# Loss of simultaneity explained by classical physics

• granpa
In summary, the concept of simultaneity is frame dependent in special relativity, meaning that different observers can have different ideas of what is happening "now" in a given event. This is due to the finite speed of light and the principle of relativity. Therefore, it is not necessary to go outside of classical physics to explain loss of simultaneity, as it is a fundamental aspect of special relativity.

#### granpa

if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity?

in other words, it is not necessary to go outside of classical physics to explain loss of simultaneity.

Yes, that's what Lorentz showed, which is why so many concepts in special relativity are named after him.

granpa said:
if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity?

in other words, it is not necessary to go outside of classical physics to explain loss of simultaneity.

No - SR results in simultaneity being a frame dependent concept, it is not simply an allowance for the time taken for light-speed communications to reach the observer.

Consider a star 1000 light-yrs away, you are seeing it as it was 1000 years ago.

You may ask, "What is the star doing now, might it have gone supernova?"

To answer this question you take the state of the star as you see it (as it was 1000 years ago) and project forward 1000 years into its future, you conclude that it was unstable and that now it has gone supernova, and now it no longer exists as it was.

Another observer happens to be passing close by you, traveling towards the star at high speed. They see the star in the same state as you do. The photons from the star reach you and them simultaneously as you both pass each other close by.

However, they measure the star's distance as, say, only 800 light years and conclude that they are seeing it as it was 800 years ago.

They ask the same question and project the state of the star 800 years into its future and conclude that it was unstable and that now it is about to go supernova, but not just yet.

Both observers have a clear idea of when now is, but one thinks that is when the star is about to go supernova and the other just after it has exploded.

The concept of now, or simultaneity, is frame dependent.

I hope this helps.

Garth

Last edited:
you didnt even touch my question at all. you answered something else entirely.

granpa said:
you didnt even touch my question at all. you answered something else entirely.

That depends on what you mean by "Classical Physics".

In classical physics both observers would think the star is at the same distance.

If you introduce Lorentz contraction and an invariable speed of light then you are back in SR.

Garth

Last edited:
what does a star 1000 light years away have to do with my q?

granpa said:
what does a star 1000 light years away have to do with my q?
One problem I have is that you have not clearly stated a question!

The title of your thread is "loss of simultaneity explained by classical physics" and you posted:
granpa said:
if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity?

in other words, it is not necessary to go outside of classical physics to explain loss of simultaneity.

You seem to be stating that the loss of simultaneity is caused by a finite speed of light that was present in classical physics. Am I right in thinking this?

If so then the answer is no- the classical physics understanding of the finite speed of light does not lead to a loss of simultaneity.

Both classical and relativistic physics have a clear idea of what is 'here and now' for the two observers passing close by in my example above, where the two physics diverge is over what 'over there and now'.

The two mutually moving observers at the moment they pass close observe the rest of the universe at a distance from them.

Their light cones coincide, they observe the same events simultaneously and realize that they are seeing these events as they were some time in the past.

However, they differ on what they think is happening now, i.e. when they project forward in time an event, observed in the present as it was in the past, to their individual concept of what is happening over there now.

That is what that supernova star 1000 light years away has to do with your question, if I have understood your question properly.

Garth

if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity within its own body?

What do you mean by: "experience a loss of simultaneity within its own body"?

Garth

exactly that.

I have explained the SR concept of simultaneity, that two events, separated by some distance, that are thought of as being simultaneous by one observer (e.g. the moment 'now') are not thought of as simultaneous by another observer moving relative to the first observer.

It's importance is that we divide time up into 'immutable past', 'present' and 'changeable future', yet moving observers disagree as to which events, distant from them, are in the past, present and future!

Your statement of "a loss of simultaneity within its own body" makes no sense to me.

Garth

how would be decide whether 2 events, one occurring at the front and one at the back of his ship (which is moving near light speed), were simultaneous?

granpa said:
how would be decide whether 2 events, one occurring at the front and one at the back of his ship (which is moving near light speed), were simultaneous?
As we are talking about events within the ship its speed (relative to what?) is irrelevant.

Set off a flash bulb at both events, if an observer elsewhere in the ship observes the flashes, allows for the light-speed delay for the flashes to reach her, and concludes they happened at the same time, then they were simultaneous in her frame of reference.

Garth

the ability to ignore the speed of the ship is part of relativity. we are assuming a purely classical universe for this thought experiment.

ok, let's look at this. how would she know that the 2 flashes were simultaneous? she knows the distance between them. she knows when they arrived at some predetermined point. so the q is how does she know the speed of light? in a classical universe the speed of light would not be constant.

Last edited:
I think granpa was initially talking about the components which make up our bodies which are all in motion and bonded by electrochemical and nuclear forces (which propagate at the speed of light).

