If there is no end to the space

[steevey]

Why and how do you think "stuff" would be moving away from each other if "space" is not expanding?

 

The fabric of "space", as far as evidence shows, is a purely mathematical representation. The only reason why we have that is so we can more easily see the patterns of how gravity acts. That's not to say that that's how the universe actually works. It isn't even that space would be expanding, it's that the position of matter is changing in a way that much matter is moving away from much other matter.

 

Somehow, although all evidences show it is a correct statement, logic shouts to me that it must be wrong.

 

 

 

 

Indeed: we don't know that.

 

From time to time I am surprised from your statements Spyman, and I really don't understand why we occasionaly disagree. I suspect I read in your text something that you didn't ment to write.

 

Actually, we know they are correct because that's how they are defined. An inch is defined as a certain amount of distance, therefore in something containing a multiple of that singular distance, all the same integers of an inch are equivalent. It's true there can be an infinitesimal way to measure distance, but mathematics accurately describes observable things using units of distance.

 

But with space, even if it doesn't exist, the properties of distance still apply in classical mechanics.

Edited February 2, 2011 by steevey

[michel123456]

(...) but mathematics accurately describes observable things using units of distance. (...)

 

Mathematics do, but physics don't. Physics need distance to expand in order to explain observation.

[Spyman]

Yes, Spyman, the difficulty in understanding is mutual, but I do understand your two scenarios above.

...

I accept that observation supports a visible cosmos which is isotropic and homgeneous.

If the expanding balloon model of cosmos is true, this does not mean that we can see beyond the "thickness" of "rubber" within our local part of the balloon and locate the center of the balloon (locus of the big bang.) Of course we can not. As you implied, even in the first example the rubber of the balloon membrane will have thickness.

("...in the first example the swarm would be spread out like a hollow shell of some thickness,...")

 

It is entirely possible that our cosmic event horizon is embedded wiithin the thickness of material expanding outward omnidirectionally from the bang.

It seems that you either don't undertand the balloon analogy or have changed it to fit your world view, in mainstream science there is no space inside or outside of the rubber, instead all dimensions of space are on the surface, like a hypersphere.

 

But more importantly you missed my point or at least you don't meantion your view of it clearly, the point is that in the first example we would be able to determine from observation that we and our neighbours are moving outward from a center, even if we are in the middle of the membrane and can't see any edges.

 

Can you explain why we wouldn't be able to determine the direction towards the centre in the first example?

 

 

As to the subjective idealism "embedded" in relativity theory, which I previously mentioned (that distance varies with observational perspective), here is a thought experiment for you.

 

If there were no intelligent life in the cosmos, we can probably agree that cosmos would not vanish simply because we were not here to observe it. (Yes?) So, the distance between, for instance, earth and sun would remain as is and sunlight would still take over eight seconds to reach earth. (Yes?)

 

Likewise rods of "equal length" would stay equal no matter how they traveled in relation to each other. Again, with no "observer" to invoke relativity, signal delay, and frame of reference difference, the rods themselves would stay the same length. Do you agree?

 

Same for time as for space and distance: Things would move around on their own and all events would happen without our observation and measurement. An earth rotation and orbit would be the same duration without our clocks disagreeing because of inertial differences in their frames of references... as we observe now, calling it "time dilation."

 

Do you see what I'm getting at here? I do not agree with the subjective idealism implicitly embedded in relativity theory.

Relativity don't need any observers, it is not about optical illusions, equal rods would physically differ in size if one was moving fast, accelerating hard or deep down in a gravity well, totally independently to if someone was watching or not. General Relativity has replaced Newtonian physics and we explain gravity with the dynamical geometry of spacetime, were the curvature of spacetime depends on placement of matter and matter moves as curvature dictates.

 

Like you say: "Things would move around on their own and all events would happen without our observation and measurement." Earth don't stop orbiting the Sun if nobody aren't looking, Earths orbit is not an illusion. The curvature of spacetime exists whether we measure it or not, greater mass have steeper curvature than lesser mass and if we would add or remove mass from a body then the spacetime curvature around it would change likewise.

