Humans and relativity - Could they take it?

Phobos · 2002-09-03 19:26:16

Even though I've known for a long time that mass increases as you approach the speed light, I've never heard anyone discuss what the effect of the mass gain might mean for the health of passengers on a ship going say 99.999999% the speed of light. Assuming they accelerated slow enough so as not to squash them, could their bodies withstand the extra mass? Since their bones, etc, would increase in mass proportionately to each other would it be a big deal? What if you had an infinite power engine and could actually obtain 100% the speed of light? Could a person survive having infinite mass?

Byron · 2002-09-04 16:31:29

Interesting question, Phobos...this is the kind of thing I've wondered about myself...lol.

I do know that the "curve" of significant mass increase doesn't dramaticaly show itself until 97% or so of c, but at 99.999999%...the ship and passengers would indeed have a lot of mass...exactly how much, I'd like to know But to answer your question of whether the extra mass would be harmful to the passengers...I think it would be, once the mass of the passengers got past, let's say two or three times normal. I don't think the strength of biological tissues (such as the heart) increases in the same proportion as mass...so once the ship reached that 99.99_something % of c, the passengers would probably begin dying of heart failure. Not something I'd want to try myself, that's for sure. If we do develop such an "infinity" engine, I'd say we send a few chimps out on a run to see what happens before we try it out on people...

B

Shaun Barrett · 2002-09-04 22:41:54

Hi Phobos and Byron!
Being a simple soul myself, it never occurred to me to wonder how people would feel on a ship approaching light speed!
I have a suspicion they might not notice any difference. I can't justify this argument with mathematics (simple soul, remember?! ), but by considering the concept of 'frames of reference'.
As we're all aware, time slows for the occupants of our craft as they speed up. At 0.9999c, or thereabouts, this time dilation effect becomes very obvious to an outside observer. But the people on board are unaware of it ... in their frame of reference, clocks are ticking at the same rate as usual and everything seems normal.
An outside observer would notice that, even though the rocket is still blazing away at the same thrust, the craft's acceleration rate is rapidly reducing. In the outsider's frame of reference, this reduction in acceleration can be explained perfectly by a corresponding increase in the 'm' term of the equations. In other words, to the outsider, the apparent mass of the craft is increasing.
But I think the crew in the spacecraft would be as unaware of any mass increase as they are of any slowing of time.
As I said, I've reached this conclusion by my own interpretation of how relativity works. And you must remember that this interpretation springs from a mind entirely unburdened by a stultifying education in theoretical physics!!
I now stoically await the thud of high calibre ammunition shredding my fuselage as I'm shot down in flames!

Phobos · 2002-09-04 22:56:41

I do know that the "curve" of significant mass increase doesn't dramaticaly show itself until 97% or so of c, but at 99.999999%...the ship and passengers would indeed have a lot of mass...exactly how much, I'd like to know

Come to think of it, I wonder if the mass change would be apparent to the people actually travelling at the speed of light or if the infinite mass would only be apparent to a "stationary" observer. But then again if the ship didn't truly obtain infinite mass it would be capable of accelerating over the speed of light so maybe everybody would be crushed to death.

Shaun Barrett · 2002-09-04 23:00:49

P.S.
I just remembered another thing about travelling at 0.9999c.
An outside observer would perceive the physical dimension of the craft, in the direction of travel, to be getting smaller.
As the craft approaches closer and closer to light speed, its forward/backward dimension would appear to approach zero! ... But only in the frame of reference of the outside observer.
Again, the crew on board our craft would be blissfully unaware of any reduction in the volume of their living quarters. In their frame of reference, their tape measures would reveal not a jot of difference in the length of their ship.
I feel this constitutes more evidence to bolster my case that no increase in mass would be noticed either.
[Still waiting for a withering attack, though!!]

Shaun Barrett · 2002-09-04 23:05:03

Yeh, Phobos!
Great minds thinking alike again!! Spooky, huh?!

Phobos · 2002-09-04 23:13:39

I now stoically await the thud of high calibre ammunition shredding my fuselage as I'm shot down in flames!

