Why is the speed of light constant?

  • B
  • Thread starter r731
  • Start date
  • #1
r731
40
6
Visualize the following scenario. A star such as the sun, and a planet such as Earth are located in a solar system. A photon travels from sun (x0) to Earth (x1), and a satellite is in the midway observing the photon. This is relates to Einstein's special relativity.

I ask this since it take light about 8 minutes to reach earth.
 

Answers and Replies

  • #2
anuttarasammyak
Gold Member
1,940
1,012
Say 8 minutes to Earth, it takes less minutes for sunlight to reach satellite.
 
  • #3
martinbn
Science Advisor
3,109
1,475
What is the question here?
 
  • Like
Likes vanhees71, Vanadium 50 and etotheipi
  • #4
Halc
Gold Member
357
299
An ant walks 5 cm/sec (3m/min) directly towards my picnic, 24m away. It will take 8 minutes for it to reach my picnic. There is a bread crumb halfway, so it will pass that breadcrumb after only 4 of those minutes.

If the ant sees the bread crumb and decides it is enough, perhaps she will take the offering and go no further. Similarly, if your satellite actually observes the photon, the photon isn't going to get to Earth at all. It can't be measured in two different places.

I did not need to invoke relativity theory to work any of that out.
 
  • Like
Likes DaveE, Delta2, vanhees71 and 1 other person
  • #5
r731
40
6
Say 8 minutes to Earth, it takes less minutes for sunlight to reach satellite.

I thought of the satellite as an observer (as in special relativity).
 
  • #6
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
29,951
15,641
What is the question here?
 
  • Like
Likes vanhees71
  • #7
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2022 Award
24,046
15,753
What is the question here?
I spy with my little eye something beginning with "p" ...?
 
  • #8
r731
40
6
An ant walks 5 cm/sec (3m/min) directly towards my picnic, 24m away. It will take 8 minutes for it to reach my picnic. There is a bread crumb halfway, so it will pass that breadcrumb after only 4 of those minutes.

If the ant sees the bread crumb and decides it is enough, perhaps she will take the offering and go no further. Similarly, if your satellite actually observes the photon, the photon isn't going to get to Earth at all. It can't be measured in two different places.

I did not need to invoke relativity theory to work any of that out.

What about small particles that travel near the speed of light?
 
  • #9
Halc
Gold Member
357
299
I thought of the satellite as an observer (as in special relativity).
Fine. It being an observer or not has no bearing on when the light gets there.
Einstein did not invent the concept of the observer.

What about small particles that travel near the speed of light?
Same as the ant. So long as it is not accelerating, a small particle traveling at near c gets to the halfway point in half the time it takes to go the whole distance.

Relativity is still not relevant here.
 
  • #10
41,311
18,943
I ask this

Ask what? There is no question in your OP, as several people have already pointed out. You do ask a question in post #8, but that question doesn't make sense by itself since it appears to be a follow up to some question you thought you were asking in the OP of this thread, but there isn't one.

If you cannot tell us what question you are asking about the scenario described in the OP of this thread, this thread will be closed, since we can't answer a question that isn't asked.
 
  • Like
Likes Halc and Dale
  • #11
eltodesukane
97
20
Why is the speed of light constant?
We don't know, we would need a deeper theory.
 
  • Sad
  • Skeptical
Likes davenn and Vanadium 50
  • #12
vanhees71
Science Advisor
Insights Author
Gold Member
2022 Award
22,472
13,406
We observe that the speed of light is constant. No theory can answer why Nature behaves as we observe her to behave, but it can describe how she does behave.
 
  • #13
shjacks45
127
49
Visualize the following scenario. A star such as the sun, and a planet such as Earth are located in a solar system. A photon travels from sun (x0) to Earth (x1), and a satellite is in the midway observing the photon. This is relates to Einstein's special relativity.

I ask this since it take light about 8 minutes to reach earth.
"C" is a constant in many physics equations and IS NOT the "speed of light" but represents(!) Maximum attainable velocity in this universe. The actual speed of light is variable depending on the optical density of a material.
 
  • Like
Likes Delta2
  • #14
Ibix
Science Advisor
Insights Author
2022 Award
10,361
11,138
"C" is a constant in many physics equations and IS NOT the "speed of light" but represents(!) Maximum attainable velocity in this universe.
I'd not phrase it that way, since ##c## is not really attainable. Things either always travel at ##c## (light, for example) or cannot attain it ever. You are correct, though, that ##c## is better thought of as a fundamental constant, the invariant speed, that light just happens to travel at. That's a consequence of its masslessness - if we found that the photon had a mass then that would imply that light does not travel at ##c## in vacuum, but would not change relativity.
 
  • #15
vanhees71
Science Advisor
Insights Author
Gold Member
2022 Award
22,472
13,406
Well, in this sense ##c## is even less: It's a conversion factor from our arbitrary human definitions of units for time and length. From a relativistic point of view it doesn't really make sense to measure time and spatial distances in different units (as from a physical point of view it doesn't make sense to measure distances and heights in different units...).
 
  • #16
I'd not phrase it that way, since ##c## is not really attainable. Things either always travel at ##c## (light, for example) or cannot attain it ever. You are correct, though, that ##c## is better thought of as a fundamental constant, the invariant speed, that light just happens to travel at. That's a consequence of its masslessness - if we found that the photon had a mass then that would imply that light does not travel at ##c## in vacuum, but would not change relativity.
We say that light does not have mass but what makes it then to change its direction when from a distant star as it reaches the sun?Why does it curve due to the effect of gravity and does not move straight?What about the fact that light cannot escape from the gravitational pull of a black hole?
 
  • #17
Ibix
Science Advisor
Insights Author
2022 Award
10,361
11,138
We say that light does not have mass but what makes it then to change its direction when from a distant star as it reaches the sun?
The geometry of spacetime, a.k.a. gravity.
 
  • #18
The geometry of spacetime, a.k.a. gravity.
Sorry perhaps the question was not needed it was quite obvious I just was not sure.
 
  • #19
41,311
18,943
Since the OP has not actually asked any question, this thread is closed.
 
  • Like
Likes davenn

Suggested for: Why is the speed of light constant?

  • Last Post
Replies
34
Views
1K
Replies
9
Views
691
Replies
64
Views
2K
Replies
13
Views
766
  • Last Post
Replies
18
Views
748
Replies
1
Views
516
  • Last Post
Replies
29
Views
2K
  • Last Post
Replies
6
Views
316
Replies
57
Views
2K
Replies
8
Views
868
Top