# Physics – Relativity

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• #591580

does anybody here understand the theory of realativity??

#790659
Bodek
Participant

Common…. Does anybody here not understand the theory of relativity?!?

#790660
volvie
Member

One aspect of Einstein’s theory (GR), postulates that the earth revolving around the sun is only relative. Nobody has proven nor even claims that it is absolute. In other words, if the Earth is the center of the universe, and the entire universe revolves around the earth, it will appear form the vantage point of anyone located within the universe that the sun is revolving around the earth, when in reality it is the opposite.

For example, lets say you throw a ball south at the speed of 60 MPH. To you and to those around you it would appear that the ball is moving and you are stationary.

But then, if you and all of those observers were actually riding on a bus traveling north at 60 MPH, then from the perspective of someone outside of the bus that ball, after you threw it, was perfectly still. It was you and your environment that were moving.

But then, what if the world was rotating at a speed of 60 MPH in the direction of south. Then, those observers outside of the bus would be the ones moving, the bus would be stationary, and the ball would indeed be traveling at 60 MPH.

Movement relative to another object depends on your perspective. And in order to know, ultimately, whether the earth revolves around the sun, because the earth is moving, or the sun revolves around the earth because the universe is moving and earth is stationary, you would have to measure form a vantage point outside of the universe, and nobody has been able to do that yet. At least not scientists.

So the idea that the earth revolves around the sun is like saying that the ball is moving inside the bus. Maybe. Or maybe everything is moving in your immediate area except the ball. You’d have to be outside the bus to know that.

Same thing here. To know whether it is the earth or the sun that is moving, you would have to take into consideration the entire universe’s movement, which no scientists has been able to do.

#790661

i understand that. but the theory runs deeper than that. thats the theory of general realativity. but then theres the deeper theory of speacial realativity. where time evolves on matter and where theres no matter there is no time. and at the speed of light there is no matter (only energy) thats y if u were to move at the speed of light u would simply dissolve into energy. therefore at the speed of light there shouldn’t be the concept of time. my question is then how can light take time to travel? as fast as it is if it were not bound to time then it would be immesurable??

#790662

and volvie correct me if im wrong- but hasnt it been proven that its actually the earth that revolvs around the sun as it spins on the axes of the universe. and its just realative as to how fast it moves. it could be im wrong but thats what i thought scientists proved.

#790663
volvie
Member

and volvie correct me if im wrong- but hasnt it been proven that its actually the earth that revolvs around the sun as it spins on the axes of the universe. and its just realative as to how fast it moves. it could be im wrong but thats what i thought scientists proved.

someonesboard, Copernicus is the one who postulated and changed sciences viewpoint, that the Earth rotates around the Sun. Before Copernicus, for most of history science said the Sun circles the Earth. Newton changed sciences viewpoint again, and said that Earth and Sun rotate about a common center of gravity. Einstein changed it again. He described the General Theory of Relativity – in which all motion is relative. In General Relativity, you can use any frame of reference. Relative to humanity, the Sun revolves around the Earth. To put it more simply, all motion is relative. Relative to humanity, the Earth is not rotating.

thats the theory of general realativity. but then theres the deeper theory of speacial realativity.

General relativity is more advanced than special relativity and is only taught in detail in certain postgraduate physics programs. Special relativity is taught earlier in less advanced programs.

#790664

Personally,I don’t particularly care either way but if you accept that the earth is stationary and the universe revolves around it, then scientifically speaking you do have to explain a certain problem:

Alpha Centauri, the closest star to our solar system is 4 light years away. It would have to travel 24 light years to make one circuit around the earth (2 x pi x radius of 4). Thus it would have to travel 24 light years in 24 hours or 1 light year per hour.

That is approx 8,000 times the speed of light

A problem that needs to be explained before you can accept that the stars revolve around the earth.It doesn’t bother me but if you wish to be scientifically consistent, it needs to be solved.

