# Spaceship is approaching the Earth with an unknown speed

• Frostman
In summary: Supposing that ##T_1 = 8## units, ##T_2 = 6## are ##T_1' = 8## units, ##T_2' = 6## (spaceship), we have:##Event\ 1:####T_1' = 8####L_1' = vT_1'=4.8## (distance from Earth for spaceship)##Event\ 2:####T_2' = 6####L_2' = vT_2'=3.6## (distance from Earth for spaceship)##Event\ 3:####L_3' = vt
Frostman
Homework Statement
A spaceship that is approaching the earth with an unknown speed, sends messages by communicating the time (of the spaceship) that is missing upon arrival, starting from the moment the message is sent.
In the first message, the missing time is ##T_1'##, in the second message it is ##T_2'##, and between the arrival of the two messages, a time interval ##\Delta T## elapses on earth.
1. What is the speed of the spaceship?
2. How long after (terrestrial), after the arrival of the second signal, will the starship arrive?
Relevant Equations
Lorentz Transformations
I started by finding the main events:
1. Sending the first message
2. Receipt the first message
3. Sending the second message
4. Receipt the second message
Now, what we know is the time by ##S'## (comoving frame with the spaceship) ##T_1'## and ##T_2'## remaining to arrive to the Earth measured at different times.
##\Delta T## is the difference between ##T_2## and ##T_1##, measured on the earth.
We can do this table

 Events​ ##S'##​ ##S##​ A: Sending 1 ##t_A'=0## ##x_A'=(c-v)T_1'## ##t_A=\gamma(t_A'+vx_A')=\gamma v(c-v)T_1'## ##x_A=\gamma(x_A'+vT_A')=\gamma (c-v)T_1'## B: Receipt 1 ##t_B = t_D-\Delta T ## C: Sending 2 ##t_C'=T_2'-T_1'## ##x_C'=(c-v)T_2'## ##t_C=\gamma(t_C'+vx_C')=\gamma(T_2'-T_1'+v(c-v)T_2')## ##x_C=\gamma(x_C'+vt_C')=\gamma((c-v)T_2'+v(T_2'-T_1'))## D: Receipt 2 ##t_D = \Delta T + t_B##

I don't know if is it correct this scheme, and it could help me to find the speed and how soon will the spaceship arrive on earth.

Last edited:
I cannot take meaning of "the time (of the spaceship) that is missing upon arrival, starting from the moment the message is sent. ". May I take ##T'##s as reading of spaceship clock when messages are sent, i.e. time of event A or C in the spaceship FR?

If I may take so
$$v=\frac{X_2-X_1}{T_2-T_1}$$
$$T_2-T_1=\gamma(T_2'-T_1')$$
$$\triangle T + \frac{X_2-X_1}{c}=T_2-T_1$$
These equation give
$$\frac{v}{c}=\frac{1-Q}{1+Q}$$ where
$$Q=(\frac{\triangle T}{T_2'-T_1'})^2$$.

If values of ##(T_1,X_1)## or ##(T_2,X_2)## are given we get the arrival time.

I should appreciate it if you check these with your results.

Last edited:
Frostman said:
Homework Statement:: A spaceship that is approaching the Earth with an unknown speed, sends messages by communicating the time (of the spaceship) that is missing upon arrival, starting from the moment the message is sent.
By missing time, do you mean the estimated time until arrival at Earth? The spaceship must know how far away the Earth is and how fast it is moving towards Earth. So, the spaceship can calculate its ETA (estimated time of arrival) on Earth. I assume that ##T_1## and ##T_2## are the ship's estimated times of arrival at Earth.

PeroK said:
By missing time, do you mean the estimated time until arrival at Earth?
Yes. It is the time to arrive on earth.
PeroK said:
The spaceship must know how far away the Earth is.
I suppose he does, he knows the time that is missing, so also how far away he is.
PeroK said:
How fast it is moving towards Earth.
It is the first request.

