Fe gravity as it relates to the speed of light

Should this thread really be stickied, it doesnt seem like it should be in my opinion, possibly a modified locked thread in which the ideas discussed herein are updated, but this one seems much too long and varied

Should this thread really be stickied, it doesnt seem like it should be in my opinion, possibly a modified locked thread in which the ideas discussed herein are updated, but this one seems much too long and varied

Originally I had locked the thread, and I'm probably going to relock it fairly soon.  For a while I've felt that a concise and complete answer to the whole speed-of-light thing is needed, since it represented at least a third of the questions we were being asked.  Since this thread has been stickied, I haven't heard the question asked at all.

My current plan is to wait a bit longer, and then lock the thread and split the discussion bits.  The resulting discussion will not be stickied or locked.

Quote from: "Erasmus"

So what about the Earth's velocity w.r.t. the CMBR?

WTR?

"W.r.t" or "wrt", not "WTR".  It stands for "with respect to", and mathematicians use it a lot because it's annoying to write "with respect to" over and over again.

Quote from: "troubadour"

Quote from: "Erasmus"

So what about the Earth's velocity w.r.t. the CMBR?

WTR?

"W.r.t" or "wrt", not "WTR".  It stands for "with respect to", and mathematicians use it a lot because it's annoying to write "with respect to" over and over again.

Sorry, i'm not a mathematician. What about the earth's velocity with respect to the CMBR?

Sorry, i'm not a mathematician.

Sorry to have been presumptuous.

What about the earth's velocity with respect to the CMBR?

I'm just curious as to what it is.

Quote from: "troubadour"

Sorry, i'm not a mathematician.

Sorry to have been presumptuous.

Quote

What about the earth's velocity with respect to the CMBR?

I'm just curious as to what it is.

I actually did not know the absolute value, only that it was moving. A quick google search yielded a result from the University of Tennessee Physics site, from an Astronomy 162 course. (http://csep10.phys.utk.edu/astr162/lect/cosmology/cbr.html)

The indication of the above image is that the local group of galaxies, to which the Earth belongs, is moving at about 600 km/s with respect to the background radiation. It is not know why the Earth is moving with such a high velocity relative to the background radiation.

There were some diagrams, but they were in flash format so I can't put them in this post. Check out the page.

A thought about the Cosmic Microwave Backround Radiation: general relativity permits us to interpret the asymmetry in the spectrum of this radiation as being due to a uniform gravitational field permeating the entire universe (not to be confused with a local gravitational field emitted by the Earth).

This is exactly how general relativity resolves the so-called "twin paradox".  Time is dilated differently for the moving twin because she is momentarily subject to a uniform gravitational field.

A theoretical physicist at CERN gave me an insight into this, from a completely different query about massless particles.

As an object approaches the speed of light, its mass increases. Therefore a force acting on it will have to increase to continue the acceleration. The formula for the extra mass of an object in motion is given by some constant divided by the speed of light less the speed at which the object is moving. This is why nothing can exceed the speed of light, as its mass will reach an infinite value, and it will require an infinite force to maintain any acceleration.

I asked about the Doppler effect, and objects moving away from each other, and he said that there is a datum point which all velocities can be measured with regard to. This is evidenced by the fact that some objects are moving away from the earth at relative speeds approaching that of light, but we can see from their effects on the objects around them, moving at lower speeds, that they do not have the gravitational properties of bodies with near-infinite mass.

In other words, from Bob's perspective, the Earth is always stationary (we are Bob), but undergoing constant acceleration.  From Alice's perspective, the Earth moves at an ever increasing rate, but the acceleration is not constant -- it decreases over time in such a way that the Earth never surpasses the speed of light.

How does the accelleration decrease without us noticing?
If the accelleration decreased, the gravity of earth would decrease as well, wouldn't it?

How does the accelleration decrease without us noticing?

It's not decelerating relative to us. Earth is only approaching the speed of light relative to someone not on its surface; consequently, Earth's acceleration only changes to someone outside of Earth's Frame of reference.

EP, please don't confuse him the way you totally confused me (for 12 pages!). What the Earth does relative to us is irrelevant, the Earth and all objects at rest on it may be considered a single object for the purposes of this scenario.
I believe what you meant to say is that, while the Earth's acceleration is decreasing relative to an object moving very slowly, say 10m/s, it's acceleration is NOT decreasing relative to an object moving very quickly, i.e., near lightspeed. If we feel 9.8m/s/s worth of simulated gravity, then when we measure our acceleration relative to an object moving at a constant velocity that has the same velocity as the Earth at the moment the measurement is made, the acceleration relative to that object will be 9.8m/s/s, even though our acceleration may be much lower relative to an object with a much smaller velocity.