#790665
volvie
Member

It is a bit harder to explain stellar parallax from the perspective of an Earth-fixed reference frame than from the perspective of a solar system barycentric reference frame. But it’s not all that hard to explain stellar parallax from the perspective of an Earth-centered frame. The motion of the stars can be explained in terms of the “third body effect”. People who model the behavior of satellites in Earth orbit vastly prefer to use an “Earth-centered inertial” reference frame (a non-rotating reference frame with origin at the center of the Earth) than a barycentric frame. From the perspective of such a reference frame, the Sun and Moon (and the planets) make the satellite’s orbit not quite Keplerian. The perturbation is explained by a pseudo-force called the third body effect.

#790666
dd
Member

The principle of relativity does not mean that all motion is relative. For that it requires an inertial frame of reference. In particular, it could allow the sun to move relative to the earth (although no scientist would ever take that seriously), but it does not allow us to say that the earth is not rotating. Sunrise and sunset is caused by the rotation of the earth and not by the sun’s movements.

To explain, we are all aware of centrifugal force – the outward force when spinning. (Think of the force that pushes you out when you make a quick turn while driving.) The earth’s rotation exerts a centrifugal force on us that pushes us away from the earth. Of course, gravity is much stronger and pulls us down toward earth. However, there is more centrifugal force when you are closer to the equater – and none when you are at the north pole. Thus, your apparent weight is lower at the equator than at the north pole – there’s nothing that relativity can do to stop this, and it shows that the earth is rotating.

#790667
WolfishMusings
Participant

Phases of Venus anyone?

The Wolf

#790668
volvie
Member

[laws of gravitation]

1. The Sun rotates around the Earth.

2. The Earth rotates around the Sun.

This is derived on a non-inertial reference frame. From the perspective of an Earth-centered reference frame, the Sun does indeed orbit around the Earth. In GR, all reference frames are equally valid.

#790669

You didn’t answer the question. How do you, or einstein, account for the necessary yet impossible speed of the celestial bodies revolving around the earth?

And besides, the principle of equivalence only pertains to linear acceleration not rotational acceleration.

I have no problem accepting that the heavens revolve around the earth, but if you want to show this is so according to the principles of science then do so honestly or admit you don’t know the answer, and hope to eventually find it.

#790670
Health
Participant

Someonesbored -I think he said that beyond the speed of light there is no time, but at the speed of light there is time. This theory makes sense how in the next world there is no such thing as time. Time is a creation from Hashem like everything else.

#790671
WolfishMusings
Participant

I think he said that beyond the speed of light there is no time, but at the speed of light there is time.

Health,

I think you might be referring to the time-dilation effects of near-light-speed travel. However, there is no such thing as “beyond the speed of light.” Nothing can travel faster than the speed of light as it would acquire infinite mass and require infinite energy to get up to that speed. Unless your last name is “Kirk” or “Picard” have a warp drive, of course. 🙂

The Wolf

#790672

no health. the theory was said at the speed of light. for if not then e=mc square, wouldnt be true. for ur using the exact speed of light to find the amount of energy. however the question that i asked remains. if light is beyond time then how can it be measured?????

#790673
mt mehdi
Member

“its actually the earth that revolvs around the sun as it spins on the axes of the universe. “

Doesn’t the Gemara say the opposite?

#790674

i dunno does it??

#790675
tomim tihye
Member

Old joke that explains theory of relativity:

You decide whether it’s the general or special or spatial theory of relativity.

But of all topics in physics, why did you choose one of the most complex?

I’m glad there’s nothing else bothering you.

#790676

there r many theories and parts of physics and quantum physics that r complex and i dont understand. its the theory of relativity however that everybody seems to understand (as stated by #2) and this question has been bothering me. so i thought id ask, but it seems that nobody has an answer…..

#790677

Just a few points:

3) If I remember correctly, once space travel became a reality, and the previously unheard-of speed of 25,000 MPH was achieved, it was proven that time actually slows down as speed increases.

#790678
WolfishMusings
Participant

ICOT,

Tachyons are, to this point, purely hypothetical. I’m not saying that they don’t exist, mind you… just that they are, to this point, purely hypothetical.