PeroK said:
So, the spaceship can calculate its ETA (estimated time of arrival) on Earth. I assume that T1##T_1## and ##T_2## T2 are the ship's estimated times of arrival at Earth.
No, ##T_1'## and ##T_2'##, they are ship's estimated times, they are sent to Earth these values.

Frostman said:
Yes. It is the time to arrive on earth.

I suppose he does, he knows the time that is missing, so also how far away he is.

It is the first request.No, ##T_1'## and ##T_2'##, they are ship's estimated times, they are sent to Earth these values.
There is a language problem here. Let's try an example to make sure we agree.

The ship calculates it is 10 days from Earth (##T'_1 = 10## days). It sends this message to Earth.

The next day the ship sends a message ##T'_2 = 9## days.

These messages are received on Earth at some times ##t_3, t_4##, say. Then ##\Delta T = t_4 - t_3## is the time difference on Earth between receiving the messages..

We then have to calculate ##v## based on ##\Delta T' = T_1 - T_2## and ##\Delta T = t_4 - t_3##.

This looks quite tricky.

One idea is to run through an example problem to see what happens. Let ##v = 0.6c##, for example. Just to see when these messages arrive on Earth. This will also give you a test for the general answer.

That's my suggestion: do the problem with ##v = 0.6c##, ##T_1 = 8## units, ##T_2 = 6## units.

Hint: use the ship frame and have events E_1 and E_2 be the sending of the messages, Events E_3 and E_4 be the messages received on Earth. And E_5 the ship arriving on Earth.

Last edited:
PeroK said:
We then have to calculate ##v## based on ##\Delta T' = T_1 - T_2## and ##\Delta T = t_4 - t_3##.
Are ##T_1## and ##T_2## prime in ##\Delta T'##? If we consider spaceship as ##S'## frame.
.
PeroK said:
One idea is to run through an example problem to see what happens. Let ##v = 0.6c##, for example. Just to see when these messages arrive on Earth. This will also give you a test for the general answer.

That's my suggestion: do the problem with ##v = 0.6c##, ##T_1 = 8## units, ##T_2 = 6## units.

Hint: use the ship frame and have events E_1 and E_2 be the sending of the messages, Events E_3 and E_4 be the messages received on Earth. And E_5 the ship arriving on Earth.
Supposing that ##T_1 = 8## units, ##T_2 = 6## are ##T_1' = 8## units, ##T_2' = 6## (spaceship), we have:

##Event\ 1:##
##T_1' = 8##
##L_1' = vT_1'=4.8## (distance from Earth for spaceship)

##Event\ 2:##
##T_2' = 6##
##L_2' = vT_2'=3.6## (distance from Earth for spaceship)

##Event\ 3:##
##L_3' = vt_3'##

##Event\ 4:##
##L_4' = vt_4'##

Making the difference we get:
##\Delta L_{43} = v\Delta T##

Now if I make a boost for the ##Events\ 1-2## in ##S## (earth) we get:
##T_1=\gamma (T_1'+vL_1')=(\gamma+v^2)T_1'##
##T_2=\gamma (T_2'+vL_2')=(\gamma+v^2)T_2'##

These are the times that Earth (##S##) saw when spaceship (##S'##) sent their messages.

But I'm getting confusing now. I don't know how to get ##T_3## and ##T_4##, I only know that their difference is ##\Delta T##. How can I get this, my datas are ##T_1'##, ##T_2'## and ##\Delta T##.

PeroK
By clarification in post #4 I would revise post #2

Please find attached space time diagrams of Spaceship FR and Earth FR.

#### Attachments

• img20201031_18584031.jpg
43.2 KB · Views: 95
Last edited:
You need to think. ##T'_1## and ##T'_2## are not times of events in the spaceship frame. If the time of event ##E_1## is ##t'_1 = 0##, then the time of event ##E_2## is ##t'_2 = T'_1 - T'_2##.

Note: personally I would not have put ##'## on ##T'_1## as this is data, not coordinates.

Second, if you can analyse a problem in one frame, why change frames? There's no reason to Transform to the Earth frame. You can use the ship frame and treat the Earth as a moving clock in that frame.

