# Interesting thought experiment.

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i was home schooled until i was 16 and did not really get a chance to learn any of this stuff, keep that in mind if you feel this subject is childs play.

before we start, lets do a warm up.

imagine a 10 ft long pole being spun in circles with a motor at a constant speed.

the part of the pole that is closest to the center of the circle is traveling less distance then the part that is further away from the center.

speed is calculated by the time it takes for an object to travel a distance.

so does this mean that the part of the pole which is further away from the center is traveling faster then the part that is closest to the center?

there for, the part that is furthest away from the center, has more mass then the part that is closest to the center?

now, if our planet was larger and had more mass, it would then have a stronger gravitational pull right?

so if our planet were to start orbiting the sun at a faster speed, this would mean that our planet would gain mass, in return, we would feel a stronger gravitational pull?

now, does this all mean that a small piece of mass can become equivalent to a larger piece of mass if the smaller travels fast enough?

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here is were this gets a little deep and i will have explain this in a form of a thought experiment.

here is what i am truly confused about, no solid matter can travel faster then light right?

so imagine in your mind the earth and the sun, make them both stationary.

now imagine there is 2 observers, one observer is the size of a normal human, imagine that observer is just floating in space, above earth, looking towards the sun.

now imagine the second observer is huge.so large that the earth would fit snugly in his palm.

(remember, speed is measured by the time and distance a object travels relative to a observer)

so, the distance between the earth and sun is vastly further away from each other from the smaller observers perspective when compared to the larger observer perspective.

now imagine yourself as the larger observer, swing your arm from the sun to the earth as fast as you can.

how long did it seem to take for your hand to travel the distance from the sun to earth from the larger perspective?

(in my mind, it took like a split second)

now, imagine all this again, except this time, imagine it from the smaller perspective, how long did it take for the larger perspective hand to cover the distance between the sun and earth?

this is where i get confused at, google says it takes about 8 mins for the light from the sun to reach earth. See the problem here?

if you are watching the the large observers hand swing from the small observers perspective, it would be impossible for the large observers hand to cover the distance between the sun and earth in the split second that it took when you observed as the large observer because that would mean his hand traveled faster then light.

what do you guys think happens?

the only thing i can think of is that, from the small perspective, the large hand would have to be moving in slow motion, really, really slowly.This means that the length

of time that elapsed for both observers is not the same.example, (and only a example) in only 1 second passing from the large guys perspective, the small guy experienced weeks passing in that very same time frame.

its like time itself will have to adjust its self so that the speed of light will remain constant from all perspectives.amazing stuff.

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so does this mean that the part of the pole which is further away from the center is traveling faster then the part that is closest to the center?

Yes. You can take a small pieces of the pole and look at their linear velocity and confirm this.

there for, the part that is furthest away from the center, has more mass then the part that is closest to the center?

No, unless you are trying to think about relativistic mass - so don't and just think in terms of rest mass of uniform pieces of your pole.

now, if our planet was larger and had more mass, it would then have a stronger gravitational pull right?

Yes, for a given distance (and by pull you mean force per unit mass on a test object). Just use Newton's gravity to see this.

so if our planet were to start orbiting the sun at a faster speed, this would mean that our planet would gain mass, in return, we would feel a stronger gravitational pull?

Nope. Forget relativistic mass in this context.

now, does this all mean that a small piece of mass can become equivalent to a larger piece of mass if the smaller travels fast enough?

Travels faster with respect to?

here is what i am truly confused about, no solid matter can travel faster then light right?

Correct, no massive particles or object can have a relativistic velocity equal to or greater than the speed of light.

so, the distance between the earth and sun is vastly further away from each other from the smaller observers perspective when compared to the larger observer perspective.

But if the two observers are stationary relative to each other then they will agree on the distance between the Earth and the Sun. You need to give more information.

now imagine yourself as the larger observer, swing your arm from the sun to the earth as fast as you can.

how long did it seem to take for your hand to travel the distance from the sun to earth from the larger perspective?

(in my mind, it took like a split second)

You would need to actually specify this to make any conclusions.

now, imagine all this again, except this time, imagine it from the smaller perspective, how long did it take for the larger perspective hand to cover the distance between the sun and earth?

