My little theory: what would happen?

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Discussion Overview

The discussion revolves around the concept of relativistic mass and how it is perceived by observers in different reference frames, particularly in the context of special relativity. Participants explore hypothetical scenarios involving an individual running at speeds approaching the speed of light and the implications for mass measurement and perception.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that as an individual approaches the speed of light, they would be observed to gain mass from an external reference frame.
  • Another participant counters that the individual in their own reference frame would not notice any increase in mass, nor would they observe effects such as length contraction or time dilation.
  • It is noted that relativistic effects are reciprocal; the runner would also observe the stationary observer as being affected by relativistic effects.
  • There is mention of multiple concepts of mass in special relativity and ongoing debate regarding which concept is most appropriate to use.
  • It is clarified that the rest mass is invariant and does not change regardless of the observer's frame of reference.

Areas of Agreement / Disagreement

Participants express differing views on the concept of mass in special relativity, particularly regarding how it is perceived by observers in different frames. There is no consensus on which concept of mass is best to use or how to interpret the implications of relativistic effects.

Contextual Notes

The discussion highlights the complexity of relativistic mass and the nuances in how different observers perceive mass and other relativistic effects. There are unresolved questions regarding the definitions and implications of mass in special relativity.

phylicity
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First post ever! Long time forum browser, but only registered yesterday! =D
I'm in my final year of high school, and have therefore only properly been doing Physics for two years, but it's something I'd like to pursue in university. Sorry if my knowledge of relativity is lacking a little...

I had this possibly stupid notion a while back... It's all hypothetical obviously:
We, happily standing in our inert reference frame, see someone start to run. They get faster and faster... in fact, they soon approach the speed of light.
Am I right in saying that we will observe them to gain mass?
BUT... in their reference frame, they are stationary. So will they be aware they have gained mass? If they were to measure their own mass, would they find it to have increased?

If anyone has any idea about this, please tell! Not sure if I'm wrong!
 
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phylicity said:
BUT... in their reference frame, they are stationary. So will they be aware they have gained mass? If they were to measure their own mass, would they find it to have increased?

No, on both counts. Nor will they notice themselves as being length-contracted, nor will they notice their clocks running slower.

To see why this has to be true, note that relativistic effects work both ways. The runner also observes you as being length-contracted, time-dilated, etc.

Now imagine that you are being observed by two runners, running at different speeds relative to you. The two runners will observe you being affected by different amounts of length contraction, time dilation and relativistic mass increase. But you obviously cannot observe yourself as simultaneously having masses of 70 kg and 100 kg!
 
Thanks!
 
phylicity said:
First post ever! Long time forum browser, but only registered yesterday! =D
Welcome to PF! Or at least welcome to the posting side of PF :smile:
phylicity said:
We, happily standing in our inert reference frame, see someone start to run. They get faster and faster... in fact, they soon approach the speed of light.
Am I right in saying that we will observe them to gain mass?
There is more than one concept of mass in SR, and some ongoing debate about which is the best mass to use. We will observe them to gain http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html" . They will not gain rest mass because it is invariant.
phylicity said:
BUT... in their reference frame, they are stationary. So will they be aware they have gained mass? If they were to measure their own mass, would they find it to have increased?
In their own frame they will obviously only measure their rest mass, which is relativistically invariant and will not change.
 
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