Let me help you get started with the specific problem. You really need to do this first, or you will get hopelessly lost with the general problem. I'll take ##c = 1##:

##v = 0.6##, ##\gamma = \frac 5 4##, ##T'_1 = 8##, ##T'_2 = 6##, ##L'_1 = 4.8##, ##L'_2 = 3.6##.

Where ##L'_1## is the distance to the Earth when the first message is send etc.

##t'_1 = 0##, ##t'_2 = 2##

These are the times that the messages are sent in the spaceship frame.

##t'_3 = \frac{d'_1}{1+v} = \frac{4.8}{1.6} = 3##

##t'_4 = t'_2 + \frac{d'_2}{1+v} = 2 + \frac{3.6}{1.6} = \frac{17}{4}##

##\Delta t' = t'_4 - t'_3 = \frac 5 4##

That's all in the spaceship frame. And, note that we haven't actually used any SR yet - except that the speed of light is ##c = 1##. That would be the same calculation in classical mechanics!

Now, treating the Earth as a moving clock, we have simply:
$$\Delta \tau = \tau_4 - \tau_3 = \frac{\Delta t'}{\gamma} = 1$$
Can you do the last bit and calculate the remaining time on the Earth clock (##\tau_R##) until the Earth reaches the spaceship?

PeroK said:
You shouldn't post the full answer!
Sorry, I deleted the lines. If it is not enough delete all the message please.

PeroK said:
You need to think. ##T'_1## and ##T'_2## are not times of events in the spaceship frame. If the time of event ##E_1## is ##t'_1 = 0##, then the time of event ##E_2## is ##t'_2 = T'_1 - T'_2##.

Note: personally I would not have put ##'## on ##T'_1## as this is data, not coordinates.

So it was what I'm saying in the first post.
##E_1## is ##t'_1 = 0##
##E_2## is ##t'_2 = T'_1 - T'_2##

They are datas, not coordinates, ok.

PeroK said:
Second, if you can analyse a problem in one frame, why change frames? There's no reason to Transform to the Earth frame. You can use the ship frame and treat the Earth as a moving clock in that frame.

Let me help you get started with the specific problem. You really need to do this first, or you will get hopelessly lost with the general problem. I'll take ##c = 1##:

##v = 0.6##, ##\gamma = \frac 5 4##, ##T'_1 = 8##, ##T'_2 = 6##, ##L'_1 = 4.8##, ##L'_2 = 3.6##.

Where ##L'_1## is the distance to the Earth when the first message is send etc.

##t'_1 = 0##, ##t'_2 = 2##

These are the times that the messages are sent in the spaceship frame.

I agree.
PeroK said:
##t'_3 = \frac{d'_1}{1+v} = \frac{4.8}{1.6} = 3##

##t'_4 = t'_2 + \frac{d'_2}{1+v} = 2 + \frac{3.6}{1.6} = \frac{17}{4}##
##\Delta t' = t'_4 - t'_3 = \frac 5 4##

That's all in the spaceship frame. And, note that we haven't actually used any SR yet - except that the speed of light is ##c = 1##. That would be the same calculation in classical mechanics!

Are these the times, always in spaceship, when Earth receipt the two messages?
Doesn't the speed you used in ##t'_3## and ##t'_4## violate the postulate that the limiting speed is ##c## (##c=1##)?

PeroK said:
Now, treating the Earth as a moving clock, we have simply:
$$\Delta \tau = \tau_4 - \tau_3 = \frac{\Delta t'}{\gamma} = 1$$

Did you call ##\tau## because the receiving events is happening in the same place? If yes, so it is a proper time.
What spaceship see is a time dilation.

PeroK said:
Can you do the last bit and calculate the remaining time on the Earth clock (##\tau_R##) until the Earth reaches the spaceship?

##\tau_R' = T_2'-t_4'=1.75##​

##T_2'## as we know is the remaining time on the spaceship clock until the Earth reaches the spacehip.
##t_4'## is the time on the spaceship clock when Earth received the second messages.
This is the remaining time on the spaceship clock (##\tau_R'##) until the Earth reaches the spacehip when Earth received the second message.
By applying the time dilation we get:

##\tau_R = \frac{T_2'-t_4'}{\gamma}=1.4##
I hope this reasoning is correct.

anuttarasammyak said:
Sorry, I deleted the lines. If it is not enough delete all the message please.
Thank you, but I need to understand deeply.