If you were able to take a moment in time and freeze it and then analyse it, you could list a huge number of events associated with my body. Say one for each molecule, using some arbitrary standard for assigning a sufficiently precise location for these molecules. If I were to be moving relative to another observer, I myself would not be simultaneous relative to that observer. That is to say, while all my molecules are simultaneous for me (inaccurately speaking, since they are moving around), they are not simultaneous for the observer relative to whom I am moving (as a whole).

It is like me being a rod, lying down parallel with the direction of relative motion. My head will not be simultaneous with my feet according to the observer not at rest relative to me. I am just taking it to molecular level, but the principle remains the same.

Talking about my body not being simultaneous with itself would rely on an arbitrary, highly precise reference frame. Say I pick a molecule in my body at random, and consider a very short period of time in which that molecule is inertial, I could say the frame in which this molecule is at rest is my "reference frame". Most of me is not at rest relative to this frame. The problem with trying to say I am not simultaneous with myself (ie relative to my arbitrary "reference frame") is that none of my other molecules are at rest relative to the majority of my body either. To have any sort of meaningful and internally consistent simultaneity I would need to have a subtantial proportion of my composite components at rest with each other. Then I could say that relative to other parts of me, that substantial proportion of me is not simultaneous.

But it is long searched.

cheers,

neopolitan

granpa said:
the ability to ignore the speed of the ship is part of relativity. we are assuming a purely classical universe for this thought experiment.

ok, let's look at this. how would she know that the 2 flashes were simultaneous? she knows the distance between them. she know when they arrived at some predetermined point. so the q is how does she know the speed of light? in a classical universe the speed of light would not be constant.
Velocities are relative in classical physics, ever since Galileo and Newton.

Within the ship, the bow the centre and the stern are all traveling at the same velocity, relative to some external reference point.

Classical physicists such as Hippolyte Fizeau and Léon Foucault in 1849 measured the speed of light.

The bow (B) and stern (S) are stationary in the observer's frame of reference, dividing their distance from her by the classical speed of light she works out the two time delays; subtracting these from the two times the flashes were observed she can tell whether B happened before A, after A, or was simultaneous with A.

What's the problem?

Garth

the ship would not perceive light to be moving at c. she can't measure the one way speed of light becaues she has no synchronized clocks (thats what we are trying to create). but she can measure the two way speed of light.

no, neopolitan, you missed my point.

Last edited:
it seems to me now that perhaps i asked the wrong q. if she can determine the velocity of light relative to her ship then she can indeed determine the absolute time of events. but the real issue of simultaneity is whether those events would transpire on board the ship moving near the speed of light the same way they would if the ship was stationary.

granpa said:
no, neopolitan, you missed my point
Okay, granpa.

Fortunately it was entertaining to think it all through, so no harm done.

cheers,

neopolitan

we need to create some kind of system of INTERACTING objects within the ship then accelerate the ship to near the speed of light and see what happens. the only system i can thingk of would be an antenna in the front and back each of which sends a pulse to the other one whenever it receives a pulse. pretty weak i know but its all i can think of.

obviously they would drift out of synch as the ship moves faster. if this happened to every part of the ship and affected everything that happened on board the ship then that would be a loss of simultaneity.

Last edited:
granpa said:
obviously they would drift out of synch as the ship moves faster.

Faster than what?

The acceleration would produce red/blue shift in the signals being sent, but this does not affect simultaneity, the changing speed during the signals time of flight can be accounted for.

Garth

faster than being at rest.

granpa said:
faster than being at rest.

Which part of the word 'relativity' do you not understand?

As I said "Velocities are (known to be) relative in classical physics, ever since Galileo and Newton.".

The is no "at rest", except relative to an observer.

Garth

not light. Galilean relativity does not include light. in classical physics light is not assumed to have constant velocity with respect to all observers.

as i said: if she can determine the velocity of light relative to her ship then she can indeed determine the absolute time of events.

Last edited:
can be accounted for.

do atoms account for it when they interact?

granpa said:
not light. Galilean relativity does not include light. in classical physics light is not assumed to have constant velocity with respect to all observers.

as i said: if she can determine the velocity of light relative to her ship then she can indeed determine the absolute time of events.

Are you proposing that if she could determine the speed of light relative to her ship she would would determine the speed of light to be anything other than 299,792,458 meters per second?

How do you propose she makes this measurement?

If she always measure the speed of light to always be 299,792,458 meters per second relative to her ship, whatever the velocity of her ship how is that going to help her determine the "absolute time of events"?

