# Tag Info

Accepted

### Would we have more than 8 minutes of light, if the Sun "went out"?

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes ...
• 152k

### 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 ...
• 122k
Accepted

### Speed of light in a black hole

It doesn't work like that. An observer at the light source (and indeed any observers anywhere else) will always see light travelling (in vacuum) at the speed of light locally. There is also a major ...
• 152k
Accepted

### Can a photon orbit around a galaxy?

No. The milky way has a lot of mass — about 1.5 trillion times the mass of the sun. For light to orbit it would have to follow a null geodesic, ie the photon sphere at 1.5 times the Schwarzschild ...
• 122k
Accepted

### What is the probability of a photon from one of these galaxies hitting the James Webb detector?

Probability without lensing The probability of any one photon of an ensemble of isotropically emitted photons is indeed proportional to ratio between the area of the detector, and the area of the &...
• 38.3k
Accepted

Accepted

### Understanding The Fluctuations In The CMB Maps

The anisotropies in the CMB are caused by four effects; three at the surface of last scattering (SoLS), and one after: Temperature differences Denser regions will be more compressed and thus hotter, ...
• 38.3k

### Can light be trapped in orbit?

"If approached from a certain angle"? No. The only circular orbit for a photon in the Schwarzschild metric is when it is emitted perpendicularly to a radial line towards the black hole and ...
• 152k
Accepted

### Energy carried by solar wind

What are the velocity, mass, and charge distribution of the solar wind. Velocity The solar wind speed has a large range of variation, between ~250–820 km/s [e.g., Chen et al., 2014; Gopalswamy, 2006; ...
• 1,021
Accepted

### Does a photon need to have EXACTLY the right energy to be absorbed by a gas molecule?

The Physics SE answer (or the part quoted) was incorrect. The photon does not have to have "precisely" the right energy to cause a transition. The reality is that there is a non-zero ...
• 152k
Accepted

### minimum size for a black hole to capture a photon

There isn't really a minimum size. All black holes have an event horizon from which nothing, not even light can escape. The (Schwarzschild) radius of this event horizon is 3km for a black hole of a ...
• 152k

### Can't pulsars and stars be used for gravitational wave measurement?

How LIGO, LISA, etc. Detect Gravitational Waves The point of instruments like LIGO and LISA is to measure time-varying changes in the distance within different arms of the instrument. In the case of ...
• 16.7k

### Does Time Exist for Photons?

Time does not exist for photons. Quoting Wikipedia's somewhat unnecessarily technical page: In relativity, proper time along a timelike world line is defined as the time as measured by a clock ...
• 7,650
Accepted

### Is the CMB the photons that were created at the birth of the atom?

The CMB consists of photons that were emitted very shortly before and during the recombination of electrons with hydrogen and helium nuclei. [Side note - why is it called the epoch of recombination ...
• 152k
Accepted

### What does less than one count from an x-ray detector mean? (Swift BAT detector)

I think there is a missing piece of information. The BAT is a coded mask telescope. The imaging is done by photons passing through a mask and falling onto an array of 32768 detectors. http://swift....
• 152k

### Energy carried by solar wind

I'm not sure such a detailed answer to your question is available. This book cites this paper as a source for the mass loss due to the solar wind: $\dot{M} \sim 2.5 \times 10^{-14}\,M_\odot/yr$. ...
• 1,044

### Can a change in the density of interstellar and intergalactic quantum fields affect the Redshift of photons?

No. Or at least such an effect has never been observed, neither in the locality of the Earth or in light detected from distant sources. If a photon has an interaction with a quantum field (such as ...
• 122k
Accepted

### Milky Way as seen by the human eye: where exactly do those photons come from?

I believe that the answer is mainly the first (directly from stars), although there will certainly be some of each of the others. The diffuseness comes from the fact that there are many more ...
• 10.3k

### Are photons aged?

This depends on who exactly is doing the measuring. There is a very real sense, despite the answers here perhaps either downplaying or not quite getting directly at it enough, in which you can ...
• 1,106

### Are photons aged?

Even if you substitute the definition of proper time by "distance traveled", you cannot guarantee you got that distance right. Suppose photon was absorbed by hydrogen interstellar gas, then re-emitted ...
• 351