light speed thought experiment

[triskaidekaphile]

for better coverage in this thought experiment i first need to ask some questions

 

relative to planet earth, would a spaceship moving at 50% the speed of light have it's time moving half as fast compared to us? if not then what speed would it take for time to be halved?

 

does radiation, such as a photon, move in a wave like the books show it? or if you were theoretically able to see it, and were looking from the side, would it bounce up and down like the books show or would it sometimes seem like a flat line as it bounces at you and away at times?

 

if we were to shoot a light beam in front of our planet and another straight behind us relative to our motion around the sun, and were to measure the speed of the light, would it be the same speed?

 

when these answers are varified then i'll proceed with the actual experiment. i'd appreciate any contributions.

[swansont]

for better coverage in this thought experiment i first need to ask some questions

 

relative to planet earth, would a spaceship moving at 50% the speed of light have it's time moving half as fast compared to us? if not then what speed would it take for time to be halved?

 

No, the factor by which the clock rate changes is [math]\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

 

does radiation, such as a photon, move in a wave like the books show it? or if you were theoretically able to see it, and were looking from the side, would it bounce up and down like the books show or would it sometimes seem like a flat line as it bounces at you and away at times?

 

The picture you see of oscillations is of the electric and magnetic field strength, It's not the trajectory of a photon.

 

if we were to shoot a light beam in front of our planet and another straight behind us relative to our motion around the sun, and were to measure the speed of the light, would it be the same speed?

 

No, since we are not in an inertial frame. We orbit the sun, so we are in an accelerating frame. It's not a big effect, though.

[md65536]

if not then what speed would it take for time to be halved?

About .866 c

 

No, since we are not in an inertial frame. We orbit the sun, so we are in an accelerating frame. It's not a big effect, though.

??? The speed of light is invariably c, for all observers, regardless of relative motion.

There's nothing you can do to change that.

Acceleration won't change the speed of light. What effect are you speaking of?

 

 

[swansont]

??? The speed of light is invariably c, for all observers, regardless of relative motion.

There's nothing you can do to change that.

Acceleration won't change the speed of light. What effect are you speaking of?

 

c is a constant in an inertial frame. If you are accelerating you are not in an inertial frame, and you will not measure the speed of light to be c. Acceleration due to rotation gives rise to the Sagnac effect.

[md65536]

c is a constant in an inertial frame. If you are accelerating you are not in an inertial frame, and you will not measure the speed of light to be c. Acceleration due to rotation gives rise to the Sagnac effect.

In the context of the original post, is the Sagnac effect equivalent to saying that...

1. Rotation of the reference frame will cause the paths of light (aimed in different directions) to curve in different ways.

2. There is no possible common path (geodesic) that the light can travel in opposite directions, from the rotating reference point.

3. Light in opposite directions travels a different distance in each direction in a given time, due to the differently curved paths.

?

 

 

 

For the purpose of the thought experiment, it might suffice to imagine an inertial frame that approximates the Earth's motion through a portion of its orbit.

Edited May 12, 2011 by md65536

[swansont]

I think you could look at the light traveling around a circumference at the Schwarszchild radius to see this s a geodesic, but it's usually considered in terms of any closed path for light, with mirrors or a fiber. If you measured the speed of light by sending it along the orbital path of the earth, you would get results of c+v and c-v under the assumption of an inertial frame. But that's one way to see that you aren't in an inertial frame.

[triskaidekaphile]

in his book ''The Elegant Universe'' Greene said that all objects are moving at light speed (i.e. relative to a frame of reference the sum of an object's speed through space and time equals the speed of light). is this an oversimplification? because if it's right then i dont understand how an object can be moving .866 c and still move at half lightspeed through time.

 

The picture you see of oscillations is of the electric and magnetic field strength, It's not the trajectory of a photon.

what is the trejectory like then?

 

If you measured the speed of light by sending it along the orbital path of the earth, you would get results of c+v and c-v under the assumption of an inertial frame.

 

does this mean that light can be sped up by gravitation or just appears to speed up?

 

if a spaceship in an inertial frame were to shoot out a beam/pulse of light in front and backward and was moving at a steady speed, would the two beams be moving at the same speed away from the ship?

