light speed & matter

[biggs]

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

[□h=-16πT]

The correct addition of velocity law is not the Gallilean addition law: the velocity isn't simply v1+v2. The speed of light remains constant for all inertial observers.

[biggs]

The speed of light remains constant for all inertial observers

does that mean light is always constant relative to an absolute space or constant relative to a third stationary observer?? i thought there was no absolute space, that everything was relative. doesnt that mean we can figure out where the centre of the expanding universe is?

[swansont]

does that mean light is always constant relative to an absolute space or constant relative to a third stationary observer?? i thought there was no absolute space, that everything was relative. doesnt that mean we can figure out where the centre of the expanding universe is?

 

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

[Janus]

does that mean light is always constant relative to an absolute space or constant relative to a third stationary observer?? i thought there was no absolute space, that everything was relative. doesnt that mean we can figure out where the centre of the expanding universe is?

 

To add to what Swansont has already stated. The speed of light in a vacuum is always c, where c is 299,792,458 meters/sec as measured by any observer.

 

Thus say you have three observes: A; standing next to the light source, B; moving away from the light source, and C; moving towards the light source. Each has a device capable of measuring the speed of the light from the source relative to themselves.

 

They will each measure that the light has a velocity of c to relative to themselves.

[biggs]

ah thanks janus, that makes a lot of sense now

 

what about my first question?

[5614]

what about my first question?

Which question? It's all been answered!

[swansont]

relativistic velocity addition

[Spyman]

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.

Relative to someone at standstill in between, You and the object are moving apart faster than light.

 

But relative to You or the object the speed will be lower than lightspeed.

 

When measuring speed You need to know the time taken to move a distance.

 

At different speeds the time and distance are different.

 

You and a person at standstill will measure the objects speed with different result, because at half the speed of light Your clock will go much slower than the person's at standstill relative You.

 

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)?

For the same reason above, the speed of light is always c relative the observer measuring it.

 

The runner with the flashlight and a person not running will both measure the speed of light to be c.

(The runners clock will go slower relative the ground.)

 

my first thread, sorry if they're stupid questions, im 16

It's not a stupid question, in fact it was my first thread in this forum too:

 

Speed of Light and Theory of Relativity ? http://www.scienceforums.net/forums/showthread.php?t=9374

 

(And age has nothing to do with it, I am way above Your's, it's the base of knowledge thats relevant.)

 

The maximum measurable speed relative two objects is c, but the maximum measurable speed between two objects is 2c, if measured by a third observer.

 

swansont's link to 'relativistic velocity addition' is very good to learn the formulas for the calculations.

[Cashio]

here is some answers:

 

1. this is not a paradox at all. when you race someone, you face the same direction. ofcourse that in relations to each other, both objects (in your example) travel faster then light, but in relations with any asteroid traveling in the same direction, none of the object travel faster then light - and that was the initial assumption (the race).

 

2. the speed of light (in vacum) is always constant. to understand that, you must first relize that the speed of light is more a proccess then just speed. the light is a short and common name for a bigger phenomenon called electro-magnetic waves. those waves are created by a magnectic field in motion causing an elctrofield to emerge causing another magnetic field and so on and on. the speed in which those waves move is the speed of light. so, the speed of the light doesn't have anything to do with the speed of the source of the light.

 

to conclude - if you were to travel at 3/4 of the speed of light in one direction and i was going at the same speed to the oposite direction, you and i could still comunicate - any electromagnetic wave (radio or light) that we will send will still reach it's destination, because the light will be faster then any of us if pointed at the same direction.

 

does this answer your questions?

[biggs]

hi

thanks for all the answers guys, i did a lot more reading and i think i understand now

i just have some questions about the equations though

in the link swansont gave me, ( relativistic velocity addition )

it has 2 equations eg. u = (v+u`)/(1+((vu`)c^2)) and the other one

could someone explain what the letters represent (eg what is u`)? the site gives vague explanations (eg u` = velocity of projectile seen by b), but is there a more universal explanation so i could apply it to any other situation? what is the velocity of the projectile seen by b (in the diagram in link)?

thanks a lot

[Spyman]

u = (v+u')/(1+((vu')/c^2)) and u' = (u-v)/(1-((uv)/c^2))

 

Observer at point A measures the projectiles speed to be u

 

Observer at point B measures the projectiles speed to be u'

 

Both observers measures their relative speed to be v

 

The fomula is for transforming what the other observer would measure from his/hers view with the knowledge of Your own view.

