Physics – Relativity

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What you copied is correct but has nothing to do with the fact that a light year is a unit of distance. It has all kinds of uses, all kinds of importance for many things but it is a unit of distance, not a unit of time

I know you want me to say you are correct so I won’t continue to try and convince you.

To YWM 80,

I don’t understand why you are confusing the issue. My original post was answering some bored guy’s question -not defining light-years! All I am proving from Wikipedia is that light takes TIME to travel. Only things with mass will lose the concept of TIME, if they travel at the speed of light!

To all,

Go back and reread my answer to his question. You will see that what I’m saying makes sense.

And I was just trying to clarify a small detail in your post.

But you are absolutely right and I should not have commented on your post.

It might have been invoked for the daily cycle, but what I’m pointing out is that if you read the language used, you’ll always see, we can’t say what goes around what. That only applies to the annual orbit which is not part of the discussion.

YW Moderator-80–

“the speed at which you can measure the time difference is of course dependent on the sensitivity and accuracy of the time measuring device. You can measure the effect on a moving snail if you have a good enough device…“

I suppose so, but you’d need an exceptionally sterile laboratory to conduct such fine measurements in order to be sure there are no outside influences.

Yes, I’m talking; “in principle” not practically.

“If you were to travel on a light beam that leaves a star 100 light years away from earth. You look at your watch as you leave the star, you look at it again when you arrive on earth, ZERO time will have passed for you and for your watch. But to the people on earth, if they could somehow have observed your voyage, they would measure that 100 years had elapsed from the time you left the star until you got here. Something like that.“

I read a sci-fi book a few decades ago where the heroes had to take an interstellar trip at near-light speed. It mentioned that the trip would be relatively short from the astronauts’ perspective, but by the time they returned to Earth, 100 Earth-years would have passed and all their relatives and acquaintances would be gone.

I never heard of Dr. Gerald Schroeder or his book, but according to most Amazon reviewers it’s quite good.

WolfishMusings–

I did a quick search on “time dilation” and “space travel”.

I was unable to find an article similar to what I remember reading, but I did find the following:

-Joseph C. Hafele and Richard E. Keating conducted “time difference” experiments by flying atomic clocks areound the world on commercial flights. Google the names for the full story.

– This quote; “Modern-day atomic clocks are so accurate that when synchronizing clocks between different observatories, the effect of time dilation due to transporting the reference clock on an airline flight must be taken into account.” Several sites mention this, but this wording was the most concise.

ZachKessin–

Wikipedia confirms Alpha Centauri is a binary star, with the less-closely located Proxima Centauri joining it to make a triple star.

I’m not sure if the term I used – “double star” – and “binary star” have different meanings; i.e. “binary” possibly denoting a closer relationship.

“First of all do not try to understand relativity without understanding Newton’s laws of motion. It would be like trying to study Kaballah without knowing Tanach first.“

Any book recommendations to give a lay-person a basic understanding? Not too dense, not overly simple, and not too boring. (I’ll leave out the “and while standing on one leg” clause.)

In the case where the object is moving slowly or not near a VERY large gravity field Newtonain mechanics and relativity give EXACTLY the same results.

Am I wrong in stating that they DO NOT give the exactly same results, only that the differences are too minute for our ability to measure them?

True there would be an effect, but it would be so small that in every practical way you can and should ignore it. For an object moving at 30meters/ second the differece would be 1/sqrt(1 – 10^14) which is in effect 1, IE no difference at all.

“ie no difference at all”

again, at least to me, it is very important to be rigorous in the use of language in the description of basic principles of the conduct of the universe.

It is, of course NOT “no difference at all” There will always be a difference, under all real circumstances.

What you mean to say is that in practical terms to humans, and according to our perceptions there is, in a practical sense, “no difference at all”

“ie no difference at all”

What you mean to say is that in practical terms to humans, and according to our perceptions there is, in a practical sense, “no difference at all”

True, but the difference is so small and in practice so much smaller than so many other things that could effect your measurement that it can be ignored. Could you measure it if you wanted? Given a million dollars of equipment in a lab maybe (I’m not up to date on this kind of lab stuff).

In many cases you are happy to get a result to 1 part in a 1000, something that will change that by 1 part in 10^14, is so small that in practice it is ignored. Now there are times when it must be accounted for but one of the things you pick up along the route to a PhD is knowing how to design an experiment so that it will account for the correct things.

A few responses.

– The speed of gravity is the speed of light, though that is not quite as firm as some of the other results in modern physics. It seems that they have stopped arguing over it.

– I can’t recommend a good book on SR, just because I don’t have any handy. You can go over the to Bad Astronomy / Universe Today forum and ask there, I’m sure someone will have several.

