- #1

- 118

- 0

Planck distance etc ? If they are,then,since in calculus a maximim/minimum is given by,for example,dy/dx = 0,then,for example,y = planck quantity when

x = some other quantity.But what is x?

- Thread starter verdigris
- Start date

- #1

- 118

- 0

Planck distance etc ? If they are,then,since in calculus a maximim/minimum is given by,for example,dy/dx = 0,then,for example,y = planck quantity when

x = some other quantity.But what is x?

- #2

turbo

Gold Member

- 3,077

- 50

- #3

- 2,226

- 9

well, one thing i think he's asking is why is the Planck Velocity the limiting velocity yet the Planck Mass is not. there are lotsa things bigger and smaller than the Planck Mass.Can you pose the question in another way?

one other way of saying that the Planck mass is large is to, instead say that the Planck mass defines the norm and ask why the masses of subatomic particles are so small. i think it becomes equivalent to asking why gravity is so weak. there's an interesting quote from Frank Wilczek at the wikipedia article on Planck units about that.For instance, why are the Planck time and the Planck length so tiny while the Planck mass is horrendously large? Is there a problem inherent in these definitions?

the Planck Mass, Planck Momentum, even Planck Charge are reasonable quantities. i s'pose that since c=1 in Planck units, then the Planck Energy is also a reasonable quantity of energy (but it's released suddenly like a bomb, i would want to be on the other side of the hill). notice that the planck energy is about as many orders of magnitude bigger than a joule as is the planck mass smaller than a kg.

- #4

marcus

Science Advisor

Gold Member

Dearly Missed

- 24,738

- 787

I don't think that is true in general. Nobody I know considers all the Planck quantities extreme.Why are Planck quantities considered to be maximum and minimum quantities -

Some are extreme, like the Planck unit of speed is the speed of light which acts as a top speed of relative motion.

Some are not extreme----for example Planck momentum.

the Planck momentum is roughly speaking the amount of momentum that you give a child on a playground swing if you give them a very gentle push.

If you give them a good firm push, then you impart maybe 3 or 4 Planck units of momentum.

====================

To answer your question about why----I think the Planck quantities which are considered extreme, are considered to be extreme for a very simple reason: they ARE extreme.

And the Planck quantities which are not considered extreme are considered to be so for an equally simple reason: they are not extreme.

Last edited:

- #5

- 372

- 0

- #6

- 118

- 0

So if Smolin is right about the Planck distance being the smallest unit of space

then perhaps the Planck distance can be derived from a relationship in Einstein's General Theory of Relativity - a relationship that relates the length of one mass to its distance from another (height in gravitational field).

- #7

nrqed

Science Advisor

Homework Helper

Gold Member

- 3,737

- 279

I would disagree. The Planck mass is meant to be a limit to the mass of anwell, one thing i think he's asking is why is the Planck Velocity the limiting velocity yet the Planck Mass is not. there are lotsa things bigger and smaller than the Planck Mass.

Patrick

- #8

grant9076

I don't claim any expertise but I think that the planck length is defined at the length where the compton wavelength is equal to the schwarzchild radius. This essentially makes it the smallest measurable distance. (I haven't done any physics in a couple of decades so please correct me if I am wrong.)

So if Smolin is right about the Planck distance being the smallest unit of space

then perhaps the Planck distance can be derived from a relationship in Einstein's General Theory of Relativity - a relationship that relates the length of one mass to its distance from another (height in gravitational field).

If you try to measure the location of a particle which is smaller than this, then (from what I understand) the outcome is likely to be one of the following:

1. The wavelength used is too long to give any meaningful measurement.

2. A short enough wavelength would have enough energy to

3. If a particle this small has enough mass to prevent condition #2 from happening, then it would be a small black hole in which case all of the light would be absorbed and no measurement would be possible.

I am not certain but I believe that a planck mass is a blackhole with a schwarzchild radius equal to the planck length. I also think that the planck time is the time that it takes light to traverse the planck length.

- #9

- 2,226

- 9

what you say is true. i'm just saying that not everything in the universe is an elementary particle (but i suppose everything is made up of them). there are objects, collections of elementary particles if you wish, that are both larger and smaller than a Planck mass which is about a speck of dust. there is no object or particle or collection of particles that has speed, relative to any observer, that exceeds the Planck unit of speed. only less (or equal if it's a photon). that's all i meant.I would disagree. The Planck mass is meant to be a limit to the mass of anelementaryparticle.

- #10

- 116

- 0

As to the 'how we know', it is simply a reflection of the observable fact that gravity is an extremely weak force compared to the other three forces. It is this weakness, which causes the Planck length to be so small (and the Planck energy to be so large).

The reason why gravity is so weak in comparison to the other forces (and consequently why the Planck length is so small) is not understood. Answer that in a convincing way (and test it with experiment) and you will win the Nobel Prize.

- #11

- 2,226

- 9

well, maybe that's the ticket. Nobel laureate Frank Wilczek had this to say about the topic:

As to the 'how we know', it is simply a reflection of the observable fact that gravity is an extremely weak force compared to the other three forces. It is this weakness, which causes the Planck length to be so small (and the Planck energy to be so large).

The reason why gravity is so weak in comparison to the other forces (and consequently why the Planck length is so small) is not understood. Answer that in a convincing way (and test it with experiment) and you will win the Nobel Prize.

http://www.physicstoday.org/pt/vol-54/iss-6/p12.html [Broken]...We see that the question [posed] is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in Natural (Planck) Units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number [1/(13 quintillion)]...

asking why the masses of the fundamental particles are so small (compared to this natural unit of mass) is essentially the same as asking why the Bohr radius is so large (compared to the natural unit of length). dunno the answer, but answering one of these 3 questions will, in effect, answer all.

Last edited by a moderator:

- #12

CarlB

Science Advisor

Homework Helper

- 1,214

- 13

The observed fundamental particles all have masses so small compared to the Planck mass that if one were to write their masses in an expansion in powers of the Planck mass, one would put their masses as zero to first order. The observed masses would be a second order effect, a splitting.

One would then wish to find a way of describing the observed particles as composites of some more fundamental particles whose charges all cancel. For example, see:

http://arxiv.org/abs/hep-th/0507032

and

http://ccdb4fs.kek.jp/cgi-bin/img_index?8103218 [Broken]

Carl

One would then wish to find a way of describing the observed particles as composites of some more fundamental particles whose charges all cancel. For example, see:

http://arxiv.org/abs/hep-th/0507032

and

http://ccdb4fs.kek.jp/cgi-bin/img_index?8103218 [Broken]

Carl

Last edited by a moderator:

- Last Post

- Replies
- 6

- Views
- 2K

- Replies
- 3

- Views
- 2K

- Replies
- 1

- Views
- 699

- Replies
- 5

- Views
- 2K

- Last Post

- Replies
- 6

- Views
- 3K

- Last Post

- Replies
- 34

- Views
- 2K

- Last Post

- Replies
- 9

- Views
- 4K

- Last Post

- Replies
- 20

- Views
- 10K

- Last Post

- Replies
- 20

- Views
- 5K

- Replies
- 2

- Views
- 3K