# Newton Or Einstein

1. Aug 31, 2013

### Hepic

As I see,I will show you something and I want to explain me who has right.

For example into the run a human walk with 5km/h while the train runs with 100km/h,in the same direction.

So Newton says: U=V1+V2=5+100=105km/h

Einstein,about his law for big speeds says: U=(V1+V2)/(1+(V1*V2)/c^2)=30/(1+(V1*V2)/c^2)=
= 30/((c^2/c^2)+(V1*V2)/c^2 )= 30/((c^2+V1*V2)c^2),so to make us 30 needs
"((c^2+V1*V2)c^2)" that to make us "1". But it will never will be 1,except If v1 or v2 is zero.
So it will be something more than 1. So with einstein theory we will have less than 30(almost 29.xxx)
because denominator is more than 1.

My question is: Who is correct and why?
A guy told me that einstein has the right,because due of speed,(e=mc^2)mass comes bigger,so will go lower at final,so the speed will be less than 30.
What you think???

Thank you !!!

2. Aug 31, 2013

### Staff: Mentor

Einstein is correct. The velocity composition formula applies at all speeds, but the deviations from Newtons formula only become apparent as v approaches c.

3. Aug 31, 2013

### Hepic

Yea but why einstein is correct?? For the reason that told me the guy as I refered or something else?
Thank you !!!

4. Aug 31, 2013

### Staff: Mentor

Einstein based his ideas on two basic principles, first that the laws of physics are the same in all reference frames, and second that the speed of light is the same in all reference frames. The velocity composition formula is a result of those two postulates.

5. Aug 31, 2013

### Hepic

Yea but you did not explain me,why law of einstein is most correct of newton?
Why the speed will be lowest? Why with einstein we find that 29.9 and with newton 30?

6. Aug 31, 2013

### Staff: Mentor

Yes, I did. Einstein is correct because the laws of physics are the same in all inertial frames and the speed of light is the same in all inertial frames. That is the reason why Einstein is correct, he made a correct assumption which Newton did not.

The relativistic velocity addition formula differs from the Newtonian formula by a factor of $1/(1+v_1 v_2/c^2)$ which is clearly always less than 1 for velocities in the same direction.

7. Aug 31, 2013

### ZapperZ

Staff Emeritus
Newton's law, it turns out, is an approximation of a more accurate description, which is Einstein's Special Relativity. Einstein found a more complete description that is valid at any speed, while Newton's description is valid only when v<<c. That's why at some point, based on the accuracy that you want, they differ.

Zz.

8. Aug 31, 2013

### bahamagreen

The OP is asking for an explanation of the mechanism that makes this true... he mentions mass increase.
That mechanism is not popular now and has been replaced by energy increase.

9. Aug 31, 2013

### Staff: Mentor

For some discussion of a similar question, namely, "why is the speed of light the same in all inertial reference frames?":

Last edited by a moderator: May 6, 2017
10. Aug 31, 2013

### Staff: Mentor

The postulates are the mechanism. They are the source of all special relativistic effects.

Suppose that I detailed some source, X, and called that the "mechanism" of velocity addition. Then, the next natural question would be, "what is the source of X". The answer would either be the postulates or some other "mechanism", Y. "Mechanism" Y could either be explained in terms of the postulates or in terms of "mechanism" Z. Etc.

Eventually, no matter how many "mechanisms" you put in between, the explanation will be the postulates.

11. Sep 3, 2013

### bahamagreen

If I observed a historical series like this:

- mechanism M1 is as deep as we know and fundamental postulate P1 is the mechanism
- discovery that mechanism M2 underlies postulate P1, new fundamental postulate is P2
- discovery that mechanism M3 underlies postulate P2 new fundamental postulate is P3...

then I would tend to consider that either:

1] there may be an eventual truly fundamental postulate, we just have not discovered the intervening layers of mechanism yet - the series ends with P(N) and M(N), where P(N) is the mechanism for M(N).
or
2] no postulates or mechanisms are fundamental - the series does not end and for every P(N) there is a subsequent underlying mechanism M(N+1) that requires a subsequent new postulate P(N+1) as its mechanism.

Both lead to the same thing in practical relation to the current understanding - that at any time in the adventure there will be a postulate P(N) that appears to be fundamental.

I agree that "Eventually, no matter how many "mechanisms" you put in between, the explanation will be the postulates."

I don't quite agree that one should think "The postulates are the mechanism. They are the source of all special relativistic effects." I would rather think that the previously held postulates P(1...(N-1)) that were subsequently understood by underlying mechanisms M(1...(N-1)) should be thought of after the fact as "not" having been fundamental and so should suggest that the status of the present "fundamental" postulate P(N) should be viewed with the same sense - that it is the latest postulate without known underlying mechanism, but is subject to a mechanism being discovered for it, which will cause the emergence of postulate P(N+1).

As an aside; this thread is about relativity... the QM side seems to have posited choice #1 above with the Bell tests formally putting a stop to the subsequent emergence of M(N+1) AKA hidden variables.
I wonder if there is a similar potential situation possible in relativity?

12. Sep 3, 2013

### Staff: Mentor

I don't think that correctly describes the actual historical series leading to relativity. The postulates are always the underlying mechanism, so the distinction between your M and P is not clear to me.

I would say that we had a set of observations O1 which were all predicted by a set of postulates P1. Eventually we found some set of observations O2 which were not predicted by P1, but found P2 which did predict O2. P1 still retains it predictive power for all O1, and P2 will always retain its predictive power for O2 even if we eventually discover some O3.

Then feel free to answer the question accordingly. I think any answer you provide will be unnecessarily complex and confusing.

13. Sep 14, 2013

### Namron Sreble

In accelerators, we keep 'surfing' charged particles on waves in the radio chambers. Each one gives some energy. Yet velocity only approaches 'c'. Where is the energy?

14. Sep 14, 2013

### ZapperZ

Staff Emeritus
What "energy"?

The energy of the charged particles is easily described via the relativistic energy. That is what we use to model the beam dynamics of these particles, and they produced accurate results, or else all the particle accelerators won't work the way we understood them!.

How is this relevant to this thread?

Zz.

15. Sep 14, 2013

### Namron Sreble

It all hangs together. Energy is being imparted, but not realized as velocity, hardly.

16. Sep 14, 2013

### ZapperZ

Staff Emeritus
Excuse me?

The charged particles did not come out of the source at such high speed and energy. The accelerating structures are the ones imparting those energy to get it up to speed. So where exactly are these "not realized"?

Disclosure: I work at a particle accelerator.

Zz.

17. Sep 14, 2013

### Namron Sreble

Cool, I did time at Brookhaven at 3 GEV. As we add energy, the "relativistic mass" rises, even tho velocity only asymptotically approaches 'c'. Total energy is 'gamma . m_o . c^2'.

18. Sep 14, 2013

### ZapperZ

Staff Emeritus
Yeah, and the problem here is? The velocity is approximately equal to the square root of the energy. It doesn't grow as fast and as quickly, and it also asymptotically approaches c.

The energy from the accelerating structures certainly are transferred to the beam! So why did you say these are not "realized"?

Zz.

19. Sep 14, 2013

### Namron Sreble

Energy is realized as I just stated.

20. Sep 14, 2013

### ZapperZ

Staff Emeritus
Then what's with your first post in this thread? Did I just wasted my time?

Zz.

21. Sep 14, 2013

### Namron Sreble

22. Sep 14, 2013