[swansont]

They're defined in the link.

 

v is the speed of frame B with respect to A. u is the velocity as seen by A, and u' is the velocity as seen by B.

[Daecon]

The faster you go, the slower time passes - so from your perspective things would still be measured at 300'000km per second - however fast you're going.

 

For instance, when people are travelling at light speed, time has essentially stopped for them, therefore all the photons (if they measured them) will still be going at "the speed of light".

 

I'm pretty sure that's how it works.

[JED3]

Actually most of you have it wrong here. Saying my clock goes slower than yours if I am traveling faster than you is silly...think about it. (A person flying by Earth at 3/4 the speed of light in a rocket, for example.) From his view in the rocket, Earth is traveling past him at 3/4 C. Speed is relative. It only matters if you "leave" Earth and her time frame, travel from "her" at a high relative rate of speed, and return to her again. In this example, you exist now at a younger age than all people you knew when you left. They might even be dead and gone many years. It is not your "speed' that caused this because, again, your speed to Earth was the same as Earth's speed to you. Speed is relative....completely. What caused you to slow in time was your "Acceleration" Or simply put, your "change" in speed There is a company called "The Teaching Company. They sell DVD courses on every main college subject. The courses are not credited, though they are given by the best professors in the world. Get "Einstein's Relativity and the Quantum Revolution," Professor Richard Wolfson. It is awesome! It answers all these questions and more.

Good Luck to all.

John jed3

[Baby Astronaut]

Actually most of you have it wrong here. Saying my clock goes slower than yours if I am traveling faster than you is silly...think about it. (A person flying by Earth at 3/4 the speed of light in a rocket, for example.) From his view in the rocket, Earth is traveling past him at 3/4 C. Speed is relative. It only matters if you "leave" Earth and her time frame, travel from "her" at a high relative rate of speed, and return to her again. In this example, you exist now at a younger age than all people you knew when you left. They might even be dead and gone many years. It is not your "speed' that caused this because, again, your speed to Earth was the same as Earth's speed to you. Speed is relative....completely. What caused you to slow in time was your "Acceleration" Or simply put, your "change" in speed There is a company called "The Teaching Company. They sell DVD courses on every main college subject. The courses are not credited, though they are given by the best professors in the world. Get "Einstein's Relativity and the Quantum Revolution," Professor Richard Wolfson. It is awesome! It answers all these questions and more.

Good Luck to all.

John jed3

That seems wrong.

 

Earth shouldn't have been at 3/4 c as if it were also speeding past you, because of a simple flaw in that logic: as far as the Milky Way and the universe are concerned, the Earth hadn't moved any faster. It's only you who are moving at 3/4 c.

 

The laws governing things don't care if your eyes seemed to view it the other way around. If you were speeding past a mountain, it would seem that it approached you and quickly passed, but to someone on the other side of the mountain, or to someone on the moon observing the mountain, the mountain's speed never changed. But yours did.

 

And while acceleration is a factor in time change, it doesn't matter if you had sped up on Earth or in a galaxy far, far away -- approaching c will make your time go slower than others everywhere who are in a less speedy frame of reference.

 

I've encountered your type of claim before on some media that explained relativity weirdness or some such, and partly based on what I know now, just don't buy into that particular aspect.

[insane_alien]

baby astronaut, you are assuming a prefered reference frame. such a thing does not exist.

[swansont]

Actually most of you have it wrong here. Saying my clock goes slower than yours if I am traveling faster than you is silly...think about it.

 

Silly, perhaps, but it's also how nature behaves. If you are moving relative to me, I will observe your clock to run slow.

 

That seems wrong.

 

Earth shouldn't have been at 3/4 c as if it were also speeding past you, because of a simple flaw in that logic: as far as the Milky Way and the universe are concerned, the Earth hadn't moved any faster. It's only you who are moving at 3/4 c.

 

The laws governing things don't care if your eyes seemed to view it the other way around. If you were speeding past a mountain, it would seem that it approached you and quickly passed, but to someone on the other side of the mountain, or to someone on the moon observing the mountain, the mountain's speed never changed. But yours did.

 

All you can say is the person and the mountain were moving relative to each other. You can't say which one is moving — the laws of physics work for both of them, and do not have a preference for either one. As insane_alien has stated, there is no preferred frame.

 

I've encountered your type of claim before on some media that explained relativity weirdness or some such, and partly based on what I know now, just don't buy into that particular aspect.

