Theory of Relativity

I think with an 8 year old kid you need to follow Gamow and try and imagine that relativity applies to the everyday, practical world the kid knows about. Imagine a particle of light as a spacehopper*. Ask the kid what it would be like to jump on that particle. Then tell him the spacehopper will be moving at 'the speed of light', so how could he jump on? If says he would run and catch it, tell him it will always be moving at the same speed away from him however fast he runs!

* Do 8 year old kids know what spacehoppers are these days?

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Originally Posted by mal4mac

I think with an 8 year old kid you need to follow Gamow and try and imagine that relativity applies to the everyday, practical world the kid knows about. Imagine a particle of light as a spacehopper*. Ask the kid what it would be like to jump on that particle. Then tell him the spacehopper will be moving at 'the speed of light', so how could he jump on? If says he would run and catch it, tell him it will always be moving at the same speed away from him however fast he runs!

* Do 8 year old kids know what spacehoppers are these days?

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Hey mal4mac yes eight year olds do know what a space hooper is.
Thank you for the post. I explained what you have just said but there is one one question they asked:
what is making the spacehopper run so fast and will it ever stop because it should eventually.
In other word the space hopper will eventually slow down to a stop right? :p

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Originally Posted by cacian

Hey mal4mac yes eight year olds do know what a space hooper is.
Thank you for the post. I explained what you have just said but there is one one question they asked:
what is making the spacehopper run so fast and will it ever stop because it should eventually.
In other word the space hopper will eventually slow down to a stop right? :p

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No! The speed of light is a constant. We don't know what is making it a constant, it's just a universal law of nature that we must accept (like bedtime rules...)

And we must accept it because it agrees with all the theories of clever physics dudes like Einstein. Thousands of experiments have either shown or assumed that the speed of light is a constant, and none of these experiments have been shown to be wrong. It's as certain as a spacehopper bouncing...

A photon might be absorbed by an atom in your retina, but whenever there's a photon around it's scurrying off at a constant speed. A light photon does not "slow down" upon entering a medium. The apparent decrease in speed in the atmosphere is because it "bounces" between particles. It's still going at the "universal constant" speed c.

(Einstein's special theory of relativity says that any particle with zero mass must always appear to move at the speed of light in any frame of reference.)

Your thread, cacian, has got me reading Richard Wolfson's Simply Einstein. He tries to explain relativity to anyone. He probably assumes his audience is older than 7, however.

One thing I would add to my original description, based on Wolfson's text, is to explain why people prior to Galileo did not feel the need for a principle of relativity. They experienced in their daily lives only one frame of reference, the earth, which was not moving while the sun, moon and constellations of stars were moving in a circular pattern around this frame of reference.

A relativity principle requires one to have at least two frames of reference.

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Originally Posted by YesNo

Your thread, cacian, has got me reading Richard Wolfson's Simply Einstein. He tries to explain relativity to anyone. He probably assumes his audience is older than 7, however.

One thing I would add to my original description, based on Wolfson's text, is to explain why people prior to Galileo did not feel the need for a principle of relativity. They experienced in their daily lives only one frame of reference, the earth, which was not moving while the sun, moon and constellations of stars were moving in a circular pattern around this frame of reference.

A relativity principle requires one to have at least two frames of reference.

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False. Galileo's was relativity. Newton clearly stated that it is impossible to establish velocity 0. Galileo had as many frames of reference as he wished.

Regarding acceleration, a very important Einstenian retardation, two spaceships traveling together, nearby, can consult each other to see if the acceleration read is external or internal.

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Originally Posted by YesNo

Your thread, cacian, has got me reading Richard Wolfson's Simply Einstein. He tries to explain relativity to anyone. He probably assumes his audience is older than 7, however.

One thing I would add to my original description, based on Wolfson's text, is to explain why people prior to Galileo did not feel the need for a principle of relativity. They experienced in their daily lives only one frame of reference, the earth, which was not moving while the sun, moon and constellations of stars were moving in a circular pattern around this frame of reference.

A relativity principle requires one to have at least two frames of reference.

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Thank you YesNo.
So one requires to have at least two but why in the grandest scheme of things?
In other words in what way does a frame of reference help me advance?

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Originally Posted by cafolini

False. Galileo's was relativity. Newton clearly stated that it is impossible to establish velocity 0. Galileo had as many frames of reference as he wished.

Regarding acceleration, a very important Einstenian retardation, two spaceships traveling together, nearby, can consult each other to see if the acceleration read is external or internal.

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I think we are in agreement.

The single frame of reference applies to the science done before Galileo's time. With Galileo one gets at least a second frame of reference, either the sun or Jupiter with its moons. Once you have two frames of reference that number would expand to as many as one wished to have.

The main credit for the idea of relativity belongs to Galileo, as I see it. Einstein did little more than affirm it for electromagnetic waves.

Although I think I read somewhere that there could be "infinitely" many frames of reference, I think that is an exaggeration. There are not even infinitely many subatomic particles in the universe nor with quantized time and space, infinitely many "points" of space-time.

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Originally Posted by cacian

Thank you YesNo.
So one requires to have at least two but why is the grandest scheme of things?
In other in what way does a frame of reference help me advance?

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The way Wolfson describes the relativity of a frame of reference is to consider the motion behavior of playing tennis or the electromagnetic behavior of cooking a meal in a microwave oven.