You need to combine the effects of time dilation here. I don't know where the two observers are exactly and if they are moving relative to each other.

this is where i get confused at, google says it takes about 8 mins for the light from the sun to reach earth.

That sounds right.

See the problem here?

Not yet... you have not formulated the problem carefully enough.

if you are watching the the large observers hand swing from the small observers perspective, it would be impossible for the large observers hand to cover the distance between the sun and earth in the split second that it took when you observed as the large observer because that would mean his hand traveled faster then light.

what do you guys think happens?

This is the no rigid body statement in special relativity. (If we ignore gravity for now)

P.S. Didn't someone else ask exactly the same question before?

Edited by ajb
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Two things here:

First the easy one: Being large or small makes no difference to how fast things move and so seems to be irrelevant.

The other one is more complicated. Yes, if you swing a long stick (or arm) then the end will move faster. You might think that if you double the length (and so double the length of the circle) then the end of the stick is moving twice as fast. That is close enough for small velocities.

But, as you say, nothing can move faster than light. So what happens is that the speed does not increase according to that simple formula (off the top of my head, I'm not sure how you do work out the speed). This was, I think, one of the examples that led Einstein to realise that space-time must be curved.

Hopefully, someone else can give a more detailed explanation.

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Hopefully, someone else can give a more detailed explanation.

The question is related to Ehrenfest paradox, which comes from a spinning disk.

The question is also related to simpler questions of rigid poles in special relativity, which will be much easier to discuss.

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Two things here:

First the easy one: Being large or small makes no difference to how fast things move and so seems to be irrelevant.

The other one is more complicated. Yes, if you swing a long stick (or arm) then the end will move faster. You might think that if you double the length (and so double the length of the circle) then the end of the stick is moving twice as fast. That is close enough for small velocities.

But, as you say, nothing can move faster than light. So what happens is that the speed does not increase according to that simple formula (off the top of my head, I'm not sure how you do work out the speed). This was, I think, one of the examples that led Einstein to realise that space-time must be curved.

Hopefully, someone else can give a more detailed explanation.

the reason why i used a small observer and a large observer is to show the differences from each ones own perspective, nothing to do about there speed, more of the difference between both of there perception of time and how they experience time differently essentially due to there mass/size.

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the reason why i used a small observer and a large observer is to show the differences from each ones own perspective, nothing to do about there speed, more of the difference between both of there perception of time and how they experience time differently essentially due to there mass/size.

This does not seem to be tied with special or general relativity. You seem to be talking about psychological perceprtion of time. In physics we like to use clocks and rulers in order to remove this personal perception of time and distance.

Edited by ajb
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Yes. You can take a small pieces of the pole and look at their linear velocity and confirm this.

No, unless you are trying to think about relativistic mass - so don't and just think in terms of rest mass of uniform pieces of your pole.

Yes, for a given distance (and by pull you mean force per unit mass on a test object). Just use Newton's gravity to see this.

Nope. Forget relativistic mass in this context.

Travels faster with respect to?

Correct, no massive particles or object can have a relativistic velocity equal to or greater than the speed of light.

But if the two observers are stationary relative to each other then they will agree on the distance between the Earth and the Sun. You need to give more information.

You would need to actually specify this to make any conclusions.

You need to combine the effects of time dilation here. I don't know where the two observers are exactly and if they are moving relative to each other.

That sounds right.

Not yet... you have not formulated the problem carefully enough.

This is the no rigid body statement in special relativity. (If we ignore gravity for now)

P.S. Didn't someone else ask exactly the same question before?

"Travels faster with respect to?"

each other.

"But if the two observers are stationary relative to each other then they will agree on the distance between the Earth and the Sun. You need to give more information.

"

no matter what they agree on, 10 feet from a giant persons perspective is more then the 10 feet from a small persons perspective.

there is no way for light particals to know that they both agreed on a distance...

i think some of you guys are missing my points o.0

it simply does not matter if its possible or not to float in space,

it does not matter if its not possible or not to be so huge.

its the principle that matters.

thats why its a thought experiment..

if u dont like notion of a large observer and a small observer, then by all means changes it to how ever u see fit, it doesnt matter.