Frostman said:
Are these the times, always in spaceship, when Earth receipt the two messages?
Doesn't the speed you used in ##t'_3## and ##t'_4## violate the postulate that the limiting speed is ##c## (##c=1##)?
Did you call ##\tau## because the receiving events is happening in the same place? If yes, so it is a proper time.
What spaceship see is a time dilation.

Yes, everything can be done in the spaceship frame. Stay in that frame!

It's simple kinematics that if light is moving at ##c## in one direction and the Earth is moving at ##v## in the opposite direction, then the distance between them reduces at a rate of ##c + v##. This is in the frame in which the Earth is moving at ##v##. Nothing is moving faster than ##c## for that to happen.

The maximum separation speed in SR is, therefore, ##2c##.

I used ##\tau## to emphasise that we are staying in the ship frame and treating the Earth as a moving clock. You could use ##t## is you like, but ##\tau## is better as it is the Earth's proper time.

Note: we can do this because events E_3, E_4, E_5 are all colocated on Earth. So, we can use the Earth's proper time at those events.

Frostman said:
##\tau_R' = T_2'-t_4'=1.75##

##T_2'## as we know is the remaining time on the spaceship clock until the Earth reaches the spacehip.
##t_4'## is the time on the spaceship clock when Earth received the second messages.
This is the remaining time on the spaceship clock (##\tau_R'##) until the Earth reaches the spacehip when Earth received the second message.

You cannot mix up proper time, data and coordinate time in one equation like that! Instead, you want:
$$\tau_R = \tau_5 - \tau_4 = \frac{t'_5 -t'_4}{\gamma}$$

PeroK said:
Yes, everything can be done in the spaceship frame. Stay in that frame!

It's simple kinematics that if light is moving at c in one direction and the Earth is moving at v in the opposite direction, then the distance between them reduces at a rate of c+v. This is in the frame in which the Earth is moving at v. Nothing is moving faster than c for that to happen.

The maximum separation speed in SR is, therefore, 2c.

I used τ to emphasise that we are staying in the ship frame and treating the Earth as a moving clock. You could use t is you like, but τ is better as it is the Earth's proper time.

Note: we can do this because events E_3, E_4, E_5 are all colocated on Earth. So, we can use the Earth's proper time at those events.
You cannot mix up proper time, data and coordinate time in one equation like that! Instead, you want:
τR=τ5−τ4=t5′−t4′γ
Ok, so what I have to find is the ##t'_5##, the time where spaceship arrives on Earth and it is simply ##t'_5=\frac{d_1}{v}=8##

##\tau_R= \tau_5−\tau_4=\frac{t'_5−t'_4}{\gamma}=3##
Correct?

Frostman said:
Ok, so what I have to find is the ##t'_5##, the time where spaceship arrives on Earth and it is simply t5′=d1v=8

##\tau_R= \tau_5−\tau_4=\frac{t'_5−t'_4}{\gamma}=3##
Correct?
Yes. Now you have a series of calculations for a given ##v = 0.6c##. You must now tackle the problem for any ##v##. This time, however, you are aiming to express ##v## in terms of ##\Delta T' = T'_2 - T'_1## and ##\Delta T = \Delta \tau = \tau_4 - \tau_3##.

What you've done so far should be very useful. And, when you get an answer you can check it for the specific problem you have done.

PeroK said:
Yes. Now you have a series of calculations for a given ##v = 0.6c##. You must now tackle the problem for any ##v##. This time, however, you are aiming to express ##v## in terms of ##\Delta T' = T'_2 - T'_1## and ##\Delta T = \Delta \tau = \tau_4 - \tau_3##.

What you've done so far should be very useful. And, when you get an answer you can check it for the specific problem you have done.