 

Doesn't matter if you "buy into" it or not. That's not the metric of validity.

Edited September 20, 2008 by swansont
multiple post merged

[Baby Astronaut]

All you can say is the person and the mountain were moving relative to each other. You can't say which one is moving — the laws of physics work for both of them, and do not have a preference for either one. As insane_alien has stated, there is no preferred frame.

I'm not sure if we're talking about the same thing. Do you mean that a clock on the mountain will also move slower because the traveler moved past it at a higher speed? Basically, the mountain is going at teh same higher speed in relation the quick traveler, thus no time change for either because their clocks are equally slowed?

 

My response was based on what I thought JED3's post implied, that time wouldn't move slower for the faster object if both were in the same "time frame" (whatever that means), because the stationary object would actually be moving at the traveler's speed in comparison to the traveler.

 

I fail to see how that's possible. First, atomic clocks which are both on Earth have shown different times when one is stationary and the other has moved quickly. Second, can the determine the boundary of a time frame even be determined?

 

 

Doesn't matter if you "buy into" it or not. That's not the metric of validity.

Certainly true.*

 

*Except for the Prez's gut, it's 100% accurate so scientific validation of its assumptions is totally unnecessary.

[swansont]

I'm not sure if we're talking about the same thing. Do you mean that a clock on the mountain will also move slower because the traveler moved past it at a higher speed? Basically, the mountain is going at teh same higher speed in relation the quick traveler, thus no time change for either because their clocks are equally slowed?

 

My response was based on what I thought JED3's post implied, that time wouldn't move slower for the faster object if both were in the same "time frame" (whatever that means), because the stationary object would actually be moving at the traveler's speed in comparison to the traveler.

 

I fail to see how that's possible. First, atomic clocks which are both on Earth have shown different times when one is stationary and the other has moved quickly. Second, can the determine the boundary of a time frame even be determined?

 

 

Then let's define our scenario more carefully. In inertial frames, observers not moving with respect to each other, at the same altitude, will see clocks running at the same rate.

 

If one observer (the traveller, T) is moving with respect to another (the mountain, M) each will see the other's clock running slow. In this situation, a third observer will not see T and M as stationary with respect to each other — they will be in different frames.

 

At "time frame" is the same as an inertial frame at a given gravitational potential (altitude). If they are not at rest with respect to each other, they are not in the same frame.

[Baby Astronaut]

Thanks, I get it now.

 

So, what JED3 said isn't really like what I had seen described before. In that explanation, I think they said when you move towards something, you aren't really moving your own body, but instead you're moving everything towards you. Is that even a real scientific view?

[swansont]

So, what JED3 said isn't really like what I had seen described before. In that explanation, I think they said when you move towards something, you aren't really moving your own body, but instead you're moving everything towards you. Is that even a real scientific view?

 

From a physics standpoint they are equivalent. Which is damn convenient sometimes, because you can often choose a frame of reference where the math is easiest.

[Baby Astronaut]

I appreciate how the math is portable between the traveler and the stationary object, but isn't that a virtual effect rather than a real world case?

 

One example to support this is the difference of how much work it takes to move your body towards a mountain, compared to how much work it takes to move the mountain towards you.

[Thales]

It brings up one of the fundamental philosophical cornerstones of scientific inquiry. That being does anything exist without an observer to observe it and perhaps even more fundamentally what is the definition of an observer. In your above example you do not simply move the mountain towards you, you need to move the entire earth, so as not to displace the mountain from its surrounds. That may seem illogical and inconceivable yet if you place your mind as the stationary point in the frame of reference you are are operating then the entire universe rotates around it, from the earth and the mountain to your limbs and muscles. Your chosen inertial frame of reference deems everything moving relative to you chosen zero point, your brain. It is an equally valid mathematical representation of reality as is choosing the mountain as your 'zero point'. It depends on what you are trying calculate and which frame involves less number crunching. For most basic mechanics problems using the earth as a frame of reference is easiest.

[swansont]

I appreciate how the math is portable between the traveler and the stationary object, but isn't that a virtual effect rather than a real world case?

 

One example to support this is the difference of how much work it takes to move your body towards a mountain, compared to how much work it takes to move the mountain towards you.

 

That involves an acceleration, and you can always tell who accelerated. It's easier, work-wise, to accelerate an individual than a large vehicle (like a car or train) containing an individual, but we often do that anyway.

 

In inertial frames, with no memory of what was accelerated, there is no difference.