Some questions about relativity

Well, there are some questions about relativity/the speed of light that I have.

We all know about the example of brother A and brother B. Brother A stays on earth and brother B goes off at the speed of light. When the brothers meet, they realize less time has passed for brother B.

But who's to say it's brother B that sped off? One could be perfectly justified by saying that brother B wasn't moving and it was in fact A that was moving away...or they were both moving. If any motion is relative, who knows who's really moving?

Or we could make it a bit different. Lets say you have some galaxy that somehow stays together while rotating at a significant portion of c. If brother B turns his ship around and goes against the motion of the galaxy, in my spaceship it looks like B is the one staying still (when B started, he was moving with the galaxy) and A is on the planet earth going at breakneck speed. So then I should see brother A as younger, and brother B should see A as older, but then you could say that brother B should see A as younger because A was moving away at almost c. Either way, outside the galaxy, I'm one hell of an old fart when they see me.

So how do you decide who is moving? And wouldn't you have the two brothers each thinking that they were the younger one, and that they were both the older one, at the same time? Hopefully I've managed to make some snese with my question, so can anyone help me out with how this works?

Thought this might help you out a little.....It's a little bit easier to read:

The Twins

Perhaps the most famous of the paradoxes of special relativity, which was still being hotly debated in national journals in the fifties, is the twin paradox. The scenario is as follows. One of two identical twins is an astronaut. He sets off in a relativistic spaceship to alpha-centauri, four light-years away, at a speed close to that of light. When he gets there, he immediately turns around and comes back. As seen by his brother on earth, the astronauts clocks ran very slowly, so although the trip took over eight years by earth time, the astronaut has only aged by, say, one year. So as he steps down out of the spaceship, he is seven years younger than his twin brother. But wait a minute-how does this look from the astronaut's point of view? He sees the earth to be moving close to the speed of light, first away from him then towards him. So he must see the clock of his brother on earth to be running slowly. So doesn't he expect his brother on earth to be the younger one after this trip?

The key to this paradox is that this situation is not as symmetrical as it looks. The two brothers have quite different experiences. The one on the spaceship is not in an inertial frame during the initial acceleration and the turnaround and braking periods. (To get an idea of the speeds involved, to get close to the speed of light at the acceleration of a falling stone would take about a year.) Suppose the two kept in touch with each other by flashing a light once a month, using their own calendars, so the other could keep tabs on his brother's age by counting flashes. After the initial acceleration, the two are parting at a constant rate, and each will see the other's flashes at some steady rate which will be less frequent than monthly, because the other guy's clock appears to be running slowly, and also the distance between them is increasing, so each succeeding flash has further to travel. When the astronaut turns around, however, halfway through his trip, he will be traveling towards the light flashes, each succeeding one has less far to travel, so he will see the flashes coming in at a faster rate, in other words, he will see his brother on earth to be aging rapidly. From the earthbound brother's point of view, after he sees the astronaut to turn around, he sees the astronaut to age at the rapid rate. But since the astronaut is traveling close to the speed of light, he arrives back on earth very shortly after his brother on earth sees him turn around! Thus for the brother on earth, watching the signals coming in from the spaceship, he sees them coming in at the slower rate for almost the entire trip, whereas the astronaut looking at the signals from earth sees them to come in at the slow rate for the first half of the trip, until he turns around, and after that, they come in at the fast rate-so the brother on earth ages more.

Another interesting perspective:

http://www.ebtx.com/ntx/ntx28c.htm

The thing that distinguishes them is the acceleration, I believe. On the other hand, is it possible to distinguish acceleration from gravity when inside a sealed box? Hmmm. :)

I suppose I wasn't comepletely clear with my first post, so I'll restate it: Basically, who is the one moving? How do you decide which one is younger? The fact that one is attached to a lump of metal that provides the acceleration isn't realy enough to say which is the one going almost c.