 

The frame of reference is a real physical 'backdrop' which includes the scale of the metric and the rate of clocks ticks. A fast moving rod will not only look shorter to us, it will also fit inside a shorter box while it passes and if it is subjected to some wear it will be preserved for a longer duration.

 

For example, heavy ions that are spherical when at rest, should assume the form of "pancakes" or flat disks when traveling nearly at the speed of light. And in fact, the results obtained from particle collisions can only be explained, when the increased nucleon density due to Lorentz contraction is considered.

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

 

A comparison of muon lifetimes at different speeds is possible. In the laboratory, slow muons are produced, and in the atmosphere very fast moving muons are introduced by cosmic rays. Taking the muon lifetime at rest as the laboratory value of 2.22 μs, the lifetime of a cosmic ray produced muon traveling at 98% of the speed of light is about five times longer, in agreement with observations.

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

 

 

I will also explain my philosophical difference with the asuumptions implicit in relativity theory.

...

Do you see what I'm getting at here? I do not agree with the subjective idealism implicitly embedded in relativity theory.

I am not an expert on Relativity but as I interpret what you are saying, you seem to think that the effects of Relativity are some kind of optical illusions and from my understanding that is wrong. You can not cherry pick half of a theory and leave out the other parts. Maybe you need to learn more of Relativity and its features before you make any final conclusions and then make a new judgement whether you accept or reject it when you have reached a better understanding.

 

IMHO, I maintain what I said in my previously post, without accepting that distances and durations actually are physically altered in different frames as Relativity predicts, you are in essence rejecting it.

[michel123456]

(...) equal rods would physically differ in size if one was moving fast, accelerating hard or deep down in a gravity well, totally independently to if someone was watching or not. (...)

 

Let's take 3 bodies A B C

 

_C moves away from A at 0,9 c.

_C moves away from B at 0,1 c

 

Observator 1 positioned on A looks at C and measures time dilation T1 & length contraction L1 for body C.

 

Observator 2 positioned on B looks at C and measures time dilation T2 & length contraction L2 for body C

 

Observator 3 positioned on C measures time dilation T3 & length contraction L3 for himself body C, where T3 & L3 equal to zero.

 

Which of the 3 observators is right? What is "really" happening?

Edited February 2, 2011 by michel123456

[owl]

It seems that you either don't undertand the balloon analogy or have changed it to fit your world view, in mainstream science there is no space inside or outside of the rubber, instead all dimensions of space are on the surface, like a hypersphere.

 

But more importantly you missed my point or at least you don't meantion your view of it clearly, the point is that in the first example we would be able to determine from observation that we and our neighbours are moving outward from a center, even if we are in the middle of the membrane and can't see any edges.

 

Can you explain why we wouldn't be able to determine the direction towards the centre in the first example?

 

 

 

Relativity don't need any observers, it is not about optical illusions, equal rods would physically differ in size if one was moving fast, accelerating hard or deep down in a gravity well, totally independently to if someone was watching or not. General Relativity has replaced Newtonian physics and we explain gravity with the dynamical geometry of spacetime, were the curvature of spacetime depends on placement of matter and matter moves as curvature dictates.

 

Like you say: "Things would move around on their own and all events would happen without our observation and measurement." Earth don't stop orbiting the Sun if nobody aren't looking, Earths orbit is not an illusion. The curvature of spacetime exists whether we measure it or not, greater mass have steeper curvature than lesser mass and if we would add or remove mass from a body then the spacetime curvature around it would change likewise.

 

The frame of reference is a real physical 'backdrop' which includes the scale of the metric and the rate of clocks ticks. A fast moving rod will not only look shorter to us, it will also fit inside a shorter box while it passes and if it is subjected to some wear it will be preserved for a longer duration.

 

For example, heavy ions that are spherical when at rest, should assume the form of "pancakes" or flat disks when traveling nearly at the speed of light. And in fact, the results obtained from particle collisions can only be explained, when the increased nucleon density due to Lorentz contraction is considered.