It appears we both had the same ideas at the same time! Relativity has a very Alice in Wonderland kind of feel to it. What I find amazing about it is that someone travelling at C would, by their own perception, appear to get to Alpha Centauri in mere seconds instead of four years! Which seems to blow away the common sense notion that they would still feel four years go away while back here on Earth we'd all be in our final resting places.

Palomar · 2002-09-05 09:58:55

Even though I've known for a long time that mass increases as you approach the speed light, I've never heard anyone discuss what the effect of the mass gain might mean for the health of passengers on a ship going say 99.999999% the speed of light. Assuming they accelerated slow enough so as not to squash them, could their bodies withstand the extra mass? Since their bones, etc, would increase in mass proportionately to each other would it be a big deal? What if you had an infinite power engine and could actually obtain 100% the speed of light? Could a person survive having infinite mass?

*Could the universe itself survive ANYTHING having infinite mass? Wouldn't EVERYTHING just get crushed?

--Cindy

clark · 2002-09-05 11:34:14

Could the universe itself survive ANYTHING having infinite mass?

Not much of a physics junkie, but isn't light going the speed of light? If so, dosen't that neccessarily imply that light has infinite mass? Now, light is a wave too which may be a reason why it wouldn't have infinite mass.

However, if light does not have infinite mass, what amount of mass does it have?

Also, the whole concept of "infinite" is a bit misleading. We consider the universe infinite only becuase we lack the means with which to actually measure it. However, our inability to actually measure something accuraetly is not the sole determinant on whether it is or is not infinite.

A reality that may exsist beyond our perception.... just thinkin out loud.

Palomar · 2002-09-05 11:53:55

Me: Could the universe itself survive ANYTHING having infinite mass?

Clark: Not much of a physics junkie,

*That makes two of us...physics, yipes! ???

Clark: but isn't light going the speed of light?

*Yes...at least that's what I've heard.

Clark: If so, dosen't that neccessarily imply that light has infinite mass?

*That's a REALLY good question!

Clark: Now, light is a wave too which may be a reason why it wouldn't have infinite mass.

*Hmmmm.

Clark: However, if light does not have infinite mass, what amount of mass does it have?

*Does light have any mass at all?

Clark: Also, the whole concept of "infinite" is a bit misleading. We consider the universe infinite only becuase we lack the means with which to actually measure it. However, our inability to actually measure something accuraetly is not the sole determinant on whether it is or is not infinite.

*Agreed.

Clark: A reality that may exsist beyond our perception

*The mind reels. ???

--Cindy

Bill White · 2002-09-05 13:54:30

What I find amazing about it is that someone travelling at C would, by their own perception, appear to get to Alpha Centauri in mere seconds instead of four years!

Have you inverted the time dilation effect?

I had always thought that going to Alpha Centauri at 99% of c would "seem" to take 4 years for the crew while centuries and centuries would pass on Earth.

= = =

clark and Cindy engaged in civil discourse?

To quote Cindy: *The mind reels. . . *

Kudos!

Pat Galea · 2002-09-05 15:07:32

Have you inverted the time dilation effect?
I had always thought that going to Alpha Centauri at 99% of c would "seem" to take 4 years for the crew while centuries and centuries would pass on Earth.

OK, let's take this one bit at a time.

The relativistic mass change is apparent only to the external observer, not to the people on the ship.

So a ship travelling past at .99c looks very massive to us here on Earth. The people on the ship don't feel a sudden urge to phone weight-watchers!

-

Time dilation works like this: ship travels to Alpha Centauri at .99c. It appears to take 4 years as viewed from Earth. To the people on the ship, it takes (quick calculation) just under 7 months.

Think about it this way. If the time dilation worked the way you said, then what would it mean to say that the ship was travelling at .99c? Light would appear to be completely stationary!

-

Light has a zero rest mass, so it doesn't have an infinite mass when it's travelling at... well, the speed of light.

Shaun Barrett · 2002-09-05 18:16:05

Hi Pat!
I am aware of light's zero rest-mass, a concept I thought of as theoretical only, until recent laboratory success in slowing light to a virtual standstill.
But the pressure of light on a solar sail obviously indicates photons have momentum. As far as I can tell, you can't have momentum without mass, right?
I've never seen a layman's guide to how the mass of photons varies with speed and have always blithely accepted that somewhere between 0 kps and 300,000 kps, light just acquires the mass necessary to exert pressure!
Perhaps you can give me (and maybe others amongst us? )
an "idiot's guide to mass variability in photons" ?!!

