B Is this an Unavoidable Collision between 2 spaceships traveling at 0.6c each?

[Acodato]

TL;DR

An inertial observer measures two spaceships on a collision course each one traveling at 60% the speed of light. From inside the spaceship can they measure how much time they have to avoid a collision?

Relativity always talks in terms of observer but fail to explain common sense problems like this. The fact that no one can measure anything faster than light does not preclude to objects moving towards each other at a higher velocity than an external observer can measure. Move the observer to one of the spaceships and now the limits of observation will render every onboard equipment useless to calculate actual time to impact.

 

[kuruman]

Move the observer to one of the spaceships and now the limits of observation will render every onboard equipment useless to calculate actual time to impact.

How do you figure? Do you understand relativistic addition of velocities?

 

[Acodato]

How do you figure? Do you understand relativistic addition of velocities?

For the observer inside the spaceship there is no relativity. His frame of reference is inert for every purpose. Nothing to add or subtract. Just unable to measure an incoming ship at 120% the speed of light.

 

[PeterDonis]

Just unable to measure an incoming ship at 120% the speed of light.

The incoming ship is not traveling at 120% the speed of light relative to the observer inside the spaceship. That was the point of the question @kuruman asked you in post #2.

 

[kuruman]

Just unable to measure an incoming ship at 120% the speed of light.

But the incoming ship will not be traveling at 120% of the speed of light. It will be traveling at a speed $v=\dfrac{0.6+0.6}{1+0.6^2}c =88\%~$ of the speed of light. That is relativistic addition of velocities.

 

[Dale]

Relativity always talks in terms of observer but fail to explain common sense problems like this

Relativity does not fail to explain this. Your lack of awareness of an explanation doesn’t mean one doesn’t exist.

As others have mentioned, this is explained by relativistic velocity addition. This is a very well known phenomenon.

 

[Acodato]

The incoming ship is not traveling at 120% the speed of light relative to the observer inside the spaceship. That was the point of the question @kuruman asked you in post #2.

If you are inside the spaceship, you agree with me you have no information about the relative motion of the two objects? Your measures will probably say it is moving 99.99% C but it will be wrong. The external observer can actually calculate time to impact. Each ship by itself can't.

 

[Orodruin]

If you are inside the spaceship, you agree with me you have no information about the relative motion of the two objects? Your measures will probably say it is moving 99.99% C but it will be wrong. The external observer can actually calculate time to impact. Each ship by itself can't.

It is clear from your posts here that you simply do not understand how the relativistic addition of velocity works. As you have already been told, you can very well measure the other spaceship's velocity relative to you. The computation was performed for you in post #5 with the result ca 0.88c. A light signal will travel faster than that so there is no issue here. Each ship by itself is perfectly capable of computing the time (in their frames) to impact. You are inventing a problem where there is none. This is extremely well known physics.

 

[PeterDonis]

If you are inside the spaceship, you agree with me you have no information about the relative motion of the two objects?

No. I disagree. If the other ship is emitting light signals at you, you can use their Doppler shift to measure its speed towards you. Which will be, as @kuruman calculated, 88% of the speed of light.

Your measures will probably say it is moving 99.99% C

No, 88%. See above.

it will be wrong.

No, you are wrong. See above.

The external observer can actually calculate time to impact. Each ship by itself can't.

The external observer has to have information about the distance between the ships as well as speed to calculate the time of impact. There's no reason why an observer on board one of the ships could not have distance information as well. And that, combined with the information about relative speed, does indeed allow an observer on board one of the ships to calculate the time to impact by their clock.

In other words, you are simply mistaken about what relativity can and cannot allow people to calculate.

 

[Acodato]

But the incoming ship will not be traveling at 120% of the speed of light. It ill be traveling at a speed $v=\dfrac{0.6+0.6}{1+0.6^2}c =88\%~$ of the speed of light. That is relativistic addition of velocities.

