# Is everything relative?

by quawa99
Tags: relative
 P: 56 Is everything we know relative or is there something absolute in this universe?
Mentor
P: 28,783
 Quote by quawa99 Is everything we know relative or is there something absolute in this universe?

Do you know about Special Relativity? Yes? Then what have you concluded from that?

No? Then maybe we can start you with that.

Secondly, what do you mean by "everything"? There are covariant/invariant values and expressions in physics that are NOT relative.

Zz.
P: 56
 Quote by ZapperZ This is rather vague. Let's start with something clearer. Do you know about Special Relativity? Yes? Then what have you concluded from that? No? Then maybe we can start you with that. Secondly, what do you mean by "everything"? There are covariant/invariant values and expressions in physics that are NOT relative. Zz.
I just have a basic idea about special theory of relativity.by everything I meant the physical quantities like energy,mass,velocity.
From special theory of relativity I have concluded that velocity of light is the same for all observers so maybe velocity of light is not relative?

P: 124

## Is everything relative?

The speed of light is absolute. So is mass. The charge of an electron is absolute. Etc
P: 56
 Quote by Hertz The speed of light is absolute. So is mass. The charge of an electron is absolute. Etc
Isn't mass relative?
Best Humor
P: 544
 Quote by Hertz The speed of light is absolute. So is mass. The charge of an electron is absolute. Etc
Mass is NOT absolute.
##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
EDIT
Mass, length, time, kinetic energy are all relative.
Charge, spin, baryon no. etc are not relative
P: 124
 Quote by quawa99 Isn't mass relative?
I believe it depends on how you learn special relativity. I learned it as mass being absolute, but I only just learned it in a classroom this semester, so I'm not an expert.

edit-
 Quote by Enigman Mass is NOT absolute. ##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
Why is this necessary?
PF Gold
P: 819
 Quote by Hertz The speed of light is absolute. So is mass. The charge of an electron is absolute. Etc
Mass?But I heard that mass increases with Speed(Kinetic Energy)

EDIT:Look at the Equation given by Enigman.
 P: 56 So bottom line velocity of light and charge are the two physical quantities which aren't relative ?
P: 124
 Quote by adjacent Mass?But I heard that mass increases with Speed(Kinetic Energy)
I hate arguing anything that I'm not too confident in, but for the sake of education!:

I learned relativistic kinetic energy as:
$T=(\gamma_u - 1)mc^2$ where mass is absolute. This is from the book "Modern Physics" second edition by Randy Harris

Also, total relativistic energy:
$E=\gamma_u mc^2$. Where mass is absolute.

edit-
 Quote by quawa99 So bottom line velocity of light and charge are the two physical quantities which aren't relative ?
There are most certainly other quantities as well..
 P: 56 Isn't charge relative because electric and magnetic feilds are relative?
C. Spirit
Thanks
P: 4,915
 Quote by quawa99 Isn't charge relative because electric and magnetic feilds are relative?
No charge is a Lorentz invariant. This follows from local charge conservation which is itself a consequence of Maxwell's equations.

Rest mass (more appropriately called invariant mass) is also a Lorentz invariant.
Best Humor
P: 544
 Quote by Hertz Why is this necessary?
Perhaps you are talking about the rest mass or the invariant mass? This the mass observed in an inertial frame where the object in question is at rest. (I perhaps should have mentioned this before)

##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
gives the mass as observed from a frame in which object in question is moving with velocity v.
But often mass and rest mass are used interchangeably
Derivation here- http://www.scribd.com/doc/98591006/S...ativistic-Mass.
(WBN beat me to it...)
Thanks
P: 2,944
 Quote by Enigman Mass is NOT absolute. ##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
The quantity ##M## in that equation is not frame-invariant, but ##M_0## is. It's something of a matter of taste which one you consider to be "mass", and that taste has changed over the years.
Best Humor
P: 544
 Quote by Nugatory The quantity ##M## in that equation is not frame-invariant, but ##M_0## is. It's something of a matter of taste which one you consider to be "mass", and that taste has changed over the years.
Yes, I was talking about relative or 'observed mass' as I thought it would be obvious from the context and not rest mass which is by definition frame-invariant.
 Quote by Hertz I learned relativistic kinetic energy as: $T=(\gamma_u - 1)mc^2$ where mass is absolute. This is from the book "Modern Physics" second edition by Randy Harris Also, total relativistic energy: $E=\gamma_u mc^2$. Where mass is absolute.
This is of course correct, provided m represents relative mass. And as ##m_{rel}=\gamma m##
The mass-energy equation reduces to ##E=m_{rel} c^2##.
##m_{rel}## is the mass that would be observed from a frame in which the object moves with velocity v and m is the mass in the frame in which relative velocity is zero.
http://www.scribd.com/doc/98591006/S...ativistic-Mass
 Mentor P: 28,783 Y'know, the more things change, the more they remain the same. This thing keeps coming back like an unwanted guest. http://www.physicsforums.com/showthread.php?t=642188 Please note this FACT: when you read the mass values of the various particles in the Particle Data Book, you'll notice that they never cite the corresponding speed. If mass is "relative", then there will not be a unique, unambiguous value. Zz.
P: 124
 Quote by ZapperZ Y'know, the more things change, the more they remain the same. This thing keeps coming back like an unwanted guest. http://www.physicsforums.com/showthread.php?t=642188 Please note this FACT: when you read the mass values of the various particles in the Particle Data Book, you'll notice that they never cite the corresponding speed. If mass is "relative", then there will not be a unique, unambiguous value. Zz.
Thanks for this link. This person has some very interesting points :)
Mentor
P: 28,783
 Quote by Hertz Thanks for this link. This person has some very interesting points :)
If by "this person" you meant Lev Okun, he is on this forum: