Gravitons... Are there anti-gravitons?

[RyanJ]

Gravitons... Are there anti-gravitons?

 

Simple question - most is not all particles have anti particles so does one exist in this case or is it the exception to the anti rule?

 

Also is they do exist would that mean you could make an anti-graity device using them

 

Cheers,

 

Ryan Jones

[CanadaAotS]

Well apparently both photons and gravitons are their own antiparticles. Since the antiphoton and antigraviton are completely indistinguishable from their counterparts they technically don't exist.

 

antiparticles travel backwards through time and have reversed charges, since gravitons and photons travel at the speed of light they do not travel through time at all. So this works out well (having no anti particle).

[CanadaAotS]

BTW all that I've just said was previously explained to me by 5614 (not just something I happened to know myself lol)

[5614]

To be technical

 

You said photons "do not travel through time at all"... well if they didn't go through time then they could never go from the light bulb to an object unless it was instant. It takes a few nano-nano-nano etc. seconds for light to travel, but that is light travelling through time.

 

What I meant was that photons don't really experience time. We know this from relativity. This is different from not moving through time...

time = distance / speed

obviously it takes light a certain time to get from A to B. The point was that as they move at c (speed of light) they don't really experience time. Due to this a photon doesn't know whether it's going back or forwards in time, to the photon it makes no difference, because of this there is no antiphoton because if you time reverse* a photon you still get a photon.

 

*Time reversing is how you get an antiparticle.

e.g. an electron travelling back in time is an antielectron aka a positron.

 

[edit] an analogy would be to take the reciprical of 1, it's still 1.

The antiparticle of a photon is still a photon.

 

(not just something I happened to know myself lol)

And I just happened to read it about a week or 2 ago in In Search of Schrödinger's Cat, not just something I happened to know myself lol!

[JonM]

how do positrons and antiprotons etc travel back in time? can you explain that a little further?

[5614]

nah, you misunderstood. (Remember an antielectron is called a positron)

 

If an electron travelled back in time it would identical to a positron travelling foward in time. This could be used as a definition of a positron. I'm not saying that the particle does travel back in time... I'm saying that it's like the particle travelled back in time.

 

ie. An electron travelling back in time is identical to a positron. Now I'm not saying that the electron is travelling back in time, I'm saying if it did then this is what you'd get (a positron).

 

What is an antiparticle? It is the opposite of a particle.

 

So take an electron, how do you get the opposite of it? You time reverse it.

What that means is you say if an electron went back in time you'd get the opposite of it, because it's doing the exact opposite of what it would be doing if it were going foward in time.

 

Put this way:

antiparticle = opposite

 

Now if "antiparticle" and "opposite" are identical then:

 

A particle going foward in time is the opposite of a particle going back in time.

 

A particle going foward in time is the antiparticle of a particle going back in time.

 

=====

 

Now if you understand that then here's a little extra.

 

If you observe an electron, how do you know it's not actually a positron going back in time?

 

Answer: You don't. They're totaly identical, you can't tell whether you've got an electron going forward or a positron going backwards.

 

=====

 

btw, if a positron travelled back in time it would be an electron. It's like an anti-antiparticle, it's the same as a particle.

[JonM]

I thought what made a positron a positron was that it is an electron with a positive charge? is that wrong?

[5614]

A positron is what would happen if an electron went back in time... ie. it would have opposite charge and spin.

 

So yes it would be positive.

 

====

 

It's hard to explain, or at least to put into words, I know what I mean!

 

Nothing actually travels back in time here. I said last post this:

 

Put this way:

antiparticle = opposite

 

Now if "antiparticle" and "opposite" are identical then:

 

A particle going foward in time is the opposite of a particle going back in time.

 

A particle going foward in time is the antiparticle of a particle going back in time.

 

A positron is a positive electron that moves forward only in time. But that isn't really a good definition. A good definition of a positron would be:

It is what you would get if an electron went back in time.