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But, as you say, nothing can move faster than light. So what happens is that the speed does not increase according to that simple formula (off the top of my head, I'm not sure how you do work out the speed). This was, I think, one of the examples that led Einstein to realise that space-time must be curved.

I would also be interested if someone could flesh that out a little,especially the "curved space" bit. Is this one of Einstein's thought experiments?

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"Travels faster with respect to?"

each other.

I don't understand.

no matter what they agree on, 10 feet from a giant persons perspective is more then the 10 feet from a small persons perspective.

Again, I don't understand. 10 feet is 10 feet... that is why we pick universally agreed on units.

there is no way for light particals to know that they both agreed on a distance...

i think some of you guys are missing my points o.0

Maybe...

it simply does not matter if its possible or not to float in space,

it does not matter if its not possible or not to be so huge.

its the principle that matters.

What principle?

It seem you have described a personal perception of time and distance ... this is no good for physics. I think you are misunderstanding the notion of observers and so on.

if u dont like notion of a large observer and a small observer, then by all means changes it to how ever u see fit, it doesnt matter.

I am not sure what to change it to. However, the closest thing I can think of is the question of a rotating disk or the simpler rigid pole.

I would also be interested if someone could flesh that out a little,especially the "curved space" bit. Is this one of Einstein's thought experiments?

I think that Einstein used the spinning disk and not an elevator in is 1916 paper on the equivalence principle.

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This does not seem to be tied with special or general relativity. You seem to be talking about psychological perceprtion of time. In physics we like to use clocks and rulers in order to remove this personal perception of time and distance.

do you really not get it?

i made it as simple as possible.

have you visualized what i was talking about?

time dilates itself.....

when einstien said he visualized himself riding a particle of light, everything had to be slowed down, do you relize why that is?

the faster one travels=the more mass it has=the more energy it has=the more time dilates.

mc2, equation in German-born physicist Albert Einstein’s theory ofspecial relativity that shows that the increased relativistic mass (m) of a body comes from the energy of motion of the body—that is, itskinetic energy (E)—divided by the speed of light squared (c2). This equation expresses the fact that mass and energy are the same physical entity and can be changed into each other.

each other.

I don't understand.

Again, I don't understand. 10 feet is 10 feet... that is why we pick universally agreed on units.

Maybe...

it does not matter if its not possible or not to be so huge.

its the principle that matters.

What principle?

It seem you have described a personal perception of time and distance ... this is no good for physics. I think you are misunderstanding the notion of observers and so on.

I am not sure what to change it to. However, the closest thing I can think of is the question of a rotating disk or the simpler rigid pole.

I think that Einstein used the spinning disk and not an elevator in is 1916 paper on the equivalence principle.

you my friend, are lost, seems like the only ones who can hold this type of convo with me are the well educated.not trying to be rude, but it seems like you "think" you understand relativity, while in reality, that notion is flying way over your head, its nothing to be botherd about though, i totally understand why alot of people have misconceptions when it comes to this, its a very tricky subject to tackle, even some of the brightest graduates have difficulties understanding the universe as it truly is.

but i would love for you to prove me wrong, here is what i propose, take your understanding of relativity, and explain it as a thought experiment, create a scenerio that will show how well you understand something, just as i did.make me understand.

Edited by xxsolarxx
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the reason why i used a small observer and a large observer is to show the differences from each ones own perspective, nothing to do about there speed, more of the difference between both of there perception of time and how they experience time differently essentially due to there mass/size.

That just changes perceptions and, perhaps, the units used. So your giant alien might say it is 3 goobers per moolifrox, while we describe it as 1 million miles per hour (or 447 km/s or 2.7 billion furlongs per fortnight). It doesn't change the physics.

I would also be interested if someone could flesh that out a little,especially the "curved space" bit. Is this one of Einstein's thought experiments?

And for the OP, here: https://simple.wikipedia.org/wiki/Rigidly_rotating_disk_paradox

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you my friend, are lost, seems like the only ones who can hold this type of convo with me are the well educated.not trying to be rude, but it seems like you "think" you understand relativity, while in reality, that notion is flying way over your head, its nothing to be botherd about though, i totally understand why alot of people have misconceptions when it comes to this, its a very tricky subject to tackle, even some of the brightest graduates have difficulties understanding the universe as it truly is.

