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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 $c$ ...
Nuclear Hoagie's user avatar
71 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 ...
James K's user avatar
  • 123k
70 votes

What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?

The light from the Sun spreads, at least initially, in a roughly isotropic fashion into the universe. As it gets further from the Sun, some of that light will interact with the interstellar medium (...
ProfRob's user avatar
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65 votes
Accepted

Why doesn't the moon twinkle?

The first handful of hits on Google actually return incomplete and even wrong answers (e.g. "Because the Moon is much brighter" which is plain wrong, and "Because the Moon is closer" which is ...
pela's user avatar
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64 votes
Accepted

How old is the oldest light visible from Earth?

The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons ...
HDE 226868's user avatar
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55 votes
Accepted

If light has no mass, why is it affected by gravity?

Another way to answer this question is to apply the Equivalence Principle, which Einstein called his "happiest thought" (so you know it has to be good). The equivalence principle says that if you are ...
Ken G's user avatar
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53 votes
Accepted

How do we have photos of galaxies so far away?

There are two reasons that often — but not always — light from galaxies millions and even billions of lightyears away make it through the Universe and down to us: Particle number and particle size ...
pela's user avatar
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52 votes
Accepted

How much of the surface of other planets is lit by the sun?

OK, let start with some assumptions: spherical objects and no atmospheric effects. Here's the relevant geometry with the object on the left representing a planet and the object on the right ...
GrapefruitIsAwesome's user avatar
45 votes
Accepted

Why does the Moon appear gray when passing between the Sun and the Earth?

That's what it really would look like if you were there with DSCOVR. The albedo of the Moon is only about 0.136, about half of the Earth's average albedo. Of course the part with clouds is higher. I ...
uhoh's user avatar
  • 30.6k
42 votes
Accepted

Is it suspicious that gravitational waves propagate at the speed of light?

It is very suspicious! It points to the fact that the speed of light isn't just some random speed that light happens to travel at, but is a fundamental property of the universe. In fact, any massless ...
James K's user avatar
  • 123k
40 votes

What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?

You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you want nature to be like has no bearing on what it is. The light from the sun is a ...
James K's user avatar
  • 123k
40 votes
Accepted

Is the Sun visible from Proxima Centauri to human eyes?

Well, there's two things we'll need for this: apparent magnitude (the brightness that an object appears to have) and absolute magnitude (the actual brightness an object has). Both of these scales are ...
Sir Cumference's user avatar
33 votes
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 ...
James K's user avatar
  • 123k
33 votes

Huge galaxies seen from earth

Actually, we don't see the whole Sun. We only see a bit less than half of it. We see the half that faces us because every point over it radiates photons in every possible direction, including exactly ...
fraxinus's user avatar
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31 votes
Accepted

Why can't moon light (reflected sun light) turn the sky blue?

The simple answer is that it does, but it's not bright enough to be visible to the naked eye. Earth's atmosphere scatters the moon light just like sunlight. The full moon (like the sun) fills about ...
userLTK's user avatar
  • 24.1k
31 votes

If light has no mass, why is it affected by gravity?

There are a couple of ways one could approach your question: Black holes are regions of space that have been deformed by a sufficiently concentrated mass. Light waves/particles always travel in a ...
Alex Hajnal's user avatar
  • 1,189
29 votes
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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 ...
Toby Speight's user avatar
29 votes
Accepted

Distance of the Crab nebula and the speed of light

You are absolutely right. The convention is that we date astronomical events by when we see them - which is verifiable, useful and absolutely certain. It wouldn’t be useful to know what year BC it ...
Martin Kochanski's user avatar
26 votes
Accepted

If dark matter bends light, how do we know the stuff in the sky is where we think it is?

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
HDE 226868's user avatar
  • 36.6k
26 votes

How do we have photos of galaxies so far away?

As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons ...
Natsfan's user avatar
  • 4,494
24 votes
Accepted

How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape?

It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon ...
James K's user avatar
  • 123k
24 votes

Can a star emit heat but no visible light?

The minimum temperature of an object classed as a "star" is something like 2700 K. Such an object, although emitting the bulk of its radiation in the infrared, would still emit something ...
ProfRob's user avatar
  • 153k
23 votes

How do we have photos of galaxies so far away?

There's a misconception in your question I don't think the other answers have addressed. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the ...
Rupert Morrish's user avatar
23 votes
Accepted

Why do stars twinkle but the Sun doesn't (I'm asking this because the Sun is also a star)

Stars twinkle because they are effectively a point of light. This point of light can be distorted and magnified by movement of patches of varying density in the atmosphere. These act as lenses causing ...
James K's user avatar
  • 123k
22 votes
Accepted

How distorted will our galaxy be if we are viewing it from several thousand light years away?

The light travel time of 100,000 years is quite small compared to the time it takes the Milky Way's spiral arms to complete an appreciable fraction of one rotation. The arms have a pattern angular ...
HDE 226868's user avatar
  • 36.6k
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 ...
AtmosphericPrisonEscape's user avatar
20 votes

How can we know if a star which is visible in our night sky goes supernova?

It is not possible to know. The speed of light is the speed of information. The information "the star has exploded" cannot travel faster than the speed of light, so there is no way to know ...
James K's user avatar
  • 123k
20 votes

Huge galaxies seen from earth

You only see the light that enters your eye! But it doesn't matter that your eye is smaller or larger than the thing you are looking at. Hold your hand up in front of your face at arms length. There ...
James K's user avatar
  • 123k
20 votes

Distance of the Crab nebula and the speed of light

Just to reiterate my comment. The approach is correct but the accuracy is spurious - the number of significant figures is not warranted by the quality of the distance information (the date of ...
ProfRob's user avatar
  • 153k

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