h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Relative to the sun I am moving at one velocity, relative to the center of the galaxy another, relative to my chair I'm stationary, do I then have multiple masses ?
Or are you trying to suggest that there can be such a thing as an absolute vale of velocity without it being relative to another point withing space ?
I may be missing something here, but its too early in the morning for me to spot it if I am.

You know, that's one of the things about relativity that has always bothered me, too, and I've no idea what the answer is. Worse, I can't remember ever even seeing the question addressed in any of the books I've read about it. It's a real poser... if there's no absolute frame of reference, how can you say "I'm doing 20% of lightspeed"? Well, yeah, but we all are right now, relative to certain bits of matter being ejected from gamma ray bursters...

I wish I knew who the best person to ask about this is. I'm only sorry it's not me. If you do find out, would you let me know?

H.

I also don't like the absolute limit of the speed of light, it doesn't make sense in similar terms.
For example, from a stationary point P two projectiles are launcehed along the same axis in oposite directions aeach at 0.75c (c is light speed) call these A and B. special relativity states that the maximum that this distance between A and B can increase at is c. I argue that this is so self evidently flawed that physicists everywhere have called it anti-intuative to defend einstein. I would suggest that maximum percivable velocity is that of light, but that this is not a limit on velocity.
Its been a few years since I have studied physics seriously, but back then I was too awed to argue with my lecturers over such an established theory, no matter that I had these thoughts back then.

A more practical example just occured to me...
Take an open light sourch say a light bulb, measure the speed of photons (which after all are considered to be energy waves and _particles_) in oposite direcyions, and sum the magnitude of their velocities, this should if special relativity is correct be c or less.

or is my thinking and understanding incorrect in some aspect ?

Yeah, I think it is. I think this is one of those things where the universe doesn't really care that it doesn't conform to common sense. That's pretty much the lesson of 20th century physics - common sense is generally OK at the human scale, but above *and* below that, the universe just ain't like that.

As for your two ships parting at 0.75c each... common sense would suggest that each would see the other recede at 1.5c. What would actually happen? I don't really know. But I DO know that it would:

a: be weird
b: involve an observer on one ship looking at the other ship and seeing it apparently shrink and get heavier, or something
c: involve that same observer seeing some sort of time dilation effect with respect to the other ship
d: all of the above being somehow provably true using particle accelerators.

The thing that always baked my noodle was this: the twin paradox, right?

Twin 1 does his astrophysics degree and volunteers to be on the first ramship to Alpha Centauri.

Twin 2 settles down with a nice girl and carves otters out of soap, or something.

Twin 1 accelerates to 0.75c on the way to Proxima, hangs about there for six months or so, and comes back at a similar speed. He's aged five years, but his twin brother is a doddering old geezer.

All fine and good, except how did the universe know which twin was moving and which was staying still? Depending on how you look at it, Twin 2 moved away from the ramship at 0.75c, using the earth...

I've always assumed that the application of an external force to generate an *acceleration* was what caused the difference - but everything I've read says it's the VELOCITY, i.e. ds/dt, NOT the acceleration, i.e. dv/dt or d^2s/dt^2, which matters in calculations of tau.

It's late, and I now have a headache. Bizarrely, I'm grateful to you for it...

H.

About the twins, the reason the universe knows is that the one who travels turns around, while the other one doesn't. If he just kept going away and didn't stop at Alpha Centauri, each twin would see the other as older. At least, thats what I read in a book by Steven Hawking or Martin Rees. Please don't ask me to explain it.

I don't study physics, but...
it seems like what people are saying is that there is no univeral constant since every thing can be going at a different speed in relation to something else. Take this example,

2 cars each going 60 mph in the same direction pass you who is standing on the side of the road, you hear a sound that is high, then drops off suddenly (the doppler effect) but the to car B the sound is flat and constant. Why does the sound seem higher and lower to the person standing on the side of the road, and constant to the other car? This is so because in car A in relation to the person is going 60 mph so the doppler effect takes place. However to the other car A in relation to car B is going zero mph because the tone stays the same.

but, oh, if, oh, ah, Hmm, I'm doing a passing impersination of Mr Dent I think my head might explode But, speed (velocity?), how's that affected by the galixy we are in moving, constantly expanding universe, and all that.... I think I need ot jump in a cold bath

I'm a second year physics student, so I should in theorey be familiar with some of the more detailed lies. In practice, I did all that before the summer, so I'll have to see how much I still remember. I think that it basically comes down to the fact that there is no universally 'correct' frame of reference. Due to the fact that you can only measure things from where you are, thingas get odd. For example, the spaceship problem:

Frame 1: that of an observer on a planet direcly in-between the two ships.

