# I Critical Energy

#### MathematicalPhysicist

Gold Member
I believe that once I asked for what is the critical speed that above it it's considered ultra-relativistic speed and below it slow speeds.
I forgot to ask about the critical energy, what is the threshold energy that below it it's considered low energy physics and above it it's considered HEP, is there such an energy? in that case how to compute it?

I would assume one needs to use the equation $E^2=p^2+m^2$, but which momentum and mass to insert?

Is this distinction between high energies and low energies really have a threshold?

Related Other Physics Topics News on Phys.org

#### sophiecentaur

Gold Member
Is this distinction between high energies and low energies really have a threshold?
No. The reason is that detecting the effects of SR would depend more on the actual experiment that the velocity involved. Take a high quality Frequency Standard with you on a trip round the Earth in a fast jet and you will detect a relativistic shift. Check the pilot's watch against the watch of his brother, who remains at the airport, and you will see no (measurable) relativistic shift.
Any "threshold" energy would be totally arbitrary. But why would that matter at all?

#### MathematicalPhysicist

Gold Member
No. The reason is that detecting the effects of SR would depend more on the actual experiment that the velocity involved. Take a high quality Frequency Standard with you on a trip round the Earth in a fast jet and you will detect a relativistic shift. Check the pilot's watch against the watch of his brother, who remains at the airport, and you will see no (measurable) relativistic shift.
Any "threshold" energy would be totally arbitrary. But why would that matter at all?
Then why is there such a distinction between HEP and Condensed Matter Physics?
I mean HEP= High Energy Physics so there's also low energy physics I guess, so where's the critical energy at?

#### sophiecentaur

Gold Member
Then why is there such a distinction between HEP and Condensed Matter Physics?
Is there "such a distinction"? How slow is the slowest 'fast car' on the road and how low is the lowest mountain that gets climbed?
Research departments have names which give a clue about what is studied but the names are not exclusive. Knowledge about condensed matter is needed in a High Energy Physics lab and you can bet that there will be a high energy specialist working somewhere in a Condensed Matter Physics if measurements involve high energy radiation.
I'd advise not wasting time on categorising. It's the least interesting part of Science, imo.

Staff Emeritus
Where do you draw the line between the largest pond and the smallest lake?

#### sophiecentaur

Gold Member
This is heading in the direction of Zen, I think.

#### Ibix

But what is the dividing line between Zen and other forms of Buddhism?

More helpfully for the OP, you can usally approximate relativistic formulae and recover Newtonian ones (e.g. relativistic K.E. is $(\gamma-1)mc^2=mv^2/2+O(v^4/c^2)$). If the neglected parts are significant to whatever precision you are measuring, you shouldn't neglect them and should consider a full-fledged relativistic treatment.

#### sophiecentaur

Gold Member
Where do you draw the line between the largest pond and the smallest lake?
If you are an estate agent (realtor?) there can be a large overlap. The seller's lake could be a buyer's pond.

#### MathematicalPhysicist

Gold Member
Where do you draw the line between the largest pond and the smallest lake?
So there's no difference between high and low energies? it's just a convention.
A condensed matter physicist can work in HEP and vice versa, since there's no difference.
I mean we have particles in both of them... :-)

#### Keith_McClary

There is the old-fashioned term "elastic scattering" which meant not enough energy to break anything.

Staff Emeritus
So there's no difference between high and low energies?
So there's no difference between ponds and lakes?

#### sophiecentaur

Gold Member
So there's no difference between high and low energies? it's just a convention.
A condensed matter physicist can work in HEP and vice versa, since there's no difference.
I mean we have particles in both of them... :-)
I'm really not sure what you want out of this thread. You are treating Physics a bit like Top Trumps.

"Critical Energy"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving