The Speed of Light: Physics and Possibilities

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Discussion Overview

The discussion centers on the nature of the speed of light, particularly whether it can be altered under various conditions, such as near black holes or when traveling through different materials. Participants explore theoretical implications and empirical observations related to the speed of light in vacuum versus in materials.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the speed of light can change when it interacts with a black hole, suggesting that it cannot escape the gravitational pull.
  • Others argue that while black holes may alter the direction of light, they do not change its speed, asserting that the speed of light in a vacuum remains constant at c.
  • A participant notes that light travels slower in various materials due to interactions with atoms, but emphasizes that this does not imply a change in the speed of light in a vacuum.
  • Another participant challenges the explanation that light is absorbed and re-emitted by atoms, citing empirical observations that contradict this view, particularly regarding the continuous nature of the index of refraction.
  • It is mentioned that there are two manifestations of "the speed of light": the speed at which light travels through different media and the constant speed limit of the universe defined by standard theory.

Areas of Agreement / Disagreement

Participants express differing views on whether the speed of light can change in the presence of black holes and in various materials. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants reference empirical observations and theoretical constructs without reaching a consensus on the nature of light's speed in different contexts. The discussion highlights the complexity of the topic, including the dependence on definitions and the nuances of light behavior in various environments.

clm321
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in physics is it possible for light to be slowed down or speed up, or is light a fixed speed?
 
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Yes it can the speed of light can change if hits a black hole, the speed of light cannot escape the pull of the black hole. therefore it must change
 
_wolfgang_ said:
Yes it can the speed of light can change if hits a black hole, the speed of light cannot escape the pull of the black hole. therefore it must change

yes but black holes change the direction. do they change the speed also?
 
clm321 said:
yes but black holes change the direction. do they change the speed also?

The answer is no, black holes do not change the speed of light.

The speed of light in a vacuum is a constant, c.

The speed of light in various materials can be different, but always lower than c. When people claim to have "stopped light", this is a claim made about the speed of light in a given material.

Note that matter is just a conglomeration of atoms. The reason the speed of light is lower in those materials is because the light is constantly being absorbed and re-emitted by the atoms. If you think microscopically, in between each absorption, the light travels in a vacuum always at c.
 
Nabeshin said:
Note that matter is just a conglomeration of atoms. The reason the speed of light is lower in those materials is because the light is constantly being absorbed and re-emitted by the atoms. If you think microscopically, in between each absorption, the light travels in a vacuum always at c.

Nope, this is wrong, ZapperZ (and other contributors) have a nice post in the FAQ on this. To quote;

"A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light."

To the OP; there are two manifestations of "the speed of light". The first is the speed at which light travels. This varies, depending on the refractive index of the medium it is traveling through (so yes, light can be slowed down, in fact there is an entire field devoted to "slow light"). The other manifestation is the constant that defines the speed limit of the universe; this is a genuine constant according to standard theory.

Claude.
 

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