Only the observer outside the spaceships has this information. No one inside can make this calculation: there are no 2 speeds from the inertial frame of reference of the spaceship.

 

[Orodruin]

Only the observer outside the spaceships has this information.

If you don’t allow the crew of the spaceship to make their own measurements, then this setup doesn’t need relativity to result in an impossibility to determine the collision time.

Otherwise your assertion is simply false. If an observer in one of the spaceships makes a measurement of the other spaceship’s velocity relative to themselves, they will obtain the result 0.88c - as described above. Not more, not less.

No one inside can make this calculation: there are no 2 speeds from the inertial frame of reference of the spaceship.

it is impossible to try to parse what you actually mean by this. All objects have easily calculable velocities in all frames. There certainly are two different speeds relevant here, the speed of the first ship relative to the original inertial frame, and the speed of the second relative to the original inertial frame.

But more to the point: They do not need to have this information. They can simply measure the speed of the other ship as described in post #9. The result will be 0.88c.

Which will be, as @Orodruin told you, 88% of the speed of light.

Just to direct credit correctly: The computation here was provided by @kuruman in post #5. I merely quoted the result.

 

[berkeman]

If you are inside the spaceship, you agree with me you have no information about the relative motion of the two objects? Your measures will probably say it is moving 99.99% C but it will be wrong.

Maybe I can help.

-Maverick- "Help me out here Goose!"

-Goose- "I got .88c closure on radar, Mav"

-Maverick- "Fox-3!"

 

[Vanadium 50]

IBTL.

What's magic about 60%? Why not 6%? Or 0.000006%? How can people walking towards each other possibly avoid a collision?

 

[berkeman]

How can people walking towards each other possibly avoid a collision?

Fox-3!

Oops, sorry, that would be anti-social. My bad.

 

[PeterDonis]

Only the observer outside the spaceships has this information.

Why not? Don't the spaceships have sensors?

No one inside can make this calculation

Why not? Don't they know how to do math?

there are no 2 speeds from the inertial frame of reference of the spaceship.

Sure there are: zero (the speed of that spaceship) and 88% of the speed of light (the speed of the other spaceship in that spaceship's frame). Which is quite sufficient.

 

[PeterDonis]

@Acodato where are you getting your information on Special Relativity? So far you appear to have nothing but misconceptions.

 

[PeterDonis]

Just to direct credit correctly: The computation here was provided by @kuruman in post #5. I merely quoted the result.

Yes, thanks for the correction! I have edited my post.

 

[Vanadium 50]

Fox-3!

An armed society is a polite society.

So far you appear to have nothing but misconceptions.

While I agree, let's see if the "why 60%" question will illuminate where the difficulty lies,'

 

[FactChecker]

Only the observer outside the spaceships has this information. No one inside can make this calculation: there are no 2 speeds from the inertial frame of reference of the spaceship.

You are missing the point. From either of the two spaceships, no calculation is needed. In the physics of his inertial reference frame, the other spaceship is moving toward him at 0.88 c. If he measures its approach speed, that is what he will get. It's as simple (for him) as that.

 

[kuruman]

While I agree, let's see if the "why 60%" question will illuminate where the difficulty lies,'

I am sure that @Acodato is aware that two cars traveling towards each other at 15 miles an hour can easily avoid a collision. If it is true that two spaceships traveling towards each other at 0.6 c cannot avoid a collision, then it must be true that somewhere between 15 mph and 0.6 c there is a threshold speed that separates collision avoidance from collision non-avoidance. Perhaps @Acodato can tell how this threshold speed is determined.

 

[phinds]

@Acodato you should consider the first rule of holes: when you find yourself in one, stop digging.

 

[Acodato]

IBTL.

What's magic about 60%? Why not 6%? Or 0.000006%? How can people walking towards each other possibly avoid a collision?

