# Tag Info

### Are photons aged?

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 ...
• 94.2k
Accepted

### Is Earth's Surface "In orbit"?

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 ...
• 14.9k
Accepted

### Does the sun cross other spiral arms in its movement around the galaxy's center?

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 ...
• 34.2k
Accepted

### It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?

If I interpreted this article correctly, then the answer to my question should be: 8 minutes is what we perceive, whereas for the photon the journey is instantaneous, due to the fact that it travels ...
• 629
Accepted

### How did Ole Christensen Romer measure the speed of light?

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 ...
• 94.2k
Accepted

### Exact measurement of a light year

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....
• 427

### It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?

When people talk about the time taken for light to go from the Sun to the Earth, they're generally just considering classical Newtonian mechanics, not relativity. So we simply divide the distance by ...
• 835
Accepted

### Which star / galaxy is moving away from us the fastest?

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 ...
• 34.2k
Accepted

### How precise are the observational measurements for the speed of gravity?

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 ...
• 995
Accepted

### Is the ratio between earth's distance from the sun and the speed of light just a coincidence?

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 ...
• 3,246

### So where are these measurements of galaxies moving faster than light?

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 ...
• 2,459
Accepted

### How fast is a comet moving when it crosses Earth's orbit?

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 ...
• 5,306

### Regarding the age / size of the Universe

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 ...
• 34.2k
Accepted

### Does near light-speed travel doppler-shift the light from the target into gamma rays?

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 ...
• 106
Accepted

### Is Darkness Faster Than Light?

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 ...

### What is the Maximum Speed that can be acheived Because of Acceleration Due to Gravity?

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 ...
• 120k

### Exact measurement of a light year

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.
• 738
Accepted

### Have more recent LIGO/VIRGO gravitational wave measurements narrowed down the speed of gravity further?

To get these kind of measurements one needs a gravitational wave detection with a electromagnetic counterpart. The big problem is, that the now quite common BH mergers are not expected to produce a ...
• 2,818

• 5,306

### Is there any cosmological evidence that neutrinos travel at a speed other than c?

A paper by Wie et al. (2016) uses the coincidence between gamma rays from Gamma ray bursts and individual 3-30 TeV neutrinos to estimate the difference in propagation speed between neutrinos and ...
• 120k