# Re: Once Again: Relativity for Thought Experiments

On Apr 6, 6:33 am, Alfonso <Alfo...@xxxxxxxxxxx> wrote:
On 05/04/11 22:03, PD wrote:

On Apr 5, 3:05 pm, Alfonso<Alfo...@xxxxxxxxxxx>  wrote:
On 04/04/11 16:46, PD wrote:

On Apr 3, 9:15 am, Alfonso<Alfo...@xxxxxxxxxxx>    wrote:
On 03/04/11 01:38, Inertial wrote:

"Alfonso" wrote in messagenews:0rCdnU7NO86R4wrQnZ2dnUVZ8oCdnZ2d@xxxxxxxxx

On 02/04/11 18:45, Inertial wrote:
"Alfonso" wrote in message
news:b9CdnT_GZafIZgvQnZ2dnUVZ7rGdnZ2d@xxxxxxxxx

On 01/04/11 19:58, Inertial wrote:
"Alfonso" wrote in message
news:R9OdnVehg6oQmAvQnZ2dnUVZ8gSdnZ2d@xxxxxxxxx
How can my change of speed w.r.t the source - 1 light year away -
affect the signal local to me.

It doesn't

It only affects your MEASUREMENT of the signal.

Similar to when you change your speed wrt a distant whistling train ..

Yes the speed of the waves is constant w.r.t the air. You change your
speed w.r.t the air you are changing your speed w.r.t the sound waves.
They are going past at a different speed so the number per second is
different. - change in frequency. The signal hasn't changed but your
speed w.r.t it has.

That's why I said similar .. not the same.

You brought train whistles into it implying that somehow that was
relevant and explained something

Yes I did. And it IS relevant .. it provides an example of how the speed
of the observer doesn't change signal itself .. only the observers
measurement of it. It renders your question moot.

The point i made was that
(like in the case of the whistle) a change in the observers speed does
NOT affect the signal itself, only the measurement of it.

If the signal itself in unaffected

Of course it isn't affected

and you are travelling at the same speed w.r.t it nothing has changed.

Wrong. Your frame of reference has changed.

And that affects your

measurement of the frequency

The FoR is a mathematical abstraction. It isn't something magical. All
FoR are identical as regards physical properties. If in the new FoR the
signal is the same and the speed of the signal is the same how can the
frequency be different?

There can therfore be no change in frequency.

Wrong. Frequency is a measurement you make of the signal. Not an
inherent property of that signal .. It is frame dependent

Saying it is "frame dependent" does not solve the problem. Why is it
frame dependent?

Why is velocity frame-dependent?
Why is momentum?
Why is kinetic energy?
Why is magnetic field?

Talking about something being "frame dependent" is simply mathematical
short hand. Very useful if all you want is to derive an answer.

I beg to differ.

Velocity is the motion of one thing *relative* to another. It is
meaningless to describe a single object as having "a velocity".
It may be convenient, but not essential, to imagine a universe with
absolute motion in which one object is "stationary" and define
everything from that view point. It makes the maths easier and it is
easier to visualise than to face the reality that everything is simply
moving relative to everything else with no fixed reference.

All of these are both easy to accept and easily verified in
measurement.

Really, the question is, "How does one determine which physical
properties are frame-dependent and which are not?" The answer is not
"intuition". The answer is "experimental observation."

The answer is in understanding *why* they are frame dependent.

What is the fundamental difference in the *why* for momentum,
wavelength, and duration?

As I say
a FoR is simply a convenience by which you imagine you have a fixed
platform in which to make measurements when there is no such thing.

So far you've offered two coupled complaints.
The first is that if a property is frame-dependent, then you have to
understand how some material body involved in the process has
performed an interaction to *alter* the property. (This is a
presumption that physical properties have one and only one true value,
and that if there is an observed change in the value of a property,
then it is a true alteration of that property.) In the cases above,
where you would have difficulty identifying where such an interaction
occurs, you simply adopt the proposition that the value of the
property is only DEFINED relative to some other material body. (Thus
your claim that momentum of a single body or system can't be defined
and only has meaning relative to another material body.) This merely
shifts the question to how you know some properties are defined
RELATIVE to some other material body and how some cannot be.

You complain bitterly that a material observer cannot possibly have
influenced the wavelength of light when the light is still very far
away from the observer.

But the very same thing is true for momentum. If a bullet is fired
from a gun, it will have a certain momentum p with respect to the gun..

As I say speed is relative. The bullet has a speed relative to the gun
and that gives you a value of the momentum. That is not the momentum of
the bullet. It is the momentum of the gun/bullet system.

Not in the usual meaning of that term. The momentum of the gun/bullet
system combines the momentum of the gun and the momentum of the
bullet, but this is distinct from the bullet's momentum and from the
gun's momentum.

I'll give you a simple example. Use YOUR definitions of the terms
anyway that you want.
A gun has a mass of 2.2 kg and a bullet has a mass of 0.028 kg.
Let's consider two states.
State 1: The bullet is not moving relative to the gun, prior to the
firing.
State 2: The bullet is fired and there is a separation velocity of 600
m/s, and it is observed that the bullet goes to the right and the gun
moves to the left.

Now, tell me, using YOUR definitions of the momentum that one should
attribute in each of these states to the gun, the bullet, and the
combination of the bullet and the gun.

of it.

Do this in a strictly *classical* manner.

Why in a strictly *classical manner* if you are talking "classically"
and I am questioning whether your approach makes sense?

Wait. Are you telling me that there is no sensible momentum of the
bullet, the gun, and the combination of the bullet and the gun in
those two states?