!

Moderator Note

This kind of behavior is unacceptable. Insulting our professors using your home-schooled ignorance is against our civility rules, it's arrogant beyond comprehension, and makes you look like a complete ass to everyone ajb has helped in his long tenure as a Resident Expert here at SFN. Feel free to attack his ideas, but refrain from senseless ad hominem.

It's no wonder why your reputation is in the negative after so short a time here. You are one rude person.

Decide if you want to stay and abide by the rules you agreed to when you joined. Don't reply to this modnote in thread, but feel free to Report it if you disagree with it.

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no matter what they agree on, 10 feet from a giant persons perspective is more then the 10 feet from a small persons perspective.

Nope.

Not unless you think that the foot is defined by the length of a person's foot ...

Edited by Strange
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now, if our planet was larger and had more mass, it would then have a stronger gravitational pull right?

Maybe, maybe not. The surface gravity is given by GM/R^2. So if you change both M and R it could increase, it could decrease, it could stay the same. If R^2 increases faster than M, the surface gravity decreases.

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Maybe, maybe not. The surface gravity is given by GM/R^2. So if you change both M and R it could increase, it could decrease, it could stay the same. If R^2 increases faster than M, the surface gravity decreases.

Examples being Saturn at 95 times the mass & 9.5 times the radius and Uranus at 14.5 times the mass & 4 times the radius of the Earth, both with a surface gravity less than that of the Earth's.

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...

when einstien said he visualized himself riding a particle of light, everything had to be slowed down, do you relize why that is?

the faster one travels=the more mass it has=the more energy it has=the more time dilates.

...

No.

It's about the constancy of the speed of light.

Say you consider yourself as "standing still" and you see someone you consider to be moving at 100 km/h towards a light that's shining at both of you.

Both of you must measure that light as moving at the same speed. That only works because while light speed is fixed, time and distance are the things that wiggle.

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I can only dream of being as 'lost' as AJB.

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I can only dream of being as 'lost' as AJB.

So lost that he can agree on the measurement of something…even with someone twice, or half, his size...

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time dilates itself.....

This does not make sense at all.

Time dilation occurs when two observers compare the duration of some event(s) that they have witnessed. I do not understand what it means for 'time to dilate itself'.

when einstien said he visualized himself riding a particle of light, everything had to be slowed down, do you relize why that is?

Okay, I have read such things before... I am not sure if he actually wrote that in a paper.

the faster one travels=the more mass it has=the more energy it has=the more time dilates.

Again, time dilation comes into play when different observers compare their clocks. You also have to be careful with 'travelling faster' - you need to say with respect to what. But okay, loosley 'moving clock run slow'.

you my friend, are lost, seems like the only ones who can hold this type of convo with me are the well educated.

Are you sure?

not trying to be rude, but it seems like you "think" you understand relativity,

I would not claim to be an expert in relativity, but I know the basics. I also know that the way you are stating things makes it difficult to translate then into meaningful phsyics.

As I said before, your notion of a 'large' and 'small' observers seems confusing. You are also mixing things like psychology perception of durations and distances with what clock and rulers measure. The closets thing I can think of to your situation is swinging a large rod or similar. You can then ask about the rod travelling faster than the speed of light as seen by the person swinging the rod.

but i would love for you to prove me wrong,

Prove you wrong on what? You first need to make clear statements that we can discuss within physics.

here is what i propose, take your understanding of relativity, and explain it as a thought experiment, create a scenerio that will show how well you understand something, just as i did.make me understand.

Maybe we can think of a simple thought experiment, one that does not involve rotation - rotation always makes it more complicated in special relativity.

Imagine I am sat somewhere on the Earth and that I have a pole that is just under 1AU in length. I line it up with the Sun and poke the Sun; a bit like a game of cosmic billiads! Suppose at exactly the same time as I move my hand to poke the Sun I also send out a photon along the pole. Will my 'poke' reach the Sun before, after or at the same time as the photon I sent?

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