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# A few questions about the Physics of light. watch

1. I've been trying to find answers to these questions which relate to the recent potential discovery of a particle which moves faster than light. (That whole neutrino thing).

I'm no physisist, but I know a fair bit about physics, but my knowledge on the physics of light is fairly small.

So, if someone could answer these questions, I'd be appreciative.

1. If an object traveling near the speed of light, fires another object traveling near the speed of light, the combined speed of the second object would never exceed the speed of light to an observer, correct? In otherwards, an observer from object A, would see object B moving at near the speed of light. But to an outside observer, seeing them pass by, would object B be seen moving at near the speed of light, or at the speed of light?

2. The other question is whether its possible to see an object hit another object before it actually hits it.

In the example given above: If object A is heading towards a planet, and fires object B at that planet, from the point of view of object A, the planet would be moving towards it at near the speed of light, and object B would be moving towards it at near the speed of light.

So assume this happens at about 600, 000, 000 m away (twice the distance light travels in a second). Object B would hit the planet after 2 seconds. Because relative to object A, thats when it would appear to hit.

However, from the point of view of the planet, the combined speeds of object A and B would not exceed the speed of light, meaning they would apparantly impact the planet at the same time.

Is this the case?

Thanks

EDIT:

Also, the speed of light is relative to the observer. What if there is no observer.

And surely, although you cannot observe anything traveling faster than light, you can infer a faster speed.

Or is it simply because of the time slowing down effect. So, (in the case mentioned before) object A only sees object B moving at the speed of light (or near it) because from its perspective, time is slower. Or something like that.
2. (Original post by W-Three)
I've been trying to find answers to these questions which relate to the recent potential discovery of a particle which moves faster than light. (That whole neutrino thing).

I'm no physisist, but I know a fair bit about physics, but my knowledge on the physics of light is fairly small.

So, if someone could answer these questions, I'd be appreciative.

1. If an object traveling near the speed of light, fires another object traveling near the speed of light, the combined speed of the second object would never exceed the speed of light to an observer, correct? In otherwards, an observer from object A, would see object B moving at near the speed of light. But to an outside observer, seeing them pass by, would object B be seen moving at near the speed of light, or at the speed of light?

2. The other question is whether its possible to see an object hit another object before it actually hits it.

In the example given above: If object A is heading towards a planet, and fires object B at that planet, from the point of view of object A, the planet would be moving towards it at near the speed of light, and object B would be moving towards it at near the speed of light.

So assume this happens at about 600, 000, 000 m away (twice the distance light travels in a second). Object B would hit the planet after 2 seconds. Because relative to object A, thats when it would appear to hit.

However, from the point of view of the planet, the combined speeds of object A and B would not exceed the speed of light, meaning they would apparantly impact the planet at the same time.

Is this the case?

Thanks

EDIT:

3) Also, the speed of light is relative to the observer. What if there is no observer.

And surely, although you cannot observe anything traveling faster than light, you can infer a faster speed.

Or is it simply because of the time slowing down effect. So, (in the case mentioned before) object A only sees object B moving at the speed of light (or near it) because from its perspective, time is slower. Or something like that.
1) c is not relative to the observer, it is a constant.
2) No. The event cannot occur after it has been observed.
3) see 1)
3. (Original post by Darkphilosopher)
1) c is not relative to the observer, it is a constant.
2) No. The event cannot occur after it has been observed.
3) see 1)
1) So in the case of an object moving near the speed of light firing something near the speed of light, surely the second object is moving twice the speed of light?

I gather that its not, or at least, not considered to be. But why not?
4. (Original post by Darkphilosopher)
1) c is not relative to the observer, it is a constant.
2) No. The event cannot occur after it has been observed.
3) see 1)
No, c is constant relative to the observer.
5. (Original post by W-Three)
I've been trying to find answers to these questions which relate to the recent potential discovery of a particle which moves faster than light. (That whole neutrino thing).

I'm no physisist, but I know a fair bit about physics, but my knowledge on the physics of light is fairly small.

So, if someone could answer these questions, I'd be appreciative.

1. If an object traveling near the speed of light, fires another object traveling near the speed of light, the combined speed of the second object would never exceed the speed of light to an observer, correct? In otherwards, an observer from object A, would see object B moving at near the speed of light. But to an outside observer, seeing them pass by, would object B be seen moving at near the speed of light, or at the speed of light?

2. The other question is whether its possible to see an object hit another object before it actually hits it.

In the example given above: If object A is heading towards a planet, and fires object B at that planet, from the point of view of object A, the planet would be moving towards it at near the speed of light, and object B would be moving towards it at near the speed of light.

So assume this happens at about 600, 000, 000 m away (twice the distance light travels in a second). Object B would hit the planet after 2 seconds. Because relative to object A, thats when it would appear to hit.

However, from the point of view of the planet, the combined speeds of object A and B would not exceed the speed of light, meaning they would apparantly impact the planet at the same time.

Is this the case?

Thanks

EDIT:

Also, the speed of light is relative to the observer. What if there is no observer.

And surely, although you cannot observe anything traveling faster than light, you can infer a faster speed.

Or is it simply because of the time slowing down effect. So, (in the case mentioned before) object A only sees object B moving at the speed of light (or near it) because from its perspective, time is slower. Or something like that.
I'll try to answer these questions as best I can, although it's been a while since I've done any special relativity:

1) Object B will be moving at nearer the speed of light to an independent observer which sees object A moving at near the speed of light. However, to A, B will be moving at near the speed of light.

2) One of the classic 'paradoxes' of SR (special relativity) is that simultaneity is frame-dependent; ie A can happen before B to one observer but B can happen before A to another observer. It's a bit complicated but see here.

3) (this is philosophical but i'll say what i think) Newton's first law can be interpreted to state that an inertial frame of reference exists. Observers have their own frames of reference. So the (/my) philosophy is that 'something always observes'.

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