72
votes
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 $c$ ...
- 2,632
41
votes
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 ...
- 129k
22
votes
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 ...
- 9,869
16
votes
Accepted
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, ...
- 39.6k
16
votes
Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?
The escape speed is defined in Newtonian physics simply by demanding that the sum of kinetic energy at launch (ballistically, with no power applied thereafter) and gravitational potential energy at ...
- 163k
15
votes
Accepted
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. ...
- 129k
14
votes
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 ...
- 918
14
votes
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, ...
- 8,037
12
votes
Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?
Yes, escape speed is an instantaneous calculation at the distance 'r' from the center of the object, as that changes you have to recalculate your escape speed.
For example, these are the escape ...
- 911
11
votes
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. ...
- 16.7k
10
votes
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, ...
- 18.4k
9
votes
Accepted
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 ...
- 21.4k
8
votes
Accepted
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 ...
- 5,334
7
votes
Accepted
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) ...
- 129k
7
votes
Accepted
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\...
- 1,681
7
votes
Accepted
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 ...
- 163k
7
votes
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 ...
- 6,401
6
votes
Accepted
Is there a rough analytical expression for the Milky Way's radial mass distribution?
For the purposes of this simple exercise, what would be an analytical expression that roughly matches the Milky Way's radial density profile, projected on to its equatorial plane?
The simplest ...
- 15.1k
6
votes
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, ...
- 163k
5
votes
Accepted
How to identify component of velocity of a star from its red shift?
The first part of this question is already answered at How Do we know about redshift?
Regarding the second part, measuring tangential velocity is much harder, and indeed impossible for stars that are ...
- 10.4k
5
votes
Accepted
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 ...
- 163k
5
votes
Accepted
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 ...
- 37.4k
5
votes
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 ...
- 18.4k
5
votes
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....
- 2,632
4
votes
Accepted
Question about doppler effect
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 (...
- 15.1k
4
votes
Can "rogue" supermassive black holes be made this way?
Yes, and in fact a mechanism somewhat like this has probably dumped a large number of BHs into intergalactic space.
Black holes tend to settle towards the center of galaxies (an effect of dynamical ...
- 7,690
4
votes
Accepted
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 ...
- 39.6k
4
votes
Can a planet with no atmosphere be orbited at extremely low altitudes?
Existence of an atmosphere simply means you'd need to supply boost power to maintain orbital speed. Maybe a lot more than, say, the ISS generates to maintain its position :-) .
As the comments point ...
- 3,883
3
votes
How can we calculate velocity of an object given the distance from an object in parsecs?
You can't, not very well anyway. That's simply because the velocity of an object isn't calculable from the Hubble constant until the so-called Hubble flow starts to dominate (which in turn only ...
- 4,861
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