P.S. Thanks for confirming for us that the mass of our
astronauts aboard their starship doesn't actually increase
... at least in their frame of reference. We can all now
sign up for that first mission to Alpha Centauri without
worrying!

Phobos · 2002-09-05 22:29:26

Not much of a physics junkie, but isn't light going the speed of light? If so, dosen't that neccessarily imply that light has infinite mass? Now, light is a wave too which may be a reason why it wouldn't have infinite mass.
However, if light does not have infinite mass, what amount of mass does it have?
Also, the whole concept of "infinite" is a bit misleading. We consider the universe infinite only becuase we lack the means with which to actually measure it. However, our inability to actually measure something accuraetly is not the sole determinant on whether it is or is not infinite.

Photons have zero mass so they don't apply to the "mass" rule. And we're also talking about infinite only in the sense that a ship that is going dead on 100% the speed of light would have infinite mass if it were to be observed by someone going at non-relativistic speeds. At least that seems to be the consensus anyway.

But the pressure of light on a solar sail obviously indicates photons have momentum. As far as I can tell, you can't have momentum without mass, right?

Actually, if I remember right, it might be a little misleading to say photons always lack mass. Photons in a high frequency state apparently show some levels of mass while those in a low frequency state show no mass. But the light that powers a solar sail would be low energy light (otherwise you'd be fried by radiation) so maybe the photons are just transferring energy to the solar sail when they collide since energy and mass are really two sides of the same coin. But I'm probably wrong on all of the points above. Anyhow considering all of these states that light and energy can change into seems to be one of the fundamental principles of string theory, that that all of these particles are only the creation of a "string" vibrating at different frequencies.

Could the universe itself survive ANYTHING having infinite mass?

Scientists seem to think that the big bang very shortly before it exploded had infinite mass. So if someone gained infinite mass in all frames of reference they'd probably big bang into a new universe.

Preston · 2002-09-05 22:46:44

ARGH! Stop the madness!

According to my current text in Classical Dynamics of Particles and Systems (4th ed by Marion and Thornton), which has a section on special relativity, physicists no longer use terms such as "relativistic mass." Likewise, there is no such thing as "rest mass," there is just mass plain and simple (new to me too). They decided that mass should be something that is considered inherent and unchanging (this is the same as what used to be called 'rest mass.' That being said, _momentum_ is the quantity that increases relativistically as you increase velocity toward the speed of light.

p=(gamma)(m)(v) for particles
where gamma=(1-v^2/c^2)^-0.5

The m in that equation is mass which does not change with respect to speed.

Photons are massless, but they have momentum. For photons, the momentum is p=h/lamda(this works for particles too), where h is planck's constant and lambda is the wavelength. For photons, the momentum is not mv.

I haven't had enough quantum or electrodynamics yet, but I should think that photons are considered massless because they are electromagnetic waves (electric and "magnetic"(electrodynamic) fields) and saying that they are "particles" only makes sense because they can push things around in a particle-like way if they are localized enough -- But they are still fields, so they don't have mass.

Shaun Barrett · 2002-09-06 00:56:03

Thanks Preston!
I didn't quite understand your drift at first ... until I filled in a few blanks myself (the biggest blank being my mind, I think! ) Let me put this in my own terms, so that anyone else interested has two explanations to read.
I was having trouble understanding why a photon should have momentum when it has no mass. But Phobos provided the solution when he mentioned that "energy and mass are really two sides of the same coin".
As Preston points out, the equation for the momentum of a particle is:-
p=mv ( .... where m is mass
and v is velocity. )
[The 'gamma' bit in Preston's explanation is a term Einstein introduced to deal with relativistic effects, which come into play when you're talking about very high velocities.]
Another basic equation of physics stems from the premise that the energy of a photon is proportional to its frequency. This is an intuitive premise because UV light has a higher frequency than visible light, and it causes sunburn. X-rays, another form of light, are yet higher in frequency, and can pass through your soft tissue. Near the top of the ladder of frequencies, are gamma rays, which will cook you to a turn in no time at all!!
Planck, did some research and turned the proportionality into a firm equation by introducing a constant (named after him), so that:-
e=hf ( .... where h is Planck's constant,
e is energy, and f is frequency)

