# Moving through spacetime

Hey guys. I've read in brian greene's book that we're always moving through spacetime at C. Is this true? Someone posted a GREAT response last time, but I think I accidentally deleted the thread (deleting first post automatically closes thread?) Anyhow, I appreciate your input.

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Yep. You at you, sitting there... aging, metabolizing, producing heat. Your watch ticks and its battery is drained. Your heart beats and burns a little sugar. You are most certainly moving through spacetime.

The phrase 'at c,' however, is a bit loaded, since c relates space to time. It is better stated 'the magnitude of your four-velocity is always the same,' but that may not be the most eloquent way of expressing things.

- Warren

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I saw an example somewhere..im not sure if this is what u mean..but..

If u consider time as another dimension, and you are physically standing still in the other 3, then your entire velocity vector (which is always C) is intirely in the time dimension...as your speed increases in the 3 physical dimensions, more of the vector's components goes toward your velocity in the physical dimensions and less to the time demension until you reach a speed of C and none of the vector is moving through the time dimension, and thus time stops for you.

Originally posted by blike
Hey guys. I've read in brian greene's book that we're always moving through spacetime at C. Is this true? Someone posted a GREAT response last time, but I think I accidentally deleted the thread (deleting first post automatically closes thread?) Anyhow, I appreciate your input.

hey, are you the same blike from scienceforums?

Anyway, yes, its true. But, as you know, time is relative. Relative to another galaxy, we are moving at c, but from our perspective, we're not. You know, the Earth is supposed to be spinning at 1600km/hr. But it could also be c from another perspective.
I know what you're thinking; light is supposed to be the same in all inertial frames. But take the Lorenz trans. for example. They describe how mass appears different when viewed from different perspectives; not light. If you take this into account, then our speed, whether it be c or not*, is relative.

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MV,

If two objects detect that they are moving at c with respect to each other, then every other observer will also agree that they are moving at c with respect to each other. Your sentiment, that velocity is relative, is correct for all velocities except c. In fact, c is an absolute velocity, upon which all observers must agree.

Also, rotation is absolute (not relative), as are all forms of acceleration. You can build a device to measure the rotation of the Earth or the acceleration of car, and thus those quantities are not relative. Uniform linear velocity, however, is relative.

Also, the Lorentz transforms do not have anything to do with mass. The Lorentz transforms convert time and space measurements in one coordinate system into corresponding time and space measurements in another.

- Warren

Don't the Lorenz transformations make mass appear to be smaller? that is, the Einstein factors relating to the trans. are to specify the appearance of a mass.
Why is rotation absolute?

Originally posted by MajinVegeta
Don't the Lorenz transformations make mass appear to be smaller? that is, the Einstein factors relating to the trans. are to specify the appearance of a mass.
Why is rotation absolute?

No the Lorentz transformations are just used for converting spacetime coordinates from one refference frame to another, mass doesn't appear anywhere in teh Lorentz transformations. I'm not sure if this is why chroot said rotation is absolute but my guess is that its becuase rotation occurs about the object itself (i.e. the Earth spins about its center) and therefore you have a definite way of measuring it unlike with velocity where it will depend on who's measuring what the value is. What really blows my mind though is that rotations of molecules are quantized! Now that is weird, but its a little off topic here, ohh well...

plus
Originally posted by chroot
MV,

If two objects detect that they are moving at c with respect to each other, then every other observer will also agree that they are moving at c with respect to each other. Your sentiment, that velocity is relative, is correct for all velocities except c. In fact, c is an absolute velocity, upon which all observers must agree.
another.

Not necessarily.

Light moves at c away from me. Another observer sees the light moving at c, but I may not be stationary with respect to him.

Counterexample.

Why is light absolute?

Originally posted by MajinVegeta
Why is light absolute?
This is Einstien's hypothesis. I believe that historically he came upon this idea while thinking about E&M versus Mechanics. Mechanics clearly shows that the speed of an object is dependent on where you're looking from and this applies to everything, including light. But what bothered Einstein was that the laws of E&M were dependent on the speed of light which would mean that the fundamental laws of E&M would be dependant on who was looking. On the assumption that the laws of physics ought to be the same for everyone Einstein came to the conclusion that the speed of light must be constant for all observers, and special relativity flowed out of this. At least I think that's the way it happened.

Greetings blike !
Originally posted by blike
I've read in brian greene's book that we're
always moving through spacetime at C.
Is this true?
Yes. Everything moves at c through space-time.
Light (EM waves) moves at c through space
and does not move through time, for example.
Matter moves through both, but if you take
yourself as the stationary reference frame for
example - then you're only moving through time.
One of the aspects of relativity is that an
event that develops in time has a light cone.
Light, for example, moves at c and hence has no
light cone and no time.
Try this site (the point at the bottom of the page):
http://users.chariot.net.au/~gmarts/timefact.htm [Broken]

Live long and prosper.

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Originally posted by MajinVegeta
Why is rotation absolute?
Why are there only two kinds of electric charge instead of one or six or thirteen?

No one knows. The universe we've been given just happens to be that way. It's a philosophical question.

- Warren

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Originally posted by plus
Light moves at c away from me. Another observer sees the light moving at c, but I may not be stationary with respect to him.
That isn't what I said. Your emitted photons are moving with velocity c with respect to you. What I said is that all observers will agree that your photons are moving with velocity c with respect to you.

