# Relativity question

Mike Hewson
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### RE: When you read that

Message 48967 in response to message 48965

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When you read that moving object's time is slower it means that standing still observer calculates (in simultanious with him reality) the moving object's time to pass slower. What he sees however depends if the moving object is going away from him or is coming to him. In first case the observer sees the object to be living slower and in the second case the object is seen to be living faster than the observer himself.

No, not in inertial/unaccelerated frames. Experiments with muon decay have shown an 'extended' lifetime before decay when they travel at a higher speed compared to when they are slower, and this is regardless of the direction in which they travel.

While Lorentz contraction depends on which aspect/face of a moving object is in the line of relative motion between it and an observer, the time dilation isn't.

The deduction of alteration of time of moving clocks is based upon the transformation of measurements of 'regular' events in another frame ( moving with respect to the observer ) brought into the observer's frame. This always involves a 'slant' distance ( upon a triangle's third side ) divided by the constant speed of light - that third/slant side will always be longer relative to the 'rest' case and goes longer with higher relative speed - quite regardless of line of approach/recession.

Now, if there is non-inertial/accelerated behaviour then it is different again.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

debugas
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### RE: muon decay have shown

Message 48968 in response to message 48967

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muon decay have shown an 'extended' lifetime before decay when they travel at a higher speed compared to when they are slower, and this is regardless of the direction in which they travel.

That's correct but this is not what i was talking about.
Moving object experiences time dilation but when you look at it you also have to take into account doppler shift.

when moving at 0.8c the dilation prolongs the life by factor of 1.(6)=1/sqrt(1-0.8^2) but due to doppler shift you see the approaching object 'living' at 300% rate and moving away object 'living' at 33% rate

Mike Hewson
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### RE: RE: muon decay have

Message 48969 in response to message 48968

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Quote:
muon decay have shown an 'extended' lifetime before decay when they travel at a higher speed compared to when they are slower, and this is regardless of the direction in which they travel.

That's correct but this is not what i was talking about.
Moving object experiences time dilation but when you look at it you also have to take into account doppler shift.

when moving at 0.8c the dilation prolongs the life by factor of 1.(6)=1/sqrt(1-0.8^2) but due to doppler shift you see the approaching object 'living' at 300% rate and moving away object 'living' at 33% rate

Well, as long as we're being careful here distinguishing between the measured times of events and the frequency of the light signals that inform you of that.

Two events in a frame moving relative to an observer ( but measured/transformed to an observer's frame ) which respectively define the start and end of a clock 'tick' will have a greater time separation with higher relative speed, and independent of direction.

Quite rightly the frequency of each light signal recieved will have Doppler shifting though. That would occur even if the observer made only a single measurement of only one event in the other frame. That signal will have Doppler shifting as described. If you compare two such shifted signals you'll get the time dilation/slowing.

The speed of light is constant ( regardless of it's frequency ) and that is what gives rise to time dilation.

Any alteration of light frequency with relative motion alters the percieved energy/momentum of recieved radiation - but not the measured time separation of distinct signals.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Mike Hewson
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### I thought I'd expand a bit

I thought I'd expand a bit here, too.

Bear in mind that we don't actually measure light frequency in the direct fashion that we may with other regular/repeating events. What happens is that we have some interferometric grating or somesuch which we throw a bunch of photons at, and a pattern emerges which we can beautifully explain on the assumption of a 'wave' model.

But each photon detection is a distinct event which either happens or it doesn't ( no 'fractional' events ), and so is particulate. We don't actually 'view' the oscillation of the electric/magnetic field as it cycles with the photon phases - so there is no counting 'up...down....up....down...up....down...' of any field strength rhythm.

However when Einstein explained the photoelectric effect ( a particulate phenomenon ), the energy of the electron ( upon escaping the metal lattice ) was measured by retardation in an electric potential. Energy from a photon, given to the electron, was responsible for it's release from the metal ( which required a minimum 'exit fee' ) and below a certain amount ( characteristic of the metal used ) it didn't happen at all. There was an observed minimum light frequency/color, as deduced by wave type measurements, that supplied this.