I'm not saying an electron can... but by saying "going back in time" all I really mean is that it's totaly reversed.

 

I suppose it could just be simpler to say that an antiparticle is the opposite of a particle.

 

This is exactly the same as saying that an antiparticle is a particle going back in time.

 

A particle going back in time = the opposite of a particle = antiparticle.

 

Is it clear what I'm saying here?

[RyanJ]

Ah, right. Makes perfect sence. There may be a particle called an antigraviton but it would be identical to that of a raviton so there can't be anti-gravity then

 

Cheers,

 

Ryan Jones

[5614]

Yep, that's right

[RyanJ]

Yep, that's right

 

Great. That make sperfect sence in a way. Although... if you could create an antigraviton that would counteract gravity then it may have explained why gravity is so muh weaker then the other forces although String Theory does a good job at that anyway

 

Thanks for clearing that up - you can explain things very well 5614

 

Cheers,

 

Ryan Jones

[5614]

Thanks

 

I think the time reversal thing may be confusing people though, or maybe it's just me!

 

An antiparticle is the opposite of a particle in every way.

 

If a particle went back in time then everything it did would be opposite.

 

Can people see the connection between those 2 lines? A particle going back in time IS the opposite of a particle which IS an antiparticle.

 

heh, I think that sounded better than all of those 2 long posts put together!

 

[edit]

 

if you could create an antigraviton that would counteract gravity then it may have explained why gravity is so muh weaker then the other forces

Remember that a particle / antiparticle anihiliation produces a lot of energy, so if there were lots of graviton/antigraviton anihiliations occuring then whilst there'd be less gravity there'd be a lot of energy!

[RyanJ]

Remember that a particle / antiparticle anihiliation produces a lot of energy' date=' so if there were lots of graviton/antigraviton anihiliations occuring then whilst there'd be less gravity there'd be a lot of energy![/quote']

 

Yea and seeing as energy can be considered to have mass through [math]E=MC^2[/math] then you'd say that the total gravity would remain unchanged in any case....

 

Cheers,

 

Ryan Jones

[5614]

Yea and seeing as energy can be considered to have mass through [math]E=MC^2[/math] then you'd say that the total gravity would remain unchanged in any case....

 

Cheers' date='

 

Ryan Jones[/quote']

Nah, this is the classical vs non-classical technicality!

 

If you take a photon and look at the numbers you'd put into the equation e=mc^2 you'd still say m=0

 

Indeed for any particle which is massless and has energy you still use m=0 in the equations e=mc^2

 

Which is ofcourse "impossible" because then e=0 (which is certainly false), until you remember that e=mc^2 is classical and it's a shortened version of the full equation.

 

[math]e^2 = (pc)^2 + (mc^2 )^2 = p^2 c^2 + m^2 c^4[/math]

 

Where p is momentum.

 

And you use:

momentum = planks constant / wavelength

p = h / lambda

not the classical definition of p because that involves m which ofcouse will return an invalid value of p.

 

Otherwise for massless particles you'd use a different equation which doesn't include m

[RyanJ]

Sorry to bump this thread after a long time but I did not want to create a new one when I had already made on on this topic already.

 

Would true anti-gravity actually work in GR (Not to say one way or another if it happens, just a question as to would it be mathematically possible)? If yes then under what conditions would it require?

 

Cheers,

 

Ryan Jones

[timo]

Would true anti-gravity actually work in GR (Not to say one way or another if it happens, just a question as to would it be mathematically possible)? If yes then what conditions would it require?

A definition for the term "anti-gravity" would certainly be a nessecary condition.

[RyanJ]

A definition for the term "anti-gravity" would certainly be a nessecary condition.

 

Hmm, good point. For this question then how about we take it as spacetime warped in the opposite way to which it is normally warped by gravity?

 

Cheers,

 

Ryan Jones

[5614]

I don't think it would work in GR.