EP, please don't confuse him the way you totally confused me (for 12 pages!). What the Earth does relative to us is irrelevant.

Well, what it does relative to us is relevant in the sense that it does nothing relative to us. That is, it doesn't move.

I believe what you meant to say is that, while the Earth's acceleration is decreasing relative to an object moving very slowly, say 10m/s, it's acceleration is NOT decreasing relative to an object moving very quickly, i.e., near lightspeed.

I think that's pretty much exactly what I said.

Has this ever been scientifically proven though? The way I see it, the theory is based on our inertia in relation to a constantly accelerating earth. Surely this acceleration would have to be with respect to an inertial reference frame?

If you can provide me with a link to a reputable site telling me this is possible, I'll take your word for it, and leave you alone!!

Here: http://www.physicsforums.com/showthread.php?t=129832

Here: http://www.physicsforums.com/showthread.php?t=129832

Hm, I wish I had noticed this when it was first brought up!  I'm confused as to exactly why the mods locked that thread but I'm sure they had their reasons.

Okay, I just happened to be reading this forum out of curiosity, because I was looking up "flat earthers" on the internet.  I was reading on creationism, which led me to geocentrists and flat-earthers also.  I was shocked enough to learn that flat earthers and geocentrists even exist(creationists are surprising enough.)  
 
In any event, I was even more shocked to see someone attack a flat earther with a flawed argument, and a flat earther defend his position with a valid response(even if it was not articulated well enough for the attacker to understand.)  I simply couldn't resist trying to clarify the issue.  

The position was that a flat platform could be continuously accelerating at a constant rate to simulate a gravitation of 9.8 m/s2 indefinilely.  This is ideally possible, even if it is logistically unlikely.  Someone incorrectly pointed out that this would lead to the light barrier being breached.  This is not true.  I am a physics graduate student, with a Bachelor of Science in Physics Reserch, so I hope I will be able to articulate the issue.  

First of all, there is an apparent contradiction in language, because people have referred to a person on the platform as being accelerating constantly in his own frame.  Technically, if his frame of reference is the platform, he is always at rest.  However, at any given instant, an inertial(nonaccelerating) observer who is at rest with respect to the platform at that instant will see him as accelerating at 9.8 m/s2.  

Suppose that I hold a golf ball over the edge of the platform.  While the ball is in my hand, it is not an inertial observer, because it is accelerating up with me and the platform.  Now, if I let it go, my and the platform's acceleration will no longer be transmitted to the golf ball, and hence it will stop accelerating up with me, and become an inertial observer.  I will percieve the ball to begin accelerating downward at 9.8 m/s2, and it will see me as accelerating upward at the same rate.  At the very instant that I let it go, it will be at rest with respect to me, but it will no longer be accelerating with me.  

However, if I look down at the golf ball with a telescope, and continue to watch it for a long enough time, I will observe that it will not continue to accelerate at 9.8 m/s2.  How could it?  If a falling body in the earth's gravitational field accelerated constantly indefinitely, it would exceed the light barrier.  Similarly, the golf ball will not see my platform as accelerating at a constant rate, but as slowly approaching the constant speed of light with ever decreasing acceleration.  

Now, is my acceleration really changing? NO.  What is changing is my relative speed with respect to the inertial observer in question.  If I now drop a second golf ball, this golf ball will initially see me as accelerating at 9.8 m/s2, just as the first one originally did.  Thus, the two inertial observers will disagree about my acceleration.  Because, at any given instant, a 'recently dropped' golf ball will see me as accelerating at 9.8 m/s2, I will always experience a force equivalent to the earth's gravity.  

I will never exceed the light barrier from the perspective of another observer, but I will continue to 'accelerate  constantly' indefinitely.  The apparent contradiction is a result of attempting to apply Newtonian(in which acceleration is invariant) thinking to a relativistic situation.  

As far as the so called professor who contradicted this view, I can only assume that the professor did not fully understand the question.  The person who posted the professor's response must not have phrased the question to the professor in quite the same way as it was claimed here.

(I hope that makes it esier to read.  Sorry about the lack of spaces on the original post, I usually do not post mesages so long.)

Could you maybe put some paragraph breaks in your post so that its legibility coefficient takes non a nonnegative value?  Thanks.

As far as the so called professor who contradicted this view, I can only assume that the professor did not fully understand the question.  The person who posted the professor's response must not have phrased the question to the professor in quite the same way as it was claimed here.