Now when Planck first correctly predicted the pattern of radiation from so called 'black' bodies, it required the assumption of quantized energy exchanges. Planck thought that was purely because the atomic oscillators in the hot body were quantised ( susbsequently somewhat true ), but Einstein went further by requiring the photons themselves ( or the electromagnetic field if you like ) to have discrete behaviour. Interestingly Planck himself never quite got his head around this - and also aspects of relativity like time dilation, no ether etc - even apologising on Einstein's behalf ( in a letter of reference ) for Einstein's 'error' with photon quantisation!

My point is that we use light's behaviour to deduce time alteration in various frames - light itself is unaffected. It's 'proper' time is zero, meaning that in it's own frame time does not pass. It travels at a constant speed in inertial reference frames.

We are able to correlate quantities which describe particle behaviour ( energy/momentum ) and those which describe waves ( frequency/wavelength ), however we cannot fully reconcile both views with any single experimental setup. In Young's double slit arrangement when you detect which slit was traversed you lose interference behaviour - and when you get the lovely pattern back you don't know which path any given photon used. From this bloody conspiracy of nature you deduce the Uncertainty Principle, which formalises that duality.

In accelerated frames there can be some subtleties about what is meant by light's velocity. After all if I throw a photon into a black hole, then it won't get to the opposite of whatever orbit I'm on, so that's hardly a constant velocity in my frame. Suppose I chuck some regular light emitter down a gravity well. Then in addition to Doppler concerns and time dilation, I will get a progressively lower frequency/energy/momentum of received photons. This is due to them having lost energy getting up the potential gradient in order to reach and be detected by me.

Light is counter-intuitive in that it can vary it's energy and momentum, but not as a function of it's speed. Ordinary material particles do.... :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Simplex0
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### RE: RE: muon decay have

Message 48971 in response to message 48968

Quote:
Quote:
muon decay have shown an 'extended' lifetime before decay when they travel at a higher speed compared to when they are slower, and this is regardless of the direction in which they travel.

That's correct but this is not what i was talking about.
Moving object experiences time dilation but when you look at it you also have to take into account doppler shift.

when moving at 0.8c the dilation prolongs the life by factor of 1.(6)=1/sqrt(1-0.8^2) but due to doppler shift you see the approaching object 'living' at 300% rate and moving away object 'living' at 33% rate

If I understand this right is the slowing down of time, the clock, on the moving rocket an explanation why the the on board detector pair measuring the light beam traveling in the same direction as the rocket will clock the speed of light unchanged. But I canÂ´t understand that the same explanation could be used to explain how the detector pair measuring the light beam traveling towards the rocket also will clock the speed of light unchanged.

Any way, tank you Debugas & Mike. I will take my time now and carefully read trough and try to understand what you have written here.

debugas
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### doh ! :( I think we need

doh ! :(

I think we need to define what is the speed of light here that we try to check
I will define it like this:

on the left side we have two detectors LA and LB (here i imagine LA to be closer to A and LB to be closer to B)

let's send our own light signal from LA and wait till it reaches LB and is reflected back by a mirror placed at LB. Then we wait while it returns back to LA

we measure total round-trip time T and devide it by distance 2*(LB-LA)

so c=T/[2*(LB-LA)]

the same can be done with right pair of detectors
we will get the same round-trip speed
c = T/[2*(RB-RA)]

note that all three observers will agree on the value of c even though
half-trip times t1, t2 and total time T (which is t1+t2 in all cases though t1t2 in some cases) and lengths |LB-LA|=|RB-RA| will be different for them

