Best Way to measure Relativistic Rocket Acceleration?

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Discussion Overview

The discussion revolves around methods to measure the acceleration of rockets moving at relativistic speeds. Participants explore theoretical and practical approaches to tracking a rocket's position over time, considering the challenges posed by relativistic effects and the limitations of current technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose using a camera positioned far from the rocket's path to capture its position over time, while others question the efficiency of this method.
  • There is a discussion about the Lorentz factor (##\gamma##) and its relevance to measuring acceleration, with some participants seeking clarification on its application in different frames of reference.
  • One participant notes that force is not well-defined in special relativity (SR) and suggests that the formula for acceleration depends on the direction of motion.
  • Concerns are raised about the feasibility of macroscopic relativistic rockets in the near future, with some participants estimating timelines of hundreds of years for such advancements.
  • Participants express uncertainty about the practical implications of measuring acceleration and tracking position, with some emphasizing the need for efficient and cost-effective methods.
  • There is a debate about the energy requirements for achieving relativistic speeds, with one participant asserting that the mass-to-thrust ratio poses significant challenges.
  • Some participants highlight the need to consider relativistic optics and the source of illumination for the rocket when measuring its position.

Areas of Agreement / Disagreement

Participants express a range of views on the feasibility of measuring relativistic rocket acceleration and the timeline for developing such technology. There is no consensus on the best method for tracking the rocket's position, and discussions about the energy requirements and implications of special relativity remain unresolved.

Contextual Notes

Participants acknowledge the complexity of measuring acceleration in relativistic contexts and the challenges posed by relativistic optics. The discussion includes assumptions about the definitions of force and acceleration in different frames of reference, which are not fully resolved.

tade
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Someday, mankind will be able to construct rockets that can move at relativistic speeds.

The acceleration is given by ##a=\frac{F_0}{γ^3m_0}##

##F_0## can be easily measured by placing a force gauge on the rocket itself.

The acceleration is much harder to measure, is has to be measured in a frame in which the rocket is zipping past at relativistic speeds.What efficient methods to measure the acceleration can you think of?

My idea is placing a camera far away from the rocket's path and taking snapshots of it to see how its position evolves over time.

But we also have to consider tricky relativistic optics.
 
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tade said:
The acceleration
The ?

And what would ##\gamma## be in this expression ?

I expect it's a lot easier to measure a Doppler shift and differentiate. Leaving cameras all over the place doesn't sound efficient.
 
BvU said:
And what would ##\gamma## be in this expression ?
The Lorentz factor? based on the speed of the rocket
 
I know. But it's relative to something...
 
BvU said:
I know. But it's relative to something...
relative to the frame which you are in, making the measurements.
 
Yeah, I looked it up . It's the expression for transformation of acceleration between two inertial frames. Have to let it sink into understand whether it's necessarily between two inertial frames or can be applied within a non-inertial frame as well. Somewhat above my pay grade (experimentalist). @Orodruin ?
 
Force in the conventional sense is not very well defined in SR. From your post, it would seem that you are using ##F_0## to be the force in the instantaneous rest frame, which would make ##F_0/m_0## equal to the proper acceleration. Your formula will then generally depend on whether or not you are accelerating in the direction of motion or not.

It is unclear to me what you want to use this for. Acceleration is easily accessible in any frame. As you say, just observe the position as a function of time an account for the travel time of light.
 
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Orodruin said:
Force in the conventional sense is not very well defined in SR. From your post, it would seem that you are using ##F_0## to be the force in the instantaneous rest frame, which would make ##F_0/m_0## equal to the proper acceleration. Your formula will then generally depend on whether or not you are accelerating in the direction of motion or not.

It is unclear to me what you want to use this for. Acceleration is easily accessible in any frame. As you say, just observe the position as a function of time an account for the travel time of light.
Yup, all the vectors are pointing in the same direction. Just linear acceleration.

Yes, that's what I said, now I need a good way to practically and experimentally carry it out. Ideas for the near future perhaps.

Engineers will have to test the relativistic performances of rockets and spaceships in the future.
 
It is still unclear what you wish to accomplish. This has already been well tested in particle accelerators.

Relativistic space ships of macroscopic size are not a near future thing.

Furthermore, you cannot neglect mass losses if you use a rocket to generate thrust and wish to reach relativistic velocities.
 
  • #10
Orodruin said:
It is still unclear what you wish to accomplish. This has already been well tested in particle accelerators.

Relativistic space ships of macroscopic size are not a near future thing.

Furthermore, you cannot neglect mass losses if you use a rocket to generate thrust and wish to reach relativistic velocities.
Yes, but the rockets still have to be tested after they have been built. I'm just wondering what methods future engineers might use to accomplish the task of tracking a relativistic rocket's position. Something efficient and as cheap as possible.

Ok, maybe the far future then. Does near-future mean in about 50 years time to you?

Certainly we can take into account the rocket's changing mass.
 
  • #11
tade said:
Does near-future mean in about 50 years time to you?

Barring unexpected giant advancements you will not see macroscopic relativistic rockets in the next 200 years. Have you taken the time to compute what portion of a rocket's mass must be converted into thrust in order for it to reach relativistic velocity? It is humongous.
 
  • #12
Orodruin said:
Barring unexpected giant advancements you will not see macroscopic relativistic rockets in the next 200 years. Have you taken the time to compute what portion of a rocket's mass must be converted into thrust in order for it to reach relativistic velocity? It is humongous.
Do you have any link regarding that? Not that I don't believe you.

Anyway, this thread is less about rocket energy and more about tracking position.
 
  • #13
tade said:
Do you have any link regarding that? Not that I don't believe you.

Anyway, this thread is less about rocket energy and more about tracking position.
The 200 years is a personal lower estimate. I would be surprised if it happened at all unless special relativity is fundamentally misguided for macroscopic objects - which we have no reason to believe.

The computation of the mass fraction needed to be ejected in order to reach relativistic velocities is a typical textbook exercise.
 
  • #14
Orodruin said:
The 200 years is a personal lower estimate. I would be surprised if it happened at all unless special relativity is fundamentally misguided for macroscopic objects - which we have no reason to believe.

The computation of the mass fraction needed to be ejected in order to reach relativistic velocities is a typical textbook exercise.
ok. but why do you think that it is impossible? someday we might find a way to harness enough energy.

and anyway for this thread I want to focus on tracking a rocket.
 
  • #15
tade said:
ok. but why do you think that it is impossible? someday we might find a way to harness enough energy.
Because the energy required is essentially the entire starting mass of the rocket. It is not a matter of finding the energy, it is the ratio between total rocket mass and useful payload that will kill you. Never mind the useful payload if you ever want to decelerate to return to the original state of motion.
tade said:
and anyway for this thread I want to focus on tracking a rocket.
You have much more pressing problems than relativistic optics then. For example, where does the illumination of the rocket come from and what frequency it will be observed at.
 
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