# Yet another (specific) speed of light question...

Page 2 - Seeking answers about space? Join the Space community: the premier source of space exploration, innovation, and astronomy news, chronicling (and celebrating) humanity's ongoing expansion across the final frontier.
Status
Not open for further replies.
H

#### heyo

##### Guest
From what I understand of reading Brian Green, and reading and pondering general relativity, the following passage should hold true.<br /><br />First off, lets say I stand at x distance from you, and you throw me a baseball at 60mph. If I stand in that spot the baseball will approach me at 60mph relative to my position. However, if I start at x distance from you, and I run backwards away from you at 20mph, if you then throw the same 60mph ball it will be approaching at 40mph relative to me.<br /><br />Not so with light, if we stand in the same spot, a fixed position relative to each other, and you turn on your flashlight, the end of the beam will approach me at C.<br /><br />However, here's the kicker, if I run backwards away from you at 1/4 C, the end of the beam will still approach me at speed C from my perspective.<br /><br />From a bystander's perspective the beam closes in on me at <C, but since my personal time is slowed because of my relative motion, that exactly makes up for it from my perspective, or it should, per Einstein's calculations<br /><br />I would see the light as redshifted, but it would still approach me at C.<br /><br />Is this premise correct?<br /><br />Heyo<br /><br />

X

#### xmo1

##### Guest
E=MC^2. Ha HA matter and energy are one and the same thing, and can be converted to each other. I was right. I was right! Show me the math. Show me the math! Whoo Hoo. <div class="Discussion_UserSignature"> <p>DenniSys.com</p> </div>

X

#### xmo1

##### Guest
So when entropy occurs the strong force of electromagnetism and the week force of gravity are both released or unbound and everything evaporates into atoms or elementary particles. <div class="Discussion_UserSignature"> <p>DenniSys.com</p> </div>

N

#### nojocujo

##### Guest
Ahh<br />I now know what FTL means. I really don't think inflation meant FTL........ It only appears to be FTL since spacetime didn't enjoy a homogeneous deflation. I.E. pre BB spacetime became convoluted in upon itself. Matter has an affinity for its' own spacetime coordinates. Spacetime inflated FTL matter retained its' own coordinate system relative to spacetime and therefore did not move FTL. PRE BB and post BB the relative coordinate systems relative to spacetime and matter remains relatively the same. <br /><br />If you blew up a balloon and marked it with dots and then crushed it while still retaining its relationshio to itself (don't tear it or poke a hole or whatever) and then blew it back up the points that had been marked retained their respective relationships during each transformation. Nothing really changed in respect to the dots you drew on the balloon. <br />I think this is what inflation is! Correct me if I'm wrong but I don't think spacetime has the limiter of FTL.

K

#### kmarinas86

##### Guest
http://www.anzwers.org/free/universe/redshift.html<br /><br /><font color="yellow">3) Comoving Distance - DC <br />The Comoving Distance is the distance scale that expands with the universe. It tells us where the galaxies are now even though our view of the distant universe is when it was much younger and smaller. On this scale the very edge of the visible universe is now about <b>47 billion light years</b> from us although the most distant galaxies visible in the Hubble Space Telescope will now be about 32 billion light years from us. <br /><br />Comoving Distance is the opposite of the Angular Diameter Distance - it tells us where galaxies are now rather than where they were when they emitted the light that we now see.</font><br /><br /><font color="yellow">(4) Light Travel Time Distance - DLT <br />The Light Travel Time Distance represents the time taken for the light from distant galaxies to reach us. This is what is meant when it is said that the visible universe has a radius of <b>14 billion light years</b> - it is simply a statement that the universe is about 14 billion years old and the light from more distant sources has not had time to reach us. <br /><br />Light Travel Time Distance is as much a measure of time as a measure of distance. It is useful mainly because it tells us how old the view of the galaxy is that we are seeing.</font><br /><br />47 billion light years / 14 billion light years = 3.357 <br />47 billion light years / 14 billion = 3.357 c

A

#### abhinavkumar_iitr05

##### Guest
Hello Heyo<br /><br />In reply to<br /><br />So now lets say the whole setup is cruising through space at .75 C, in the laser-beam-pointed-forward direction. You hit your beam switch, and you do your calculations based on the times that the detectors went off, would your calculations still tell you that the end of the beam passed through the detectors at speed C?<br /><br />Yes the answer is that you will again find the speed to be equal to that of c.This is because in the reference of the scientist the flow of time will be slowed.The new flow of time is given by Einstein's Special Theory of Relativity.Thus u will actually measure the speed to be equal to c.<br /><br />For more clear concept,let's solve the following problem<br /><br />Let the length of the bar be equal to L when the bar was still.Speed of light be equal to c in this frame.Therefore the time taken by the light to travel L is given by (L/c).<br /><br />Now let's consider the case when the bar was moving with speed .75c<br />In this case length contraction will take place which is given by relativity theory.At the same time the flow of time will be slowed also given by relativity theory.The increase & decrease is in the same ratio say Y=(1-v^2/c^2)^0.5<br />Now when calculating the speed the ratio cancels each other giving the speed equal to c.All this takes place in the moving frame wrt to the bar i.e. for the observer outside.For the observer inside there will be no change & the speed of light comes out to be equal to c.<br /><br />This is the reason for having the speed of light constant in all the reference frame.<br />Also what I would like to mention here is that <br /><br />Relative speed=(v+u)/(1+vu/c^2)<br /><br />for all the speed.

S

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>I would see the light as redshifted, but it would still approach me at C.<p><hr /></p></p></blockquote><br />Surely you can have no means of measuring this, just relying on mathematics.<br />If it was a ball, that's visible before it's arrival, you'd calculate the second differential of the hyperbolic tangent of your angle of gaze but light isn't visible until it's arrival, precluding any measurement <img src="/images/icons/smile.gif" /><br /><br />

Status
Not open for further replies.