# Questions on the Theory of Relativity?

I’m a novice to Science and have some questions on the theory of relativity concerning the twins idea. Where one twin leaves Earth and comes back and finds he’s twin twenty years older then him. Is this becasue of the speed he travelled through space or gravity. Or something to do with both? I would be very happy if you could awnser these questions in particuler and what the theory of relativity is in general?

It’s the speed, Pieinsky. That motion through space comes at the cost of “local motion”. If you’re moving through space real fast, things like atoms and clocks can’t move as fast as when you’re standing still, because a slice of their motion is diverted into the motion through space. Here’s something that partway explains it : Simple inference of time dilation due to relative velocity. The Lorentz factor is just Pythagoras’ theorem really.

Gravity is similar but different, because of “the principle of equivalence”. Things move slower down near the surface of a planet, not because yyou’re moving fast, but because space is different. Everything moves slower, even microwaves and atomic clocks. That’s why we have the GPS clock adjustment. Check out the threads I’ve started or relativity+ for more. It’s really simple, and you can understand it, easily. Gravity works something like this:

Imagine a swimming pool. Every morning you swim from one end to the other in a straight line. In the dead of night I truck in a load of gelatine powder and tip it all down the left hand side. This starts diffusing across the breadth of the pool, imparting a viscosity gradient from left to right. The next morning when you go for your swim, something’s not right, and you find that you’re veering to the left. If you could see your wake, you’d notice it was curved. That’s your curved spacetime, because the pool is like the space round a planet, the viscosity gradient is like Einstein’s non-constant g[size=85]μν[/size], and you’re a beam of light. As to how the gradient attracts matter, consider a single electron. We can make an electron along with a positron from light, via pair production. Since the electron also has spin, think of it as light trapped in a circular path. So if you’re swimming round and round in circles, whenever you’re swimming up or down the pool you’re veering left. Hence you find yourself working over to the left. That’s why things fall down.

Excuse someone who’s just a novice in their weak understanding but are you basically saying relativity in terms of speed is different entirely from the one of gravity. That their completely unrelated or that if they are related even so when one is on its own the rules of relativity still apply.
So say there was no gravitational bodies( I know this doesn’t make sense) and a twin was on a planet (say ridiculously) there’s no gravity and the other came back after traveling very far through space would he be younger then he’s twin?

Pieinsky, you are dealing with multiple concerns all at once and perhaps getting them confused.

Galileo introduced the idea of relativity back in the 1600’s as he expressed the simple logic involved (“Galilean Invariance”). The simple logic is that addition of speeds is addition of speeds regardless of where the observer might be. From a gun’s perspective a bullet is leaving at the same speed whether the gun is moving or not. Yet from an observer watching a gun racing by and a bullet flying out of it, the bullet is moving at the gun’s speed PLUS the speed that the bullet was propelled from the gun. But back at that time, the idea that light always travels at the exact same speed regardless of how fast its source was moving hadn’t occurred to anyone. The light that rushes out of a flashlight leaves the flashlight at light speed only, whether the flashlight was speeding along or not (unlike the gun and bullet). They could have deduced it, but I suspect they were too busy arguing.

Much later, Einstein more precisely approached the concept realizing that simple addition of speeds can’t work if light is to always be constant speed (which it must be). In working with detailed physics relationships, equations can be worked out that show how to more accurately add speeds when it comes to something moving near the speed of light and thus reaching a speed that cannot be allowed to be added past (“Special Relativity”).

But it was finally realized that actually everything is literally made of the exact same “stuff” (electromagnetic waves) as light and if light has a constant speed, then the inner bits and pieces of every atom and particle must also be spinning at a constant speed. That thought leads to having to apply Einstein’s equations to every tiny particle and every motion within anything (its aging).

The end effect of such a study reveals that when an object is moving close to the speed of light, every tiny motion within the object is reaching a foreboding maximum and thus everything within the object must be approaching the exact same speed as everything else in the object. But if everything is moving at the exact same velocity, that means that time is not progressing for that object because time is merely a measure of the relative changes in position of the bits and pieces and in such a case, there is no relative changing in position going on - time stands still - nothing ages for that object.

But of course, we said the object was moving near the speed of light, but in reference to what? Well that is your other twin proposed to not be moving. So you have one person aging normally and the other person speeding so fast that every particle within is not changing its positional relationship to every other particle within and thus not aging.

All of that is the concern of “Special Relativity”. But as it turns out, light doesn’t really always travel at the exact same speed. If light encounters a gravitational field, it slows. This concern has little to no effect on our aging issue because all of the bits within the person would be encountering the same gravity field. But trying to be even more exact in calculating what is really to be expected as high velocity effects take place, we must include the fact that gravity changes the speed of light and we end up with “General Relativity”.

So, to sum it up a little;

1) Galilean Invariance is the simple logic that an observer must add speeds relative to himself.
2) Special Relativity reveals that relative changes (time) slow down when anything is moving near the speed of light.
3) General Relativity includes for us the concern that light (and thus all things) slow down even more when in a gravitational field.

The twins issue is really only a concern of special relativity.