I could be wrong on this, but I’m fairly certain you need to go faster than 25K MPH to see the effects of time dilation.

Lastly, of course it’s possible that FTL travel can happen one day through technological means. However, your analogy to supersonic travel* is flawed. No one denied that an object could move faster than the speed of sound, they merely theorized that a structural object (such as an aircraft) could not withstand it. I’m sure they would have agreed that with better engineering and materials, it would be possible.

However, nothing (as we presently understand it) can go FTL. Any technological advances that will allow FTL travel will probably not involve altering this law of physics, but rather with warping space and/or time in some way.

The Wolf

* Bonus question: What was the first man-made object to move faster than the speed of sound?

#790679

WolfishMusings-

I could be wrong on this, but I’m fairly certain you need to go faster than 25K MPH to see the effects of time dilation.

Unfortunately, I don’t remember where I saw it, but I recall reading that a small but measurable time difference did occur. B”N I’ll do a little research (google) later.

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…80

Any technological advances that will allow FTL travel will probably not involve altering this law of physics, but rather with warping space and/or time in some way.

I’ve heard the term “worm hole” used with regards to this, similar to “tesseract” re: time travel, but who knows?

Bonus question: What was the first man-made object to move faster than the speed of sound?

1) The X-1 (level flight)

2) The P-38 (in a dive – this is where it was proven that an aircraft could withstand supersonic flight, although the controls froze, due to the shock wave, IIRC).

3) A bullet (no human aboard [except for in the book From the Earth to the Moon, speaking of books]).

4) A cracked whip’s tip (not the entire object, but a portion of it).

5) Anything man-made, since the Earth orbits at @66,000 mph, and rotates at @1,025 mph at the equator.

6) ?

#790680

there is no valid reason to say that one can not move at the speed of sound. however moving at the speed of light is imposible, for that is part of e=mc square. that at the speed of light there is no mass therefore it will turn into energy.

and about particles that move faster than light- as alan guth- theoretical physicist at MIT, explains that “C in speacial relativity is not just the speed of a certain object thats called light, C is the limiting velocity of any motion..” meaning to say that it is not possible to move faster than the speed of light, according to speacil relativity.

#790681
Health
Participant

To the wolf and who ever’s bored,

I did a net search about this. First of all, a couple of years ago, a few German scientists got some photons to move faster than the speed of light. And I stand corrected, at the speed of light there is no time for matter, but since light has no matter, it still would be considered within “time”. (Because of this, no one has got anything with mass to go the speed of light, because it would require an infinite amount of energy. Also, the mass would be infitismly small.) LIGHT- It was there a little bit ago -now it’s here. It was there 10 light years ago -now it’s here; so we can see that star or whatever. Do a net search – there are a lot of easy explanations of his theories to be found. I’m so happy my college didn’t require me to have Physics.

#790682

It was there 10 light years ago

A light year is a measurement of distance, not time

#790683

health,

r u coming to answer my orriginal question??

#790684
Health
Participant

To YWM-80,

It’s also denoting a certain amount of time. It takes time for the light emitting from that planet or star to be seen here on Earth.

#790685
Health
Participant

To the guy who’s bored,

I’m not. (Also, I’m not bored.) But I think others answered your original question. If that doesn’t satisfy you -do a net search. There are many sites that put it on an easy level. I’m coming to answer your second question.

#790686

A light year is by definition a measurement of distance. I don’t think you could find a single physicist who would not agree indisputably with this statement (maybe a few high school science teachers).

Since the distance measurement is based on the speed of light you can easily infer a time measurement from it as regards light, but it is a distance unit. It is not a unit of volume, weight, mass or time.

This is confusing for many people so I wanted to clear it up.

from Wikipedia: “A light-year, also light year or lightyear, (symbol: ly) is a unit of length” It does not mention that it is also a unit of time.

