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TheQuestionGuy14
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I was curious, is the speed of light in a vacuum really constant to all observers no matter their speed or movement? Is it possible for someone to somehow see light travel slower?
Yes, to the best of our ability to measure.TheQuestionGuy14 said:I was curious, is the speed of light in a vacuum really constant to all observers no matter their speed or movement?
Kind of. You can prepare unusual single photon states that don't propagate at the speed of light, but this is (roughly speaking) because they correspond to expanding waves and the average speed of an expanding wave is always lower than the wave speed. It's cheating, in a sense. And (in any sense) it doesn't change the invariance of ##c##.TheQuestionGuy14 said:Is it possible for someone to somehow see light travel slower?
Ibix said:Yes, to the best of our ability to measure.
Kind of. You can prepare unusual single photon states that don't propagate at the speed of light, but this is (roughly speaking) because they correspond to expanding waves and the average speed of an expanding wave is always lower than the wave speed. It's cheating, in a sense. And (in any sense) it doesn't change the invariance of ##c##.
TheQuestionGuy14 said:I was curious, is the speed of light in a vacuum really constant to all observers no matter their speed or movement? Is it possible for someone to somehow see light travel slower?
Because the word "speed" can mean multiple different things in relativity, broadly related by being the rate at which the distance between two things grows in some sense or other. The speed limit only applies to local measurements. So you will never, under any circumstances, see anything overtake a pulse of light (excepting the odd "slow" photon states I mentioned above).TheQuestionGuy14 said:Also, apparently things can't go faster than the speed of light, but apparently galaxies far away travel faster than the speed of light, how is this possible?
TheQuestionGuy14 said:how is this possible?
Ibix said:Because the word "speed" can mean multiple different things in relativity, broadly related by being the rate at which the distance between two things grows in some sense or other. The speed limit only applies to local measurements. So you will never, under any circumstances, see anything overtake a pulse of light (excepting the odd "slow" photon states I mentioned above).
But once you try to measure the speed of something that isn't right next to you, there isn't a unique way to do this. And there isn't necessarily a restriction on the speed you can obtain - because it's not the same thing as the local measurement.
As a trivial example, turn 360° on the spot. In a frame where you were stationary, Alpha Centauri just moved about twenty five light years in a circle, in only a second or two. Relativity doesn't break. The reason for supra-luminal recession speeds is harder to explain without maths, but it's a similar effect.
Vanadium 50 said:A good answer is at https://www.space.com/33306-how-does-the-universe-expand-faster-than-light.html Oops...that's the same link you posted. Did you read it? If there is something you don't understand, could you tell us what it is?
To the extent that "really" means anything, yes, really it is moving faster than light, by some measures.TheQuestionGuy14 said:So it's not really going faster than the speed of light, is it?
Because "nothing can travel faster than light" isn't true for all definitions of speed in curved spacetime. As V50 points out, the article you linked explains in some detail.TheQuestionGuy14 said:I don't understand how the universe expands faster than the speed of light, if nothing can surpass light.
Ibix said:Because "nothing can travel faster than light" isn't true for all definitions of speed in curved spacetime.
I'm not sure your way is any less confusing to a beginner, but it's certainly a more precise statement than my answer.PeterDonis said:This way of putting it might be confusing.
Right. However, the number you get for such a "rate of separation" is not a relative velocity. That is, it is not the velocity of the one thing in the rest frame of the other.Justin Hunt said:However, wouldn't you calculate that the spaceship and the light are not separating at the speed of light?
TheQuestionGuy14 said:So it's not really going faster than the speed of light, is it?
As far as the people in the ship are considered, there is no time dilation or length contraction. They and all parts of the ship are at rest and the flash of light is moving at speed c relative to both the front and back. The calculation is easy: the clocks at both ends of the ship are synchronized because they're at rest relative to one another; so take the time at which the light arrives at one end, subtract the time at which it was emitted at the other end and we have the flight time; the distance traveled is the length of the ship; divide one into the other and we have the speed. The numbers are the same whether the flash is traveling from front to rear or rear to front.Justin Hunt said:@jbriggs444 The part I am trying to understand is why wouldn't people in the ship wouldn't notice light traveling faster from front to back versus back to front. The only thing i can think of is that it is impossible for it not to be a round trip (impossible to do only one way experiments). This would allow length contraction to hide it.