The more interesting question then becomes;

If motion is relative, then we can look at the same scenario from the eyes of the twin in motion (twin B). From that perspective, it is the other (formally non-moving twin A) that is moving. But if that is the case, then twin B sees twin A racing at near the speed of light (rather than the other way around as earlier) and when he stops, the twin that was moving, twin A, is now much older, not younger. Emm… what happened?

Thanks guys, I think you answered my main question that I desired to know which was that special relativity can be separate and happen without general relativity.
I have another question does special relativity only happen when one being goes near the speed of light. Do they have to go that fast?
My other question is there only one closeness to speed of light that involves this relativity. Or are there different degrees of it?

Special relativity actually applies to all relative speeds, even zero. But it is a waste to calculate with special relativity equations until the object is getting close to the speed of light because the difference in using or not using special relativity equations at lower speeds is minuscule and not worth the trouble. Special relativity does not suddenly come into effect. It is always in effect. It just becomes significant enough to bother with once you are dealing with near speed of light objects. The closer you get, the more relevant the special relativity concerns become.

Another thing that is almost never pointed out is that the time or aging for the speeding object does not actual get to zero even if the object were to travel at 100% light speed because it can only travel in one direction at a time. The time related to that one direction would become zero, but the average time involved in aging for example, deals with travel in all directions at once thus the entire aging process would never actually stop even if the person were truly at 100% light speed (which he couldn’'t do anyway).

Similar time dilation can occur due to one of the twins being surrounded by an extreme gravity field compared to what the other twin is in. Neither has to be moving relative to the other, but the one in the high strength field will age slower anyway. I think the TV series Andromeda was based on the idea of a star ship that had become caught in the event horizon of a black-hole and held in stasis for 300 years until someone pulled it out.

No. If you’re going fast all subatomic local motion has to be going slower, because the speed of light is the overall limit. Hence you end up time dilated. If you’re down near the surface of a planet, local motion is going slower too, and again you end up time-dilated. So they aren’t entirely different. They’re related by the principle of equivalence.

Yes. The travelling twin is time dilated because his motion through space was at the cost of local motion. He has lived a slow-motion life whilst travelling through space.

Like James said, there are different degrees of it, but if your motion is slow compared to the speed of light, you don’t notice much time dilation.

I have to point out that in fact, the twins paradox is an issue of GR. Take two twins, called A and B: both are living on the International Space Station. Twin A decides to take a trip somewhere across the galaxy at close to the speed of light. In order to do so, his spaceship must accelerate. Once he reaches his ‘cruising’ velocity (say 99% of c, the speed of light) he stays at this velocity for a while. Let’s assume he has a telescope pointed back at the ISS, and can (somehow, despite the massive red-shift) see twin B’s correspondingly zeroed clock. If he compares the rate of his clock with that of twin B’s clock, he notices that B’s clock’s hands are turning much slower than his own clock’s. Let’s also assume that B has a telescope trained on A’s spaceship, and can see A’s clock. Here’s the weird bit: Twin B notices that Twin A’s clock’s hands are also moving slower than his own clock’s! The twins are both moving relative to each other at 99%c (as there is no absolute frame of reference), and are both noticing the same time dilation effect. Freaky, huh?

Twin A now decides that he’s travelled far enough, and wants to return to the ISS and see his brother again. So he has to decelerate his spaceship, and then accelerate in the opposite direction (both of which could be modelled as a single acceleration pointing backwards in the direction of the ISS). As he approaches the ISS, he again decelerates, then comes to a stop and docks with the space station. But when he gets off his spaceship and is greeted by his brother, he’s horrified to find that twin B is now an old man, whilst A is still in his early 20’s! The ‘paradox’ of the age differences can only be resolved when one considers the accelerations that have been applied to twin A; having been in an acceleratory field (which is equivalent to a higher gravitational field) for a portion of his journey, his clock has measured less time than the unaccelerated clock, and he has correspondingly aged less.

So, in order to really consider and think about this paradox, one cannot ignore the acceleratory effects on the one twin’s real time dilation, whereas if we were to consider it just in terms of Special Relativity, we would be forced to conclude that both twins suffer the same dilation with respect to each other.

I hope I made some sense. I’m aware of the hideous complications and headaches involved with thinking about these things, and I really don’t claim to understand General Relativity completely; only to just about get my head around the basic principles of what goes on.

My first post on ilovephilosophy.com I’m liking the high level of thought and intelligence that exists in this forum.

There is an easier solution… Resolve to the Twin Clocks Paradox

The thing that is required to solve the paradox is memory. Either they remember what has transpired concerning the accelerations or they remember who left the “Frame of Origin”.

Without memory, they are pretty much screwed regardless. But that should be expected since it is time they are trying to measure.

Regardless, James, it’s still clear that because acceleration is involved, General Relativity Theory must be invoked to resolve the true nature of what goes on. Neither twin has the ‘true’ picture; speed and time are subjective measures, governed by the observer’s position in an acceleratory field.

To me, it seems that this “Frame of Origin” simply complicates it further - there is no ‘real’ inertial rest frame, as that would require there to exist an absolute frame of reference. The only considerations one needs to take into account are spatial velocity, time dilation and acceleration. (Acceleration can be measured from one’s own frame of reference: it’s merely velocity that cannot be.)

