# Mass/Density General Relativity Question

CasualCalculus
Is the increase in mass/density of an object in motion purely relativistic?

What I mean is if you were in a spaceship approaching the speed of light would you be able to measure a change in your mass/density or would the change only be observable to someone at rest. I'm aware of the Lorentz formulas so I understand that mass/density changes are relativistic but are they purely relativistic like time?

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Is the increase in mass/density of an object in motion purely relativistic?

What I mean is if you were in a spaceship approaching the speed of light would you be able to measure a change in your mass/density or would the change only be observable to someone at rest. I'm aware of the Lorentz formulas so I understand that mass/density changes are relativistic but are they purely relativistic like time?

First, you are taking about relativistic mass, which is an outdated concept. See:

https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

Second, there is no such thing as absolute velocity, only velocity relative to something else. There is no such thing as a spaceship "approaching the speed of light"; only approaching the speed of light relative to something like the Earth, or approaching the speed of light in a given reference frame.

There could be, therefore, no such thing as an absolute change in mass depending on motion. For example, your velocity relative to a high-energy particle hitting the Earth is a significant fraction of the speed of light. But, you are moving at a far lower speed relative to the centre of the Earth. So, if you could measure your relativistic mass, would you measure a low mass based on your velocity relative to the centre of the Earth or a large mass based on your velocity relative to a high-energy particle?

Third, what you can measure is something's Kinetic Energy (KE). If something is at rest relative to you it has zero kinetic energy. If it is moving at close to the speed of light relative to you, then it has a very high kinetic energy.

The old concept of relativistic mass tended to associate some of the increase in KE to an increase in mass. This always seemed highly illogical to me.

CasualCalculus
CasualCalculus
Okay all the above points accepted (I'll read that thread - I was not aware relativistic mass was an outdated subject).

But for my benefit (as an interested layman) as time of a object slows as it approaches the speed of light (relative to an object at rest) - say a person in a spaceship - they would not perceive that time for them is slowing (relative to a person at rest).

I understand from GR that as an object approaches the speed of light (relative to an object at rest) its mass also increases. Whereas you can't perceive how time for you is slowing (relative to an object at rest), can you perceive your own mass increasing?

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... can you perceive your own mass increasing?

No, no, no.

By the way, there is no such thing as an object "at rest". Only at rest relative to something else or in a given reference frame. This concept of relativity of all motion goes back to Galileo, 1564-1642. A man ahead of his time.

CasualCalculus
Sorry I've seen the term 'at rest' used in several forums and always assumed it as a short hang for 'at rest relative to something else' - I thought that was implied in what I wrote.

Secondly so I'll take it that the change in your own mass and density is not something you could say physically measure yourself - by observing your own mass/density at a lower velocity and at a higher velocity you wouldn't perceive a physical change.

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Sorry I've seen the term 'at rest' used in several forums and always assumed it as a short hang for 'at rest relative to something else' - I thought that was implied in what I wrote.

Secondly so I'll take it that the change in your own mass and density is not something you could say physically measure yourself - by observing your own mass/density at a lower velocity and at a higher velocity you wouldn't perceive a physical change.

The problem is that everything you write is based on the tacit assumption that motion is absolute. By dropping the "relative" you've fallen into the trap of seeing all motion as absolute.

So, let me ask you: as you currently sit at your computer, what is your velocity? Is it low or high?

CasualCalculus
I fully get where you're coming from, the basis for asking my question is I'm trying to understand if there are any physical properties of spacetime and I didn't want to assume that mass/density follows the same relativistic nature as time, although it seemed like it should.

From the looks of it I need to have a better understanding of the stress-energy tensor to understand this

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I fully get where you're coming from, the basis for asking my question is I'm trying to understand if there are any physical properties of spacetime and I didn't want to assume that mass/density follows the same relativistic nature as time, although it seemed like it should.

From the looks of it I need to have a better understanding of the stress-energy tensor to understand this

You need to have a better understanding of Galilean relativity; then Special Relativity. None of this has anything to do with stress-energy.

You didn't answer my question about your current velocity, for example. You're missing the basics at the moment.

CasualCalculus
I don't need to answer that question as I get your point and I understand Galilean physics thank you. I asked a deliberately simple question - it would have been better phrased as 'what are the physical properties of space-time other than gravity? - I.e. what effects does space-time have on physical matter' - however that question had too broad a brief. I thought I would ask the simpler question of 'are the changes of an object's properties in motion entirely relativistic?' which you've answered. And thank you for that. It may seem entirely obvious that yes, they are entirely relativistic but I didn't want to simply assume. Whereas it's easy to find explanations to the relativistic nature of time, considering other properties such as density, mass etc. doesn't have the same depth of answers online.