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Author Topic: Physics questions: Consequence of relative velocity in relativity  (Read 1212 times)

LordBucket

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Was trying to explain relativity to someone and realized that apparently I don't understand it as well as I thought I did. So, couple questions for you physics buffs:

Hypothetical scenario 1:

There's a planet. Two rockets sit together on a launchpad on the planet. Their relative velocity is zero. They then launch at the same time on parallel trajectories at identical speeds. Because the rockets have mass, they are gravitationally attracted to one another. And with friction from the launchpad no longer holding them in place, this causes them to now (slowly) accelerate towards each other.

The rockets now both accelerate to arbitrarily large, but identical fractions of the speed of light.

 * Does this change in velocity relative to the planet cause their gravitational attraction to each other to increase?


Hypothetical scenario 2:

An object accelerates away from a reference point at 99% of the speed of light. We now measure the mass of the object. The object then comes to a complete stop, and then accelerates to 99% of the speed of light back towards our reference point. We again measure the mass of the object.

 * Will the two measurements be equal?


Hypothetical scenario 3:

The entire universe consists of two objects. They are at rest. Object 1 accelerates to 50% of the speed of light in a direct line away from object 2. Call the amount of energy required for this acceleration, Energy A. Object 2 now accelerates to 50% of the speed of light in a direct line towards object 1. This causes them to now be at rest. Object 1 now accelerates to 50% of the speed of light in a direct line away from object 2. Call the amount of energy required for this acceleration, Energy B.

 * Are Energy A and Energy B equal?
 * Is A+B equal to, or less than infinity?


Hypothetical scenario 4:

There sits in space an indestructible platform. Sitting on the platform is a spaceship with a reactionless drive. Due to the reactionless drive, it will not be pushing against the platform as a consequence of its launch. The ship then accelerates to a sufficiently large fraction of c that its mass becomes arbitrarily large.

 * What happens? The ship, by virtue of its gravitational force upon the platform, will tend to pull it along. But the fact of it being pulled along means its speed relative to the platform is reduced...which reduces the relative mass and therefore the force of gravity exerted by the ship upon the platform. How is this resolved?

Sheb

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #1 on: December 13, 2014, 07:59:47 pm »

PTW
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TheDarkStar

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #2 on: December 13, 2014, 08:57:43 pm »

As I understand it, there are two concepts of mass that you need to remember - rest mass/intertial mass is the one that is tied to gravity, and relativistic mass describes how difficult it is to change the trajectory of something as it get close to the speed of light.
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LordBucket

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #3 on: December 13, 2014, 09:04:49 pm »

As I understand it, there are two concepts of mass that you need to remember - rest mass/intertial mass is the one that is tied to gravity,
and relativistic mass describes how difficult it is to change the trajectory of something as it get close to the speed of light.

http://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitational_mass

"Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. "

http://en.wikipedia.org/wiki/Equivalence_principle

"...the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference."

Karlito

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #4 on: December 13, 2014, 09:08:52 pm »

So, there might be some people on this board with physics expertise, but if you really want good explanations it'd probably be better to ask a science forum, rather than a gaming one.
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smjjames

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #5 on: December 13, 2014, 09:14:14 pm »

1. Um, I don't think so?

2. I'm pretty sure that going at 99% the speed of light does not change your resting mass. What you would definetly observe though, is the doppler effect since the light coming from the ship will be redshifted when it goes away from you and blueshifted as it goes towards you.

3 and 4. I don't know.
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10ebbor10

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #6 on: December 14, 2014, 03:53:52 am »

3.
 a) Yes
 b) Less

This can most easily be explained by introducing a third observer, which doesn't change speed.

In the first situation, both spaceship A and spaceship B accelerate to have a relative velocity of 0.5c compared to C.

Next, spaceship A accelerates to have a relative velocity of 0.5 c compared to B, however, this acceleration only means a relative velocity of 0.8 c compared to C (I hope I didn't screw up a velocity there somewhere). Hence why the energy required isn't infinite. While you continually execute the same speed increase in your own reference frame, in a constant frame each successive increase will be smaller.
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Sergarr

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #7 on: December 14, 2014, 04:51:27 am »

From that wiki article on mass: "Because the relativistic mass is proportional to the energy, it has gradually fallen into disuse among physicists"

If physicists stopped using relativistic mass, it probably means that the relativistic mass is not an actual physical concept. Hence, in all these examples the mass doesn't actually increase, it just gets harder to accelerate things when their velocity is close to c.

EDIT: The problem with relativistic mass is that it's actually different for if you want to move along or perpendicular to your velocity vector. So if you wanted to use a relativistic mass in more than just simple cases, you would have to write it down like some sort of vector value.
« Last Edit: December 14, 2014, 05:00:46 am by Sergarr »
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Il Palazzo

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #8 on: December 14, 2014, 05:18:08 am »

Hypothetical scenario 1:

There's a planet. Two rockets sit together on a launchpad on the planet. Their relative velocity is zero. They then launch at the same time on parallel trajectories at identical speeds. Because the rockets have mass, they are gravitationally attracted to one another. And with friction from the launchpad no longer holding them in place, this causes them to now (slowly) accelerate towards each other.