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

 

A comparison of muon lifetimes at different speeds is possible. In the laboratory, slow muons are produced, and in the atmosphere very fast moving muons are introduced by cosmic rays. Taking the muon lifetime at rest as the laboratory value of 2.22 μs, the lifetime of a cosmic ray produced muon traveling at 98% of the speed of light is about five times longer, in agreement with observations.

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

 

 

 

I am not an expert on Relativity but as I interpret what you are saying, you seem to think that the effects of Relativity are some kind of optical illusions and from my understanding that is wrong. You can not cherry pick half of a theory and leave out the other parts. Maybe you need to learn more of Relativity and its features before you make any final conclusions and then make a new judgement whether you accept or reject it when you have reached a better understanding.

 

IMHO, I maintain what I said in my previously post, without accepting that distances and durations actually are physically altered in different frames as Relativity predicts, you are in essence rejecting it.

Regarding your statement:

..."'in mainstream science there is no space inside or outside of the rubber, instead all dimensions of space are on the surface, like a hypersphere."

What would you say would be inside and outside this "baloon?" If "nothing" then that is empty space.

Can you imagine an "end of space?" What boundary might describe such an "end?" What might be beyond that imagined boundary? More space, ad infinitum is the only possible answer that I can find.

 

Now to your question,"Can you explain why we wouldn't be able to determine the direction towards the centre in the first example?"

Yes. If the visible cosmos is within one small sphere of "rubber" the whole "membrane" of which is a vastly larger cosmos that we can see, and the whole thing is expanding outward from the bang, all we can see is local movement, neither beyond the inside or outside of the "membrane."

 

I think most of you other questions are answered in what I just posted in the "Ontology of Time" thread.

No, not an optical illusion but rather that the world/cosmos has an objective existence without the limitations of our measurements, so the latter do not describe "reality as it is." A rod may be physically compacted by sufficient accelleration, but I don't think that is what you mean. Observing it change length does not actually make it change length, which is what I think relativity (and you) assert. That is the subjective idealism component of my criticism of relativity, philosopically speaking.

 

The difference in "lifespan" of natural vs labatory muons, for example does not mean that there is some local time environment which is extended in the one case and shortened in another. They just "live longer" (or shorter) in different energy environments. (See my comments on time ontology in the other thread mentioned.

I hope this addresses most if not all your questions. BTW, I don't think you responded to my specific questions in previous post. "Yes?"... "Do you agree?" Does cosmos disappear lest we observe it?

 

PS, Re: "A fast moving rod will not only look shorter to us, it will also fit inside a shorter box while it passes..."

The same observation (frame of reference) by which the rod looks shorter applies also to the box... or maybe I again misunderstand you.

 

Thanks.

Owl

[steevey]

We don't actually know space itself is expanding and we aren't 100% sure there's a center, and the reason is from the fact that through the observations of Hubble and other telescopes, the center of the universe is through observable evidence, everywhere. All matter is moving away from all other matter in a way so that no matter what piece of matter you look at, there's always things expanding away from it. In every single direction you look, there's both old and young galaxies expanding away from every other galaxy.

The center of the universe is either everywhere or nowhere.

Edited February 3, 2011 by steevey

[Iggy]

If the expanding balloon model of cosmos is true, this does not mean that we can see beyond the "thickness" of "rubber" within our local part of the balloon and locate the center of the balloon (locus of the big bang.)

 

What would you say would be inside and outside this "baloon?" If "nothing" then that is empty space.

 

A lot of people that don't have a background in science or math find the balloon analogy confusing.

 

The surface of a balloon is two dimensional. It represents a 2-manifold that is not embedded in a higher dimensional space. It has what is called intrinsic curvature.

 

In one dimensional space there is one set of directions that you can move--forward/backward. In two dimensional space there are 2 sets--forward/backward and left/right. In three dimensional space there are three--forward/backward, left/right, and up/down.

 

Because the surface of the balloon represents a 2-manifold there are only two sets of meaningful directions--forward/backward and left/right. There is no third spatial dimension either filled with nothing or filled with empty space.

