# What is Planck length: Definition and 81 Discussions

In physics, the Planck length, denoted ℓP, is a unit of length in the system of Planck units that was originally proposed by physicist Max Planck, equal to 1.616255(18)×10−35 m. The Planck length can be defined from three fundamental physical constants: the speed of light, the Planck constant, and the gravitational constant. It is also the reduced Compton wavelength of a particle with Planck mass. Regardless of whether it represents some fundamental limit to the universe, it is a useful unit in theoretical physics.

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1. ### For fun: How big of a Microscope is needed to see a Planck Length?

If you had--infinite--resources, material, manpower, energy, but only with current technology, or something that could be learned in a short time, how big of a microscope would it take to see a Planck Length? If there's a current limit, still with infinite resources, how small could you see and...
2. ### I Between a Zeptometer and the Planck length

What exists between approximately 1 X 10 E-21 meters (preon length?) and the Planck Length of 1.6 X 10 E-35 meters?
3. ### B Planck Length 1 Detector? Emitter?

Do we have the ability to detect the gamma ray of Planck Length 1? Is there any known phenomenon, natural or man-made, that could emit on Channel 1? How dangerous to man and machine would Channel 1 be / at what density (is that the right term?) would it become dangerous? Would there be any...
4. ### B Is The Planck Length the smallest length possible?

So, I was wondering is the Planck length the smallest length possible for measurement in physics? Please give as much details as to why or why not.
5. ### I To measure a Planck length would require a black-hole photon?

I was viewing this video in which the narrator says that the energy of a photon that could discern a Planck length would require a photon of such high energy that it would be a de factor black hole. Is this accurate?
6. ### B Is There a Connection Between Planck Length and Planck Time in Relativity?

If Planck length is 10^-35 of a meter and Planck time is 10^-43 of a second, doesn't that mean that as the relative speed gets closer to the speed of light and time acceleration and length contraction are happening, they are happening at a different rate, since length contraction has to reach...
7. ### I Comparing Planck's Mass, Length, Time and Energy

Sean Carroll gives the Planck's set of four dimensioned quantities: Planck's mass, length, time and energy. I wanted to compare them with actual things.\begin{align} m_p=\sqrt{\frac{\hbar c}{G}}&=2.18\times{10}^{-8}\rm{kg}&\rm{{10}^{7}\ E. coli}\phantom {100000000000000000000}&\phantom...
8. ### I Planck Length & Singularity: Point or Sphere?

Since Planck length is a distance unit of space-time which cannot be further subdivided, would that not imply that an electron, or singularity, or any other point particle cannot actually be a “point”, but must rather be a spherical volume with a Planck length diameter? i realize this may well...
9. ### B Schwarzschild radius of an object is smaller than Planck length

I had this idea when some people said that LHC can produce black hole. Based on the calculation of Schwarzschild Radius, any mass than 9.375×10^7 kg have a Schwarzschild radius smaller than the plank length. Particles inside LHC or other particle accelerator have clearly radii smaller than that...
10. ### B Can a wavelength be smaller than a Planck length?

According to this article https://physics.stackexchange.com/questions/321229/why-dont-we-have-proton-neutron-microscopes, they seem to hint that heavier objects have a shorter wavelengths and give better resolution. If I kept increasing the mass of a particle and kept it's speed at .9c would I...
11. ### B Planck Length: Upper Limit on Photon Frequency & Momentum

Is it known how much momentum a photon possesses if it's wavelength is at the Planck length, and what happens if it's momentum is somehow increased from that?
12. ### I Is it possible for something to be smaller than a Planck length?

After watching a youtube video 2 years ago, the video said the smallest thing known is at Planck length, but could there be something smaller than Planck length
13. ### I The Planck length and string theory

String theorists frame much of their studies in the context of Planck length. The theories are meant to fold together QM and general relativity. The equation for Planck length includes the gravitational constant, G. It seems to me the theorists are assuming the gravitational laws extend to...
14. ### I Wavelength approaching the Planck length

If I heat up an object,the object emits a electromagnetic wave with a shorter and shorter wavelength. But if I heat an object up to Planck temperature, and making the EM wave that it emits has a wavelength of Planck distance,what will happen when I heat it up even more? Will the wavelength...
15. ### How many Planck lengths are there in a parsec?

