light speed & matter

[Tsadi]

hi i have 2 questions

 

1) if an object is traveling at more than half the speed of light, and i am traveling in the opposite direction on the same 1d plane at more than half the speed of light, then relative to me, wouldnt the object be traveling faster than the speed of light? i thought nothing could travel faster than light.

 

2) whats the speed of light like? is it relative to the source? like if i am running and i shine a light, will that light be speed of light + running speed, (when measured from the still ground i am running on)?

 

ps. is this thread in the right section? my first thread, sorry if they're stupid questions, im 16

 

1) No. You see, the speed of light is the same for all observers. You are simply adding velocities, and that cannot be done with relativity to allow some violation of speed. If it was, a great many things in the universe would be violating relativity.

 

2) If you run with with a torch, your velocity is not added to light speed. You might think, that if you where moving at the speed of light, holding a mirror, you would be moving so fast, the light could not strike the mirror, and you would not see your reflection. Truth be told, again, the speed of light is the same for all observers, so a little thought would indicate that light would still strike the mirror, as bizarre as this might sound.

[Baby Astronaut]

If you run with with a torch, your velocity is not added to light speed. You might think, that if you where moving at the speed of light, holding a mirror, you would be moving so fast, the light could not strike the mirror, and you would not see your reflection. Truth be told, again, the speed of light is the same for all observers, so a little thought would indicate that light would still strike the mirror, as bizarre as this might sound.

It's really not that bizarre at all. Very simple in fact.

 

Since time has slowed down for you, but not for light, then when you are moving at the speed of light, time is at a standstill for you, but not for light which therefore still hits the mirror at c speed.

 

If you were going at half the speed of light, just your time would slow down to half, thus light reaches the mirror at c speed.

 

Is that correct?

[Mr Skeptic]

The speed of light is constant to all the observers, not just to some frame of reference.

 

What if the observer is accelerating? Will he measure a different speed of light?

[Baby Astronaut]

What if the observer is accelerating? Will he measure a different speed of light?

Based off what I've learned here, I'm going to say no.

 

I'm not sure if my last post (above yours) is correct in principle, but that's my opinion of how light maintains a constant speed in all frames of reference. In other words, between different frames the math problem readjusts so that light speed always measures c. For example, let's say c were 20, and your speed through both space and time is 10. If you increased your space speed to 15, then your speed through time would adjust to 5. If you increased your space speed to 17, then your speed through time accordingly drops to 3.* In any case, light emitted by you is still c -- you might've increased your space speed, but it takes you that much longer to catch up to the light, whose experience of time doesn't change, while for us it does.

 

I could be wrong on that aspect, but won't know for sure until someone knowledgeable responds to that post.

 

*Math is oversimplified and possibly not accurate.

Edited September 29, 2008 by Baby Astronaut

[Mr Skeptic]

Hm, my question was badly phrased. There could be a distinction depending on whether the acceleration is homogeneous or inhomogeneous. For example, falling through a gravitational potential, versus being in a spaceship with thrusters engaged. I'd expect that the speed of light would be measured to be constant in free fall, but not in an accelerating spaceship (where the instruments would be accelerating but not the light).

[swansont]

What if the observer is accelerating? Will he measure a different speed of light?

 

Yes, he can. That's one way you can tell you're in an accelerating frame of reference, and can be seen in the Sagnac effect (a rotating frame)

 

Based off what I've learned here, I'm going to say no.

 

I'm not sure if my last post (above yours) is correct in principle, but that's my opinion of how light maintains a constant speed in all frames of reference. In other words, between different frames the math problem readjusts so that light speed always measures c. For example, let's say c were 20, and your speed through both space and time is 10. If you increased your space speed to 15, then your speed through time would adjust to 5. If you increased your space speed to 17, then your speed through time accordingly drops to 3.* In any case, light emitted by you is still c -- you might've increased your space speed, but it takes you that much longer to catch up to the light, whose experience of time doesn't change, while for us it does.

 

I could be wrong on that aspect, but won't know for sure until someone knowledgeable responds to that post.

 

*Math is oversimplified and possibly not accurate.

 

The velocity four-vector is the invariant quantity, so the numbers get combined in quadrature, but the idea is basically right. If someone observes you to be moving through space, your motion through time is also affected to make your velocity vector through spacetime remain at c.