[cheryl j (OP)]

I'm sorry if this is a dumb question but physics is not really my area. I've been listening to the CBC Massey lectures by physicist Neil Turok, which I quite like. Anyway, when he talks about mass increasing at higher speeds, how does that happen? Is there actually an increase in the amount of matter or atoms or particles? Or does it just take more force to accelerate it? I had always thought that mass and matter were the same thing.

[simplified]

I'm sorry if this is a dumb question but physics is not really my area. I've been listening to the CBC Massey lectures by physicist Neil Turok, which I quite like. Anyway, when he talks about mass increasing at higher speeds, how does that happen? Is there actually an increase in the amount of matter or atoms or particles? Or does it just take more force to accelerate it? I had always thought that mass and matter were the same thing.

If you stop photon then mass increases.

[Soul Surfer]

when you try to accelerate a particle or a larger object towards the velocity of light.  As you get close to this velocity it gets much harder to accelerate as if its mass had increased so that however hard you try you cannot actually increase the velocity of light but you are increasing its kinetic energy.  The famous equations of Einstein tell you how this increase works. 

The reason for this is that Einstein realised that the laws of physics cannot change when you are moving, because you can only find out if you are moving  (or something else is moving )  by looking at something else.  Also if you only have one other thing to look at to tell you that you are moving you cannot tell which of you (or both of you ) are moving.

This results in the fact that light must travel at the same speed to you whatever speed you are doing because a famous experiment done 125 years ago "The Michelson Morley experiment" was designed to measure the true velocity of the earth through space by measuring the velocity of light in different directions throughout the year failed to measure the change brought about by the velocity of the earth in its orbit round the sun.  This should have easily been detectable and came as a total shock to physicists at the time.

The non variability of the velocity of light with motion had has been proved experimentally to be true to one part in 10**-17 in recent years

[yor_on]

Relativity is always about about two 'frames of reference', measuring one relative the other. So when we say that somethings mass has increased due to a velocity, or speed, we always define it relative some other object, the observer. The fact is, as far as I can see that is , that all uniform motions is the same, no matter what 'speed' they will seem to have relative you. So the 'mass' you find should then be a representation of the energy a collision (between you and what you measure against) would produce, and also about directions.

Locally, as long as you're uniformly moving, it shouldn't change anything in your local experiments, although you might find starlight outside your solar system to blue shift. Accelerations are another thing in that they are locally (intrinsically?) measurable whereas a uniform motion always will need a referent to measure a speed.

[Pmb]

I'm sorry if this is a dumb question ...

Don't worry. It's not a dumb question, so your safe.

Note: relativistic mass is often also referred to by the name inertial mass.

Anyway, when he talks about mass increasing at higher speeds, how does that happen? Is there actually an increase in the amount of matter or atoms or particles? Or does it just take more force to accelerate it? I had always thought that mass and matter were the same thing.
[/quote]
Yes. It does require an increased force by an amount which is greater than the
Newtonian expression.

For a list of jouirnal articles on this subject please see
http://home.comcast.net/~peter.m.brown/ref/mass_articles/mass_articles.htm

Meanwhile I can give you the gist of it. The increase in mass is a result of the properties of spacetime. The combination of the relationship between space and time between two different inertial frames, one the rest frame of the observer. m and the other the rest frame of the particle M, shows that M = m/sqrt(1 - v^2/c^2).

The derivation can be found at my website which is at
http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm

If you're really randy about this subject I studied this aspect of relativity in great deal and summarized it in this (unpublished) paper. Why would you consider reading an unpublished paper? For the same reason you'd consider reading a post I'd post in a thread. With that in mind the article is at http://arxiv.org/abs/0709.0687

[Phractality]

I'm gradually seeing the fallacy in my reasoning about planetary orbital periods. If you have a continuous ring of tiny planets in a circular orbit about their barycenter, and you observe that from a frame moving at .866 c relative to the barycenter, the ring of orbiting planets will appear elliptical. It will be half as wide in the direction of relative velocity. However, the individual planets are not moving in an ellipse in that reference frame. Instead, they are following a cardoid path. To determine their acceleration in that reference frame, you have to analyze the cardoid motion, and that probably requires GR.

[a_dark_knight]

I vote that it's actually inertia which increases, not mass. To say mass is incredibly misleading, in my opinion. And the inertia doesn't *really* increase, it's just that there's kind of a gap between two perspectives moving very fast relative to eachother. They disagree about time flow which results in these weird effects, like objects seeming heavier and harder to push than usual. It's because in a sense they're moving faster than they appear, so the rest of that momentum goes into this apparant "weight".

[Pmb]

I vote that it's actually inertia which increases, not mass.

These are not independant things. Mass is the measure of something's inertia.

[lightarrow]

How does mass increase at higher speeds?

It doesn't.

