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Let's assume that we have a stationary observer in an inertial frame who looks at a space rocket moving to the right (say in the +x direction) at relativistic and constant speed. We know that there will be time dilation given the by the Lorentz factor formula.

Assume that the space rocket is sending a rhythmic light pulse with a fixed time interval (according to the space rocket) directly back towards the stationary observer.

Will the time interval of the laser pulse clock (with a 1 second time interval according to the space ship) received by the stationary observer ALWAYS be greater than 1 second due to time dilation, even if the spaceship is moving towards the stationary observer?

joseph h
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Stallmp
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    Don’t confuse the doppler-effect and its k-factor (which depends on velocity), with time-dilation and its $\gamma$ (which depends on speed). – robphy Jul 19 '23 at 15:32
  • Can you elaborate? Do you mean to say that the distance getting larger or smaller (resulting in the change of the time interval between the pulses perceived by the stationary observer) is a result of the Doppler effect and not time dilation? How do you separate the two effects exactly? Since the only thing that can be measured here is the time interval between the pulses by the stationary observer. – Stallmp Jul 19 '23 at 15:58
  • (See https://physics.stackexchange.com/questions/689569/time-dilation-in-an-approaching-object-contradictory-to-dopplers-effect for details). Doppler compares proper-times from a common event to two lightlike related events (related by signaling), whereas time-dilation relates proper-times from a common event to two spacelike-related events (related by simultaneity according to the measurer). – robphy Jul 19 '23 at 19:28
  • But in this case, doesn't both time dilation and Doppler effect play a role at the same time regarding the received time interval between the pulses? – Stallmp Jul 20 '23 at 02:16
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    When separating or approaching with speed v, the time dilation factor is the same. When separating with speed v, the received frequency of signals is decreased…. But when approaching the frequency is increased. (Alternatively, one could consider the Doppler effect used for sound, but with light speed signals. Including time dilation gives the relativistic Doppler effect.) – robphy Jul 20 '23 at 03:32
  • But what confuses me is that as the rocket ship approaches and the frequency increases, doesn't that counteract the effect of time dilation? – Stallmp Jul 20 '23 at 05:19

2 Answers2

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Here's a spacetime diagram that shows a ship (moving with $v=(3/5)c$ in the lab frame) sending periodic signals to the lab frame. Time runs upwards.

I've drawn the diagram using my "rotated graph paper" method so that it is easy to count ticks along segments. The light-clock diamonds are traced out by the light-signals in a light-clock along the timelike diagonal of the diamond. The important feature is that all light-clock diamonds have the same area.

RRGP-robphy-TimeDilation-vs-Doppler

  • You can see the velocity of the ship is $\displaystyle v=\frac{T_1E_1}{OT_1}=(3/5)c$ (count 5 ticks up , then 3 space-ticks ["sticks"] over). By construction (using these numerically-nice numbers), there are 4 diamonds along the ship worldline segment $OE_1$.
    The time dilation factor is $\displaystyle\gamma=\frac{OT_1}{OE_1} =\frac{5}{4}$, which you can check using the formula $\displaystyle\gamma=\frac{1}{\sqrt{1-(v/c)^2}}$.

  • You can see the Doppler factor $\displaystyle k=\frac{OR_1}{OE_1}=2$ when they are separating,
    which you can check using $\displaystyle k=\sqrt{\frac{1+(v/c)}{1-(v/c)}}$ with relative-velocity $v=(3/5)c$.
    You can see the Doppler factor $\displaystyle k=\frac{OR_{-1}}{OE_{-1}}=\frac{1}{2}$ when they are approaching,
    which you can check using $\displaystyle k=\sqrt{\frac{1+(v/c)}{1-(v/c)}}$ with relative-velocity $v=-(3/5)c$.

    In other words, when the ship is emitting with a period of 4-ticks (in the ship frame),
    the lab receives the signals (on the lab-worldline) with period 8-ticks when they are separating [thus, redshift],
    but period 2-ticks when they are approaching [thus, blueshift].

    In both the separating and approaching cases,
    the lab regards (by, say, a radar measurement to determine the events on the lab-worldline are simultaneous with distant events on the ship worldline) the elapsed time between consecutive ship-emission-events to be 5 ticks.

So, by proportions, the answer to

Will the time interval of the laser pulse clock (with a 1 second time interval according to the space ship) received by the stationary observer ALWAYS be greater than 1 second due to time dilation, even if the spaceship is moving towards the stationary observer?

is no.

As mentioned in my comment to the OP, consult Time dilation in an approaching object - contradictory to Doppler's effect?
for some details on "time-dilation" vs "Doppler effect"
with regard to spacelike-related events vs lightlike-related events.


If we turn-off time dilation (assuming absolute time), we get a diagram like this:

RRGP-robphy-TimeDilations-vsDoppler-noTimeDilation

For simplicity, we keep the emission signals the same.
So, now, assuming no time-dilation (pure classical Doppler effect), the emission-period by the ship is now 5 ticks.

