energy $$ E = \frac{1}{2}v^2 - \frac{GM_\odot}{r},\qquad\qquad(*...


8

The expansion rate of space is not itself the reason that the radius $R_\mathrm{Uni}$ of the observable Universe is larger than 14 billion lightyears (Gly). Just the fact that space expands is the reason. If space did not expand, then $R_\mathrm{Uni}$ would be the expected 14 Gly, as this is the distance that light can travel in the 14 billion years (Gyr) ...


8

You have neglected length contraction in your analysis. You see the initial distance of the star contracted by the same factor that the elapsed time is shortened. So, from the perspective of the Earth, you take 1.0101 years to travel a distance of 1 light year at a speed of $0.99c$. From your perspective, the destination comes to you. It arrives after 0.1425 ...


7

Actually, the light hitting us from Proxima Centauri is not necessarily 4.243 years old. Perhaps some of the photons arriving here were created in the photosphere of Proxima. But some of them will have been created in the center of the star, and these photons may take many years to arrive at the photosphere, where they are then "emitted". For our sun, ...


7

The equation for Doppler shifting towards the blue spectrum (i.e. when you are traveling towards it) is: $\lambda_b = \lambda_c/(c + v_b)$ where $\lambda_b$ is the shifted wavelength, $\lambda_c$ is the original wavelength, $c$ is the speed of light and $v_b$ is your velocity towards the source. BUT! When you get closer to the speed of light you need to ...


7

The darkness does not have a speed, as it's just the space the light travels through. It's always there until the brief instant a photon whizzes through. It would be analogous to asking what speed the road is travelling at, rather than the car.


7

It all depends on your frame of reference, but the biggest speeds caused by gravity will involve black holes or perhaps neutron stars. From the point of view of a far external observer, an infalling object (from infinity) reaches a maximum inward speed of $0.385c$ (where $c$ is the speed of light) towards the black hole at 3 times the Schwarzschild radius. ...


7

You're forgetting that one year is not 365 days, but 365.2422 or something close to that. That change will give you a number much much closer to the google provided number.


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