[md65536]

because if it's right then i dont understand how an object can be moving .866 c and still move at half lightspeed through time.

If for example it's a rocket moving at .866 c relative to "our inertial reference frame", its velocity is .866 c according to us.

 

No velocity is halved in this example. The Lorentz factor is 2.0, which means that according to us, a clock on the rocket ticks at a half rate.

 

However, lengths in the rocket's frame are also contracted by the same factor of 2.0, so both distance and time in the rocket's frame are scaled by the same amount, and the rocket's velocity = d/t remains unchanged at 0.866c despite time dilation and length contraction.

 

Lengths in the rocket's frame (including distance to it) are halved, and its time passes half as quickly, according to us.

 

what is the trejectory like then?

 

if a spaceship in an inertial frame were to shoot out a beam/pulse of light in front and backward and was moving at a steady speed, would the two beams be moving at the same speed away from the ship?

If we're in an inertial frame, its trajectory follows the curvature of space, which is a straight line in the absence of a gravitational field (essentially straight in this example. Einstein believed we'd never be able to detect spacetime curvature in our local weak gravity fields so let's say it's negligible).

 

Yes, the speed of light of the beams would be c in both directions, relative to us.

(They'd also both be c according to the rocket, or any other inertial frame.)

Edited May 16, 2011 by md65536

[swansont]

in his book ''The Elegant Universe'' Greene said that all objects are moving at light speed (i.e. relative to a frame of reference the sum of an object's speed through space and time equals the speed of light). is this an oversimplification? because if it's right then i dont understand how an object can be moving .866 c and still move at half lightspeed through time.

 

He's referring to the velocity four-vector. The spacetime interval four-vector is comprised of the three spatial dimensions and the fourth being ct, such that [math]s^2 = x^2+y^2+y^2-c^2t^2[/math]

It's an invariant — it has the same value in any inertial frame.

 

The velocity is the derivative of that and has a length of c. If the object is moving then a spatial velocity contribution is present and its clock will run slow by an equal amount to maintain the length at c. The numbers might not be intuitive because the Lorentz transform is not linear, and these are vector components.

[IM Egdall]

The following link gives a picture of how everything travels at the speed of light through spacetime:

 

 

 

[triskaidekaphile]

sorry that its taken a while for me to respond.

 

for now i guess i'll have to learn a lot more to understand how speed is shared between space and time. but i can continue with the thought experiment.

 

in an area devoid of significant gravitational disturbances, lets say that we are in a spaceship that to us is motionless,. then a space ship passes in front of us traveling at half light speed, at the exact moment they pass in front of us they flash a short burst of light directly forward and a short flash directly behind themselves, now, the light moving forward would be moving at a speed that i dont know exactly, i would have thought half lightspeed compared to us but now i'm not sure, anyhow, it stands to reason (for me at least) that the light moving backward would have to be moving at the same speed relative to the moving ship as the front flash of light, and if it was half light speed then the speed of the light would be canceled by the speed of the source ship and therefore look like a motionless peice of light.

 

i'm sure i'm missing something but i don't know what.

 

and thank you for your time.

 

and thanks to I ME for the video link

Edited June 1, 2011 by triskaidekaphile

[Spyman]

There is no absolute space to measure speed against. Motion is measured relative the observers frame of reference.

 

The special principle of relativity states that physical laws should be the same in every inertial frame of reference, but that they may vary across non-inertial ones. This principle is used in both Newtonian mechanics and the theory of special relativity. Its influence in the latter is so strong that Max Planck named the theory after the principle.

 

The principle requires physical laws to be the same for any body moving at constant velocity as they are for a body at rest. A consequence is that an observer in an inertial reference frame cannot determine an absolute speed or direction of travel in space, and may only speak of speed or direction relative to some other object.

http://en.wikipedia.org/wiki/Principle_of_relativity

 

 

It doesn't matter which spaceship emitts the lightbeams since observers in both the spaceships consider themselves to be the one standing still. If the "moving" spaceship is intersecting perpendicular to the front view of the "standstill" spaceship, then observers in the "moving" spaceship see the "standstill" spaceship moving sidewise towards their front and then passing by towards their rear direction, with half the speed of light.