Ok, I can rewrite all in terms of ##T'_2##, ##T'_1## and ##\Delta T##:

##t_1'=0##
##t_2'=T_1'-T_2'##
##t_3'=\frac{vT_1'}{1+v}##
##t_4'=T_1'-T_2'+\frac{vT_2'}{1+v}##

##\Delta t' = t_4'-t_3' = T_1'-T_2'+\frac{vT_2'}{1+v} - \frac{vT_1'}{1+v}=T_1'-T_2'+\frac{v}{1+v}(T_2'-T_1')=(\frac{v}{1+v}-1)(T_2'-T_1')##

Now remembering that

##\Delta T = \frac{\Delta t'}{\gamma}=(\frac{v}{1+v}-1)(T_2'-T_1')\sqrt{1-v^2}##

##\Delta T = (\frac{v}{1+v}-1)(T_2'-T_1')\sqrt{1-v^2}##

##\frac{\Delta T}{(T_2'-T_1')}=(\frac{v}{1+v}-1)\sqrt{1-v^2}##

##(\frac{\Delta T}{(T_2'-T_1')})^2=\frac{1-v^2}{(1+v)^2}##

Naming:

##Q=(\frac{\Delta T}{(T_2'-T_1')})^2##

We have:

##\frac{1-v^2}{1+v^2+2v} = Q##

##1-v^2=Q+Qv^2+2Qv##

##v_{1,2}=\frac{-Q\pm\sqrt{Q^2-(1+Q)(Q-1)}}{1+Q}##

##v_{1,2}=\frac{-Q\pm 1}{1+Q}##

##v_1 = \frac{-Q - 1}{1+Q}=-1##
##v_2 = \frac{-Q + 1}{1+Q}=\frac{1-Q}{1+Q}=\frac{(T_2'-T_1')^2-\Delta T^2}{(T_2'-T_1')^2+\Delta T^2}##

Why do I have two speeds? Is it all correct?

That all looks fine. ##v = -1## is clearly a spurious solution from solving a quadratic. We have ##v > 0##.

What about the remaining time until the ship reaches Earth?

Frostman said:
Why do I have two speeds? Is it all correct?
1+v is canceled out both from denominator and numerator of the equation so
$$\frac{1-v}{1+v}=Q$$

Frostman and PeroK
PeroK said:
That all looks fine. ##v =1## is clearly a spurious solution from solving a quadratic. We have ##v > 0##.

What about the remaining time until the ship reaches Earth?

##\tau_R=\frac{t_5'-t_4'}{\gamma}={(T_1'-T_1'+T_2'-\frac{vT_2'}{1+v})}{\sqrt{1-v^2}}##
##\tau_R=(T_2'-\frac{vT_2'}{1+v}){\sqrt{1-v^2}}##
##\tau_R=T_2'\frac{\sqrt{1-v}}{\sqrt{1+v}}##
##\tau_R=T_2'\frac{\Delta T^2}{(T_2'-T_1')^2}##

Is it ok?

PeroK
Frostman said:
##\tau_R=\frac{t_5'-t_4'}{\gamma}={(T_1'-T_1'+T_2'-\frac{vT_2'}{1+v})}{\sqrt{1-v^2}}##
##\tau_R=(T_2'-\frac{vT_2'}{1+v}){\sqrt{1-v^2}}##
##\tau_R=T_2'\frac{\sqrt{1-v}}{\sqrt{1+v}}##
##\tau_R=T_2'\frac{\Delta T^2}{(T_2'-T_1')^2}##

Is it ok?
You have one mistake there. The last formula shouldn't have squares. Also, note that ##T'_1 > T'_2##.

And what happens if you try ##T'_1 - T'_2 = 2## and ##\Delta T = 1##? Do you get ##v = 0.6## and ##\tau_R = 3##?

anuttarasammyak said:
1+v is canceled out both from denominator and numerator of the equation so
1−v1+v=Q
Yes, thank you.

##Q=\frac{\Delta T^2}{(T_2'-T_1')^2}##

##\frac{\Delta T^2}{(T_2'-T_1')^2}=\frac{1-v}{1+v}##

##v=\frac{(T_2'-T_1')^2-\Delta T^2}{(T_2'-T_1')^2+\Delta T^2}##

Can you confirm?