#790687

from wikipedia

Tachyon

In special relativity, while it is impossible in an inertial frame to accelerate an object to the speed of light, or for a massive object to move at the speed of light, it is not impossible for an object to exist which always moves faster than light. The hypothetical elementary particles that have this property are called tachyons. Their existence has neither been proven nor disproven, but even so, attempts to quantize them show that they may not be used for faster-than-light communication.[23] Physicists sometimes regard the existence of mathematical structures similar to tachyons arising from theoretical models and theories as signs of an inconsistency or that the theory needs further refining.[24]

#790688

#790689

the question was: “does anybody here understand the theory of realativity??”

#790690
WolfishMusings
Participant

Health,

I think you’re a bit confused on this. A light-year is a measure of distance — nothing more. It is the amount of distance that light can travel in one year (hence the name). It is equal to 4.5 trillion miles (approximately).

Also, the mass would be infitismly small.

Actually, the opposite is true. Objects *gain* mass as they approach c. I used to joke that if I excersized (by running) I would actually gain weight (because I was a bit closer to c.*

4) A cracked whip’s tip (not the entire object, but a portion of it).

Yep — that’s the answer I was looking for.

The Wolf

* Yes, it’s a joke. Obviously I can’t run that fast for it to make a bit of difference.

#790691

Even if you could run fast enough, I think you would lose the weight as you stopped running. But you would still be physically younger (or older?) than you would have otherwise been.

#790692

haha. that u understand. but the question that if at the speed of light u exceed the boundaries of time then how can it take time (which is within the boundary of time) to travel??

#790693

I don’t understand it at all, it may be that no one understands it in a real sense, but this is what I understand from Dr. Gerald Schroeder’s book: “The Science of G-d”

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.

Depends on the persective of the system you are in relative to the other system of interest. I think that is also kind of the nutshell of the theory in general

#790694
WolfishMusings
Participant

ZachKessin has a degree in physics. I’m waiting for him to weigh in.

The Wolf

(get it? — relativity, mass dilation, weigh in…)

#790695

that i understand. i actually heard him (Dr. Gerald Schroeder) speak at a seminar. but what i dont understand is how can it take time for us?? like this- if theres no time then theres no now and later, before and after. if so how can the beam get to earth after it was set off even in our sight??

#790696

I understand he is quite a distance away from his special computer having a general eye check up with Dr. Einstein, which is taking some time as his pupils have to dilate first.

#790697

So you want to understand Relativity

Okay, here’s a basic introduction to the subject

#790698

#790699
ZachKessin
Member

Sorry I would have commented sooner but I had a crazy busy day.

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. (though with more math) Every physics department in the world starts by teaching Newtonian physics, I know mine did.

One important point about both SR and GR is that they do not get rid of Newtonian mechanics, but extend it. Newtonian mechanics are a special case of relativity. 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. In terms of the solar system the only place where they give different results is in terms of the precession of the perihelion of the planet mercury, and that effect is measured in 10’s of arc-seconds per century.

It terms of time dilation the amount time slows down on a moving body is with a factor of the sqrt(1-v^2/c^2). So the closer you can get to the speed of light the easier that is to measure. In terms of subatomic particles which can get VERY close to c it is quite easy to measure. In terms of more macroscopic particles it is harder. But the best atomic clocks can be accurate to 1 part in 10^14, so for example an atomic clock on a GPS sat in orbit will not run the same as a clock on the ground. The effect is small but it is there.

GR came about when Einstein decided to ask a very simple question, he noticed that gravitational mass and inertial mass are always the same and wondered why that should be so. He then spent 10 years working out the math to turn it into something that you could actually use, and missed the chance to predict the big bang 15 years before Edwin Hubble found it.

If people have specific questions please ask them and I will try to respond (or ask someone who would know)

#790700
ZachKessin
Member

Oh one more FYI, Alpha Cent is actually a Triple star not a double, well probably its not 100% sure that proxma is actually in orbit about the A&B stars.

And if you postulate a Earth centered system you still have to explain the orbits of the other planets (remember them) which from the point of view of a fixed Earth are just strange. Oh and the Coriolis force, and the phases of Venus, and stellar parallax, and orbital dynamics.