– If you want a good introduction to Relativity and much of physics go watch “The Mechanical Universe” It was a set of videos done at Cal Tech in the 80’s and goes over most or all of what would be in freshman physics. You can find them over at Google Video.

volvie, one point you are missing, there is a very important difference between linear motion (IE in a line) and accelerated motion including rotation. Lets say you have a pickup truck and you put a bowling ball in the back, if you drive down a the street without turning or speeding up or breaking the ball will stay in one place relative to the bed of your truck. But if you should turn then the ball will feel an inertial force as it tries to maintain its momentum while the truck moves under it. This force will be equal to mv^2/r. Relativity postulates that all frames of reference are equivalent if and only if they are not accelerated.

Similarly the Earth turns around its axis once each day. As long as scales are small we don’t feel a force from that turning but in some cases it can be measured and is important. An object moving from the Earth’s equator to the north will find that it has too much angular momentum and will feel a force, exactly the same as our bowling ball in the truck above, and will start to deflect to the East. If the Earth was not rotating this would not happen.

All non-inertial frames are accelerating with respect to all inertial frames.

First point acceleration can mean not just a change in speed but also a change in direction. It is a vector quanity. So a child on a merry-go-round is experiencing constant acceleration which is at right angles to his direction of motion. (The acceleration points to the center of the rotation, while the motion is going forward)

True but in any accelerated field you can always tell what the acceleration is because it will produce a force that you can measure. That child on the merry-go-round will feel a force pushing him outwards to the outside of the merry go round. Now on a child’s ride it will be a light force but you could say get in a fighter jet and pull a tight turn and feel a very large force. In this case the jet is accelerating and i assure you that you can feel it. (OK I’ve only been up to about 2 g’s and it was in a very small prop aircraft)

Third you must understand that GR is at its heart a theory of Gravity, so in the Limit of low gravity it must agree with Newton, and it does.

A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton’s equation of motion in a gravitational field, written out in full, it is:

(Inertial mass) (Acceleration) = (Intensity of the gravitational field) (Gravitational mass).

[end quote]

The equivalence principle proper was introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s2 being a standard reference of gravitational acceleration at the Earth’s surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus:

“we […] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.” (Einstein 1907).

That is, being at rest on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being accelerated by its engines. From this principle, Einstein deduced that free-fall is actually inertial motion. By contrast, in Newtonian mechanics, gravity is assumed to be a force. This force draws objects having mass towards the center of any massive body. At the Earth’s surface, the force of gravity is counteracted by the mechanical (physical) resistance of the Earth’s surface. So in Newtonian physics, a person at rest on the surface of a (non-rotating) massive object is in an inertial frame of reference.

The original equivalence principle, as described by Einstein, concluded that free-fall and inertial motion were physically equivalent. This form of the equivalence principle can be stated as follows. An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g.

Quoted from “Principles of Equivalence: Their Role in Gravitation Physics and Experiments that Test Them”:

The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.

Yet I’ve quoted how this discussion relates to the Earth specifically.

Yet I’ve quoted how this discussion relates to the Earth specifically

Your quote refers to LINEAR MOTION in relation to the earth, NOT to the movement of the earth itself (rotational or linear)

That’s very interesting. Although that paper (from mathpages.com) is referring to particle motion, if you will look at the math a little further down

Some may find this graphic illustration interesting:

http://www.astro.utoronto.ca/~zhu/ast210/both.html

Here are a few quotes from Hans Reichenbach, who was Einstein’s student in Berlin, when he lectured on the theory of relativity. (In 1926, with the help of Albert Einstein, Reichenbach became assistant professor in the physics department of Berlin University. After Hitler came to power he left Germany, eventually moving to the United States and accepting a professorship at the University of California):

The cosmologies of Copernicus [Heliocentrism] & Ptolemy [Geocentrism] are kinematically equivalent; both of them are descriptions of the same facts, and Ptolemy’s epicycles of the planets are the kinematic equivalents of the circular orbits of Copernicus.

Reichenbach, Hans- Philosophy of Space and Time- p210-211

The Copernican conception is indeed simpler, but this does not make it any “truer” since this simplicity is descriptive… One description may be simplest for some phenomena while a different description may be simplest for others; but no simplest description is distinguished from other descriptions with regard to truth. The concept of truth does not apply here, since we are dealing with definitions.

Reichenbach, Hans- Philosophy of Space and Time- p219

He [Copernicus] was able, in fact, to cite as a distinct advantage only the greater simplicity of his system. … Here lies one of the reasons which led scientists to accept the Copernican system, even though it must be conceded that, from the modern standpoint, practically identical results could be obtained by means of a somewhat revised Ptolemaic system.

Hans Reichenbach- From Copernicus to Einstein- p18

Motion is change in position; it is clear, however, that it cannot be observed unless it is a change in position relative to a certain body and not relative to an ideal space point. Is it meaningful, under these circumstances, to speak of absolute motion or of motion relative to space, if motion relative to other bodies only can be observed? According to this principle there exists only a motion of bodies relative to other bodies, and it is impossible to distinguish one of these bodies as being at rest, because rest means nothing but rest relative to another body, i.e. rest is itself a relative concept.

