# Regarding the relation between the mas of an object and its speed and based on the theory of relativity.

Simplex0
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Topic 192780

Assume that there are a moving star with a density so close to being a black hole that if the speed of the star increases with 1 m/s the increased mass will transform it in to a black hole. Will that star continue to be a black hole if the speed is reduced with 1 m/s?

Would it appear to be a black hole to a observer regardless the velocity of the observer relative the star?

Tomas

Keck_Komputers
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### Regarding the relation between the mas of an object and its spee

Once it colapses it will not revert to normal matter. According to most theories once it passes the required density it collapses all the way down to a point.

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Odysseus
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### RE: Assume that there are a

Quote:
Assume that there are a moving star with a density so close to being a black hole that if the speed of the star increases with 1 m/s the increased mass will transform it in to a black hole. Will that star continue to be a black hole if the speed is reduced with 1 m/s?

Speed, being relative, doesnâ€™t affect gravitation; otherwise any observer could turn your star into a black hole just by travelling at 1 m/s in the opposite direction! Anyway, itâ€™s not fashionable to talk about â€œrelativistic massâ€? any more; I gather that the current tendency is to regard mass as invariant, while applying the â€˜gamma factorâ€™ to momentum.

Simplex0
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### RE: Speed, being relative,

Message 67098 in response to message 67097

Quote:
Speed, being relative, doesnâ€™t affect gravitation

Is there a thing as 'Absolut speed'?

Chipper Q
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### RE: Assume that there are

Quote:

Assume that there are a moving star with a density so close to being a black hole that if the speed of the star increases with 1 m/s the increased mass will transform it in to a black hole. Will that star continue to be a black hole if the speed is reduced with 1 m/s?

Would it appear to be a black hole to a observer regardless the velocity of the observer relative the star?

Tomas

My guess is that there's no way to impart a uniform acceleration upon the whole of the star, even using another star, as you must consider tidal forces arising from the inverse-square-law nature of the gravitational force over the interval of spacetime that includes the whole star. So I don't think a black hole would form in that fashion, but possibly after whatever the star's merging with, although for something like a white dwarf gaining mass from a companion star, they both explode violently (if I'm not mistaken, but I'm not sure what's left from that type of event, although it's probably not a black hole but rather planetary nebula at that point?).
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Quote:
Speed, being relative, doesnâ€™t affect gravitation; otherwise any observer could turn your star into a black hole just by travelling at 1 m/s in the opposite direction!

Hmm... but is there some aspect of the metric that changes for the earth when a muon is detected at sea-level (from a cosmic ray interaction thousands of meters up), in the direction of propagation of the muon?

Odysseus
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### RE: Is there a thing as

Message 67100 in response to message 67098

Quote:
Is there a thing as 'Absolut speed'?

Not according to the theory of relativity: the notion had to be discarded along with that of the â€œluminiferous Ã¦therâ€?. One of Einsteinâ€™s basic working principles was that every inertial (non-accelerated) reference frame must be like every other.

Odysseus
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### RE: Hmm... but is there

Message 67101 in response to message 67099

Quote:
Hmm... but is there some aspect of the metric that changes for the earth when a muon is detected at sea-level (from a cosmic ray interaction thousands of meters up), in the direction of propagation of the muon?

Iâ€™m not sure what you mean, but I donâ€™t think so. ;)

In order to model the muon correctly using an Earth-based clock you have to apply the Lorentz transforms according to the particleâ€™s relative velocity, but that doesnâ€™t affect any other measurements made by the same clock.

Simplex0
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Thank you for your answers, seams that it has ben a change in consensus regarding the relation between mas and speed.

Klimax
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### Hi all One thing.Spacetime

Hi all
One thing.Spacetime itself is supposed to be absolute.As far as we do not try to go below Planck time and length,then space and time loses their meanings.

Chipper Q
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### Thanks Odysseus. Not sure if

Thanks Odysseus. Not sure if the muon example simplified it for Tomas, but there are always 12 full-sized inches to the foot, and when comparing measurements between one inertial frame of reference to another one relative to it, the measurements will be the same in both frames only by transforming them properly from one frame to the other (which involves just some algebra, along with the principle that the speed of light is the same in both frames). If I have that right, then a star having some critical density will not measure (or experience) any change in its own density, regardless of how it might accelerate, despite what might be measured in any other different inertial frame of reference. If it experiences no change in its density, then it wouldn't collapse into a black hole.

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Klimax posted:

Quote:
One thing.Spacetime itself is supposed to be absolute.As far as we do not try to go below Planck time and length,then space and time loses their meanings.

But where does one single region of this fundamental interval begin? Or at what coordinates in any frame of reference can you identify with any certainty that no events take place in adjacent halves of two such regions? What is meant by 'absolute'?

Ben Owen
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### Tomas, It's basically what

Tomas,

It's basically what Odysseus said: You see yourself as still and the star as moving at 1m/s, but the star sees you as moving and itself as still.

In relativity we tend to talk about "mass" meaning the quantity as measured in the rest frame of the object. Observers moving with respect to the object will measure higher values of the mass, or equivalently the energy. The former is called "rest mass" in introductory textbooks, but those of us who work with it tend to get sloppy with language after a while because when you're used to working with it it's easy to tell which one you mean.

Kind of like "speed" vs "velocity" - in school they emphasize the difference (the latter includes direction as well as a number), but working physicists tend to get sloppy with the words because it's clear to us from the context whether we mean the scalar or the vector.

Now the star will feel an effect if another star goes by it - or, the more astrophysically common scenario, orbits it. That is the tidal effect Chipper Q mentioned. To a first approximation it doesn't change the density. But the density does change a little, and for a while there was a big argument over whether it increased or decreased. After many very different calculations, the verdict is that it decreases unless you arrange the currents in the star just right.

The thing with the currents is called "gravitomagnetism". It means in relativity you get gravitational interactions between moving masses (mass currents) a lot like the magnetic interaction between electric currents.

Hope this helps,

Ben