# B Is Special Relativity Universally Accepted?

1. Jan 1, 2018

### Jimmy87

Hi pf,

When I was taught about SR, I was told it was one of the most successful theories in science and has been verified over and over again. However, I have been watching some videos online and wanted some guidance. I initially came across this:

This guy claims to have a PhD in physics but seems to talk complete nonsense in my opinion. He claims Einstein said that mass increases as your velocity increases near the speed of light. He then says the proof of Einstein being wrong is that all particle accelerators observe no mass increase. My understanding is that relativistic mass increases but this is not your true mass anyway so surely you should not observe any mass increase should you? What did Einstein actually say about mass and approaching light speed?

After reading through some of this guys threads, some people said that SR has been proved through muon decay, which I have studied a bit. His response to these comments is that this is not universally accepted and gave links to some other theory that can explain it (<link deleted>) and a video by a physics researcher that claims that time is invariant (unless you are accelerating):

I just wanted to know if any current researching physicists debate whether or not SR is a complete theory? It frustrates me that if I came across people like this I wouldn't know what to say in response because we are never taught the extent to which these theories are tested and if there are any anomalies at all.

Thanks for any information given.

[Moderator's note: links to sites that are not acceptable sources per the PF rules have been deleted.]

2. Jan 1, 2018

### phinds

Yes, SR is universally accepted and anyone saying it's wrong is full of crap.

3. Jan 1, 2018

### phyzguy

Even the idea that the Earth is round is not "universally accepted". There are always nut jobs and crackpots. There is no debate among serious scientists on the validity of special relativity, but that does not stop the crackpots.

4. Jan 1, 2018

Good point

5. Jan 1, 2018

### Staff: Mentor

He is confusing relativistic mass with rest mass. Relativistic mass was a commonly used concept in the early days of SR, which is why you will find Einstein and others, in those early days, saying that mass increases with velocity--they meant relativistic mass. But as time went on it became clear that this concept caused more problems than it solved, so it was abandoned. Modern treatments of SR use the term "mass" to mean rest mass. That's why modern particle accelerator experiments report no mass increase: the rest mass of the particles has not increased.

The link is to a crackpot site. Muon decay is perfectly consistent with SR; this has been established both by observing muons formed in the Earth's upper atmosphere by cosmic rays, and by observing muons in laboratories around the world.

No reputable physicists doubt that SR is correct within its domain of validity. To be clear, though, that domain is limited: SR does not deal with cases where gravity is significant. So SR is not a "complete" theory in that sense; if gravity is significant, you have to use general relativity.

6. Jan 1, 2018

### Staff: Mentor

Moderator's note: two off topic posts have been deleted. Posts that potentially violate the PF rules should be reported, not responded to.

7. Jan 1, 2018

### Jimmy87

Sorry, I didn't realise. I truly apologise if I offended anyone. My intent was never by any means to support anything these people said (and I thought I made that clear). In fact it made me very angry when I came across these and was the reason for my post.

8. Jan 1, 2018

### Jimmy87

What did Einstein actually say about mass and approaching the speed of light. Did he know that the true mass would stay as it was?

9. Jan 1, 2018

### Mister T

That's true, and you didn't mention the Global Positioning System. The engineers who synchronize the various clocks aboard satellites and on Earth must take SR as a fact. If they didn't the synchronization would be so far off that the GPS on your phone wouldn't be able to tell you what city you're in, but as it is it can tell you the street corner you're on.

Physics journal editors will tell you that they receive a steady stream of "proofs" that SR is wrong. Apparently it's a favorite among the crackpots, along with the 1st and 2nd Laws of Thermodynamics.

10. Jan 1, 2018

### Staff: Mentor

Einstein knew that rest mass (or "invariant mass"--the term "true mass" is not used) does not change with the speed of an object. I don't have any specific quotes of his about relativistic mass, but he knew that it was not the same as rest mass.

11. Jan 1, 2018

### Mister T

There is a concept known as relativistic mass that increases with velocity, but Einstein was no fan of it. You can invent the concept of relativistic mass, and as a consequence of that invention claim that mass increases with speed. But the notion of mass increasing with speed is not a consequence of the postulates of SR. Time dilation, length contraction, and relative simultaneity are consequences of the postulates, consequences that have been verified quite extensively by experiment and observation.

Particle physicists do not use the concept of relativistic mass, so that explains the reason why they do not observe an increase in mass.

In the last 25 years or so the concept of relativistic mass has all but disappeared from the textbooks, it simply causes too much confusion, and the recently deceased Russian physicist Lev Okun was perhaps instrumental in providing the seed for that reform.

12. Jan 1, 2018

### pervect

Staff Emeritus
Professionally, special relativity is well accepted - accepted enough to be used as the fundamental definition of the SI meter. And you already know what your professor said.

From the National Institutes of Standards (NIST) website:

You are possibly already quite familiar with SI units. I'll say a bit about the history of the SI system anyway, in case you are not familiar with them. An international body, the General Conference on Weights and Measures (the CGMP) was established in 1857, and the meter has evolved through a long history. At one point the meter t was defined by a physical artifact as the above quote mentions, but nowadays it's _defined_ based on how far light travels in a specified amount of time. I believe there was some interesting and famous discussions when the decision to make the switch was made, but I'm afraid I don't recall the details well enough to relate, and my sources on that might not be the best. (And a lot of the question here is about sources).