From this observer's point of view, both of the ships are traveling at .75c, and the distance between tehm is increasing at a rate of 1.5c. This is perfectly acceptable, as, in this frame of reference, nothing is moving faster than 1c, they are just moving away from one another.

Frame 2: that of an observer on one of the ships... this is where it gets odd.

as an observer in ship A cannot get data on ship B instantly, it must wait for a transmission to travel from A to B and back again, during which time, A has moved forwards a fraction, affecting the data. The net upshot of all this, is that from the point of view of an observer at A, the ship B would be traveling at probably around .85c, I can't remember the actual equations, and I'm about 120 miles from my text book.

Note that although the movement of the ships affects the data, this does not invalidate it. As I said earlier, there is no 'correct' frame of reference from which to take 'absolute' readings, so the data gathered in the frame of reference of the ship is as valid as that from the frame of reference of the planet. In fact, it should be noted that the speed of the ships is merely measured as being .75c in that frame, just as it is measured to be a different value in the frame of the ship.

There is a lot of complicated maths associated with converting between two inertial (constant velocity) frames of reference which are moving relative to one another, which results in a short (about 5 or so) list of equations for translating between the two.

hope this helps/confuses/provokes some discussion/isn't as innacturate as I have a feeling it might be.

Sir! Sir!;
Permision to jabber insainly

Permission granted

apparently this is something that isn't counter-intuitive once you've done a bit of it... I'm not sure whether I want to reach that stage or not

Physicists use the model of the light-cone, or time-cone (a pictographic representation of human perception, with the point of the cone pointed downward) to explain the effect of two objects moving away from each other at a relative speed greater than c.

The point at the bottom represent your current point in time (which of course continues moving downward), the sides of the cone reaching upwards represents your perceivable portion of the universe.

An object moving away from you at 1.5c relative will be outside your 'cone of perception' because light (or any information) from that source can not reach you. Ever. Therefore, relatively speaking, it does not exist (in your perceivable Universe), and Universal Constant is maintained.

Remember, we are not really measuring the Universe in physics, we are measuring Human Perception only. We can never really know ‘what’s out there' because we are limited to measuring it through our programmed human perception.


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I put the mass increasing with velocity thing to my Physics teacher a while ago, and he said that's actually another lie that was made up so that people could keep using F=ma (or Ft=mv-mu, if you like). Mass is, apparantly, only dependent on the type and number of atoms.

As for the photons moving at opposite directions, remember that both time and space are distorted. As time dilates, space is compressed or something along those lines.

Urgh.

Remind me not to do a Physics degree.

Mass distorts spacetime.
Fast moving objects distort spacetime more than slow moving ones.
Therefore fast moving objects have more mass than slow moving ones.
QED - I hope

Noggin

My understanding is that there are several types of mass. Rest mass never changes and had to do with the number and types of atoms.

Inertial mass increases with velocity. Inertial mass has more meaning to the universe, but rest mass is used so that all objects can be compared without consideration of velocity.

I think this is true.

Well, I'm afraid that's an appalling example of logic there Nogin. Never mind though, the inertial mass and rest mass certainly seems to make sense.

How do we know inertial mass increases with velocity? Because mass *is* the distortion of spacetime and the distortion of spacetime increases with velocity.

And that's an appalling example of spelling, Boncy.

Noggin

I think I agree, but I'm not sure I understand.

"I don't understand a word you say, but I will defend to the death your right to say it."
--Paraphrase of Voltaire.

Sorry about mis-spelling your name.

Yes, the logic works with the word "is" instead of "causes". Thanks, and sorry for being pedantic.

Oops, missed the final reply. No need to apologise, and my tongue was firmly in my cheek on the spelling.



Noggin

Frames of reference are only equally valid if the do not accelerate and decelerate. Thus, the twin who went on the spaceship had to accelerate to get up to 0.75c, and then decelerate again to return to Earth (assuming that he didn't stop when his ship turned around). Because of this, his frame of reference (that the Earth twin should've only aged 5 years) is not valid. According to the Universe, the twin on the spaceship moved because of the acceleration.
Hope this helps.