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1

$begingroup$

1) In Minkowski space, coordinates which satisfy

x²=t²-X²>0

Region of spacetime is time-like.

2) If its

x²=t²-X²<0

Region is space-like.

3) But if

x²=t²-X²=0

then its "trajectory of light-like particles".

I have understood the first two points about time and space like region but I could not get the third one about "light like particles".
My confusion is about why just light like particles? there are many other particles at quantum level.

special-relativity mass metric-tensor causality

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edited 1 hour ago

Qmechanic♦

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asked 1 hour ago

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1

$begingroup$

1) In Minkowski space, coordinates which satisfy

x²=t²-X²>0

Region of spacetime is time-like.

2) If its

x²=t²-X²<0

Region is space-like.

3) But if

x²=t²-X²=0

then its "trajectory of light-like particles".

I have understood the first two points about time and space like region but I could not get the third one about "light like particles".
My confusion is about why just light like particles? there are many other particles at quantum level.

special-relativity mass metric-tensor causality

share|cite|improve this question

edited 1 hour ago

Qmechanic♦

107k121991239

asked 1 hour ago

sk9298sk9298

595

New contributor

sk9298 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

$begingroup$

1) In Minkowski space, coordinates which satisfy

x²=t²-X²>0

Region of spacetime is time-like.

2) If its

x²=t²-X²<0

Region is space-like.

3) But if

x²=t²-X²=0

then its "trajectory of light-like particles".

I have understood the first two points about time and space like region but I could not get the third one about "light like particles".
My confusion is about why just light like particles? there are many other particles at quantum level.

special-relativity mass metric-tensor causality

share|cite|improve this question

edited 1 hour ago

Qmechanic♦

107k121991239

asked 1 hour ago

sk9298sk9298

595

New contributor

sk9298 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|cite|improve this question

edited 1 hour ago

Qmechanic♦

107k121991239

asked 1 hour ago

sk9298sk9298

595

New contributor

sk9298 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|cite|improve this question

edited 1 hour ago

Qmechanic♦

107k121991239

asked 1 hour ago

sk9298sk9298

595

New contributor

sk9298 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited 1 hour ago

Qmechanic♦

107k121991239

edited 1 hour ago

Qmechanic♦

107k121991239

asked 1 hour ago

sk9298sk9298

595

New contributor

sk9298 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 1 hour ago

sk9298sk9298

595

2 Answers
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My confusion is about why just light like particles? there are many other particles at quantum level.

You are correct. The terminology is historical in nature. Light was the first massless particle to be discovered. The terminology “lightlike” was established before any other massless particles were discovered. Once other massless particles were discovered it was shown that they also travel along lightlike geodesics, but by then the term “lightlike” was well established.

An alternative term with the same meaning as “lightlike” is “null”. If you prefer then you can always use “null” and just understand that people saying “lightlike” mean the same thing.

share|cite|improve this answer

answered 1 hour ago

DaleDale

6,5871829

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1

$begingroup$

Only particles with zero mass can travel between two events which are separated by a light-like distance. The trajectory is called light-like because photons (light) are massless, and historically the first example of a massless particle, as well as the only example in the 1910's. There are other massless particles, like gluons which would also be able to travel between two events separated by a light-like distance.

The reason why only massless particles are able to travel between two events separated by a light-like distance is that it requires you to travel at exactly the speed of light. You can see this by considering the equation $t^2-x^2=0$, this means that $x=pm t$. These equations are with the units such that the speed of light $c=1$. Thus the particle taking this trajectory is travelling at the speed of light.

share|cite|improve this answer

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

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2

$begingroup$

My confusion is about why just light like particles? there are many other particles at quantum level.

You are correct. The terminology is historical in nature. Light was the first massless particle to be discovered. The terminology “lightlike” was established before any other massless particles were discovered. Once other massless particles were discovered it was shown that they also travel along lightlike geodesics, but by then the term “lightlike” was well established.

An alternative term with the same meaning as “lightlike” is “null”. If you prefer then you can always use “null” and just understand that people saying “lightlike” mean the same thing.

share|cite|improve this answer

answered 1 hour ago

DaleDale

6,5871829

$endgroup$

add a comment | 

2

$begingroup$

My confusion is about why just light like particles? there are many other particles at quantum level.

You are correct. The terminology is historical in nature. Light was the first massless particle to be discovered. The terminology “lightlike” was established before any other massless particles were discovered. Once other massless particles were discovered it was shown that they also travel along lightlike geodesics, but by then the term “lightlike” was well established.

An alternative term with the same meaning as “lightlike” is “null”. If you prefer then you can always use “null” and just understand that people saying “lightlike” mean the same thing.

share|cite|improve this answer

answered 1 hour ago

DaleDale

6,5871829

$endgroup$

add a comment | 

$begingroup$

My confusion is about why just light like particles? there are many other particles at quantum level.

You are correct. The terminology is historical in nature. Light was the first massless particle to be discovered. The terminology “lightlike” was established before any other massless particles were discovered. Once other massless particles were discovered it was shown that they also travel along lightlike geodesics, but by then the term “lightlike” was well established.

An alternative term with the same meaning as “lightlike” is “null”. If you prefer then you can always use “null” and just understand that people saying “lightlike” mean the same thing.

share|cite|improve this answer

answered 1 hour ago

DaleDale

6,5871829

$endgroup$

share|cite|improve this answer

answered 1 hour ago

DaleDale

6,5871829

answered 1 hour ago

DaleDale

6,5871829

answered 1 hour ago

DaleDale

6,5871829

1

$begingroup$

Only particles with zero mass can travel between two events which are separated by a light-like distance. The trajectory is called light-like because photons (light) are massless, and historically the first example of a massless particle, as well as the only example in the 1910's. There are other massless particles, like gluons which would also be able to travel between two events separated by a light-like distance.

The reason why only massless particles are able to travel between two events separated by a light-like distance is that it requires you to travel at exactly the speed of light. You can see this by considering the equation $t^2-x^2=0$, this means that $x=pm t$. These equations are with the units such that the speed of light $c=1$. Thus the particle taking this trajectory is travelling at the speed of light.

share|cite|improve this answer

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

$endgroup$

add a comment | 

1

$begingroup$

Only particles with zero mass can travel between two events which are separated by a light-like distance. The trajectory is called light-like because photons (light) are massless, and historically the first example of a massless particle, as well as the only example in the 1910's. There are other massless particles, like gluons which would also be able to travel between two events separated by a light-like distance.

The reason why only massless particles are able to travel between two events separated by a light-like distance is that it requires you to travel at exactly the speed of light. You can see this by considering the equation $t^2-x^2=0$, this means that $x=pm t$. These equations are with the units such that the speed of light $c=1$. Thus the particle taking this trajectory is travelling at the speed of light.

share|cite|improve this answer

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

$endgroup$

add a comment | 

$begingroup$

Only particles with zero mass can travel between two events which are separated by a light-like distance. The trajectory is called light-like because photons (light) are massless, and historically the first example of a massless particle, as well as the only example in the 1910's. There are other massless particles, like gluons which would also be able to travel between two events separated by a light-like distance.

The reason why only massless particles are able to travel between two events separated by a light-like distance is that it requires you to travel at exactly the speed of light. You can see this by considering the equation $t^2-x^2=0$, this means that $x=pm t$. These equations are with the units such that the speed of light $c=1$. Thus the particle taking this trajectory is travelling at the speed of light.

share|cite|improve this answer

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

$endgroup$

share|cite|improve this answer

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312

answered 1 hour ago

LucashWindowWasherLucashWindowWasher

30312