Quote:

---

Originally posted here by thesecretfire
I suppose I wasn't comepletely clear with my first post, so I'll restate it: Basically, who is the one moving? How do you decide which one is younger? The fact that one is attached to a lump of metal that provides the acceleration isn't realy enough to say which is the one going almost c.

---

Hmm. I hate trying to answer these questions, I end up confusing myself over time. The thing is, brother B experienced acceleration when he sped up and left and when he slowed/changed direction to come back. The stationary brother didn't. I guess you could say that motion is relative, but inertia is not.

And then we get into how acceleration can be indistinguishable from gravity, under certain conditions... bleh.

Your point about acceleration is good, I hadn't considered that. But then again, the whole thing with time (and space and weight) dilation is just based on velocity, so acceleration shouldn't matter, should it?

Acceleration? What?

Really, relativity can't be applied in terms of a quantity of matter like us. It's more available at particles level. The greater the size, the less appliability (is it the word?) of relativity. Anyway, you can continue... ;)

When you accelerate, you change your speed. But it isn't speed itself that matters, it is the acceleration. Acceleration uses energy, and that energy pools to mass.... blah blah blah changing metersticks blah blah blah. "Speed" is just a byproduct, in a sense...

Remember all, speed=distance/time
Acceleration is just the rate of increase in speed.

What I will say is that, everything is relative. That is the one thing that must be accepted to first begin to understand Physics. Also remember that the speed of light is a constant. Its speed is always that of c. This constant is not changed by velocity of the object which it leaves. Also spacetime is determined by the position that one is relative to the other object. This is the other end of the constant theory. You can never see time "go faster" than it is currently moving for you. If you are moving away from a point "let's say earth" in a space ship, earth's time will appear to be moving slower than your own, this would be the same as that of the earth observers watching you. So to answer your question, the 2 brother theory remains correct regardless the relative speed of the object the observer is on "earth". And no, you will not age slower on earth because you are moving relative some point outside the galaxy. The brother that left will age slower because he left the point you are on, and you will age faster becuse your speed remained slower than his, as you stayed on the earth.

Quote:

---

The brother that left will age slower because he left the point you are on, and you will age faster becuse your speed remained slower than his, as you stayed on the earth.

---

If "everything is relative", how do you know that "the brother who left" was the one
actually moving? If he was (relatively) stationary, while the earth "left him"
would the results be different?

No, the answer is that the brothers aged at the same rate since
each experienced time in his own frame of reference and there
is no way to simultaneously measure two events happening
widely separated by distance, and moving relative to each other.

As one brother is "outbound", they may indeed age at different rates,
but when he turns around and accelerates back toward earth,
the process reverses, so that by the time he gets back
they are the same age once more
:cool:

Quote:

---

Originally posted here by rcgreen

As one brother is "outbound", they may indeed age at different rates,
but when he turns around and accelerates back toward earth,
the process reverses, so that by the time he gets back
they are the same age once more
:cool:

---

Uhm... no. Do you want me to elaborate on that? :D I'll have to use the ship-and-earth light clock example.

Quote:

---

Originally posted here by Terr
When you accelerate, you change your speed. But it isn't speed itself that matters, it is the acceleration. Acceleration uses energy, and that energy pools to mass.... blah blah blah changing metersticks blah blah blah. "Speed" is just a byproduct, in a sense...

---

Well, okay, but you CAN still say that acceleration is relative. The velocity of their difference in speeds is increasing in a negative direction. (For example, two cars heading going the same direction have a difference of speeds (velocities). If one accelerates, the difference of speed is increasing/accelerating. Just put this in to clear up my extremely unclear way of stating things)
So if their difference of speeds is changing, it's still hard to say who is accelerating. Also, it's important to note that brother B doesn't see his own spaceships mass changing (I don't think so, anyway...he doesn't observe it getting shorter, so I don't think he'd observe it's increased mass) , he sees brother A's mass changing and the earth's mass changing.

*SecretFire is struck down by the grammar gods for bracket abuse*

Well.. To put it as simply as I can...