 

Whilst I do not know enough maths to even guess if you think about it GR was made on the fundemental assumptions that all mass/energy etc. warps spacetime in a very specific way.

 

Antigravity in GR would involve changing the fundemental assumption that all matter warps spacetime in specific ways dependant on the amount of matter etc. I don't think you could change something like that and still get GR to work.

 

In fact the more I think about it the more I think that if some matter interacted with spacetime differently (e.g. has an antigravitational field) then GR would be incorrect. Making the safe assumption that GR is correct means antigravity cannot occur.

[RyanJ]

I don't think it would work in GR.

 

Whilst I do not know enough maths to even guess if you think about it GR was made on the fundemental assumptions that all mass/energy etc. warps spacetime in a very specific way.

 

Antigravity in GR would involve changing the fundemental assumption that all matter warps spacetime in specific ways dependant on the amount of matter etc. I don't think you could change something like that and still get GR to work.

 

In fact the more I think about it the more I think that if some matter interacted with spacetime differently (e.g. has an antigravitational field) then GR would be incorrect. Making the safe assumption that GR is correct means antigravity cannot occur.

 

I see... come to think of it what you said earlier in the thread makes it difficult (if not impossible) for it to exist in QM because a graviton and an anti-graviton would act the same way...

 

I can't seem to find any interesting papers taking about this so if anyone has any links or papers etc. please share them with us

 

Cheers,

 

Ryan Jones

[timo]

Hmm, good point. For this question then how about we take it as spacetime warped in the opposite way to which it is normally warped by gravity?

I think you are taking the pictures about the sun lying in the middle of an elastic membrane too serious: The dimension that the membrane is deformed in (down) doesn´t exist (4D curved spacetime is not embedded into a higher-dimensional flat space). So you´d have trouble defining the "opposite way to which it is normally warped". In fact, strictly speaking your question would become "within GR, can spacetime be warped differently that it is warped within GR", anyways.

[RyanJ]

I think you are taking the pictures about the sun lying in the middle of an elastic membrane too serious: The dimension that the membrane is deformed in (down) doesn´t exist (4D curved spacetime is not embedded into a higher-dimensional flat space). So you´d have trouble defining the "opposite way to which it is normally warped". In fact, strictly speaking your question would become "within GR, can spacetime be warped differently that it is warped within GR", anyways.

 

Unfortunatly I can't seem to even imagine something in 4D (As I''m told its almost impossible to do so anyway) so if the 3D version is not sifficient can we think about anti-gravity as the reversal of gravity in the sence it would be repulsive instead of attractive? I'm trying to find a point from where I can try to start to understand the concept...

 

Cheers,

 

Ryan Jones

[john5746]

Wouldn't anti-matter produce anti-gravity? Matter produces gravity, so the opposite would be anti-gravity?

 

Just an idea from left-field.

[ydoaPs]

no. antimatter has positive mass.

[timo]

Unfortunatly I can't seem to even imagine something in 4D (As I''m told its almost impossible to do so anyway)

Imagining flat 4D is simple. If you´re out for a date you agree upon place AND time. Curved 4D, yes, I think that´s pretty impossible to imagine. Luckily, physics is about the governing laws and equations, not about imagination ...

 

is not sifficient can we think about anti-gravity as the reversal of gravity in the sence it would be repulsive instead of attractive?

I´d have to think a moment whether attractive and repulsive are properly defined in an arbitrarily curved spacetime, so I cannot really answer that. But e.g. Dark Matter which is in agreement with GR causes "distances" between points to increase with a positive "acceleration". So losely speaking, you might have what you call anti-gravity there.

 

I'm trying to find a point from where I can try to start to understand the concept...

Imho, starting to learn differential geometry and GR would be a great starting point for trying to understand the not-yet-defined concept of "anti-gravity within the scope of GR". GR actually isn´t as hard as people normally think. It just nessicates to give up or generalize a few concepts familiar from linear algebra.

[john5746]

no. antimatter has positive mass.

 

Doh!!