[Mentor's note: moved to GD because this is a thread about Alexa, not parsecs or Planck lengths] I know it's a strange question. Here is Alexa's answer: https://drive.google.com/open?id=1hXYFT_zBnFy-N0BotiTSDL8_S6nHK3ns
16. ### I Does the Planck Length Experience Length Contraction at High Speeds?

Imagine I had a ruler 1 Planck length long, I then send it on a rocket close to the speed of light, does its length contract? It seems to my simple intuition, relativity would say yes and Qm would say no. Is there a simple solution to this ?
17. ### I Could the original size of the Universe be the Planck length

If you guys haven’t noticed through my posts so far is that I am interested in the Big Bang. I am considering the concept of the Planck length, but I may have some confusion. How did Space time start? Did the universe start from nothing and created units of spacetime? Could the original size...
18. ### B Planck Length and Lorentz Contraction

I've heard that Planck length is the smallest length ever! but if something that his length is equal to Planck length and moving by speed dv which is infinitesimal change in speed or higher than that , then according to special relativity his length must be equal to L'=L \sqrt1-v^2/c^2 which...
19. ### B Surface volume of 3-sphere with radius of 2 Planck length?

What is the surface area ("surface volume") of a 3-sphere having a radius of 2 Planck lengths? Is the product of the Planck's constant, Einstein's proportionality constant and Planck time also equal to this volume? Does this equivalence signify anything? What does it signify?
20. ### Is there any proof to show that gravity works at the Planck length?

Is there any experimental proof which shows that the force of gravity works at Planck length? Has it been verified already? Is it verifiable?
21. ### I Is the Planck length a proper length?

The reciprocal of the Planck length, ##\Lambda=1/l_P##, is used as a high-frequency cutoff in the particle-physics estimation of the vacuum energy. For example in "the cosmological constant problem" Steven Weinberg says that summing the zero-point energies of all normal modes of some field of...
22. ### I Planck length and space expansion/contraction

Imagine a cubic volume of space, defined today, with edges of the order of the Planck length. As cosmological time goes forward this volume expands so that the edges of the cube expand with the scale factor. This seems ok. However, as physics is time-reversible, it should be valid to imagine...
23. ### B What is the significance of the Planck length in physics?

The definition of the Planck length I have found is that it is the shortest length that has meaning. What is meant by that? Can someone give be a better definition of what it is?
24. ### I Does the Planck length expand in a FRW universe?

By using particle physics natural units with ##\hbar=c=1## so that Planck's length ##l_P=(8\pi G)^{1/2}## we can express Einstein's field equations as $$G_{\mu\nu}=l_P^2\ T_{\mu\nu},$$ where ##G_{\mu\nu}## has dimension ##[\hbox{proper length}]^{-2}##, ##l_P## has dimension ##[\hbox{proper...
25. ### I Has Planck length been derived rigorously?

ħello. Here is my question: Has the so-called "planck length" (~1.61622837 * 10^(-35) meters) been derived with mathematical rigor within standard quantum field theory? If so, I need help finding this proof. Any literature would be appreciated. Also: If it has, then doesn't this imply a...
26. ### I Is the Planck Length the Ultimate Limit of Measurement?

If I understand correctly, the Planck length, or maybe a tenth of it, is considered the point at which quantum effects mess up any attempt to apply our present physical laws, and so one could not even in theory make a dependable measurement of something smaller. However, I was wondering what...
27. ### B How have we figured out the Plank length of the universe?

how have we figured out plank length of the universe ?and know its that size ?
28. ### I Subbing Planck length into length contraction equation?

I was wondering if it is possible to work out the maximum amount of energy an object with mass can have using the length contraction equation (i.e. "actual" length divided by Lorentz factor). The way I thought of doing this was by rearranging e = mc^2 to get c^2 = e/m. Then, substitute e/m into...
29. ### B Is it possible to do experiment on the Planck scale?

Hi. I was wondering if experiments on the Planck length (about 10-35 m) are possible or not. If it is possible, what kind of experiment would that be? If it's not, then what is the problem? Or what is need to be done? Thanks.
30. ### B What is the relationship between the Planck length and black holes?

