72
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 ...
- 129k
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 $c$ ...
- 2,632
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 ...
- 37.4k
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 ...
- 5,334
54
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
...
- 39.6k
53
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 ...
- 2,359
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 ...
- 129k
41
votes
Accepted
Why does Mars have a jagged light curve?
My first guess was that this was "something to do with the moon" since there seems to be roughly a monthly periodicity. But looking more closely suggests that my first guess was wrong. The &...
- 129k
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 ...
- 129k
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 ...
- 2,879
32
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 ...
- 1,199
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 ...
- 1,840
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 $0.001$-$0.01M_{\odot}$ ...
- 37.4k
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 ...
- 4,504
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 ...
- 162k
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 ...
- 330
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 ...
- 129k
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 ...
- 37.4k
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 ...
- 9,869
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 ...
- 129k
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 ...
- 129k
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 ...
- 162k
18
votes
Accepted
Can you see the starting and the ending of a light beam passing in the distance?
Nothing forbids this, and it is actually observed astronomically. You need a very bright source of light: such as a supernova (which isn't a beam, but radiates in all directions) and very large ...
- 129k
18
votes
Accepted
Problem regarding the absorption lines of the Sun
Possibly you are under the misapprehension that the number of photons is a conserved quantity? That isn't true, there are more photons at any given wavelength when you are deeper into the star because ...
- 162k
18
votes
Does bending of light around the Sun depend on the wavelength?
The amount of "gravitational light bending" is independent of the photon energy (light wavelength).
The reason is that the light follows a path through spacetime that is appropriate for a ...
- 162k
18
votes
How can redshifted light be detected?
In a redshift (whether that be caused by relative motion, gravitation or cosmological expansion), all wavelengths are increased by the same factor.
Redshift is determined by identifying features in a ...
- 162k
17
votes
If an exoplanet transit we are seeing is 13000 light years away, are we seeing a 13000-year-old orbit?
In one word, yes. Anything and everything we see, we see the way it was a certain time ago—about 1.3 seconds for the Moon, about 13,000 years for your hypothetical planet. Like @Richyt pointed out, ...
- 7,659
17
votes
How distorted will our galaxy be if we are viewing it from several thousand light years away?
Yes, there would be some distortion, but not enough to make a visible difference. Let's do a rough calculation.
Assume the galactic disc has a diameter of $100,000$ LY, and we are viewing it from a ...
- 16.7k
17
votes
Why does Mars have a jagged light curve?
To follow up on @JamesK's excellent answer and observations we can calculate the period of apparent oscillations due to the aliasing that arises from sampling at a frequency close to or below the ...
- 30.7k
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