Can We Travel Close to the Speed of Light?

AI Thread Summary
Traveling at the speed of light is impossible for objects with mass, but they can approach it closely with sufficient energy. The discussion highlights a common misconception regarding the Planck length, emphasizing that time and space are not quantized in a way that limits speed changes to this scale. Theoretical speeds can be measured without reference to Planck units, and the energy required to accelerate objects near light speed is immense. Understanding these concepts can help clarify the limitations of speed and energy in physics. Overall, the Planck length does not impose a practical limit on speed changes.
Mederman
Messages
4
Reaction score
3
Hello!
My kid asks if this is theoretical idea is correct and I just don't know this stuff very well:
It is impossible to travel at light speed but not impossible to travel just below. So the highest theoretical speed should be:
"The distance light has traveled in one second" minus "One Planck length"

I ask: How about "The distance light has traveled in on hour" minus "One Planck length", isn't that closer to the speed of light?

He didn't like that because he think that a Planck length is the smallest thing and cannot be divided. How should we think about this?
 
Physics news on Phys.org
Or how about the distance light traveled in one year minus a Planck length? Or the distance light traveled over 13.8 billion years minus a Planck length, etc ad nauseum...

What exactly is the point in all this?

Zz.
 
The highest possible speed is the speed of light in a vacuum (c). However, this is only achievable for particles without mass. For everything that has a mass, c can theoretically be approached as close as one has energy to accelerate the mass. But that gets soon in ranges where the total energy of the universe isn't sufficient anymore. So there is a practical limit below c, depending on the mass. The difference between any actual speed and c has nothing to do with the Planck length. The value is c, regardless whether measured in km/h, mph, lightseconds/second, lighthours/hour or whatever. This difference is a difference of speeds, hence measured in the same units. If it is close to c, then it is something like 0.999999999999999991c and the difference will be 1c-0.999999999999999991c=0.000000000000000009c and thus has the same unit as c has. There is no Planck length ansywhere.
 
Mederman said:
speed should be:
"The distance light has traveled in one second" minus "One Planck length"

Presumably also divided by one second :wink:
 
  • Like
Likes berkeman
I asked him again what he meant. He says that speed cannot change less than one Planck-length per second.
I think this is the question I should have asked to begin with.
 
This doesn't make sense. Second is an arbitrary unit. Why should it have any physical relevance? If you say Planck-length per Planck-time you end up exactly with c.
 
Ok, I'll try this: Can something travel less than one Planck length.
 
Mederman said:
I asked him again what he meant. He says that speed cannot change less than one Planck-length per second.
I think this is the question I should have asked to begin with.
The Planck length and Planck time are irrelevant. It's a common misconception that you have picked up that they have some significance in this respect. Time and space are not quantised like that.
 
  • Like
Likes russ_watters and etotheipi
  • Like
Likes russ_watters and etotheipi
  • #10
PeroK said:
The Planck length and Planck time are irrelevant. It's a common misconception that you have picked up that they have some significance in this respect. Time and space are not quantised like that.
I'll use this answer and tell him that Planck length may be a smallest unit, but distances aren't a fixed number of Planck units and speeds can change less than one Planck length per second.

Thanks for helping me out, you guys are great!
 
  • Like
Likes etotheipi and PeroK
  • #11
Mederman said:
I'll use this answer and tell him that Planck length may be a smallest unit

It may. Or the smallest length could be something else. Or there may be no smallest length.
 
  • Like
Likes DennisN
  • #12
Mederman said:
Summary:: The maximum speed in therory

It is impossible to travel at light speed but not impossible to travel just below. So the highest theoretical speed should be:
It might be helpful for your child to understand the energies required to accelerate an object to near light speed. That might help to end the "how super close" type questions. Calculate the energy it takes to accelerate even a small particle close to light speed, and you start to get an appreciation for how hard it is. Even for "theoretical" questions, you cannot exceed reasonable amounts of energy input.
 
  • #14
Mederman said:
I'll use this answer and tell him that Planck length may be a smallest unit, but distances aren't a fixed number of Planck units and speeds can change less than one Planck length per second.

Thanks for helping me out, you guys are great!
Vanadium 50 said:
It may. Or the smallest length could be something else. Or there may be no smallest length.
Well, it's even worse: due to time dilation and length contraction the chosen units/intervals won't necessarily be consistent with each other for different observers, or the same observer at different points in a trip.
 
  • #15
russ_watters said:
due to time dilation and length contraction

This is sort of fixable, but of course there is always cost. For now I would say it's an idea that lacks both experimental evidence and theoretical motivation.
 
  • Like
Likes russ_watters

Similar threads

Back
Top