 

The same analogy can be made with a 1-manifold. A string that stretches would represent one dimension of expanding space. A circle that expands also represents one dimension of expanding space. A line and a circle are both one-dimensional manifolds.

 

Maybe I don't understand the question. I assume that we agree on lightspeed as a universal constant (even if the light source is speeding, as per special relativity.) So we we can say that light travels a given distance in a given unit of time. The sun is 8+ light minutes from earth (give or take its position in elliptical orbit.) It is the same distance regardless of time unit used or frame of reference differences among different observers. Relativity stays intact, but that distance doesn't actually vary with observer frame of reference.

 

Unfortunately, it looks like you don't understand relativity.

 

The distance measured in light minutes or any other units depends on the reference frame. The speed of light is constant. If two reference frames have different velocities and they both agree on the speed of the same photon then they won't be able to agree on the distance the photon travels or the time it takes to travel.

 

A good example of this is to think of the photon as something slow like a baseball. The catcher is at home plate and he isn't moving. The batter is running at 5 miles per hour toward first base after bunting the ball. The catcher grabs the ball and throws it toward first base--it flies through the air past the runner.

 

If the speed of the baseball is constant then both the runner and the catcher need to agree on the speed of the ball. The runner should figure that the ball is moving away from himself at 10 miles per hour and the catcher should look at the ball and say that it is moving away from himself at 10 miles per hour.

 

A couple quick questions then--if the baseball is moving away from the catcher at 10 miles per hour and the runner is moving away from the catcher at 5 miles per hour then how fast does the runner think that the baseball is moving away from himself? If the runner is going half the speed of light and the baseball is instead a laser then how fast does each think the laser is going?

 

Understanding the solution to this thought experiment is the first step in understanding relativity. It can give a lot of people trouble.

Edited February 3, 2011 by Iggy

[Spyman]

Which of the 3 observators is right? What is "really" happening?

All 3 are correct, none can claim superiority. They are in different environments where the scale of the metric and seconds differs between them.

 

 

Regarding your statement:

..."'in mainstream science there is no space inside or outside of the rubber, instead all dimensions of space are on the surface, like a hypersphere."

What would you say would be inside and outside this "baloon?" If "nothing" then that is empty space.

I say again, empty space is not the same as nothing, empty space can be measured in three dimensions: length, width and height. What can be outside of our dimensions? If I say nothing that is NOT empty space, because space only exists within our three spatial dimensions. If there is something outside of space then it includes a new dimension which would have other properties than distance as we know it.

 

 

Can you imagine an "end of space?" What boundary might describe such an "end?" What might be beyond that imagined boundary? More space, ad infinitum is the only possible answer that I can find.

I continue to fail to visualize a saddle shaped Universe, but I have no problems whatsoever to imagine a flat infinite Universe and sometimes I can succesfully create an mental image of a closed finite Universe that curves back on itself like a hypersphere. All three of these shapes of the Universe has boundaries built into them that comes with the dimensions but none of them have boundaries somewhere inside space.

 

 

Now to your question,"Can you explain why we wouldn't be able to determine the direction towards the centre in the first example?"

Yes. If the visible cosmos is within one small sphere of "rubber" the whole "membrane" of which is a vastly larger cosmos that we can see, and the whole thing is expanding outward from the bang, all we can see is local movement, neither beyond the inside or outside of the "membrane."

The local movement would be in the direction away from the center, wouldn't it? And as such we would be able to point backwards and say that the center should be there.

 

 

I think most of you other questions are answered in what I just posted in the "Ontology of Time" thread.

I might read through it later, when I have more time.

 

 

No, not an optical illusion but rather that the world/cosmos has an objective existence without the limitations of our measurements, so the latter do not describe "reality as it is." A rod may be physically compacted by sufficient accelleration, but I don't think that is what you mean. Observing it change length does not actually make it change length, which is what I think relativity (and you) assert. That is the subjective idealism component of my criticism of relativity, philosopically speaking.

There might be a 'real' cosmos hiding behind our observations but our measurements describe the reality as we perceive it. The rod does not actually change length but it is measured to have different length from different environments and in each and every one of these environments the measured size of the rod describes the reality exactly as it is, in that environment as the observers there perceive it, as such the rod actually have a real different length in another frame.

