# B Light speed travel (now "understanding the difference between weight and mass")

1. Apr 12, 2018

### Doubleclick

So an object unaffected by any gravity free floating in space has zero mass. Now could this object reach light speed. Or am I confused?

2. Apr 12, 2018

### Buzz Bloom

Hi Doubleclick:

Yes, you are confused. What have you been reading that gave you the idea that gravity is involved with the speed a mass-less object/particle travels? A mass-less particle/object is always moving at the speed of light for as long as it exists. It does not have to "reach" the speed of light.

Regards,
Buzz

3. Apr 12, 2018

### jbriggs444

An object unaffected by gravity has zero weight. Its mass is whatever its mass is.

e.g. a 1 kilogram mass far from any gravitating object still masses 1 kilogram.

4. Apr 12, 2018

### Doubleclick

Sure
So mass without weight is still mass?

5. Apr 12, 2018

### sophiecentaur

Correct. Google "Weight and Mass" and you will find more hits than you would ever have time to read.

6. Apr 12, 2018

### Staff: Mentor

Yes. Weight is the force of gravity acting on a mass. Welcome to the PF.

7. Apr 13, 2018

### ZapperZ

Staff Emeritus
Mass isn't defined by its weight (i.e. what it "weighs" in gravity). If the object is in a zero-gravitational field, it still has a mass. If you try to push on it, it will exert an inertia, with or without gravity.

Zz.

8. Apr 13, 2018

### Khashishi

Nothing is unaffected by gravity. Gravity is a curvature of spacetime, and it affects everything equally no matter the mass. Even light, which is massless, is affected by gravity.

9. Apr 13, 2018

### EN2

Mass is how much stuff is inside an object (matter).
Even a "weightless" astronaut is made of matter and has mass. (Same goes for a grain of sand deep in interstellar space)
If you are up there with him and you give him a shove, it requires energy, and you and he start moving apart with velocities inversely proportional to your masses.

10. Apr 13, 2018

### John1945

I suspect one of the problems for many, including some academics, is the use of the term force when discussing gravity.. It is more accurate to describe gravity as an effect. Everything in the Universe is made of the same stuff, energy, but is clumped in space at different densities. The greater the density, the greater the mass, the more it bends the surrounding space time, also energy. The Universe is a mass of energy, some stretched thin and others such as black holes, very dense.

11. Apr 13, 2018

### phinds

No, the greater the MASS the greater the mass. Density is just how the amount of mass is arranged but does not change the amount of mass.

12. Apr 14, 2018

### John1945

Technically, yes, mass is mass. My point though was that the more mass that cumulates in space, such as planets, stars and other large objects, the denser they become at their core as the accumulated mass is compressed inward. This is the very reason stars begin fusion. At the quatum level, it is all made of the same stuff, including the fabric of space time and the accumulated mass warps all the space time around it. There are many misconceptions about gravity and I wonder at times what they are teaching these days in colleges. There seems to be a lot time spent on writing equations that no one understands but those who wrote them, but the more we have learned about the real observable universe, it is much more simple. It is only complicated when people start in to trying to prove science fiction and their imaginations.

13. Apr 14, 2018

### weirdoguy

There is no fabric of spacetime...

Well, apparently

14. Apr 14, 2018

### phinds

There is no such thing as the "fabric" of space-time. That's pop-science nonsense.

EDIT: Ah, I see weirdo beat me to it.

15. Apr 16, 2018

### John1945

Physicists have been trying to describe the nature of the space between the objects they can observe in the Universe for centuries, variously described as the ether or other terms. Because we still can't accurately describe energy or quantum particles, many refer to the energy that permeates the Universe as space time. All energy and densities affect all surrounding energies/densities in some way. Even the description of light itself is continued to be debated, even whether particles move or transmute along a path. Whatever it is, we know it exists. Space is not empty and we know that it is totally filled with energy, whatever its form and it is what most of us refer to as space time. This is what Einstein was speaking about when he spoke of the fabric of space. It is what we describe when explaining gravitational lensing.

16. Apr 16, 2018

### Staff: Mentor

Agreed. This thread has deviated from the OP's question (which has been answered, thanks folks). This is a good time to end this thread, I think.