The thing your saying about who is moving is a vlaid point. If you and a friend were in cars next to each other and one started moving, would you know who is moving? Of course. How? Because you will see the car moving RELATIVE to the ground. Now, if you and your friend were in two space ships in space, and one started to move, who is actually moving? You can't tell because you have no reference point like the ground. So yes, everything is relative.

Now, the reason of the twin paradox. The reason behind the age difference has to do with mass. The brother on Earth (A) remains the same mass because he is not moving, (relative to the earth that is). Now, lets take brother B getting wisked off at c (constant of light). Here.. As an object approaches the speed of light, its mass approaches infinity. Thus why the speed of light can't be broken, you would have an infinit mass and that would take infinit energy + 1 to push you past it, which doesn't happen. Okay, so, brother B continues towards the speed of light, therefore his mass is increasing. Now according to the theory of gravitational time dialation, time is slower near higher gravity sources. Since brother B's mass is increasing, so is his gravity, therefore warping time and making his time slow down compared to that of the brother A on Earth.

Well, thats what I think.. Hope you could follow it.

Uhm... no, acceleration is NOT relative... because... here it comes... you can feel it. If I am in a ship, and another ship with person B is nearby, and one or the other starts to accelerate, I can tell if it is me because my inertia will cause me to push on the ship as I accelerate with it.

You can detect acceleration, or changes in speed. You can't detect (from within a system) constant speed... unless you have some atomic clock that was already calibrated at a known speed... which takes us back to relativity again. But I digress.

Okay here we go...

Quote:

---

Some excerpts from The Universe in a Nutshell by Stephen Hawkings
In June of 1905, Einstein wrote a paper that pointed out that if one could not detect whether or not one was moving through space, the notion of an ether was redundant.

Ether: A hypothetical nonmaterial medium once supposed to fill all space.

Instead, he started from the postulate that the laws of science should appear the same to all freely moving observers. In particular, they should all measure the smae speed for light, no matter how fast they were moving. The speed of light is independent of their motion and is the same in all directions.

This required abandoning the idea that there is a universal quantity called time that all clocks would measure. Instead, everyone would have his or her own personal time. The times of two people would agree if the people were at rest with respect to each other, but not if they were moving.

This has been confirmed by a number of experiments, including one in which two accurate clocks were flown in opposite directions around the world and returned showing very slightly different times. This might suggest that if one wanted to live longer, one should keep flying to the east so that the plane's speed is added to the earth's rotation. However, the tiny fraction of a second one would gain would be more than canceled by eating airline meals.

Einstein's postulate that the laws of nature should appear the same to all freely moving observers was the foundation of the theory of relativity, so called because it implied that only relative motion was important. The theory of relativity, although not at the time, is now completely accepted by the scientific community, and its predictions have been verified in countless applications.

Although the theory of relativity fit well with the laws that governed electricity and magnetism, it was not compatible with Newton's law of gravity. This law said that if one changed the distribution of matter in one region of space, the change in the gravitational field would be felt instantaneously everywhere else in the universe. Not only would this mean one could send signals faster than light (something that was forbidden by relativity); in order to know what instantaneous meant, it also required the existence of absolute or universal time, which relativity had abolished in favor of personal time.

Einstein realized that there is a close relationship between acceleration and a gravitational field. Someone inside a closed box, such as an elevator, could not tell whether the box was at rest in the Earth's gravitational field or was being accelerated by a rocket in free space.

If the Earth were flat, one could equally well say that the apple fell on Newton's head because of gravity or because Newton and the surface of the Earth were accelerating upward. This equivalence between acceleration and gravity didn't seem to work for a round Earth, however --- people on the opposite sides of the world would have to be accelerating in opposite directions but staying at a constant distance from each other. This would literally rip the planet in quarters.

---

Hope that helps. Let me know if you have any other physics questions. I have a library full of the stuff.

There is a piece of news about this at the moment, regarding the use of the International Space Station to verify Einstein's theory. A summary is at http://news.bbc.co.uk/hi/english/sci...00/2033324.stm