My point is on non-relativistic notions just as you mentioned. If one spaceship is 1.2 light years apart from a planet, how long does is take for a 0.6C spaceship to reach the planet? 2 years. Now if another spaceship is launched from the planet at 0.6C how long until they meet (collide)? 1 year. Because it is what it is. Now you do the funky mathematics of relativity, doppler, redshift, whatever, and you say they will meet only after 1.4 years due to relativity, that is exactly the inevitable collision.

Why 0.6C? Because any speed below 0.5C could actually be detected.

 

[PeterDonis]

My point is on non-relativistic notions just as you mentioned.

Non-relativistic physics is not correct; relativistic physics is. That has been shown by many, many experiments over the past century and more. So if you are reasoning from non-relativistic notions, to the extent those notions contradict relativity, you are reasoning from factually wrong premises.

If one spaceship is 1.2 light years apart from a planet, how long does is take for a 0.6C spaceship to reach the planet? 2 years. Now if another spaceship is launched from the planet at 0.6C how long until they meet (collide)? 1 year.

These times are times relative to an observer at rest with respect to the planet.

Now you do the funky mathematics of relativity, doppler, redshift, whatever, and you say they will meet only after 1.4 years due to relativity

Who has said that? Have you done a relativistic calculation? In what frame is this claim supposed to be made? Not in the frame of the observer above (at rest relative to the planet); for that observer the time for the spaceships to meet is 1 year. That is what relativity says.

 

[Acodato]

Non-relativistic physics is not correct; relativistic physics is. That has been shown by many, many experiments over the past century and more. So if you are reasoning from non-relativistic notions, to the extent those notions contradict relativity, you are reasoning from factually wrong premises.


These times are times relative to an observer at rest with respect to the planet.


Who has said that? Have you done a relativistic calculation? In what frame is this claim supposed to be made? Not in the frame of the observer above (at rest relative to the planet); for that observer the time for the spaceships to meet is 1 year. That is what relativity says. But in the frame of either spaceship, it will take less than 1 year for them to meet, not more. That is what relativity says.

If you agree from the observer at rest it will take one year to meet half way then they are traveling at 1.2C. Observer can measure 0.6 and 0.6. The spaceship can't measure 1.2C. That is all light is: a measure.

 

[berkeman]

The spaceship can't measure 1.2C. That is all light is: a measure.

Did you read the link supplied early in your thread?

That is relativistic addition of velocities.

If so, what are your thoughts? If not, why not?

 

[PeterDonis]

If you agree from the observer at rest it will take one year to meet half way then they are traveling at 1.2C.

More precisely, their "closure rate" in the frame in which the planet is at rest is 1.2C.

Observer can measure 0.6 and 0.6.

Yes.

The spaceship can't measure 1.2C.

The relative speed of the other spaceship in that spaceship's frame is not 1.2C. It is 0.88C. The spaceship can measure that, as has already been pointed out.

If all you are going to do is keep repeating the same wrong claims, this thread is going nowhere and will be closed.

 

[Acodato]

More precisely, their "closure rate" in the frame in which the planet is at rest is 1.2C.


Yes.


The relative speed of the other spaceship in that spaceship's frame is not 1.2C. It is 0.88C. The spaceship can measure that, as has already been pointed out.

If all you are going to do is keep repeating the same wrong claims, this thread is going nowhere and will be closed.

I already said, at 0.88C the spaceship calculates about 1.4 years to collide. Can you prove this wrong?

 

[PeterDonis]

I already said, at 0.88C the spaceship calculates about 1.4 years to collide.

How did you come up with this number?

 

[phinds]

But in the frame of either spaceship, it will take less than 1 year for them to meet, not more. That is what relativity says.

I already said, at 0.88C the spaceship calculates about 1.4 years to collide. Can you prove this wrong?

HE JUST DID (well, he didn't show you the math but he could). You aren't listening to what people are telling you. You just can't stop digging, can you?

 

[Acodato]

How did you come up with this number?

At 1.2C takes 1 year, what will take at 0.88C?