The rockets now both accelerate to arbitrarily large, but identical fractions of the speed of light.

 * Does this change in velocity relative to the planet cause their gravitational attraction to each other to increase?
No it doesn't. The source of gravity in GR is not just (relativistic) mass, but the stress-energy tensor. One way of putting it, is that velocity and acceleration also play a role in determining the strength of gravity (and their contributions are not attractive). It's important not to try and carry over Newtonian concepts to GR.
One rule of thumb to remember this particular situation by, is that a black hole in one frame is a black hole in any frame. That is, you can't change the dynamics of the system by changing reference frames.
Another way to think of it (but a grossly oversimplified one), is that the relativistic mass increase is offset by the relativistic effects on time and length.

Quote
Hypothetical scenario 2:

An object accelerates away from a reference point at 99% of the speed of light. We now measure the mass of the object. The object then comes to a complete stop, and then accelerates to 99% of the speed of light back towards our reference point. We again measure the mass of the object.

 * Will the two measurements be equal?
Yes. The two are symmetrical.
Quote
Hypothetical scenario 3:

The entire universe consists of two objects. They are at rest. Object 1 accelerates to 50% of the speed of light in a direct line away from object 2. Call the amount of energy required for this acceleration, Energy A. Object 2 now accelerates to 50% of the speed of light in a direct line towards object 1. This causes them to now be at rest. Object 1 now accelerates to 50% of the speed of light in a direct line away from object 2. Call the amount of energy required for this acceleration, Energy B.

 * Are Energy A and Energy B equal?
 * Is A+B equal to, or less than infinity?
Yes, A and B are equal. You've basically changed the reference frame half way through the thought experiment (when you said they're at rest again). From that moment on it's exactly the same experiment again. If I were to guess, the confusion probably arose from still thinking as if the reference frame was the original one, in which case the two objects are never at rest apart from the initial setup, so you can't treat them as if they were (e.g., you can't accelerate an object moving at 0.5c by another 0.5c).
No reason why they should sum to infinity.
Quote
Hypothetical scenario 4:

There sits in space an indestructible platform. Sitting on the platform is a spaceship with a reactionless drive. Due to the reactionless drive, it will not be pushing against the platform as a consequence of its launch. The ship then accelerates to a sufficiently large fraction of c that its mass becomes arbitrarily large.

 * What happens? The ship, by virtue of its gravitational force upon the platform, will tend to pull it along. But the fact of it being pulled along means its speed relative to the platform is reduced...which reduces the relative mass and therefore the force of gravity exerted by the ship upon the platform. How is this resolved?
This looks like a much more complicated problem than it seems at a first glance. I'd take it to somewhere like physicsforums rather than ask here.

Why are you asking about such things on B12 anyway? You might get a few informed answers as long as it's SR, but these questions dip into GR, and that's a something you'll need more than just geeks for.

If physicists stopped using relativistic mass, it probably means that the relativistic mass is not an actual physical concept. Hence, in all these examples the mass doesn't actually increase, it just gets harder to accelerate things when their velocity is close to c.
No, it's not exactly that. It's a useful concept, as long as you know what you're doing.
Have a read through these:
http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html
https://www.physicsforums.com/threads/what-is-relativistic-mass-and-why-is-it-not-used-much.783220/#post-4919337

The problem with relativistic mass is that it's actually different for if you want to move along or perpendicular to your velocity vector.
That seems to be true for the relativistic equivalent of the inertial mass, not for the source of gravity. The first link above has some discussion on the subject.
« Last Edit: December 14, 2014, 05:22:50 am by Il Palazzo »
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Sergarr

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #9 on: December 14, 2014, 06:26:27 am »

Relativistic mass concept is bad because it invites much, much confusion as seen in the OP of this very thread.

It's better to use invariant values, because they're more intuitive. The concept of "mass" as being something depended on the "speed" you move relative to the object I'd argue to be very counter-intuitive.
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Leafsnail

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Re: Physics questions: Consequence of relative velocity in relativity
« Reply #10 on: December 14, 2014, 11:11:36 am »

Reading around it seems like yes, inertial mass does contribute to gravity... but you also have to consider the fact that gravitational waves travel at a finite speed and that objects moving relative to each other change the gravitational force experienced too.  The actual maths behind it looks pretty complicated but that's the basic reason why the paradoxes you're describing don't apply.

e: I guess you could think of it being sortof like electricity and magnetism.
« Last Edit: December 14, 2014, 05:18:05 pm by Leafsnail »
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