Would we expect to play tennis differently on a planet in a galaxy traveling very fast away from us? If we were on that planet, would we expect to heat up a meal differently? The principle of relativity claims that we wouldn't have to do anything differently. The laws of physics, both the motion laws as well as the electromagnetic laws, are the same in that planet's frame of reference as our own.

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Originally Posted by YesNo

One thing I would add to my original description, based on Wolfson's text, is to explain why people prior to Galileo did not feel the need for a principle of relativity. They experienced in their daily lives only one frame of reference, the earth, which was not moving while the sun, moon and constellations of stars were moving in a circular pattern around this frame of reference.

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What about a ship? In fact, Galileo used a ship to explain his theory of relativity. And they had ships before Galileo! So you, or Wolfson, need to explain why people didn't think of this before Galileo. I can't see that Galileo had more "need" to explain this than other people. Given the Spanish Inquisition perhaps more of a need not to :angel:

http://physics.ucr.edu/~wudka/Physic...ww/node47.html

I think the validity of infinity resides in division by two and is inescapable. There cannot occur an object without some measure of inside or, in the macro, some measure of beyond. by inference, or what has been named "weak proof." I don't see it as weak. But, of course, you can't get a baby to have a natural vocation for imaginary Cheerios, except if it is a convenient way of gaining the actual, occurring, Cheerios in three dimensions.
In the large scheme of things we could find as little solution as in the microscopic. This will always trigger unending and useless speculation as to where God is hiding, is, exists, or occurs.
But we must accept that as scientists we only can be annoyed at the impossibility of postulating occurrence other than as being and existence in the imagination. But because of these vicious, circular arguments I am exposing, where the existence or being of God couldn't possibly help science, beyond the mental kind, why should but a psychologist elebarote on it. The rest of the people is subjected to faith as the only solution and the psychologist as a volunteer at the mental stage. Don't forget to have some fun and firm faith in love and hope.

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Originally Posted by mal4mac

What about a ship? In fact, Galileo used a ship to explain his theory of relativity. And they had ships before Galileo! So you, or Wolfson, need to explain why people didn't think of this before Galileo. I can't see that Galileo had more "need" to explain this than other people. Given the Spanish Inquisition perhaps more of a need not to :angel:

http://physics.ucr.edu/~wudka/Physic...ww/node47.html

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I think you're right that there was opportunity to come up with a principle of relativity long before Galileo. Perhaps some cultures actually did.

The following about infinite speeds in a vacuum and everything naturally slowing down from the article are things I didn't know about Aristotle's mechanics, but I don't know much about Aristotle.

This directly contradicts the Aristotelian philosophy which claimed that

all objects on Earth, being imperfect, will naturally slow down,
that in a vacuum infinite speeds would ensue,
and that perfect celestial bodies must move in circles.

I can see the part about things naturally slowing down, but why the infinite speeds in a vacuum?

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Originally Posted by cafolini

I think the validity of infinity resides in division by two and is inescapable.

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As far as mathematics goes, that makes sense. One could keep dividing by two as often as one liked and get closer and closer to having a pile with an infinite number of smaller and smaller pieces.

However, if one actually wanted to have that infinite pile of pieces one would have to assume that one could complete such an infinite process of dividing something even if that something were imaginary. I think that assumption was labelled the "axiom of choice", but it has been a while.

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Originally Posted by YesNo

The way Wolfson describes the relativity of a frame of reference is to consider the motion behavior of playing tennis or the electromagnetic behavior of cooking a meal in a microwave oven.

Would we expect to play tennis differently on a planet in a galaxy traveling very fast away from us? If we were on that planet, would we expect to heat up a meal differently? The principle of relativity claims that we wouldn't have to do anything differently. The laws of physics, both the motion laws as well as the electromagnetic laws, are the same in that planet's frame of reference as our own.

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Thank YesNo for explaining this most helpful and I am now beginning to have a feel for it.
If I may add then, this simply platonic reasoning because using a microwave or playing tennis in a different planet/ galaxy is not going to happen.
Unless of course one wishes to make a film out of it to demonstrate what one means.

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Originally Posted by YesNo

As far as mathematics goes, that makes sense. One could keep dividing by two as often as one liked and get closer and closer to having a pile with an infinite number of smaller and smaller pieces.

However, if one actually wanted to have that infinite pile of pieces one would have to assume that one could complete such an infinite process of dividing something even if that something were imaginary. I think that assumption was labelled the "axiom of choice", but it has been a while.

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I think the problem with this argument begins before the "completion of an infinite process" problem.

Doesn't mathematics only make sense in "our world"? And in our world you can't just keep chopping things in half. How would you chop a Higgs boson in half?

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Originally Posted by YesNo

As far as mathematics goes, that makes sense. One could keep dividing by two as often as one liked and get closer and closer to having a pile with an infinite number of smaller and smaller pieces.

However, if one actually wanted to have that infinite pile of pieces one would have to assume that one could complete such an infinite process of dividing something even if that something were imaginary. I think that assumption was labelled the "axiom of choice", but it has been a while.

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True. However, if it were possible, or you found the means and the technique to do it, you would. That there is a smallest particle is a worse assumption. Humanity has been claiming smallest particles for ages and always found smaller ones. They can't pull that one anymore against historical evidence of their scoundrelship.