Light is never stopped in the sense you mean. It's true that light can be trapped at the surface of a black hole, but to anyone in a position to investigate the event horizon is moving outwards at lightspeed (which is one explanation of why you can't cross the horizon outwards - you can't catch it). So light trapped at the event horizon will, indeed, pass an observer at lightspeed despite being trapped.uservt2018 said:I wonder what would happen to a photon trapped inside a black hole or somehow stopped by any kind of gravitational force.
Ibix said:Light is never stopped in the sense you mean. It's true that light can be trapped at the surface of a black hole, but to anyone in a position to investigate the event horizon is moving outwards at lightspeed (which is one explanation of why you can't cross the horizon outwards - you can't catch it). So light trapped at the event horizon will, indeed, pass an observer at lightspeed despite being trapped.
Spacetime geometry near a black hole is fun.
The speed of light in materials is variable, yes. That's been known at least as far back as Fermat, who died in 1665. It's the speed of light in vacuum that is invariant.uservt2018 said:Check lab experiments that stopped photons using fluids and crystals I think that this idea of constant speed of light is over...
You are misunderstanding what people mean when they say that the speed of light is constant. We are talking about the speed of light in a vacuum here, and no one is expecting or suggesting that the speed of light in a medium is necessarily ##c##. This qualification is so well understood that people generally leave it off.uservt2018 said:Check lab experiments that stopped photons using fluids and crystals I think that this idea of constant speed of light is over.
Again, you are misunderstanding what is meant by saying that the speed of light is constant. If a flash of light passes you, you will measure its speed to be ##c## regardless of whether you're moving or not, and regardless of whether there is a gravitational field present. The "non-constant" speed that you're thinking of is what is called a "coordinate velocity" and it has no physical significance - it's just an artifact of the way that we assign times and positions to distant events in a curved spacetime... (it's not constant when influenced by a gravitational field)
Ibix said:The speed of light in materials is variable, yes. That's been known at least as far back as Fermat, who died in 1665. It's the speed of light in vacuum that is invariant.
Edit: I have my doubts about "stopped", too. Do you have a reference for that?
This googling exercise will find many non-technical descriptions of these experiments in question, but these oversimplified descriptions are not acceptable references under the physics forums rules. There's a reason for this: If you run down the actual peer-reviewed papers describing the real physics, you will find that these popular explanations written for laypeople are incomplete to the point of being misleading (often they've glossed over the crucial distinction between phase and group velocities). In any case, they can't be used to support your argument here - you'll need to cite the peer-reviewed papers for that.uservt2018 said:I have read about an experiment of Lene Hau about this, Google more and you will find news about German scientists using crystals to completely paralize photons...
That logic works (with a bit of tweaking to phrase it more precisely) for coordinate velocities, but as I said above... we aren't talking about coordinate velocities here.By logic if it's influencing curvature it's possible to influence the speed too...
The postulate - and topic under discussion - is that the speed of light in a vacuum is constant. We know there are ways of slowing it down in media.uservt2018 said:Check lab experiments that stopped photons using fluids and crystals
Yes it is, when in a vacuum.uservt2018 said:(it's not constant when influenced by a gravitational field and when manipulated by other means)
No. It does not work like this. If opposing gravitational fields cancel out - indeed, anytime net gravity is essentially zero - it just goes straight.uservt2018 said:By logic if it's influencing curvature it's possible to influence the speed too, you can imagine a situation in which a photon traveling at constant speed reaches a point of coincidence of gravitational fields and it can't take a destination, it would be stucked in vacuum without movement because there wouldn't be a dominant force,
The speed of light is the speed at which electromagnetic radiation (such as light) travels in a vacuum. It is approximately 299,792,458 meters per second.
According to the theory of relativity, the speed of light is constant for all observers, regardless of their relative motion or velocity. This is known as the principle of constancy of the speed of light.
The constancy of the speed of light was first proposed by Albert Einstein in his theory of special relativity. This theory was based on the observation that the laws of physics should be the same for all observers, regardless of their relative motion. Experiments, such as the Michelson-Morley experiment, have also confirmed the constant speed of light.
In a vacuum, the speed of light is constant and does not change. However, it can be slowed down when passing through certain materials, such as water or glass. This is because the material's atoms and molecules can absorb and re-emit the photons of light, causing it to travel at a slower speed.
The principle of constancy of the speed of light is important because it is a fundamental principle of physics. It has implications for our understanding of space, time, and the universe. It also allows for the development of theories, such as special and general relativity, which have been crucial in advancing our understanding of the physical world.