That was my interpretation as well. While your demonstration of the point is eloquent and concise, could you please put in some line breaks to aid in legibility?

That was my interpretation as well. While your demonstration of the point is eloquent and concise, could you please put in some line breaks to aid in legibility?

Wow, ten points for out-diplomacying me.  Yeah, sorry if I was harsh -- you (mrscience) do raise some good points but I was finding it hard to get through your whole post.

Okay, I just happened to be reading this forum out of curiosity, because I was looking up "flat earthers" on the internet.  I was reading on creationism, which led me to geocentrists and flat-earthers also.  I was shocked enough to learn that flat earthers and geocentrists even exist(creationists are surprising enough.)  

In any event, I was even more shocked to see someone attack a flat earther with a flawed argument, and a flat earther defend his position with a valid response(even if it was not articulated well enough for the attacker to understand.)  I simply couldn't resist trying to clarify the issue.  

The position was that a flat platform could be continuously accelerating at a constant rate to simulate a gravitation of 9.8 m/s2 indefinilely.  This is ideally possible, even if it is logistically unlikely.  Someone incorrectly pointed out that this would lead to the light barrier being breached.  This is not true.  I am a physics graduate student, with a Bachelor of Science in Physics Reserch, so I hope I will be able to articulate the issue.  

First of all, there is an apparent contradiction in language, because people have referred to a person on the platform as being accelerating constantly in his own frame.  Technically, if his frame of reference is the platform, he is always at rest.  However, at any given instant, an inertial(nonaccelerating) observer who is at rest with respect to the platform at that instant will see him as accelerating at 9.8 m/s2.  

Suppose that I hold a golf ball over the edge of the platform.  While the ball is in my hand, it is not an inertial observer, because it is accelerating up with me and the platform.  Now, if I let it go, my and the platform's acceleration will no longer be transmitted to the golf ball, and hence it will stop accelerating up with me, and become an inertial observer.  I will percieve the ball to begin accelerating downward at 9.8 m/s2, and it will see me as accelerating upward at the same rate.  At the very instant that I let it go, it will be at rest with respect to me, but it will no longer be accelerating with me.  However, if I look down at the golf ball with a telescope, and continue to watch it for a long enough time, I will observe that it will not continue to accelerate at 9.8 m/s2.  How could it?  If a falling body in the earth's gravitational field accelerated constantly indefinitely, it would exceed the light barrier.  Similarly, the golf ball will not see my platform as accelerating at a constant rate, but as slowly approaching the constant speed of light with ever decreasing acceleration.  Now, is my acceleration really changing? NO.  What is changing is my relative speed with respect to the inertial observer in question.  

If I now drop a second golf ball, this golf ball will initially see me as accelerating at 9.8 m/s2, just as the first one originally did.  Thus, the two inertial observers will disagree about my acceleration.  Because, at any given instant, a 'recently dropped' golf ball will see me as accelerating at 9.8 m/s2, I will always experience a force equivalent to the earth's gravity.  I will never exceed the light barrier from the perspective of another observer, but I will continue to 'accelerate  constantly' indefinitely.  

The apparent contradiction is a result of attempting to apply Newtonian(in which acceleration is invariant) thinking to a relativistic situation.  As far as the so called professor who contradicted this view, I can only assume that the professor did not fully understand the question.  The person who posted the professor's response must not have phrased the question to the professor in quite the same way as it was claimed here.

---

Not my post, just reposted with paragraphs so people can read it better

(I hope that makes it esier to read.  Sorry about the lack of spaces on the original post, I usually do not post mesages so long.)

Hey no worries.  Now that I've read the whole thing I agree that your exposition of the situation is pretty clear.

Do you have any input on the CMBR issue?  It was claimed that the CMBR is a preferred frame of reference, and so we can determine our absolute velocity and acceleration by observing it.  I disagree; I do not  think that you can just pick any reference frame and call it preferred.

Do you have any input on the CMBR issue?  It was claimed that the CMBR is a preferred frame of reference, and so we can determine our absolute velocity and acceleration by observing it.  I disagree; I do not  think that you can just pick any reference frame and call it preferred.

Yes, I do.  You are correct that there is no absolute frame of motion.  The reason why cosmic backround radiation was brought up is because of a misunderstanding in how it is used in the literature.

The universe is not like the earth in that it is not an object of constant volume, and a clear, definable path of motion.  Ordinarily, when determining the age of an object, (such as the earth), we use the object's 'proper frame of reference', that is, the frame of motion in which that object is considered at rest.  (Because motion affects how we measure time).