Frostman said:
Can you confirm?
That is fine.

PeroK said:
You have one mistake there. The last formula shouldn't have squares. Also, note that ##T'_1 > T'_2##.

And what happens if you try ##T'_1 - T'_2 = 2## and ##\Delta T = 1##? Do you get ##v = 0.6## and ##\tau_R = 3##?
I get ##\tau_R = -3##, because in denominator I have ##-2## and numerator is ##6##. Have I forgotten any signs?

Frostman said:
I get ##\tau_R = -3##, because in denominator I have ##-2## and numerator is ##6##. Have I forgotten any signs?
You used ##T'_2 - T'_1##, which (as I pointed out) is negative. And this became a problem when you took the square root.

You need to be more careful. Note that also, you used ##\frac{v}{1+v} - 1##. This simplies to: ##-\frac{1}{1+v}##, which is also negative.

You need to start noticing these things at this level of physics.

Frostman said:
##\Delta t' = t_4'-t_3' = T_1'-T_2'+\frac{vT_2'}{1+v} - \frac{vT_1'}{1+v}=T_1'-T_2'+\frac{v}{1+v}(T_2'-T_1')=(\frac{v}{1+v}-1)(T_2'-T_1')##
For example. That is not wrong. But, you should notice that:
$$(\frac{v}{1+v}-1)(T_2'-T_1') = \frac{T'_1 - T'_2}{1+v}$$
Which is better, in my opinion.

Frostman
Okay, I understand. I'll make all the counts on my book and I'll repost the result, now with too many posts I lose the my thread.

It's just a sign as you pointed out.
The results are:

##v=\frac{(T_1'-T_2')^2-\Delta T^2}{(T_1'-T_2')^2+\Delta T^2}##

##\Delta \tau_R = T_2'\frac{\Delta T}{T_1' - T_2'}##
Thank you PeroK and anuttarasammyak. I always make a mistake in setting the problem and I take absurd paths when the easiest way is not there. I think I have to do more and more practice.

## 1. How will the spaceship's unknown speed affect its approach to Earth?

The speed of the spaceship will greatly impact its approach to Earth. If the speed is too slow, the spaceship may not have enough momentum to enter Earth's atmosphere and could potentially crash. If the speed is too fast, the spaceship may not be able to slow down enough to safely land on Earth.

## 2. Can we determine the speed of the spaceship as it approaches Earth?

Yes, scientists can use various methods such as radar or telescopes to measure the speed of the spaceship as it approaches Earth. They can also analyze the trajectory and distance of the spaceship to calculate its speed.

## 3. What are the potential risks of a spaceship approaching Earth with an unknown speed?

The biggest risk is a potential collision with Earth or other objects in space. The unknown speed could also make it difficult for the spaceship to safely land on Earth or enter Earth's atmosphere, causing damage to the spaceship and potential harm to those on board.

## 4. How can we prepare for the arrival of a spaceship with an unknown speed?

Scientists and space agencies can prepare for the arrival of a spaceship with an unknown speed by closely monitoring its approach and trajectory. They can also have contingency plans in place in case the speed is too fast or too slow for a safe landing.

## 5. What factors could affect the speed of the spaceship as it approaches Earth?

There are many factors that could affect the speed of the spaceship as it approaches Earth, such as the propulsion system of the spaceship, the gravitational pull of other celestial bodies, and any potential obstacles in its path. The spaceship's trajectory and angle of approach can also impact its speed.

• Introductory Physics Homework Help
Replies
6
Views
889
• Introductory Physics Homework Help
Replies
2
Views
882
• Introductory Physics Homework Help
Replies
38
Views
3K
• Introductory Physics Homework Help
Replies
10
Views
3K
• Introductory Physics Homework Help
Replies
1
Views
994
• Special and General Relativity
Replies
12
Views
2K
• Special and General Relativity
Replies
75
Views
4K
• Introductory Physics Homework Help
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
2
Views
4K