- Warren

If u consider time as another dimension, and you are physically standing still in the other 3, then your entire velocity vector (which is always C) is intirely in the time dimension...as your speed increases in the 3 physical dimensions, more of the vector's components goes toward your velocity in the physical dimensions and less to the time demension until you reach a speed of C and none of the vector is moving through the time dimension, and thus time stops for you.

It means something different than what one normally associates with velocity. In fact it shouldn't be called that. It should be referred to as the 4-velocity.

What it means is that in spacetime there is a worldline associated with your body. Draw two axes on a piece of paper. Label one axis "x" which represents your postition in S, label the otherone "t" which represents the time as measured in S. If you are at rest in the frame of referance S then in S the worldline is the time axis since at t = 0 you're at x = 0. At t = 1 you're at x = 0 etc. The curve of postition versus time is thus a line - the time axis.

You can represent any point in that diagram as a combination of the two vectors

Using the notation P = (t,x) for a spacetime point there is a one-to-one corresponce with the point and a spacetime vector.

Unit vector on time axis = (1,0) = "T"
Unit vector on space axis = (0,1) = "X"

Then

P = (t,x) = (t,0) + (0,x) = t(1,0) + x(0,1)

P = t"T" + x"X"

If x = 0 for al time then

P = t"T"

That's what that means.

Pete

No the Lorentz transformations are just used for converting spacetime coordinates from one refference frame to another, mass doesn't appear anywhere in teh Lorentz transformations.

The Lorentz transformation does more than that. Let X = (ct,x,y,z) be a point in spacetime. Then in another frame of referance the point X has coordinates X' = (ct',x',y',z'). Let L = Lorentz transformation matrix. Then

X' = LX

By definition: Anything quantity A which satisfies

A' = LA

is called a Lorentz 4-vector. Energy and momentum are components of such a vector. I.e. Let p = mechanical momentum of a particle and let E = the energy of the particle. Define P as

P = (E/c, p)

Then it can be shown that

P' = LP

P is called the "Energy-Momentum 4-vector." "E/c" is called the "time-component".

Mass = M = E/c^2. So you can see that the role of mass is similar to the role of time. Each is relative.

Pete

Why are there only two kinds of electric charge instead of one or six or thirteen?

Seems to me that charge, as we know it, can either be the same or the cannot be the same. In that sense there are only two possibilites for charge.

That's the way I see it anyway.

Pete

the mass of the electron is 9.1E-31kg

the charge of the electron is -1.6E-19C

in SR, GR, QM, SST, MT, GUT, EM or any other:

does the charge and the mass of the electron change?

if so will that poor particle still be an electron?

if not then why there are conservations of charge and mass?

until you answer those Qs you are forbiden to discuss spacetime, gravitons, photons, strings, speed of the lights,even light it self, etc...GOT IT?

?ti tog.ti teg ll'uoy ti teg i nehw

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Originally posted by pmb
Seems to me that charge, as we know it, can either be the same or the cannot be the same. In that sense there are only two possibilites for charge.

That's the way I see it anyway.

Pete
However, three are three kinds of strong charges (called red, green, and blue) and only one kind of gravity charge (called mass). Certainly two is not the only acceptable number.

- Warren

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Originally posted by dr-dock
until you answer those Qs you are forbiden to discuss spacetime, gravitons, photons, strings, speed of the lights,even light it self, etc...GOT IT?
Seek professional help.

- Warren

dr-dock wrote

re - "the mass of the electron is 9.1E-31kg"

The "proper-mass" of the electron is 9.110E^31 kg - Yep. The relativistic mass of the electron depends on speed. What's your point?

re - "the charge of the electron is -1.6E-19C" - Yep. Charge of the electron is invariant. What's your point?

re - "in SR, GR, QM, SST, MT, GUT, EM or any other:" - Yep!

re - "does the charge and the mass of the electron change?" - The charge is invariant. What's your point?

re - "if so will that poor particle still be an electron?" - ?

re - "if not then why there are conservations of charge and mass?"

Just becase the proper-mass of a particle is **invariant** it does **not** imply that the relativistic mass of a particle is. In fact the relativistic mass in covariant. Similar to time - proper-time is invariant and time itself is covariant.

However just because the relativistic mass of a particle depends on the speed it in no way implies that "total" relativistic mass is not conserved.

For example: If a particle of proper mass M_o disintegrates into two particles, each of which have the same proper mass m_o, each will also have the same speed. let g = 1/sqrt[1 - (v/c)^2]. The relativistic mass of each particle is m = = g*m_o. Therefore the total mass after the disintegration is M = 2m = 2g*m_o

The initial energy is E_i = M_o*c^2
The final energy is E_f = M*c^2

Energy conservation implies E_i = E_f, and thus

M_o = M

Therefore energy is conserved. If you push the particle to accelerate it then you're adding energy.

re - "until you answer those Qs you are forbiden to discuss spacetime, gravitons, photons, strings, speed of the lights,even light it self, etc...GOT IT?" - Who died and made you king?

Sorry bub! You'll have to tell that to someone who actually cares.

Pmb

Originally posted by chroot
However, three are three kinds of strong charges (called red, green, and blue) and only one kind of gravity charge (called mass). Certainly two is not the only acceptable number.

- Warren

that's a different ball game. Those aren't electrical charges.

Matter of semantics I guess

Pete