Now Tomas, the key point:
suppose a light signal from A that shows 12 o'clock reaches the detector LA
and we issue our own signal. Will both signals travel together ? Yes!
when they reach LB what light signal that was traveling from A will be at detector LA (simultanious in rocket's frame of reference)? It will be the light signal showing time 12 o'clock and 1/3 second (here i assume LB-LA = one light second and rocket moving at speed of 0.8c)

now we wait for our own light-signal to return back to LA. When it returns back it will be at LA together with the observer's A light-signal showing time 12 o'clock and 2/3 of a second and mind you it took us 2 seconds to have it travel from LA to LB and back from LB to LA ( T = 2 sec )

Simplex0
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### RE: doh ! :( I think we

Message 48973 in response to message 48972

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doh ! :(

I think we need to define what is the speed of light here that we try to check

Don't get me wrong debugas. I am grateful for your efforts to help me to understand this, it's just that I want to take my time to carefully read and try to understand your explanations and how they are related to different parts of the question and especially what is measured on the rocket.

debugas
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### Now on the right side with

Now on the right side with the beam coming towards us from observer B:

suppose a light signal from B that shows 12 o'clock reaches the detector RB
and we issue our own signal from RB back to RA. Will both signals travel together ? Yes!
when they reach RA what light signal that was traveling from observer B will be at detector RB ? It will be the light signal showing time 12 o'clock and 3 seconds. Well observer A and B will disagree with us about it being the moment simultanious with reaching RA but it is their problem about what they think simultanious means :) - at least they will agree with us on the following part:

now we wait for our own light-signal to return back from RA to RB. When it returns back it will be at RB together with the observer's B light-signal showing time 12 o'clock and 6 seconds (everyone agrees about it) and mind you it took us 2 seconds to have it travel from RB to RA and back from RA to RB ( T = 2 sec )

Simplex0
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### RE: doh ! :( I think we

Message 48975 in response to message 48972

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doh ! :(

I think we need to define what is the speed of light here that we try to check
I will define it like this:

on the right side we have two detectors RA and RB (here i imagine RA to be closer to A and RB to be closer to B)

let's send our own light signal from RA and wait till it reaches RB and is reflected back by a mirror placed at RB. Then we wait while it returns back to RA

we measure total round-trip time T and devide it by distance 2*(RB-RA)

so c=T/[2*(RB-RA)]

the same can be done with left pair of detectors
we will get the same round-trip speed
c = T/[2*(LB-LA)]

note that all three observers will agree on the value of c even though
half-trip times t1, t2 and total time T (which is t1+t2 in all cases) and lengths |RB-RA|=|LB-LA| will be different for them

I have no problem to understand that case.

I want to restrict the experiment as follow...
First you measure the time it will take for the light to from RA to RB, the light is moving in the same direction as the rocket in this case.

The second measurement is the time it takes for the light sent from RB to go to RA, the light is moving in the opposite direction to the rocket in this case.

Seen from a stationary person standing at A the time between a reflection from the 2 detectors in the first case will be 2 years.

In the second case the time difference will be a fraction of a second.

I can logically understand what the person standing at A measure here. The hard thing to understand is that on the rocket, according to the theory, the clock will show the same time elapsed in both case.

debugas
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### RE: I can logically

Message 48976 in response to message 48975

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I can logically understand what the person standing at A measure here. The hard thing to understand is that on the rocket, according to the theory, the clock will show the same time elapsed in both case.

rocket observer thinks the light has reached the mirror when observer A still thinks light is moving towards the mirror and has not reached it yet. The fact is neither of them knows if mirror is there (maybe it was broken already or removed by some alien?). In other words the moment at which you think light has reached the mirror is your speculation about what there now is and you base your guess on the time light will need to cover the distance (it is your imaginary 3D space you think to be simultanious with you)- whether it is true you will only know when the light comes from there and tells you what really happened.

When observer A thinks light reached the mirror, the rocket observer thinks light is already for some time on its way back after the event.

rocket observer and observer A have different understanding of what the "simultanious with me" reality is. One defines the moment of a distant event based on the distance and the time light needs to cover it so no wonder it depends on your relative motion to/from that event