# Tag Info

29

Photons are blue-shifted when attracted by gravity (I mean - moving towards a mass, not moving at right angles to the gravitational field like in an orbit). They can't go faster, but their energy goes up.

23

You don't feel acceleration. When onboard the ISS, you are accelerating towards the earth (down) due to gravity: if you didn't, you would just fly away from the planet. Because you and the ISS are accelerating exactly the same way, you don't feel a thing. You don't feel a force if it's accelerating you: you feel pressure caused by opposing forces. Here ...

6

It's because photons are massless and so they can and must move at the speed of light with any none zero amount of momentum. This means that even though they move at the cosmic speed limit they can have very little amounts of momentum.

5

As Wikipedia and nowadays google will inform you, the speed of light is 299,792,458 m/s. Similarly Wikipedia and google put the radius of the Earth at 6,371 km. You can easily do the calculation in Wolfram Alpha to find a trip around the Earth at the speed of light takes about 133ms. A similar query shows that 26,930 such orbits fit inside an hour. Or, in ...

5

A tensile pulse travels through the rope at the speed of sound. This speed depends on the density and the bulk modulus of the material - a rope strong enough to support its own weight would probably have a very high bulk modulus. The equation is $$v = \sqrt{\frac{K}{\rho}}$$ Where $K$ is the bulk modulus and $\rho$ the density.

4

Now there is a light ray moving outward at the speed of light. I'm afraid that isn't the case; within the event horizon of a Schwarzschild black hole, the radial coordinate is timelike and so, moving 'outward' toward the horizon is as impossible as moving 'backward' in time. This plain to see in the Kruskal–Szekeres coordinates: Image credit See ...

4

Size is relevant. Even weak photons can be absorbed by knocking an electron to a higher energy level. If you were that small you would be ripped apart.

3

Let's say we're sending a scouting mission to an Earth-like planet 100ly away to see if it's suitable for colonization. We could send the scouts out at near light speed, and due to time dilation they could easily survive the trip without dying of old age. If we send them out at .9c then the entire round trip will only take 20 years in their frame of ...

3

To talk about acceleration in space is a little bit dangerous without exact definition. One has to separate free fall and acceleration from an impulse. Imagine, you are inside the ISS during an orbit correction. The impulse from the rocket engine you could feel, you get some weight, and and this is an acceleration. In all other time you are weightless and ...

3

As far my limited knowledge go, things in space aren't slow down unless something interferes with them, so what prevents me to build a spaceship powered by nuclear power that will keep accelerating until we get to the limits of physics? Like the voyager ship that is now outside our solar system, it had by know plenty time to accelerate to be ...

2

The rules of relativity theory basically tell you the following about velocity-related transformations: First, the vacuum speed of light is an invariant and second, the "inside" of the light cone (slower-than-light motion) and the "outside" of the light cone (faster-than-light) never get mixed up. So no matter how you manipulate your velocity, no matter what ...

2

For better clarity, let's define the following: Axial direction = the direction the person & light beam are drawn into the BH. Radial direction = the direction perpendicular to the axial direction. If we, looking in the same direction as the person & light are being drawn into the BH, watch the light beam as it is drawn into the BH, we will see the ...

2

thats an interesting question, but photonic fields are fundamental and cannot really be compared to say a baseball. If you throw a baseball at near the speed of lightit would probably break a lot of stuff, but photons don't really effect things very much unless they are high energy. Note that high energy photons like x-rays actually do have the ability to ...

2

From the perspective of a photon: There is no such perspective. I was trying get an understanding of the universe from the photons perspective. There is no such perspective. Consider the following excerpt from I am driving my car at the speed of light and I turn on my headlights. What do I see?: Sometimes people persist: What would the ...

2

I think a possible analogy would be to imagine that the singularity is a waterfall. By emitting light, you are trying to send a signal upstream using a tame fish. Outside the event horizon the fish is able to make headway against the current. But the river flows so fast within the event horizon as it approaches the waterfall, that your fish ends up going ...

2

Determining what is happening "right now" on a planet 63 light years away is exactly the same as determining what will be happening 63 years in the future on Earth. Both problems are, technically speaking, impossible. We can't know exactly what the future holds 63 years from now, as things may change in surprising ways, just as we can't know what is ...