Perhaps the following thoughts might help. Let's say the clocks of her ships are initially synchronised and she is initially at rest. When she accelerates the clocks at the front and rear of her ship will no longer look synchronised to her. Even more surprising (this was shown in a in old thread) is that depending on the acceleration scheme used to accelerate her ship the clocks may not synchronised after she stops accelerating and settles down a new inertial reference frame. (For example it was shown that Born Rigid acceleration will cause the clocks to go out of sync during the transfer from one inertial frame to another. She would then have to go round and re-synchronise her clocks for her new "state of rest". Now although she can determine by this "loss of synchronocity" of her own clocks that something has changed she can not come to any conclusions that related to absolute time of events or absolute motion.

For example, let's imagine she happened to be initially at rest in an absolute reference frame. Her de-synchronised clocks after she accelerates tells her something has changed. (The acceleration she felt and measured would have told her same thing.) With some careful calculations based on the difference in times of her de-synchronised clocks she might even determine her new velocity relative to her initial velocity. (She could equally well determine her new velocity by looking at the recording of her onboard accelerometers.) Has she determined her absolute velocity? The answer is no. For all she knows ,she might have had a negative velocity relative to the absolute reference frame initially and accelerated to an absolute stop. Everything she she can measure can not give her any clue of her true velocity relative to the absolute reference frame.

i hate having to explain myself over and over again. go back and read the op. we are assuming a purely classical universe here.

granpa said:
if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity?

in other words, it is not necessary to go outside of classical physics to explain loss of simultaneity.

In classical physics the speed of light emmited by an object moving at v relative to the observer would have been been assumed to be v+c in the forward direction and v-c in the backward direction and there would not have been any loss of simultaneity. Once you you postulate that light has a constant finite velocity independent of the speed of the emmiter velocity you leave classical physics behind.

[EDIT] Hmmm.. Let me restate that. Classical physics assumed light propagated in an ether type medium so they probably already had a concept that light velocity was independent of the emmiter velocity, but without length contraction and time dilation they would have seen a loss of simultaneity where there isn't any. For example classical physics fully expected the Michelson Morley experiment to measure the light signals on the two arms of the apparatus to be out of sync and the null result came as a bit of a shock.

Last edited:
granpa said:
where are you getting that crap? light was assumed to be a wave.

in any event it certainly isn't leaving classical physics behind.

I was just editing my post when you posted your response. Does the part I added make more sense?

they would have seen a loss of simultaneity where there isn't any?

would a classical object moving through a classical universe experience a loss of simultaneity within its own body or not? its not clear what you are saying.

i believe it would.

in a classical universe there is an absolute time so the q is 'will events on board the ship that are simultaneous when the ship is stationary still be simultaneous when the ship is moving near the speed of light'?

Last edited:
granpa,

the ability to ignore the speed of the ship is part of relativity. we are assuming a purely classical universe for this thought experiment.
Even in Gallilean relativity, a state of rest cannot be distinguished from a state of uniform motion.

granpa said:
they would have seen a loss of simultaneity where there isn't any?

would a classical object moving through a classical universe experience a loss of simultaneity within its own body or not? its not clear what you are saying.

i believe it would.

Using classical assumptions, they probably would have predicted a loss of simultaneity within a moving body but they also would have have incorrectly predicted the speed of light is different for different observers.

I gave the example that in a Michelson Morley type experiment they would have predicted that a signal emmited from the centre of the two arms, when reflected off mirrors at the ends of the arms transverse and parallel to the motion of the apparatus, would not return simultaneously, whereas the experiment proved the the signals do return simultaneously. Hense "they would have seen a loss of simultaneity where there isn't any".

Mentz114 said:
granpa,

Even in Gallilean relativity, a state of rest cannot be distinguished from a state of uniform motion.

classically one has only to measure the speed of light to determine ones velocity.

Last edited:
kev said:
Using classical assumptions, they probably would have predicted a loss of simultaneity within a moving body but they also would have have incorrectly predicted the speed of light is different for different observers.

I gave the example that in a Michelson Morley type experiment they would have predicted that a signal emmited from the centre of the two arms, when reflected off mirrors at the ends of the arms transverse and parallel to the motion of the apparatus, would not return simultaneously, whereas the experiment proved the the signals do return simultaneously. Hense "they would have seen a loss of simultaneity where there isn't any".

all i am trying to say is that loss if simultaneity can be explained classically. time dilation and length contraction can't as far as i know.

granpa said:
classically one has only to measure the speed of light to determine ones velocity.

On the incorrect classical assumption that the speed of light would be different for different observers they probably would have incorrectly concluded that "one has only to measure the speed of light to determine ones velocity".

Classical physics largely assumed a static absolute medium that light propagated in, so it would not have been surprising if they incorreectly predicted a notion of absolute motion. If they had taken Gallileo a bit more seriously when he said there is nothing that can be measured in the closed moving cabin of a ship that would indicate your motion or lack of motion they might have come up with relativity as we know it sooner.