# 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 ...
• 123k
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 ...
• 17.1k
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 ...
• 38.5k
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 RĂ¸mer 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 ...
• 123k
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 (31.5576 ...
• 501

### 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 ...
• 975
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 ...
• 38.5k
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 ...
• 1,005
Accepted

### Why speed of light is considered to be the fastest?

The recession rates relevant to Hubble's law are not relative velocities, so there's no reason to compare them to the speed of light. They can be defined as follows. Conceptually, imagine a chain of ...
• 4,694

### Why do we say that gravitational waves travel at the speed of light, when they can escape a black hole and light cannot?

Gravitional waves CAN'T escape a black hole; nothing that carries information can escape. The event Horizon is one way. The gravitational waves don't come from the singularity but instead come from ...
• 987

### 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 ...
• 2,539

### 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 ...
• 38.5k
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 ...
• 3,216

• 6,673

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

I can find plenty of evidence for electrons travelling at speeds less than c. The neutrino is said to be a fermion like an electron. But I can't find any evidence of neutrinos travelling at speeds ...

### 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 ...
• 153k

### Is Darkness Faster Than Light?

Darkness is not a thing. It is just the absence of light. It doesn't have speed. Try observe the image: In this image this is shown that suppose there is darkness on any many light year far object if ...
• 497
Accepted

### Does the edge of the Universe travel faster than the speed of light?

The universe has no edge, as far as we can tell. Now, the observable universe does have an "edge" - it's the most distant places we can see. But that's just us and our limited instruments. Due to ...
• 18.3k

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

Imagine you are in orbit around the earth, several 100 km upwards. What happens when you slow down? That's right, you fall down until some force stops your fall. That force is the pushback from the ...
• 673

### Are we already moving faster than light?

Short Answer NO. Long Answer - The Earth "wobbles" on its polar axis. This motion is not relevant (IMO) to the calculations needed to return to earth from interstellar space as it is an "in situ" ...
Accepted

### The light of which star is next to reach the earth?

I think you may be thinking of Olbers' paradox . This supposes that if the universe were infinite in time and extent, and stars were more or less randomly placed, then every line of sight would end at ...
• 123k
Accepted

### Speed gaseous planets

There isn't such a great difference between Earth and Mars (at about 25 hours) and the gas/ice giants (between 10 and 20 hours). Earth had a day of about 6 hours in the early Solar system, but has ...
• 123k