# B Can an object in every frame of reference have zero velocity

1. Feb 27, 2016

### frankin garcia

If im sitting, not moving, my velocity can be zero along with my acceleration but in truth the earth is moving along with the solar system and so on but i wanna know theoretically if a massless object in space can have zero velocity and zero acceleration, in essence what i am askimg is can something, have zero velocity/acceleration in every frame of reference possible? ( excuse me if this doesn't make sense im only in high school and im still tryimg to teach myself this stuff, so if this sounds stupid im sorry lol)

2. Feb 27, 2016

### Staff: Mentor

In whatever frame I'm in, I will observe objects that are either stationary with me or moving closer or farther away.

If I then start moving then other objects will become stationary with me but not necessarily the ones that I observed earlier unless they started moving when I started moving.

You could think of it as being in a bicycle race, some folks are moving at your speed and so appear stationary whereas everyone else is moving faster or slower. If you decide to race a bit faster then some of those racing past you will appear to now be stationary and everyone else is still moving faster or slower.

3. Feb 27, 2016

### HallsofIvy

If your question is "can there be an object which has 0 speed in every reference frame", the answer is clearly "no". Suppose we have one frame in which the object has speed 0. Then there certainly exist another frame which has speed $v\ne 0$ with respect to the first frame and that same object will have speed -v in that frame.

4. Feb 27, 2016

### Orodruin

Staff Emeritus
First of all, there is no such thing as "having zero velocity" unless you specify relative to what you have zero velocity. This is true in classical mechanics as well as in relativity and is really not anything particular for SR. In order to understand SR, you first need to understand what things are conceptually new.

Second, a massless object can never be at rest relative to something else it will always move at the speed of light.

5. Mar 1, 2016

### the_emi_guy

Expanding on other replies, relativity is all about motion in space-time, not motion in space alone. If you move 1 mile in space in one second, you may consider this to be "fast" but during that same second you have traveled 186,000 miles in space-time.

6. Mar 1, 2016

### HallsofIvy

In my answer, above, I assumed that your question was "can there exist an object which has velocity 0 in every frame". The answer to that is, clearly, "no" because we can have frames that are in motion relative to each other. Say, frame A has velocity non-zero v relative to frame B. An object that has velocity 0 relative to Frame A also has velocity v relative to frame B.

However, it occurs to me that you might have meant "in every frame, can there exist an object that has velocity 0 in that frame" (perhaps different objects in different frames). The answer to that is just as clearly "yes".

7. Mar 1, 2016

### alw34

You have a number of different concepts hidden in your post. It takes a while to learn how to sort them out from one another. I'll tackle a few here:

The correct way to state this would be to posit " ..my velocity can be zero along with my acceleration with respect to the room I'm in", for example, if that is what you were thinking....or "the class room I'm in". That provides your reference frame.

But even reference frames are tricky at first: Usually they are discussed as either inertial, meaning moving at a constant velocity, or 'non inertial', meaning accelerating. A 'room' is not such a simple reference as you might think.

When you are seated, what do you feel? a 'force' pushing against your backside, right? That is, the seat is pushing against you. That means you are actually accelerating! So even though you may not be moving with respect to your local reference frame [whatever room you are in] you are actually accelerating.

So sitting in a room, not only are you moving with a velocity as you understood, you are also accelerating which may be different that you assumed.

What is interesting is that when a massless object is emitted [created], say a photon [a 'particle' of light' emitted from an atom, as far as is known it appears at light speed and stays at light speed until it is absorbed somewhere else.

8. Mar 1, 2016

### Staff: Mentor

As others have already pointed out, "moving with a velocity" is meaningless unless you specify relative to what. You are not moving at all relative to the room. The room is a non-inertial frame, as you point out, but that doesn't make it invalid as a reference relative to which velocity can be defined.

This also means that you can have zero velocity even though you are accelerating in the sense you have defined "accelerating"; that's because the sense of "accelerating" that you are using, i.e., that you can feel a force pushing on you, does not imply anything about the derivative of your velocity with respect to time. For clarity, the term "proper acceleration" is often used to refer to feeling a force, and the term "coordinate acceleration" is used to refer to the derivative of your velocity with respect to time.

9. Mar 1, 2016

### alw34

frankin, in case my post was not clear I was referring to your original post:

where you used a reference frame of sorts.

So even though it superficially seems you are 'sitting still in a room' you seemed to understand with respect to some other frame of reference, say the sun in our solar system, you are actually moving. And of course the sun is moving within our galaxy and so forth.

I found the idea of "proper acceleration" very powerful because it is easy to relate to our everday experience when starting out trying to understand relativity. Other parts of relativity are NOT normally experienced at our everyday slow speeds.

There is a nice introduction of some terms here:

https://en.wikipedia.org/wiki/Proper_acceleration

Frankin, if you are interested, consider exploring each term in a source of your choice. Wikipedia is great on some and rather obscure on others.

Here's a nice example of proper acceleration:

"In an accelerating rocket after launch, or even in a rocket standing at the gantry, the proper acceleration is the acceleration felt by the occupants..."

Here is a good source I found recommended in these forums: http://mathpages.com/rr/rrtoc.htm [Reflections on Relativity]

And you have probably experienced a similar series of accelerations sitting sit in a car, where you feel a 'force' while seated without the vehicle moving along the road, then an additional 'force' as the car accelerates up to highway speed. And there is another acceleration you 'feel' as the car rounds a curve in the road.

If you have studied derivatives in math, many here can assist you in further understanding.

10. Mar 1, 2016

### Orodruin

Staff Emeritus
Well, perhaps because it is not a relativistic concept?