This article is posted on twitter with the byline: "You, me, and even aliens from Alpha Centauri could arrive at the same Planck units." but does not explain why. http://www.symmetrymagazine.org/article/the-planck-scale I believe I have read (but cannot find the reference now) the following...
31. ### Stellar-mass black hole formation sequence

I feel like this could go in quite a few of the Physics subforums (Quantum Physics, Beyond the Standard Model, Special and General Relativity, or High Energy, Nuclear, Particle Physics) instead of Astronomy and Cosmology, but hopefully this will work. This is my first question I've posed here...
32. ### Insights A Hand-Wavy Discussion of the Planck Length - Comments

klotza submitted a new PF Insights post A Hand-Wavy Discussion of the Planck Length Continue reading the Original PF Insights Post.
33. ### The Impact of Cosmological Expansion on the Planck Length: A Quantum Perspective

I get that Planck length is derived from h, G and c. But as a theoretical measure of length how is it affected or not affected by cosmological expansion?
34. ### Dimensional Analysis of Planck Length

Hello! So in looking at black hole thermodynamics, I came across the equation ## l_p = \sqrt{G\hbar} ## But in doing a dimensional analysis of ## \sqrt{G\hbar} ## I get ## [\sqrt{G \hbar}] = \sqrt{ \frac{Nm^2}{kg^2} \frac{m^3}{kgs} } ## This obviously doesn't amount to a length. What...
35. ### Understanding the Planck Scale

Hi, So I am a little confused on the Planck scale. Correct me if I'm wrong (I hope this doesn't sound idiotic) but I believe that any field on a classical scale should be able to be quantized and work on a quantum scale/Planck scale, where the Planck length and the Planck time are the...
36. ### How can it be deduced general relativity from planck length

How can it be deduced general relativity from Planck length?
37. ### Speed of light and planck length

I was thinking about the special theory of relativity and how as one approaches the speed of light, ones length contracts. what I would like to know is: If I had an object and I accelerated it towards the speed of light and it contracts in the direction of travel, what happens when it reaches...
38. ### What happens when matter pops into and out of existence at Planck length scales?

All - my first post, and as a lay person interested in quantum physics, forgive me if my questions are naive or ill-informed. Is it possible that the Universe inflated then exploded into being from a singularity smaller than Planck length? If I understand the concepts properly, the Planck...
39. ### Is the Planck Length a 3d area?

If so, is 'its half' also not a real size of space? What I am asking is pretty much; what is the meaning of the notion of a 'smallest possible' 3d area? What is the meaning of that cusp between 2d and 3d? Almost like asking what is in between 1.999999999999(repeating) and 2. Must...
40. ### Planck length and quantized position

After reading an article on Planck length, I began to wonder whether or not the theoretical limit implied that position could be quantized in whole integer multiples of Planck length? To demonstrate what my question is asking mathematically I hope you will scrutinize the equations below: If...
41. ### Exploring the Smallest Measurable Units: Planck Length & Pi

Hi all. This is my first time posting so forgive me If I am doing something wrong. I am a year 7 student interested in all types of physics and my question is, if nothing can be smaller than Planck length then wouldn't past a certain point the digits of pi become obsolete? Simply because the...
42. ### Koch snowflake and planck length

surely if you keep adding smaller and smaller sides to the snowflake they will become even smaller than the Planck length and so how can the perimeter be infinite- or is the infinite perimeter only theorteically possible with maths, but not actaully acheivable?
43. ### What Happens When the Universe is Reduced to the Planck Length?

What would happen if the entire universe would be reduced in size by several orders of magnitude. I mean not just the distances between the galaxies but also the size of every planet, every atom, every nucleus and so on. That also means the strength of all forces would have to become smaller...
44. ### Gravity affects quarks at the Planck length and Planck energy

This comes form the wiki article on quarks: Does that mean that when two Quarks come as close to each other as the Planck distance 10-33m then gravity must be taken account if one is to understand their movement? I know vaguely that the Planck energy is the point at which a black hole...
45. ### Is a Planck length really the smallest length?

I am just an interested high school physics student so I don't really know much about the subject, but I am given to understand that a Planck length is the smallest length. It cannot be further subdivided and no length smaller can be measured even with a great advancement in measuring...
46. ### Proof of Planck Length: A Pedological Derivation

Hi all, Does anybody can provide a source that can prove Planck's length from scratch ? I mean from classical mechanics to quantum mechanics, a pedological proof/derivation ? Thanks in advance.
47. ### Can We Move in Planck Length and Dimension at the Time of the Big Bang?

Hello all . We know Planck length is and universe was in that density at big bang . Is that mean there was dimension at that time ? I mean , can we move in Planck length ? like up , down, right, left, forward, backward ؟