 

 

The difference in "lifespan" of natural vs labatory muons, for example does not mean that there is some local time environment which is extended in the one case and shortened in another. They just "live longer" (or shorter) in different energy environments. (See my comments on time ontology in the other thread mentioned.

I hope this addresses most if not all your questions.

This is where you clearly disagrees with Relativity which claims that it is the local time/space environment that is different.

 

 

BTW, I don't think you responded to my specific questions in previous post. "Yes?"... "Do you agree?" Does cosmos disappear lest we observe it?

I think our Universe will continue to exist independently of sentinent observers.

 

 

PS, Re: "A fast moving rod will not only look shorter to us, it will also fit inside a shorter box while it passes..."

The same observation (frame of reference) by which the rod looks shorter applies also to the box... or maybe I again misunderstand you.

The box is not moving relative us and as such it is not subjected to lenght contraction like the fast moving rod.

Edited February 3, 2011 by Spyman

[michel123456]

I am confused:

 

Spyman wrote:

(emphasis mine)

All 3 are correct, none can claim superiority. They are in different environments where the scale of the metric and seconds differs between them.

 

And later:

There might be a 'real' cosmos hiding behind our observations but our measurements describe the reality as we perceive it. The rod does not actually change length but it is measured to have different length from different environments and in each and every one of these environments the measured size of the rod describes the reality exactly as it is, in that environment as the observers there perceive it, as such the rod actually have a real different length in another frame.

 

I don't understand anything. What do you mean? Is there only one reality, where " the rod does not actually change length", or a multiple "reality" where "the rod actually have a real different length in another frame".

 

IMHO these 2 concepts of Reality are mutually exclusive.

[Iggy]

It's always the same length in its own frame. It is different lengths in different frames.

[IM Egdall]

This is the world of relativity. It doesn't make sense because it violates our "common sense". But common sense is a result of our experiences. And we only experience speeds which are a tiny fraction of the speed of light. (Even at airplane speeds, we are only going at about a millionth the speed of light relative to the ground.) At these speeds, relativity effects are so small that they we don't notice them. But they are real, as demonstated in thousands of observations and experiments.

 

So our limited life experience forms how we think about time and space, and limits our perception of reality. Here's another mind-bending prediction of special relativity. Say you are moving with respect to me. And I see two events occuring at the exact same time. In other words they are simultaneous to me. But due to your relative motion, you see these same two events happening at different times! Check it out at:

 

http://www.marksmodernphysics.com/ (Click on selected animations, The Relativity of Simultaneity)

Edited February 4, 2011 by I ME

[Spyman]

I don't understand anything. What do you mean? Is there only one reality, where " the rod does not actually change length", or a multiple "reality" where "the rod actually have a real different length in another frame".

 

IMHO these 2 concepts of Reality are mutually exclusive.

Sorry if I expressed myself badly, I don't seem to find the correct words to use...

 

I agree with what IGGY says: "It's always the same length in its own frame. It is different lengths in different frames."

 

What I ment is that the rod doesn't grow or shrink, it stays the same size in its own frame. It doesn't suddenly change size because someone is observing it from another frame, but it will be of a different size as measured from the other frame. Likewise the rod can't have more than one length simultaneously, it is the observers rulers that are in different scales and therefore measures the rod's length to be different. But it is more than a optical difference, the physics of every frame demands that the rod is of the measured length in that frame to sustain the laws of nature.

 

I guess one could say that the rod would be in a more relaxed or natural state when measured from its own frame when no forces are acting on it and it might be tempting to argue that it would represent the rods true and not 'warped' size, but every other frame are equaly valid or real and in every frame the laws of physics are preserved. There is no preferred frame where measurements would be special or somehow priviliged.

 

Viewing it as multiple realities might help as long as you remember that they are all observing the ONE and SAME story, there is only one rod which all observers are measuring and it only goes through one set of events once. Although they can disagree on the order of events, timestamps and distances, they are still observing the one and same occurrence from different viewpoints.