[Bill S]

Hi Cheryl J; having worked through this thread I find myself wondering if your original question was answered.  I think it may have been, but that could be because I had my own pre-conceived idea as to what it should be.

I would be fascinated to know your thoughts.

[JP]

I vote that it's actually inertia which increases, not mass.

These are not independant things. Mass is the measure of something's inertia.

How does mass increase at higher speeds?

It doesn't.

Both are right.  Like many things in modern physics, we've realized that when we go to extremes that are way outside of our daily experience, seemingly simple concepts like mass can have multiple definitions. 

In paticular, at high speeds two types of mass can be defined: invariant mass and relativistic (or inertial) mass.  At slow speeds, both are equal and reduce to our everyday idea of mass. 

When someone says mass increases at high speeds, they're talking about relativistic mass.  Both Soul Surfer and pmb gave good explanations of why relativistic mass increases: it gets more energy to boost something's speed by the same amount if it's going fast than if it's going slow. 

When someone says mass doesn't change at high speeds, they're talking about invariant mass, which by definition stays the same at all speeds.  Both have their uses in physics, and so long as you're clear about which one you're using, you won't have problems.  You can get to one from the other.  However, most physics courses and physicists mean invariant mass when they say mass, since that's become the standard concept taught in class.

[lightarrow]

Does a human being become a massive black hole, if moving fast enough? Then we all should be, since we all are moving fast enough, with respect to some very fast particle or with respect a far galaxy receding from us at relativistic speed.

[JP]

That would make sense if there was a unique definition of mass that was defined in terms of the formation of black holes, but it's not...

I actually tend to agree with you, lightarrow, that invariant mass is the better definition to use for many things, including when teaching students relativity.  It's certainly the only definition I found useful in a bit of work I did in graduate school.  I don't oppose experts in the field using relativistic mass if they tell me it's useful for them.  I don't know enough cosmology to intelligently criticize or support its use there, for example.

[a_dark_knight]

These are not independant things. Mass is the measure of something's inertia.

But mass creates a gravitational field. Whereas inertia doesn't, in my opinion. That's the distinction I'm referring to. Mass also implies the amount of "stuff" (or matter) whereas inertia is just resistance to a force. So does that mean that things moving near the speed of light have a larger gravitational field than they would otherwise? Maybe it would normally be negligible but the whole point of science is to be accurate.

[yor_on]

That's a really good point Dark
Relativistic mass and 'gravity' are not the same.
But if we use 'energy' as our measure then?
=

Although, you have uniform constant accelerations to consider too?
But if we're talking 'speeds' as something uniformly moving.

[JP]

That's a really good point Dark
Relativistic mass and 'gravity' are not the same.
But if we use 'energy' as our measure then?

You have to use the stress-energy tensor: http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
This is a reason why arguments that only involve mass break down, e.g. why don't moving masses become black holes since they get heavier?

A simpler argument is that it's really the invariant mass that matters if it's possible to catch a ride on the mass.  Then you'd see the mass at rest with respect to you and its field would be entirely due to its invariant mass.  You could then figure out its gravitational field in any reference frame you chose by using GR to change reference frames.

[yor_on]

Let's take a example that gives us a totally new, and well earned, headache

Consider yourself heating up a gram of some, very, temperature resistant metal. You've weighted it before but after it gets heated you weight it again, finding it to weight more. One way to describe it might be to consider the particles making the material accelerating inside the metal as they gain 'energy' from heat, moving agitatedly. Can we then discuss those particles as gaining a relativistic mass, or not?

If they have we also find that this relativistic mass indeed have changed the 'proper' mass, that is if we would confine a proper mass to be whatever constitutes of that piece metal macroscopically..

And is it 'accelerations' that do it, or 'uniform motion'

[JP]

Yor_on, here's the same question, but in a slightly more extreme form:  If you have a box made of perfect mirrors and you inject some light into it, the box's energy has now increased.  If it's sitting still next to you, its mass increases (by E=mc², which holds for stationary objects).  So clearly its mass, measured at rest, went up.  Since invariant mass is supposed to not change with reference frame, and the rest frame is a reference frame, its invariant mass also went up.  Additionally, if you try to push it, you'll find its inertial mass went up. 

But photons individually have no mass?  How did it gain mass?

[cheryl j (OP)]

Hi Cheryl J; having worked through this thread I find myself wondering if your original question was answered.  I think it may have been, but that could be because I had my own pre-conceived idea as to what it should be.

I would be fascinated to know your thoughts.

I think I'm more confused than ever.

[cheryl j (OP)]

okay, what about that weird meter stick thought experiment where different observers pass it going different relative speeds. Does the meter stick really become shorter as the observers approach the speed of light. Is there "less" of the meter stick? Because in this experiment it doesn't sound like the meter stick's mass, matter, or inertia has changed at all, the observers are different.