 

 

Observers in both spaceships measures the two lightbeams to propegate with full lightspeed in their frame.

 

Einstein postulated that the speed of light in free space is the same for all observers, regardless of their motion relative to the light source.

http://en.wikipedia.org/wiki/Introduction_to_special_relativity

 

 

The "standstill" spaceship measures the other spaceship to pass with half lightspeed chasing the "forward" lightbeam with full lightspeed which makes them calculate a differenting speed of half lightspeed between the "forward" lightbeam and the other spaceship in the direction of movement. In the other direction they measure the "backward" lightbeam to recede with full lightspeed and calculate a speed differance of one and a half lightspeed between the other spaceship and the "backward" lightbeam.

 

The "moving" spaceship measures the other spaceship to pass with half lightspeed chasing the "backward" lightbeam with full lightspeed which makes them calculate a differenting speed of half lightspeed between the "backward" lightbeam and the other spaceship in the direction of movement. In the other direction they measure the "forward" lightbeam to recede with full lightspeed and calculate a speed differance of one and a half lightspeed between the other spaceship and the "forward" lightbeam.

 

 

It seems like an impossibility but the trick is that since the two spaceships are moving relative each other, they measure distance and time differently and therefore also ends up with different values for observed speed.

 

 

Observers in both spaceships measures the clock in the other spaceship to tick to slow compared to their own.

 

Time dilation is an observed difference of elapsed time between two observers which are moving relative to each other, or being differently situated from nearby gravitational masses. An observer will see the other observer's clock ticking at a slower rate than his/hers. This effect doesn't arise from technical aspects of the clock or the fact that any signal needs time to propagate, but from the nature of space-time described by theory of relativity.

http://en.wikipedia.org/wiki/Time_dilation

 

 

Observers in both spaceships measures the other spaceship to be shorter in direction of movement than their own.

 

In physics, length contraction - according to Hendrik Lorentz - is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer. This contraction (more formally called Lorentz contraction or Lorentz-Fitzgerald contraction) is usually only noticeable at a substantial fraction of the speed of light; the contraction is only in the direction parallel to the direction in which the observed body is travelling.

http://en.wikipedia.org/wiki/Length_contraction

Edited June 1, 2011 by Spyman

[SSDS]

There is no absolute space to measure speed against. Motion is measured relative the observers frame of reference...

 

(and so on)

 

That isn't so, in principle absolute speed can be measured - see http://arxiv.org/abs/0706.3979

(and http://arxiv.org/abs/0812.2819 , till (including) section 4.2.2)

 

Some comments additionally.

 

First of all - there isn't, in certain sense, in realty such a thing as "reference frame" except to "absolute frame". All other "frames" exist only as systems where components are concatenated by some forces; i.e. - constitute "a wagon" . "Free bodies" don't belong to the "frame".

 

So, e.g. for a couple of satellites moving at the same orbit and having practically the same (orbital) speed (and so, of course, belonging to the same "reference frame") the speed of this frame can be measured without reference to any other body, e.g. - Earth; only by using the instruments in the satellites.

 

Though to measure the "absolute speed" is impossible in this case since the satellites, Sun, Earth, etc. constituted the "wagon" under gravity force.

 

But for "free" satellites - e.g. somewhere in space where gravity is weak comparing the forces that moved the satellites in this space two-point, the absolute speed of the satellites can be measured.

More - see the links.

 

Cheers

 

P.S. The physical model in the links above is grounded on the informational conception ( http://arxiv.org/abs/1004.3712) which sometime seems as not too real. But now the conception got some unexpected publicty - Google shows more then 100 000 links answering on

 

"You’re living in a computer simulation, and math proves it" .

 

Though the "sensation" is a full rubbish, it resembles in certain sense the informational conception.

[mooeypoo]

!

Moderator Note

SSDS, this is a mainstream physics thread, requiring mainstream physics answers. Do not hijack the thread with speculative subjects.
If you want, you're welcome to open your own thread in the speculation forum and discuss why the "absolute frame" is the next best answer to this current question.