# Tag Info

### Why does light accelerate instantaneously to c, while no other phenomena do it?

"Accelerate instantly" would imply that a photon takes many different velocities at the same point in time. In fact, it would imply that a photon takes on every velocity between $0$ and $c$ ...

### Can a planet with no atmosphere be orbited at extremely low altitudes?

Yes, but. Firstly the 400km orbits of the ISS are already extremely low, in comparison to the 6400km radius of the Earth. See https://what-if.xkcd.com/58/ for pictures of orbits. So if you rephrase ...

### Why does light accelerate instantaneously to c, while no other phenomena do it?

I am not sure this is a problem of visual communication. My incling would be to think this is a problem of language communication. The equations of relativity tell us that anything with zero rest-mass ...
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### What does the velocity dispersion of a galaxy mean?

Definition of the velocity dispersion From the title of your question, I'm unsure whether you actually know what "dispersion" means: The dispersion of some numbers is the spread around their mean, ...
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### How to find the shape of an orbit

If you want just the "shape" you need one number: the eccentricty. The shape of an ellipse is determined by the eccentricty. It is a measure of how "uncircular" an ellipse is. ...

### Why does light accelerate instantaneously to c, while no other phenomena do it?

You are looking for a way to visualize the fact that a photon is created traveling at the speed of light. Remember that a photon is actually a perturbation of the electromagnetic field. That field is ...

### Can a planet with no atmosphere be orbited at extremely low altitudes?

An example of a planetary mass object or planemo that is almost airless is the Moon. It does have an atmosphere, but very, very thin. The Moon has an atmosphere so tenuous as to be nearly vacuum, ...
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### What (actually) is the " deprojected half-light radius" of this almost-all-dark-matter Galaxy?

The half light radius is the radius from within which half the luminosity emerges. "Deprojected" means that the authors must have fitted some model to the 2D distribution of light, which can then be ...
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### Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?

The Ca triplet in the near infrared are extremely strong resonance absorption lines. They are by far the strongest features in the near infrared spectra of cool G,K,M type dwarfs and giants, which ...

### Why does light accelerate instantaneously to c, while no other phenomena do it?

Take a look at this diagram from Feynman's lectures at Caltech on angular momentum. Here, an atom with angular momentum $m=1$ starts out in an excited state on the left hand side of the diagram. ...

### Can "rogue" supermassive black holes be made this way?

I assume you're asking about central supermassive black holes (SMBHs, one per galaxy), not stellar-mass black holes. The answer is yes, but what actually happens is the two SMBHs have to merge first, ...
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### What does "unremarkable transverse peculiar velocity" mean exactly, and how is it calculated here?

"Peculiar velocity" is a fixed term and describes the velocity of an object relative to a defined rest frame. Astronomy has the problem that you need different methods to measure the 3D ...
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### Which celestial object in the solar system has the fastest tangential velocity at its surface?

Hands down it's Jupiter. It has both the largest radius and faster rotational velocity of the planets, and smaller objects will not be able to compete with that huge radius. Jupiter's rotational ...

### Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?

The ESA states it pretty clearly (although their figure of 855.2 nm is incorrect; it should be 866.2 nm): The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-...
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### Is blue light not traveling faster then red light in space?

Photons are massless. This doesn't depend on their energy, so doesn't depend on their frequency or wavelength. Massless particles travel at the speed of light. Even if we abandon particles and look ...
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### Calculating object velocity at perihelion

The vis-viva equation is commonly written like this: $$v^2 = GM\left(\frac{2}{r} - \frac{1}{a}\right)$$. For $r=a(1-e)$: v = \sqrt{GM\left(\frac{2}{a(1-e)} - \frac{1}{a}\right)} = \sqrt{GM\...
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### Binary Star Initial Speed

You will always get a "stable" orbit if the stars have less than escape velocity relative to each other. (unless you are modelling the stars as having non-zero radii so they can collide) ...

### Why does light accelerate instantaneously to c, while no other phenomena do it?

I think your core misunderstanding is this : In physics, it always takes some time for a particle to move from rest to some speed. If a particle breaks up into two other particles, then the moment ...

### Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?

According to Cropper and Katz 2011 part 2.2, the RVS working group considered other bands, but the ~850 nm band is relatively unaffected by absorption in the Earth's atmosphere, facilitating ground-...

### Calculating object velocity at perihelion

You can do this without having to know or derive the vis-viva equation, just by applying conservation of energy and angular momentum. At perihelion and aphelion the velocities are purely tangential, ...
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### Calculating obital velocity from radial velocity

You can't without assuming something about the overall velocity. The radial velocity is one component of a velocity vector; you are missing the other two components, which could in principle be ...
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### Why does the neutral hydrogen velocity have this characteristic behavior in the galactic plane?

The reason is detailed in depth in this pdf, which contains the following diagram: Some key quantities: $R_0$: Distance from the observer to the center of the Milky Way $R$: Distance from target gas ...

### Why collisions between earth orbiting satelites do occur?

"All earth orbiting satellites should have the same velocity" is not true. Kepler's Laws merely state that an object in a circular orbit at a particular altitude must have a particular speed....
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### What is the standard reference point for measuring speed?

The rest frame for measuring (astronomical) speeds and velocities depends on the context and purpose. A geocentric frame, based on the Earth's centre of mass might be appropriate for objects in orbit ...
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### What is the fastest a galaxy or other body has been measured to (or can) travel?

Hubble velocities The Universe expands and carries galaxies away from each other with a relative velocity proportional to the distance between them. This is Hubble's law, and if the Universe is ...

### Does Absolute Velocity Exist?

Chris, you are actually on the verge of understanding how special relativity works. You're very close. You only need to take one extra step. to state that all speed is relative to an object is to ...
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Before I start, let me just say that this topic is vastly more complicated than you've presented and what I will be showing. The trouble here is that ultimately, everything you've done and I will do (...