So, what you need to do is ask yourself. Who or what is the arbiter of what's "well accepted". Do you believe things you read by random, unknown posters on the world wide web, or do you believe your professor? How does your professor get his information? Does he read the WWW and tell you what the WWW says, or does he go to professional published peer-reviewed papers and well respected institutions?

More generally, does one determine what's "true" by taking an opinion poll? And if so, whom does one poll? Everyone? Or does one give more weight to some opinions than others? Is that approach (of finding truth by polling) even a good idea?

If you study the history of science more, you might decide that in order to determine what's true, it's a good idea to rely on experiment. But you still need to figure out what experiments you believe, and why. Unless you are in a position to personally perform such experiments as are necessary, I suppose, but that seems to be rather demanding (though it seems ideal). For instance, if you ran across this youtube video, <<link>> , should you believe it, or not? Who were the authors? Was it ever published in a peer reviewed paper? Do you trust them? Do the arguments it makes make sense to you?

There are certainly some interesting social phenomenon here. My take on it is that a lot of the objections to relativity come from people who don't understand it. I'm not quite sure why the objectors feel such a strong need to talk about things they apparently (in my opinion at least) don't understand. I can also add that I believe that the relativity of simultaneity is the usual stumbling block for understanding special relativity.

But if you're listening to the rest of what I said, you should realize that I'm just another voice on the WWW. Though naturally I believe that what I write is true - except when I make an unfortunate error. (It happens). In the end, you really need to learn about what sources are credible, and what sources are not, and why. Experience may help.

13. Jan 1, 2018

### Staff: Mentor

That is more of a question of the changing usage of the word “mass”. As you probably already know, the meaning of a word varies over time.

There are several formulas in relativity where you find the expression $m\sqrt{1-v^2/c^2}$ in the place where you would expect to find $m$ from Newtonian physics. So in the early days of relativity people thought that it might be convenient to refer to this expression as “mass”.

However, there remain many places where $m$ appears without the factor $\sqrt{1-v^2/c^2}$, or with several of them, so calling it “mass” doesn’t help that much overall and the practice has fallen out of favor.

So where Einstein said $m\sqrt{1-v^2/c^2}$ he has been proven correct, regardless of whether he called it “mass” or not. Today we don’t call it “mass”, but that doesn’t make early authors wrong when they did. The usage of the word simply changed between then and now.

Last edited: Jan 1, 2018
14. Jan 1, 2018

### Staff: Mentor

I'm a particle physicist: We see the effects of special relativity directly in the accelerators and detectors all the time. None of these machines would work if any of the predictions of special relativity would be wrong. But it is much more than that. All of modern physics is based on special relativity. General relativity is a generalization that includes gravity, and quantum field theory is fully based on special relativity. Without special relativity, fundamental physics of the last 100 years wouldn't exist. None of the measurements that confirm general relativity and QFT over and over again would make sense. We couldn't have built atomic clocks, NMR scans wouldn't have a theoretical explanation, we wouldn't have an idea where antimatter came from, could not explain PET scans, wouldn't have an idea why radar works, couldn't calculate the Doppler effect seen in wireless communication, and so on. Special relativity is used everywhere.

One of my favorite examples of precision measurements: There is a property called Lande-factor of the electron, and it can be both predicted and measured with extraordinary accuracy using quantum field theory. The theoretical prediction for its value is 2.002,319,304,363 where the last digit has an uncertainty of 2 (simplified: it should be in the range of 1 to 5, larger deviations are unlikely). The experimental measurement is 2.002,319,304,361,4 where the last digit has an uncertainty of 6.
The first 12 digits agree, and the deviation at the 13th (!) digit is of the size of the uncertainties of the prediction.

15. Jan 1, 2018

### Staff: Mentor

You might want to read over this summary.

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

Occasionally crackpots will come up with an alternative that explains one or two experiments, but any alternative must explain all of these.

Note that there are continuing careful scientific experiments to detect ever smaller and smaller deviations from SR. A deviation in one of the Standard Model Extension parameters would give us a great clue about the direction that future theories need to take. None has been detected to date

16. Jan 2, 2018

### Staff: Mentor

Science Adviser (yes he is one here - just does not post a lot) John Baez wrote an excellent article of relativistic mass etc:
http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

Well worth a read - it gets the concepts right.

Regarding the truth of SR we have derivations these days its very hard to ague with eg:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Of course in science correspondence with experiment is everything - and it has stood up to every test. However a derivation like the above shows if its wrong something rather drastic is incorrect with our understanding of things. It may be - but it would be a breakthrough in a sense if it happened - as Dale said.

Thanks
Bill

17. Jan 2, 2018

### Staff: Mentor

I wouldn't say that. It has been tested to a high degree of accuracy, and we have derivations its very hard to argue with. I would say - as Issac Asimov said - extraordinary claims require extraordinary evidence. Although remembered as a science fiction writer he was also a professor of Biochemistry so knew his science.

Thanks
Bill

Last edited: Jan 2, 2018
18. Jan 2, 2018

### vanhees71

There's a very clear statement by Einstein in a private letter to use the invariant mass only since the concept of relativistic mass leads to confusion. The issue is very clearly analyzed in

L. B. Okun, The Concept of Mass, Physics Today 06/1989
https://doi.org/10.1063/1.881171

19. Jan 2, 2018