# Why is the speed of light constant?

• B
• r731
In summary: Einstein did not invent the concept of the observer).Einstein did not invent the concept of the observer.
r731
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.

Say 8 minutes to Earth, it takes less minutes for sunlight to reach satellite.

What is the question here?

vanhees71, Vanadium 50 and etotheipi
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.

DaveE, Delta2, vanhees71 and 1 other person
anuttarasammyak said:
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).

What is the question here?

vanhees71
What is the question here?
I spy with my little eye something beginning with "p" ...?

Halc said:
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?

r731 said:
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.

r731 said:
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.

r731 said:

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.

Halc and Dale
Why is the speed of light constant?
We don't know, we would need a deeper theory.

davenn and Vanadium 50
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.

r731 said:
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.

Delta2
shjacks45 said:
"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.

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...).

Ibix said:
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?

universe function said:
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.

Ibix said:
The geometry of spacetime, a.k.a. gravity.
Sorry perhaps the question was not needed it was quite obvious I just was not sure.

Since the OP has not actually asked any question, this thread is closed.

davenn

## 1. Why is the speed of light considered a constant?

The speed of light is considered a constant because it has been measured to be the same value in all inertial reference frames, regardless of the relative motion of the observer and the source of light. This has been confirmed through numerous experiments and is a fundamental principle in the theory of relativity.

## 2. How was the speed of light first measured?

The speed of light was first measured by Danish astronomer Ole Rømer in 1676. He observed the varying time intervals between eclipses of Jupiter's moon Io as the Earth moved closer to or further away from Jupiter in its orbit. By comparing these time intervals, he was able to calculate the speed of light to be approximately 220,000 kilometers per second.

## 3. Is the speed of light truly constant in all situations?

According to Einstein's theory of special relativity, the speed of light is constant in a vacuum. However, it can be slowed down when passing through certain materials, such as water or glass. This is due to the interaction of light with the molecules in these substances, causing it to travel at a slower speed.

## 4. Why is the speed of light considered the universal speed limit?

The speed of light is considered the universal speed limit because according to the theory of relativity, it is the maximum speed at which all matter and information can travel in the universe. As an object approaches the speed of light, its mass increases and more energy is required to accelerate it further, making it impossible to reach or exceed the speed of light.

## 5. Can the speed of light ever change?

According to our current understanding of physics, the speed of light is a fundamental constant and cannot be changed. However, some theories, such as string theory, suggest that the speed of light may have been different in the early universe. This is still a topic of ongoing research and debate among scientists.

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