2

No your argument is not correct. Firstly, velocities do not add linearly like 3-vectors in Euclidean space: the relativistic sum of two velocities always has a speed of less than $c$ if both the velocities' magnitudes are less than $c$ (no matter what their direction). Secondly, photons have no rest frame: that's a basic property of things that have zero ...

2

What you are confusing here is speed and velocity. Light speed is constant, but the velocity, which takes into account the direction as well as the speed is not. As an example of how something can accelerate without changing speed, consider the case of circular motion, where the acceleration of an object moving at a speed $v$ in a circle of radius $r$ is ...

1

I am sure there was a delay when Nixon talked to the Apollo crew. But how long was the delay? The distance from the Earth to the Moon is about 360000 km, if I remember correctly, so the one-way delay is about 1.2 s (plus maybe a smaller but possibly comparable delay for processing and retranslation) - that is noticeable, but it is not a showstopper. As for ...

1

I don't know the "formal" proof, but here is my proof: Time dilation and length contractions are given to us by the Lorentz transformations by: t’ = t/(1-v2/C2)1/2  and d’ = d/(1-v2/C2)1/2 (in other words “same” or proportional to each other) where: t = distance/length traveled through the T dimension in observers own frame of ...

1

Take as an example the electron positron annihilation Feynman diagram: Before time t' there exists no photon. After that the photon exists conserving energy, momentum and angular momentum for the reaction, and has to have velocity c in special relativity, because of its zero mass. The concept of acceleration needs the photon to exist before it does, ...

1

A key to understanding this is realizing that it's not always true. In fact, at x-ray frequencies, refractive indices are typically less than 1, so that the phase velocity is faster than the vacuum speed of light. The key difference is that x-ray frequencies are well above the natural frequencies of most of the electronic excitations that are involved in the ...

1

As I know, Field Theory, that to what appeals the topic creator cannot explain the very powerful gravitation fields . So trying to understand what happens with a photon there are inside the Black Hole in meaning of Field Theory, or Special Relativity, isn't a good idea. The Nature has no the alone space , and the alone time , you can abstractly image ...

1

speed is $$\frac{\text{distance}}{\text{time interval}}$$ but at the event horizon of a black hole, time interval becomes $0$. Imagine a flashlight flashes periodically 1 flash/s (in flashlight's reference frame). As flashlight getting close to the event horizon, someone far away from the event horizon will see flashlight flashing $0.1$ flashes/s, ...

1

Your assumption of a perfect mirror is something that will make this difficult to answer (also assuming you can close the door fast enough to keep some amount of the light in before it escapes). If the mirror is perfect and no light escapes, then it should contain the light, bouncing around forever. In reality, a small portion of the light is absorbed upon ...

1

Immediately. They start to flow immediately. When you connect a resistor to the negative terminal of a battery and a wire to the positive terminal of the battery the whole resistor gets to lower potential and the whole wire gets to a higher potential. So when you start to connect that wire to the resistor you literally bring a positive voltage wire towards ...

1

Time dilation: linear or exponential or other? Other $$\Delta t' = \gamma\Delta t = \frac{\Delta t}{\sqrt{1 - \frac {v^2}{c^2}}}$$ Lorentz factor $\gamma$ as a function of speed (in natural units where $c=1$) - Image by Zayani CC BY-SA 3.0

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Wouldn't that greatly reduces the time you need to get to a distant star, since you can increase your speed exponentially? In addition to hft's answer, due to relativistic kinetic energy $KE$ , assuming you use high constant thrust your actual acceleration would start tailing off quickly at relativistic speeds, so you certainly couldn't "increase your ...

1

so what prevents me to build a spaceship powered by nuclear power that will keep accelerating until we get to the limits of physics? Now matter how long the spaceship is able to accelerate (as measured by an accelerometer attached to the spaceship), there is always an inertial reference frame in which the spaceship is instantaneously at rest. Indeed, ...

1

as far as i could imagine the ball would be vibrating, it would have frequency, because it has a very large momentum or better to say that it has energy which is very huge with respect to its mass, you can't see that ball because it is too fast to be observed by our necked eyes, you can only see it with a super super slow camera. You see what the ...

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