In the case of the universe, this presents a problem.  The universe is expanding, with different galaxies moving in different directions, at different speeds.  So what do we do?  Physicists who want to express the age of the universe use CMBR as a way of defining an agreed upon reference frame from which to judge the age of the universe.  

The point is, however, that the laws of physics DO NOT recognize any frame of motion as prefered. CMBR is something that HUMANS have chosen as a convienient sort of 'cosmic clock'.  I hope that this clarifies the issue.

The point is, however, that the laws of physics DO NOT recognize any frame of motion as prefered. CMBR is something that HUMANS have chosen as a convienient sort of 'cosmic clock'.  I hope that this clarifies the issue.

This really does clarify the issue; thanks.  I agree that the CMBR is useful for descriptions of the universe on cosmological scales.

Hi.  ....

Where to start?

How do you get  v = g / sqrt(1/t^2 + g^2/c^2)  from gt / sqrt( 1 + g^2t^2 / c^2 )?

Starting with gt / sqrt( 1 + g^2t^2 / c^2 )
I can see simplifying to gt / sqrt( 1/t^2 + (g^2/ c^2) * t^2 )
But when you try to factor out the T  you get:
g / (sqrt( 1/t^2 + (g^2/ c^2) * t^2 ))/T  

You can not simplify the t^2 by saying Sqrt (t^2)/T = 1 because the value (g^2/ c^2) * t^2 ) is added to another value within the square root function.  Try substitutingg some real numbers fo T and you should see what I mean.

Likewise, you remove the time factor from your description of relative motion.  Acceleration is change of velocity over time.  To use your description of why you don't exceed light speed, Starting velocity is zero, final velocity is zero.  There is no acceleration, therefore no gravity like effect.  

Another way of looking at it is the acceleration is an increase of kinetic energy.   By accelerating the entire system, you are increasing the kinetic energy in the system.  So the comparative kinetic energy is not increased because the earth and the object on it are increasing kinetic energy at the same rate, due to your acceleration.  Inertia causes object not directly acted upon by your "Dark Energy" to experience the gravitational affect.

They absorb some of the energy, and if acceleration were to stop they would equalize and no longer experience gravity.  But their kinetic energy would stay the same.  If you accelerate again, you start to add to the kinetic energy again.   In a relativistic sense the speed of light barrier sets a maximum Kinetic energy that an object can be imparted with.  

Relativity also shows that as you approach C, the amount of energy needed to increase kinetic energy in an object increases.  The faster you go, the less linear.  Theoretically you would reach a point where a person on the earth could not lift anything because you would be so close to C that a couple of fps increase of velocity would require extraordinary energy.


Enough for now, I'll try to compose my other objections in the next few days.

You don't get drunk smoking weed.

edit: don't I look stupid when I comment on a troll and then his post is deleted

You don't get drunk smoking weed.

Maybe you can't.  :lol:

Hmm... I wonder if you could ferment weed... thus making 'dopeahol' that you could get drunk AND high off of at the same time.

Problem... it would still be illegal.

Solution! Ferment tobacco to make 'Nicahol' It would probably taste like crap, but it would be very addictive...

If you use natural tobacco you grow yourself, it probably won't be half as addicting without the massive amount of nicotene that the corporations put in their cigarettes.

Anyway, BACK ON TOPIC!  (Not that I have anything to add to it either.)

Theoretically you would reach a point where a person on the earth could not lift anything because you would be so close to C that a couple of fps increase of velocity would require extraordinary energy.

Kinetic energy is relative. From our frame of reference we will never reach a point where a person can't lift an object because the energy required to lift it will never change.



Edit: trying to get my signature to work

How do you get  v = g / sqrt(1/t^2 + g^2/c^2)  from gt / sqrt( 1 + g^2t^2 / c^2 )?

Algebra.  Pretty straightforward:


             g t            1/t                  g                       g
v = --------------------  * --- =  -------------------------- = ---------------------
     sqrt( 1 + g²t²/c² )    1/t     1/t * sqrt( 1 + g²t²/c² )    sqrt( 1/t² + g²/c² )


The first = is justified by the fact that I multiplied the right-hand side by (1/t) / (1/t), which is 1.  The second = is justified by cancelling out t and 1/t in the numerator.  The third = is justified by the fact that a sqrt(b) = sqrt(a²b), q.e.d.

Likewise, you remove the time factor from your description of relative motion.

No I don't.  That's what the "t" is for.

Another way of looking at it is the acceleration is an increase of kinetic energy.

What EP said.  Don't forget that this analysis is done from the perspective of an inertial observer.  To him, the Earth's speed increases asymptotically, as does its kinetic energy.

*edit* math alignment fixed