Hot answers tagged

112 votes
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 ...
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70 votes

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 ...
  • 93.8k
31 votes
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 ...
  • 120k
30 votes
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 ...
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27 votes
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 &...
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15 votes
Accepted

Can light be trapped in orbit?

In principle yes, but in practice no. At the so-called photon sphere, gravity is exactly so strong that a photon on a tangential trajectory would stay in orbit. For a non-rotating black hole of mass $...
  • 34.2k
12 votes
Accepted

Can a photon have a stable orbit around a black hole?

For any massive object the gravitational potential energy is given by Newton's law: $$ V(r) = -\frac{GMm}{r} $$ The gravitational potential energy is due to the attractive gravitational force, but ...
  • 1,569
11 votes

Can light be trapped in orbit?

There does exist a solution for a massless point particle ("photon") that asymptotes to the light ring. It has a surprisingly simple closed form. $$r =\frac{GM}{c^2}\left(3+\frac{9}{-2+\cosh\...
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10 votes
Accepted

Is the resulting light from a supernova a product of photons bouncing around in the Sun?

First, the Sun will not end up as a supernova - only a star $>8$ times the mass of the Sun will end its life in that way. You also have the wrong idea about "trapped light" (photons bouncing ...
  • 120k
9 votes

Speed of light in a black hole

You cannot exceed the speed of light "locally". But you can -see- imagine* distances increase quicker than the speed of light. If by traveling you mean "moving compared to local space time", then ...
  • 1,402
9 votes

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

The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "...
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9 votes
Accepted

Based on the smallest number of photons that can deliver information about a star, what is the largest size of the universe?

So, assuming my assumptions aren't assinine (they very well could be) ... Your words, not mine. Your "very well could be" is the case. Assume the universe is infinite The universe might well be ...
  • 28.7k
8 votes
Accepted

How does gravity interact with a photon?

It is simply not true that gravity can only interact with mass. Rather, any long-range spin-2 force interacts with all energy-momentum equally, and it source is the stress-energy-momentum tensor. That ...
  • 7,684
6 votes
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, ...
  • 34.2k
6 votes

Are photons aged?

There is a quantity in relativity of $s^2$ which is defined as $t^2-x^2$, where $t$ is the difference in time between two events, and $x$ is the difference in position (measured in units such that $c=...
6 votes
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 ...
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5 votes
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; ...
5 votes
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 ...
  • 120k
4 votes

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 ...
  • 14.9k
4 votes

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 ...
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4 votes

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 ...
  • 120k
4 votes

How does gravity interact with a photon?

What we call "gravity" is really just the distortion of spacetime. If spacetime was completely straight, there would be zero gravity. But the existence of a massive body is one of the things that can ...
4 votes
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 ...
  • 120k
4 votes
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....
  • 120k
4 votes

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 ...
  • 93.8k
4 votes
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 ...
  • 9,928
3 votes

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: $\dot{M} \sim 2.5 \times 10^{-14}\,M_\odot/yr$. ...
3 votes

Do neutrinos have as much information as photons do?

This is a very broad question and though a comprehensive answer lies outside the scope of this simple Q&A format, I give you a couple of examples where "neutrino telescopes" would revolutionary. ...
  • 120k
3 votes

Does The ISW Effect Predict Net Redshift Over Distance?

The (late time) ISW is caused by the evolution of cosmic structures as photons of the cosmic microwave background traverse them on their way to our detectors. It may cause a redshift or blueshift with ...
  • 120k

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