Are you telling me that momentum conservation makes no sense?

I've given you the data you need to make any (very simple)
calculations you need. Can you show with numbers what those values

Speed is relative but acceleration is not so an increase in speed due to
acceleration is a valid concept. Both the gun and the bullet are
accelerated in proportion to their mass. One will have increased its
speed and hence its momentum in one direction and the other will have
increased its speed and hence its momentum in the other and the net
change in momentum will be zero.

Let me put one to you. You have two masses M1 and M2 on a collision
course with a closing speed of v.
Q1/ If there is a non elastic collision what is the energy dissipated?
Q2/ Why would you want to suggest that one mass rather than the other
has the kinetic energy?

Can you not see that the concept of a mass having Kinetic energy is a
nonsense? The *system* has kinetic energy. It may be useful shorthand to
talk in terms of a FoR but what you are in fact doing is ascribing to a
mass the hypothetical energy which would be dissipated if it was brought
to a halt in that FoR by a collision with a hypothetical infinite mass.

If the bullet's
speed relative to the target is different to the bullets speed relative
to the gun the Momentum of the bullet/target system will be a different
value. Your mistake is to imply that momentum is a property of the
bullet. It isn't because speed isn't absolute. If there was an absolute
reference - something one could truly describe as stationary then the
bullet would have an absolute speed and an absolute momentum, so would
the gun and so would the target.

If there is a target that the bullet eventually hits, the bullet may
well have a different momentum p' with respect to the target -- even
though there was NOTHING that happened to the bullet between gun and
target. Given that, then when exactly did the bullet's momentum change
from p to p'? And how did it do that? And if the bullet's momentum is
defined relative to a material object, then when does this definition
SHIFT from being relative to the gun to being relative to the target?

This is *classical* physics, Alfonso, nothing complicated.

You seem to be the one having difficulty with a universe where there is
no absolute reference. You seem incapable of thinking without the
mathematical shorthand of a FoR where you can imagine yourself as
stationary. A FoR is simply a mathematical abstraction to make
visualisation easier.

You have a problem with *physical* quantities being dependent on
something you consider to be only a mathematical construction. But as
you can see in the bullet case, it is nothing extraordinary. It is
just something to be learned.

You really do have problems. Go back to basics. Start with Speed is
relative so you cannot attribute a speed, or momentum, or kinetic energy
to an isolated object in a universe without absolute motion.

Sure you can. It's just a frame-dependent velocity.

Because the value of the velocity depends *entirely* on the choice of frame, there is no
way in which this number can be taken as absolute. Unless you mean by
"absolute" = "the value as it happens to have in the frame you're
considering".

The maths
*work* if you arbitrarily assume you are stationary but every time you
change what you arbitrarily consider is stationary everything naturally
changes because the reality is that you never were stationary. The speed
never was v it was v relative to what you had arbitrarily decided to
call stationary.

What has changed which affects your measurement?

You frame of reference. Frequency is frame-of-reference dependant.

Saying it is "frame dependent" does not solve the problem. Why is it
frame dependent? A FoR is a mathematical abstraction. It isn't something
magical. Talking in terms of frames of reference is mathematical
shorthand useful when doing the maths as it avoids thinking things
through from scratch. You label something "frame dependent" when you
have understood why it is otherwise you are simply accepting it as an
article of faith.

Forget waves for a moment and consider a continuous stream of equally
spaced flashes of light. Doppler applies just the same. The frequency is
simply the number passing you in a given time. Frequency is the inverse
of time. The "period" is the time interval between pulses and is the
reciprocal of the frequency - the number passing you in a second.
If you increase your speed towards the direction they are coming from
the number passing you in a given time increaes and the interval between
pulses gets shorter - why? The intuitive answer is that they are now
travelling at a higher speed. That the pulses have the same physical
spacing (the signal hasn't changed) but are passing you faster so there
is more per second and the interval between one passing you and the next
is shorter. Relativity says no (second postulate). The rate they are
passing you is the same. How can that be?

Lorentz theory explains it. The signal hasn't changed. You have changed
your speed relative to the signal which is why the frequency has changed
such that if you measure the speed it appears not to have changed.

SR does not explain Doppler.

.

## Relevant Pages

• Re: Once Again: Relativity for Thought Experiments
... difference in the *why* for momentum, wavelength, and duration? ... respect to the gun. ... the momentum of the bullet. ... bullet has a mass of 0.028 kg. Let's consider two states. ...
(sci.physics.relativity)
• Re: Once Again: Relativity for Thought Experiments
... momentum of a single body or system can't be defined ... momentum p with respect to the gun. ... That is not the momentum of the bullet. ... A gun has a mass of 2.2 ...
(sci.physics.relativity)
• Re: Once Again: Relativity for Thought Experiments
... difference in the *why* for momentum, wavelength, and duration? ... respect to the gun. ... the momentum of the bullet. ... bullet has a mass of 0.028 kg. Let's consider two states. ...
(sci.physics.relativity)
• Re: Once Again: Relativity for Thought Experiments
... difference in the *why* for momentum, wavelength, and duration? ... respect to the gun. ... the momentum of the bullet. ... bullet has a mass of 0.028 kg. Let's consider two states. ...
(sci.physics.relativity)
• Re: Once Again: Relativity for Thought Experiments
... difference in the *why* for momentum, wavelength, and duration? ... respect to the gun. ... the momentum of the bullet. ... bullet has a mass of 0.028 kg. Let's consider two states. ...
(sci.physics.relativity)