prompter: Is it true to say that light cannot travel at the speed of light since a perfect vacuum cannot exist?

assistant: The speed of light in a vacuum is a constant and is considered to be 299,792,458 meters per second. In a perfect vacuum, light can travel at c (the speed of light in a vacuum). 
However, a perfect vacuum cannot exist in reality. Even in the most perfect vacuum, there are still residual particles such as cosmic rays and other subatomic particles. These particles can affect the speed of light. 

However, the effect is negligible and the speed of light in a vacuum is considered to be constant and equal to c.

assistant: No, that is inaccurate.
The simple explanation of why travels more slowly in substance is that the light is absorbed and reemitted by the atoms,
but between them light does travel at the conventional speed of light.

photons are massless particles and had they moved slower then the speed of light they would have no energy.

prompter: but if light always travels at the conventional speed, why can't it escape black holes at all? if it was due to there being too much matter in them, wouldn't the photons still make it out eventually?

prompter: Fair enough. Speaking of high speed collisions, what would you estimate the general destruction was to the human body during last year's eruption in Tonga?

prompter: Ok, thank you. Let's assume we could build a rocketship that could travel at the speed of light and it had headlights that we could turn on. Would the light of the headlights travel at double the speed of light?

prompter: If light has no mass does that mean it cant be affected by gravity(plz answer yes or no)

prompter: Okay, say for example you've got a 1 billion light years of vacuum and light travels through it, would it take exactly 10 billion years to cross it? Or would the small amount of things that are actually in that vacuum add a couple of seconds on to the total time? What sort of numbers are we talking about here?

prompter: Can you then explain why photons should always move at the speed of light?

prompter: Why would light have no energy if it didn't travel at the speed of light?

assistant: Yes it is true. Achieving a perfect vacuum is not possible according to current theoretical physics and this is needed to achieve 299,792,458 m/s which is the speed of light. Therefore, light cannot reach the speed of light in the known universe.

prompter: How is the currently assigned value for the speed of light actually calculated or measured if it cannot be achieved within the known universe?

assistant: While it is technically true that a perfect vacuum cannot exist, light can still travel through a vacuum at its maximum speed, which is 299,792,458 meters per second (m/s). Light in a vacuum travels at its fastest speed because the medium through which it travels isn't interacting with the light and slowing it down. This constant velocity of light in a vacuum, which is often referred to simply as the speed of light, is one of the most universal and well-established principles in physics.

It is important to note that a vacuum is not devoid of energy and particles, but it is still considered to be a vacuum because it is a region of space where the average density of matter and energy is much lower than normal. Even in a vacuum, there are tiny bits of matter and energy present, such as cosmic rays or vacuum fluctuations, but these are not enough to significantly affect the speed of light.

prompter: If you did have a hypothetically perfect vacuum, would light travel through it faster than the imperfect vaccuum of space?

prompter: How would someone be able to prove that light goes at the speed of light in outer space?

prompter: How was the speed of light in a relative vacuum measured?

prompter: Why doesn't light move faster when bent by gravity?

prompter: How do vacuum tubes work?

prompter: What impact does the average vacuum have on the speed of light?

assistant: Light can be slowed by particles and transparent mediums like air, water, and glass. The ratio by which light is slowed is called the refractive index of the medium, this value is always greater than one. Outer Space is considered to be a near-perfect vacuum. The refractive index of outer space is generally considered to be one, which means that light isn't slowed down. Therefore, light can be considered to be travelling at the speed of light with outer space as the medium.