[michel123456]

I agree on everything.

I have the feeling that it is not the way Relativity is commonly understood. We should start a thread on this.

Edited February 5, 2011 by michel123456

[Iggy]

I think an easy way to consider that length really changes with frame of reference is to compare it to other things that change with frame of reference. Momentum, for example. No one would doubt that momentum is real or that it is different depending on the speed of the observer.

 

The most satisfying consideration for me personally is that length is a 3D property in a 4D space time, or, it might be more exact to say that it is a spatial property in a space time world.

 

If you look at the a book so that the face of it is toward the front it could be 15 cm wide. The 3D book has a certain 2D width. Rotating the book in 3D so that the spine is facing front will give it less 2D width. It isn't as wide when looking at the spine. The 2D width depends on how the book is positioned in 3D.

 

Changing the velocity of something in 3D amounts to rotating it in 4D space time, so it makes sense that the 3D property of length changes depending on its velocity, or its angle of rotation in space time. Just like all of the 2D images one could take of a 3D object from whatever angle are real and none are preferred, so too are all the possible 3D lengths of something in space time real and not preferred.

 

The Lorentz transformation is, as I said above, a rotation in spacetime. It is, admittedly, different from the rotations we are used to, but it is (especially when expressed mathematically) the same kind of animal.

Living in a 4-Dimensional World

[michel123456]

(...) If you look at the a book so that the face of it is toward the front it could be 15 cm wide. The 3D book has a certain 2D width. Rotating the book in 3D so that the spine is facing front will give it less 2D width. It isn't as wide when looking at the spine. The 2D width depends on how the book is positioned in 3D. (...)

 

I agree with this description.

This description corresponds to perspective, or optical deformation: the book intrisically does not change because someone somewhere is observing it. What is changing is the point of vue of the observer and his measurements of the phenomena "book".

I suppose that some other people here will disagree, saying that following Relativity, the book "really" contracts.

 

What happen with Relativity is that it is not an optical deformation (only), it is also a duration deformation, and a gravitational deformation. As a sum, it is a deformation of all measurements and as such, physicists may consider it is a physical deformation. But IMHO the book does not change and the rod does not contract.

[Sisyphus]

I agree with this description.

This description corresponds to perspective, or optical deformation: the book intrisically does not change because someone somewhere is observing it. What is changing is the point of vue of the observer and his measurements of the phenomena "book".

I suppose that some other people here will disagree, saying that following Relativity, the book "really" contracts.

 

What happen with Relativity is that it is not an optical deformation (only), it is also a duration deformation, and a gravitational deformation. As a sum, it is a deformation of all measurements and as such, physicists may consider it is a physical deformation. But IMHO the book does not change and the rod does not contract.

 

But no one is saying it is a physical deformation, just that no measurement is more "real" than the other, which is to say that they are both real. It is 20cm long, and it is 10cm long, and it does not change.

 

That's not a contradiction.

 

I think a good way to think of it is to keep in mind that objects simply do not have single, absolute properties like length, but rather length in a given reference frame. For some reason people find this a lot easier to grasp for some properties than others. What is your velocity? Not an answerable question unless you specify a reference frame. What is your length and mass? Also not answerable, for the same reason.

 

I suppose it's just a matter of direct experience. People can handle measuring a car's speed relative to a road while knowing the road is on a spinning planet orbiting a star, or measuring walking speed inside an airplane they know is moving far faster than that with relative to the ground. But we have no direct experience with speeds and masses large enough for relativity to matter, and so "what is your mass relative to X" still sounds strange to most people, who insist that one answer is right in some absolute sense while others are just illusion.

[michel123456]

I have read several times from other forum members that length contraction "actually happens".

 

I agree that it is all a question of measurements. But usually when I say that someone comes and hit my fingers.

 

Discussion was about space & time.

You talk about mass.

 

What is your length and mass? Also not answerable, for the same reason.

 

and so "what is your mass relative to X" still sounds strange to most people, who insist that one answer is right in some absolute sense while others are just illusion.