But, with any wave form, multiplying its wavelength by its frequency gives you its velocity. So:-
fL=v ( ... where f is frequency
L is wavelength, and
v is velocity.)
Of course, we're dealing with light. So we can replace the term v with c, Einstein's designation for the speed of light.
Manipulating the equation, gives us:-
f=c/L
Therefore e=hf becomes:-
e=hc/L
But mass and energy are equivalent ... e=mc*2

Therefore, hc/L=mc*2

From which, h/L=mc

This is now a very similar equation to the one for the momentum of a particle, p=mv.
Thus, for a particle which is also a wave i.e. a photon, its momentum can be expressed:-
p= h/L (Planck's constant divided by the
wavelength.)
... Which is what Preston told us in the first place!!

I know this is really a bit sneaky, because it actually skirts around the problem of the photon's mass 'm'. We simply see two similar expressions, mv and mc, assume they must both refer to momentum, and therefore assume the equivalence of p and h/L !
The term 'm' is still there in the equation h/L=mc, and it still refers to the mass of a photon, but we just simply leave that bit alone and use the h/L bit for our momentum calculations when dealing with light!!
The amazing part is ... in practice, IT WORKS!

Pat Galea · 2002-09-06 03:23:14

According to my current text in Classical Dynamics of Particles and Systems (4th ed by Marion and Thornton), which has a section on special relativity, physicists no longer use terms such as "relativistic mass." Likewise, there is no such thing as "rest mass," there is just mass plain and simple (new to me too). They decided that mass should be something that is considered inherent and unchanging (this is the same as what used to be called 'rest mass.' That being said, _momentum_ is the quantity that increases relativistically as you increase velocity toward the speed of light.

True, but the concept you use always depends upon what problem you're trying to solve. Sometimes you really do need to be thinking about the relativistic mass.

Photons are massless, but they have momentum. For photons, the momentum is p=h/lamda(this works for particles too), where h is planck's constant and lambda is the wavelength. For photons, the momentum is not mv.

Well... the momentum is still mv, but again it's the relativistic mass.

I'll agree that the concept becomes a little bit useless when we're talking about photons.

I should think that photons are considered massless because they are electromagnetic waves (electric and "magnetic"(electrodynamic) fields) and saying that they are "particles" only makes sense because they can push things around in a particle-like way if they are localized enough -- But they are still fields, so they don't have mass.

Clarification point: photons are not fields, they are field mediators. Actually, to be really precise, we should start talking about virtual photons, but that's getting a bit heavy. (Or massless, if you like )

They are considered to be particles because they behave in a way that we'd expect particles to behave. i.e. when you fire them at a screen, you get discrete 'lumps' of light hitting it, rather than a gradual smooth accumulation.

Also, there's no fundamental requirement that field mediators (vectors) have to be massless. The strong nuclear force vectors are quite massive, in fact.

The idea that photons are massless comes merely from the fact that they travel at the speed of light. Any non-zero rest mass would give them an infinite relativistic mass.

Of course, photons never rest, so the very concept of 'rest mass' in this case is a theoretical one i.e. it is derived from the observed behavior.

Caveat: although we have excellent math to describe electromagnetic waves, we still have absolutely no idea what the hell is actually 'waving', or even if such a question actually makes any sense at all. So it may be that there are fundamental reasons for these things, but I don't know what they are.

-

EDIT: Removed some stray tags.

Pat Galea · 2002-09-06 03:30:21

The term 'm' is still there in the equation h/L=mc, and it still refers to the mass of a photon, but we just simply leave that bit alone and use the h/L bit for our momentum calculations when dealing with light!!

If you wanna make things get really hairy, start thinking about General Relativity.

Light is merely following the shortest distance between two points; a geodesic (analagous to a great circle on Earth).

So the fact that light bends when it passes by the Earth is just a geometric effect of the curvature of the space-time continuum.

So what does that tell you about the mass of the photon?

The amazing part is ... in practice, IT WORKS!

Yup, it sure is amazing!

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Re: Humans and relativity - Could they take it?

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