Hans Reichenbach- Philosophy of Space and Time- p210

And here are quotes from some others:

The commonly held view is that Copernicus’s heliocentric model vanquished the competition, especially the geocentric view of Ptolemy, because it yielded better predictions of the positions of the celestial bodies. In actual fact, the predictions of the Copernican model were a little worse than those obtained via the complicated series of epicycles… the real selling point of the Copernican model was that it was much simpler than the competition yet still gave a reasonably good account of the observational evidence.

Today we cannot say that the Copernican theory is ‘right’ and the Ptolemaic theory ‘wrong’ in any meaningful physical sense.

Fred Hoyle, Nicolaus Copernicus, Heinemann Educational Books Ltd., 1973, p78

And hence this affirmation: “the earth turns round” has no meaning, since it cannot be verified by experiment; … such an experiment … cannot even be conceived of without contradiction …

Henri Poincare- Science and Hypothesis- p117

There is one more implication that modern science has perceived in the work of Copernicus. The same observational data that Ptolemy organized in his geocentric theory of deferent and epicycle can also be organized under the heliocentric theory of Copernicus. Despite the belief of the latter that the new theory was true, the modern view is that either theory will do and that there is no need to adopt the heliocentric hypothesis except to gain mathematical simplicity. Reality seems far less knowable than Copernicus believed, and today scientific theories are regarded as human inventions.

Morris Kline- Mathematics and the Search for Knowledge, p85

… in fact simplicity of the mathematical theory was the only argument Copernicus & Kepler could advance in favor of their heliocentric theory as opposed to the older Ptolemaic theory.

Is the path of the earth around the sun an ellipse? No. Only if the earth & sun are regarded as points and only if all other bodies in the universe are ignored. Do the four seasons on earth repeat themselves year after year? Hardly. Only in their grossest aspects, which are about all men can perceive anyway, do they repeat.

Morris Kline- Mathematics: The Loss of Certainty- p 344, 350

volvie, volvie

your quotes, though interesting are regarding another issue entirely.

The issue of what is the effective center of the SOLAR SYSTEM.

heliocentric (helio meaning sun) and geocentric (geo meaning earth) refers to the two perspectives of whether the earth REVOLVES around the sun or whether the sun REVOLVES around the earth. This can be decided either way. It has nothing to do with the question of whether or not the earth ROTATES or is still and the universe revolves around the earth.

But please keep trying. I’m hoping you will turn up something.

Yes, that’s what im saying, but if you are now going to argue about this im not interested, while a totally unrelated issue is on hand here.

Have you actually heard the combined gravity terutz? If not, i wouldn’t say it is farfetched until you know what it is. Perhaps it has to do with the fact that the distribution of mass within the universe is not homogeneous. My suspicion is that it probably doesn’t hold water but I won’t comment on it’s validity until I’ve heard it’s explanation.

As far as the third problem you have not answered it at all.

Just as light travels at the speed of light toward an observer moving away from the source, although a stationary observer would say that the light traveled faster than the speed of light; so too, if by definition you can say that either object is revolving around the other, even if that causes you to speculate that it would mean travel faster than light, it still makes sense.

Well, if the stationary observer asks the moving observer how fast the light is moving, he will answer, at the speed of light. The stationary guy can then do the math and figure out that for himself the light heading toward the moving guy is going faster than the speed of light.

I would ask you to do it the easy way, and turn around.

From the star’s point of view, it’s staying put (radialy). From our point of view we can say that it went around that fast as long as it has nothing to do with us. Relative movement means that there is no absolute space. Although there is the rule that nothing will accelerate faster than the speed of light, one object turning around may, by definition, cause a distant object to be said to have gone around it. The idea is that it means the same thing. The motion won’t be initiated by the distant object, but the effect and the meaning is the same.

You have to be Mechaleck between the Pe’ula and the Chalois.

Well, if the stationary observer asks the moving observer how fast the light is moving, he will answer, at the speed of light. The stationary guy can then do the math and figure out that for himself the light heading toward the moving guy is going faster than the speed of light.

Ah no, light moves at the speed of light. The math here is the Lorenz transformations which give a correct result. If you try to do simple vector addition what you get will not match reality.

The merri go round is a good analogy to prove that something does spin in relation to itself. I wholly agree to that point. I therefore agree to mod’s first two questions. It is only the third point which I am addressing.

You are obviously saying that because you feel that the Torah implies it. That is why I mentioned the Maharal who, it seems, embraced the new model.

The Sefer Habris, after stating emphatically that whoever believes this, is absolutely not an heretic, goes on to explain that personaly he believes the old way. He brings two physics proofs and two Torah proofs. His physics proofs aren’t proof. He misunderstood centrefugal force.

His Torah proofs are

1) The Pesukim make more sense that way

2) The Arizal describes the earth as being from the “straight” world, while the sun, moon and stars are from the surrounding world. How can it be that the straight revolver around the surrounding.