(emphasis mine)

 

Are you talking about rest mass?

 

Because IMHO rest mass corresponds exactly to the "answer is right in some absolute sense".

[Spyman]

For other frames, the relativistic mass (of a body or system of bodies) includes a contribution from the "net" kinetic energy of the body (the kinetic energy of the center of mass of the body), and is larger the faster the body moves. Thus, unlike the invariant mass, the relativistic mass depends on the observer's frame of reference.

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

[Iggy]

(...) If you look at the a book so that the face of it is toward the front it could be 15 cm wide. The 3D book has a certain 2D width. Rotating the book in 3D so that the spine is facing front will give it less 2D width. It isn't as wide when looking at the spine. The 2D width depends on how the book is positioned in 3D. (...)

...This description corresponds to perspective, or optical deformation: the book intrisically does not change because someone somewhere is observing it.

 

It looks like I used a not-too-helpful analogy.

 

The front of the book is wider than the spine. It isn't an optical deformation. It really is wider. If, for example, you had a hole in the wall just a few cm wide and you want to fit a book through that hole then you might have to rotate the book just right to fit it through.

 

The idea is that rotating a book in three dimensions changes its two dimensional width, or length.

 

In the same way, rotating an object in 4 dimensional space time (which is the same thing as giving it velocity or changing its frame of reference) changes its length in three dimensional space. If something rotates relative to me (or I rotate relative to it) in space time then I should expect its spatial property of length to change.

 

This may not be a very helpful analogy, it's just what helps me when thinking about the reality of length contraction.

 

 

Probably more helpful is to realize that length is not an inherent property of an object. The way that an object's momentum or kinetic energy depends on velocity is like how its length depends on velocity. Momentum, kinetic energy, and length are all three dimensional, frame dependent, things.

 

You could probably see how saying "that object has a lot of kinetic energy" only makes sense if you're really saying "that object has a lot of kinetic energy relative to our frame of reference (or relative to some other specified frame of reference)". Spatial length is the same way. It really does not make complete sense to say "the distance between those things is very large" unless you're really saying "the distance between those things is very large relative to our frame of reference (or some specific frame of reference"

 

If you leave off the "relative to" part of the description then you don't have a meaningful description--it's incomplete.

[Sisyphus]

I have read several times from other forum members that length contraction "actually happens".

 

Yes, the object actually is 10cm in one frame and actually is 20cm in another.

 

I agree that it is all a question of measurements. But usually when I say that someone comes and hit my fingers.

 

Perhaps you're not clear or consistent about what "a question of measurements" is supposed to mean.

 

Discussion was about space & time.

You talk about mass.

 

 

 

(emphasis mine)

 

Are you talking about rest mass?

 

Because IMHO rest mass corresponds exactly to the "answer is right in some absolute sense".

 

I'm talking about relativistic mass.

[michel123456]

Yes, the object actually is 10cm in one frame and actually is 20cm in another.

 

I cannot accept this concept.

 

You said before:

 

(...)sounds strange to most people, who insist that one answer is right in some absolute sense while others are just illusion. (...)

 

Well, I am like "most people".

To me, there is only one Reality, with a big "R". It is when the FOR of the observer is the same as the FOR of the object.

The "right" length of the rod is the one measured by the observer who keeps the rod in his hand.

All other observers, thousands of ligth years away, will make other measurements.

 

The same goes with mass: the observer at rest with the rod will measure a specific, absolute rest mass, and another observer thousands of LY away will measure another mass. The difference between the 2 masses will be called "relativistic mass", and will be the result of a deformation of measurements together with length contraction & time dilation.

 

As Spyman wrote, the rod does not contract, and as I say, its mass does not change. What change are measurements made by different observers from different FOR.

[Iggy]

I am like "most people".