But anyway, I think we’re both on the same track of understanding. I was just attempting to clarify it further for the sake of Pieinsky.

Not once you see how it works, MrMermaid. It ends up something like perspective. If I look at you when we’re separated by distance, you look smaller than me, but when you look at me I look smaller than you. Freaky huh? No. It’s similar when we’re separated by relative motion. When we look at each other’s local motion, it looks slower for both of us.

You need the accelerations to break the symmetry, but that’s all. If Twin A had employed the same accelerations but had cruised for longer, there’s a bigger difference in their ages.

Sure, you can’t ignore it. But IMHO it’s better to think of it as a change in motion rather than something that takes us into GR.

Thanks. But don’t think all this stuff is complicated. It really is simple really. There’s only really two things you need to know about special relativity, and the rest follows naturally. One is the simple inference of time dilation due to relative velocity. The √(1 - v²/c²) Lorentz factor comes straight out of Pythagoras’s theorem, where the hypotenuse is the light path, and is length one because we use the c=1 natural units. The base of the triangle represents your speed as a fraction of c. The other thing you need to know is that this applies to matter because matter is “made of light”, see pair production. Think of electron spin as light going round and round instead of reflecting back and forth, and you’re just about there. A lot of people aren’t keen on this “made of light” thing, but ask them about proton/antiproton annihilation to gamma photons, and they’ll go all evasive on you and say it’s too complicated to explain. It isn’t.

hat blog post has nothing to do with memory. And there is a flaw in that blog post: the writer says that syncing the clocks between two frames requires identifying a specific origin frame. However, this is not the case. We can always arbitrarily replace two frames with frames co-moving with those frames in order to have those two frames share an origin (or any other arbitrary spacetime location). This operation does not single out one frame as the origin frame.

The difference in the time of one twin is due to the different path length taken between the start and end point of the scenario. The travelling twin must change rest frames at some point in their journey, whereas the stay at home twin does not have to change rest frames. This limitation means that the travelling twin must have a longer spacetime length between the starting event and the ending event. GR is not required to resolve this issue. (A full discussion of the scenario can be found in B.F. Schutz, A first course in General Relativity.)

And do note that a reference frame is an artefact of measurement. You can’t actually point up to the night sky and say “Oh look, a reference frame!” When people are talking about SR and talk about being in a reference frame, what they usually mean is being in some state of motion. And that motion alters the way you see the world.

Thank you for pointing out that you don’t actually believe in relativity theory but rather your own pet theory (unproven, without the ability to describe a single physical scenario) in which there is an absolute reference frame (that we can never measure).

But it is not required merely to know which got older. If you want to know exactly how much older, you have to know every acceleration detail throughout the entire journey considering both parties.

Nonono… the Frame of Origin is not an absolute frame. It is merely whatever frame they started in when their clocks were in sync. They could have been zipping through space together at that time.

I see that you are struggling with reference frames, so i decided to lend a machette to your thicket, so to speak… -It is possible that there is a reference frame - space fabric. Think of it this way - what if the whole “Space” is not space at all, but instead it is packed full with infinitesimal particles that never move - just oscilate… When energy is applied to a certain point, it is actually applied to one of those particles, which continues to transfer the energy to another particle through transverse oscilation - if the oscilation frequency is such, we and our measuring instruments will detect that transverse wave as light, another time when the frequency is different it will seem as an x-ray… This would explain the dual nature of light, in the sence that light is neither wave nor a particle - but a wave of particles moving similar to the surface of the sea. It could be the reason why light can’t travel faster than c - just as the speed of sound is limited to the density of the medium the same principle would apply here - we could think of c - not as speed but as a transfer constant, the ability of the vacuum to convey energy through transverse waves…

The best analogy I have heard of is that light is like a crime wave - which is also a a probability wave.

If you look at a crime wave in terms of discrete crime events - it’s the particle view, but a(n idealized) crime wave also has a frequency, velocity and amplitude - the wave view. A crime wave - like light, is both and neither.

What you’re describing is essentially a luminiferous aether. Sorry, but it was disproved by Michelson and Morley in their famous experiment in 1887. The speed of light is constant in all directions and at all velocities, i.e. in all reference frames. There can’t be any fundamental reference frame in the universe: the closest thing to it is the CMB radiation, and the measurement of its perceived red/blue-shift from the observer’s own relative velocity.

No, i wasn’t talking about aether - if i remember correctly, aether relates only to space between material objects - I included matter into the equation in the sence that it is created by the mentioned waves - The objects themselves appear to move in much the same principle as the waves that created them, by shifting the peaks of the waves… it’s still a work in progress (I’m currently brushing up on my physics notes from school) but the idea is to think lateral, to attempt to find a new approach to an old problem… Actually this isn’t anything unheard of - it is based on the fact that matter=energy - and as energy travels through waves, and particles are made of energy, visible objects are made of particles - It follows that as composing parts are shifted through waves, the whole object is nothing else but a wave. So objects are not travelling through this quazi-aether - they ARE the same stuff…