  • When they are separating, $\displaystyle k_{classical,sep}=\frac{OR_1}{OE_1}=\frac{8}{5}$,
    which is consistent with $(1+(v/c))$ with $v=(3/5)c$.
  • When they are approaching, $\displaystyle k_{classical,app}=\frac{OR_{-1}}{OE_{-1}}=\frac{2}{5}$,
    which is consistent with $(1+(v/c))$ with $v=-(3/5)c$.

[NOTE: Without time-dilation, this experiment will not be in agreement with the principle of relativity.
Consider light-signals emitted by the lab frame every 5 ticks. When does the ship receive those signals? ]


UPDATE to address the OP's question in the comment.

Here's a radar measurement by the lab frame.

  • Wristwatches zeroed when the lab and ship meet at event O.

  • After 2 ticks on the lab-wristwatch (event $S_1$), the lab sends a signal (encoded with "2"), which meets the ship worldline (at $E_1$) and is reflected back to the lab, received when the lab-wristwatch reads 8 ticks.(at $R_1$).

  • Data collected by the lab: $OS_1=2$, $OR_1=8$

  • Data analyzed by the lab: $k=\frac{OE_1}{OS_1}$, $k=\frac{OR_1}{OE_1}$ (with $E_1$ possibly unknown).
    So, $k^2=\frac{OR_1}{OS_1}=\frac{8}{2}=4$. Thus (Doppler Effect) $k=2$, predicting $OE_1=(k)OS_1=4$.

    The lab assigns time-coordinate $OE_t=(OR_1+OS_1)/2=5$ to the event $E_1$.
    The lab assigns space-coordinate $OE_x=(OR_1-OS_1)/2=3$ to the event $E_1$.
    Thus (Time Dilation) $\displaystyle \gamma=\frac{OE_t}{OE_1}=\frac{5}{4}$.

    (We could suppose that when the ship receives the light-signal, it sends its wristwatch reading "4" with the reflection [of the "2" from the lab]. Thus the reception event at the 8th-tick has the original "2-tick" timestamp carried in the light-signal from the lab and possibly the "k*2-tick" timestamp from the light-signal from the ship.)

RRGP-robphy-radar-1

As an exercise, you should carry out an analogous radar measurement done by the ship. Here is the diagram.

RRGP-robphy-radar-2

You should also use the very first diagram to analyze the situation when they are approaching, using a radar-measurement beginning at an event 8-ticks before they meet (when their wristwatches read $-8$).

(You are encouraged to construct analogous diagrams for $v=(4/5)c$. [Velocities with rational Doppler factors lead to relatively-simple arithmetic with fractions.])

robphy
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  • Thanks a lot for the very clear answer! So basically there is both time dilation and the Doppler effect involved here simultaneously. I am wondering now: how can ever the time dilation factor for a moving object be observed for a stationary observer? Because the moving object is always moving relative to the stationary observer, so the Doppler effect will always come into play. How can the stationary observer confirm the time dilation factor in practice? – Stallmp Jul 21 '23 at 16:24
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    @Stallmp (I’m traveling now so a more complete answer will have to wait.) Each observer can use radar to determine which local event is simultaneous-according-to-the-measurer with a distant event. One could imagine emitted light-signals encoded with the wristwatch time of the emitter. Upon collection by a measurer, one could determine the coordinates of all events… and thus confirm time dilation. – robphy Jul 21 '23 at 18:42
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    @Stallmp I updated my answer to address your question with a detailed description of a radar measurement. – robphy Jul 22 '23 at 23:50
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Minkowsky diagram of spaceship wit v=c/2 The best way to illustrate your question is a diagram like here. red is light signal coming from the spacecraft and reaching the stationary earth the outgoing spaceship in blue, returning grenn. You see that earth gets the signal in equal time intervals as they are send in equal time intervals.

trula
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    By "red", do you mean "orange"? I see only orange, green & black lines along with blue & black circles. Could you amend your text to coincide with what is actually depicted? – Kyle Kanos Jul 19 '23 at 15:49
  • yes my red turned orange, but this you should see for yourself – trula Jul 19 '23 at 17:21
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    I'm not sure what that is supposed to mean. – Kyle Kanos Jul 19 '23 at 18:59
  • it just says the light pathin the picture is orange – trula Jul 19 '23 at 22:08
  • How does this graph take into account time dilation as well, as this seems to be only based on the Doppler effect? – Stallmp Jul 20 '23 at 02:01
  • @Stallmp : The picture shows you what happens in the earth frame, and confirms that the signals from the outbound ship arrive at equally spaced intervals. It can't explicitly show time dilation because it shows only one frame. To see the time dilation, add the outgoing (or incoming) ship's frame to the picture. – WillO Jul 20 '23 at 02:22