#790701
HaLeiVi
Participant

Let’s try to clear up a couple of things.

First off, regarding the orbit of Earth, I always notice a big confusion. There are two aspects: The yearly orbit around the sun, and the daily revolution.

When speaking about how relativity reconciles the two, we must realize that that applies to the yearly orbit. That doesn’t help anybody stick up for Chazal since they are never referring to this orbit. For all you know, they would agree that Earth revolves around the sun once a year.

As far as the daily revolution, dd already (finally) explained that we actually can prove that the earth rotates by testing the effects of centrifugal force.

Now, back to relativity. Let’s understand, that the whole thing started when it was noticed that light always seems to be traveling at the same speed, even when you are traveling away from it.

Based on this, Einstein decided that the speed pertains to how fast it will get to you. Since light, he says, travels at top speed, it will always get to you at the speed of light. This should answer someonesbored and Mod-80.

If it makes sense in our frame of reference then it makes sense. As far as we are concerned, we are going around the sun and everything else stays put.

#790702

Okay, I think I can explain the answer to your question pretty simply:

Since time is relative to considerations of space as applied to a particular system which is fixed relative to the other system to which it can be compared, the perception is relative to the observer and his location both in terms of time AND space (leaving out gravitational effects), considering his absolute frame of reference relative to a moving frame of reference that can be considered absolute only relative to the other frame of reference when moving in an absolute time-space continuum that is relative to the originally considered time-space continuum. Of course this is true only if you consider both frames of reference as independent only in a relative sense, or absolute if you consider both frames of reference to be linked in a universal manner, with totally independent matrices on both relevant axes.

Now assuming you have understood me so far the conclusion can be easily reached that therefore the two perspectives will be separated by a differential consideration of perspective equal to the sum of the speed of each system as measured from the other one, minus C as measured from a third unrelated system.

:-;

this was a joke of course

#790703

what i still dont understand is that. how can light be first here then there if its not bound to time?

#790704

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?

Do you mean to state that if a ball going exactly 30 miles an hour from my perspective, passes another ball going in the opposite direction of the first at exactly 30 miles an hour from my perspective, that an observer on one ball (with a perfect measuring device) will measure the speed of the other ball at 60 mph EXACTLY?

#790705
HaLeiVi
Participant

By the way, the issue Mod-80 brings up reminds me of Rav Saadya Gaon’s Kasha/Raya that if the the universe is infinite, then that would mean that the far ends of the universe travel an infinite distance every day. Traveling an infinite distance would, of course, take forever. Therfore, it would never get back.

Another way to by; although people today still discuss the issue of what revolves around what, based on Mamarei Chazal, it would be interesting to note that the Maharal, who wasn’t that much later than Copernicus actually embraced the idea. That is without all the present day evidence. He writes in the fifth (I think) Perek of Nesiv Hatorah, that “someone recently came up with a new neat Astronomy that turned the whole Astronomy of all the years on its head.”

That, to me, sounds like he liked it. And we all know that the Sifrei Maharal are dedicated to honoring the words of Chazal. I know that bringing up this topic causes the thread to be closed, but I couldn’t help it. I never got a chance to mention these points anywhere before the thread closes.

So, quick! Somebody, change the topic back.

#790706

HaLevi you have it exactly backwards. The whole controversy is in trying to advance the notion that the earth doesn’t rotate, but the universe revolves around it. The orbit of the earth around the sun is not the issue. The question is does the earth revolve or does the sun rise and set in a real sense. It’s not a question of a yearly phenomenon but a daily one.

As far as centrifugal force and other measurements, the proponents of this theory explain that it is the combined gravitational force of the universe as it revolves around the earth that causes those effects, not centrifugal force. I have no idea if this makes sense or not.

The notion of whether the sun revolves around the earth or vice versa is another issue entirely, though also interesting it has nothing to do with einsteins relativity, only with the general notion that movements are relative to one’s perspective.

#790707
Health
Participant

To all,

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