 

I agree that your understanding is common--even among those studying physics. Studies show:

 

Villani and Pacca have demonstrated that university students’ reasoning in relativistic contexts is similar to that observed by Saltiel and Malgrange in Galilean contexts. A case study by Hewson with a physics graduate student illustrated the importance of “metaphysical beliefs” (e.g., time is absolute) to his understanding of special relativity The student in the study classified certain relativistic effects (including length contraction) as distortions of perception. Posner et al. report similar results in interviews with introductory students and their instructors. O’Brien-Pride has conducted interviews and administered early versions of some of the research tasks described here in which university students appear to believe that the order of events depends on observer location. Her preliminary results provided impetus for the investigation detailed in this paper.

 

http://arxiv.org/ftp/physics/papers/0207/0207109.pdf

 

To me, there is only one Reality, with a big "R".

 

Can I please ask--how much kinetic energy does an airplane have?

 

If there is only one reality does that mean there is only one real value of kinetic energy? Since kinetic energy depends on velocity, I don't think it has to mean that.

 

It is when the FOR of the observer is the same as the FOR of the object.

The "right" length of the rod is the one measured by the observer who keeps the rod in his hand.

All other observers, thousands of ligth years away, will make other measurements.

 

Two observers who are 1,000 lightyears apart will make the same measurement for the length of something if they have the same velocity.

 

To me, there is only one Reality, with a big "R". It is when the FOR of the observer is the same as the FOR of the object... the rod does not contract

 

I think a lot of people have the same view.

 

The problem with that view that I have is the distance to the Andromeda galaxy. If a person travels the distance with a velocity 0.9999c relative to the galaxy then it will take them 35,357.99 years. Distance is speed times time. The distance to the Andromeda galaxy is therefore 0.9999 lightyears / year times 35,357.99 years = 35,354.5 light years.

 

We on earth measure the distance at 2.5 million light years. The person actually making the trip measures the distance at about 35 thousand lightyears. Who are we to tell them that their distance isn't real if they are the ones traveling the distance?

 

I believe that you are correct that there is only one reality. But, that reality is four dimensional. There are many different 3 dimensional versions or perspectives of the four dimensional reality. Spatial distance is a 3 dimensional property and there are many different equally correct and equally real spatial lengths of a four dimensional thing--just like there are many different equally correct and equally real lengths of a 2D cross section of a 3D object.

 

Distances in space are relative to reference frame. Distances in space time are invariant--they do not change from reference frame to reference frame.

[michel123456]

I guess Iggy's post was intended to make me change my mind...

 

From the .pdf provided page 2:

"The student in the

study classified certain relativistic effects

(including length contraction) as distortions

of perception. Posner et al. report similar

results in interviews with introductory

students and their instructors."

 

It seems I agree with the student.

 

Can I please ask--how much kinetic energy does an airplane have?

 

Woops, you are sliding from a subject to another. If we cannot even agree on what distance is, it is useless to jump into further considerations.

 

I believe that you are correct that there is only one reality.

 

That is something.

 

But, that reality is four dimensional. There are many different 3 dimensional versions or perspectives of the four dimensional reality. Spatial distance is a 3 dimensional property and there are many different equally correct and equally real spatial lengths of a four dimensional thing--just like there are many different equally correct and equally real lengths of a 2D cross section of a 3D object.

 

Distances in space are relative to reference frame. Distances in space time are invariant--they do not change from reference frame to reference frame.

 

O.K. distances in space are relative to reference frame.

It means from some other FOR, the rod I have in my hand can be observed as contracted.

Can you define some FOR where the rod in my hand is expanding?

[Spyman]

Can you define some FOR where the rod in my hand is expanding?

Between different gravitationally or accelerating frames.

 

Diagram 1. Changing views of spacetime along the world line of a rapidly accelerating observer. In this animation, the vertical direction indicates time and the horizontal direction indicates distance, the dashed line is the spacetime trajectory ("world line") of the observer. The lower quarter of the diagram shows the events that are visible to the observer, and the upper quarter shows the light cone- those that will be able to see the observer. The small dots are arbitrary events in spacetime. The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how the view of spacetime changes when the observer accelerates.

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

[michel123456]

Excellent.

I don't expect you to argue that the small dots left & right are actually dancing like that just because an observer happened to travel accelerating rapidly?

 

And I don't understand where is the answer to my question at the end of post #48.