# A few simple questions (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

#### The_Thinker

Hey...
Its been a looong time since i posted here... :uhh: but there have been a few nagging questions in mind that i needed clarified. And i would appriciate it if someone were nice enough to answer them... Anyway... here they are a few of them are very basic though..

1)Consider a spacecraft in space or vaccum, would its mass matter there? as in would you require more force to accelerate a heavier rocket than one with lesser mass?

2)If a mass is accelrated (hypothetically) with infinte energy.... does it gain infinte mass? and does its time stop? relative to the rest of us...( of course we won't be there cause it the mass will be infinte and there won't be any space for us... but anyway... )

3)If light has no mass but it has energy given by hf, taking energy and mass equilence, how did they prove that light had no mass?

4)Is it wrong to equate newtonian mathematics with einstiens equations?

5)Does a mass in vaccum or free space truly feel no resitance whatsoever?

6)Does the energy of light reduce when it travels through long distances? If not... then why does red shift occur?

7)No mass is allowed to travel beyond the speed of light... But does that apply to its relative velocity as well? as in is an objects relative velocity not allowed to exceed the speed of light? (i know the answer for this is no... its relative velocity doesn't matter here... but i just wanted to make sure....)

8)Light travels at a constant velocity... but does it travel, can! it travel below the constant speed of light (in vaccum) i mean. Im not talking about the speeds in other mediums...

9)Why is it that when a mass is accelarated towards the speed of light that it gains mass? i mean should it not lose mass so that it can travel at the speed of light massless... i know its because it follows the equations... but what is the theory backing it up?

10)Could anyone let me know of some good books to read so that i can improve my knowledge in the fields of quantum theory, general and special relativity and string theory? I want books that go deep but not deep enough that a non physicst cannot understand.

Thats it... if you reply to this... thx a lot... i really appriciate it... and thank you for your patience... #### russ_watters

Mentor
The_Thinker said:
1)Consider a spacecraft in space or vaccum, would its mass matter there? as in would you require more force to accelerate a heavier rocket than one with lesser mass?
F=ma. Don't confuse weight and mass.
2)If a mass is accelrated (hypothetically) with infinte energy.... does it gain infinte mass? and does its time stop? relative to the rest of us...( of course we won't be there cause it the mass will be infinte and there won't be any space for us... but anyway... )
Basically yes - and don't roll your eyes: Don't confuse mass and volume.
3)If light has no mass but it has energy given by hf, taking energy and mass equilence, how did they prove that light had no mass?
Don't combine equations that don't match (just because the units match, doesn't mean the equations can be combined).
4)Is it wrong to equate newtonian mathematics with einstiens equations?
That's not very specific, but generally yes.
5)Does a mass in vaccum or free space truly feel no resitance whatsoever?
No. Again, don't confuse weight and mass. f=ma
6)Does the energy of light reduce when it travels through long distances? If not... then why does red shift occur?
Yes, but its not the distance but the speed of the source.
7)No mass is allowed to travel beyond the speed of light... But does that apply to its relative velocity as well? as in is an objects relative velocity not allowed to exceed the speed of light? (i know the answer for this is no... its relative velocity doesn't matter here... but i just wanted to make sure....)
Since all velocity is relative, yes.
8)Light travels at a constant velocity... but does it travel, can! it travel below the constant speed of light (in vaccum) i mean. Im not talking about the speeds in other mediums...
Light travels at constant speed, not velocity.
9)Why is it that when a mass is accelarated towards the speed of light that it gains mass? i mean should it not lose mass so that it can travel at the speed of light massless... i know its because it follows the equations... but what is the theory backing it up?
The equations are the theory. Others can give more detailed explanations of how it works, but "why" tends to head toward philosophy.

#### pervect

Staff Emeritus
Why don't you read the sci.physics.faq about the two common (and different!) defintions of mass in common use in relativity

http://www.math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

and trim your questions down to just the single most important one (or perhaps a couple of closely related questions).

As is, with so many questions (and so many assumptions on your part), I'm not sure where to start.

#### The_Thinker

k... cool will do... thx for the prompt replies though... :)

#### The_Thinker

k got it... most of the confusions gone... but....
russ_watters said:
No. Again, don't confuse weight and mass. f=ma
k... in general do objects feel any sort of resistance when travelling through vaccum irrespective of their mass... as in... is there any data that indicates that objects do tend to lose some energy when travelling long distances?

russ_watters said:
Yes, but its not the distance but the speed of the source.
k got it.... so what you mean is that redshift occurs because of the speed of the source and has nothing to do with its distance?

russ_watters said:
Since all velocity is relative, yes.
k cool... so its relative velocity also must not exceed that of c, is that right? then what would happen if one photon comes in one direction and the another comes opposite to it... what would be the relative velocity that these two would see then? or is it erronous to compare light as an observer?

article about relativistic mass pointed to by pervect said:
"Ouch! The concept of `relativistic mass' is subject to misunderstanding. That's why we don't use it. First, it applies the name mass--belonging to the magnitude of a four-vector--to a very different concept, the time component of a four-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself."
Does this mean that with added energy to the object the space time around the object curves more than if it was at rest?

and also does relativistic mass... add to its weight?

thx for your patience guys... #### Jheriko

The_Thinker said:
then what would happen if one photon comes in one direction and the another comes opposite to it... what would be the relative velocity that these two would see then
This is one thing that is quite easy to understand about relativity imo. "The speed of light appears constant to every inertial observer" is a postulate of relativity. This implies that for anyone moving without accelerations (e,g, a particle of light) that any particle of light appears to move with a speed c. Even if the observer is a photon moving at c and another photon is hurtling towards it at c from the opposite direction.

If you want to know how to deal with the more general case of objects moving at any given velocity, you should look up the addition of velocities.

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving