List of common misconceptions about relativity?

bcrowell

Staff Emeritus
Gold Member
I'm wrapping up 3 weeks on relativity in my freshman calc-based physics course at the community college where I teach. At my final meeting, I want to spend most of the time having the students break up into their lab groups and discuss certain specific misconceptions, questions, and confusions about relativity. Each group will have certain questions that are assigned to them. I'll mostly just walk around the room and listen, maybe giving corrections or help if they're stuck or messing up. At the end, they'll present their answers to the rest of the class. I've been looking over PF's relativity and cosmology FAQ lists, etc., for possible questions to assign, and have come up with the following that seemed to be reasonable ones to expect students at this level to be able to attack:

How can light have momentum if it has zero mass?
Accelerate a baseball to ultrarelativistic speeds. Does it become a black hole?
What does the universe look like in a frame of reference moving at c?
Where did the Big Bang happen?

I would like to at least double the length of this list. Does anyone have any others to add? I'm hoping that such a list would be helpful to other people besides me.

To give an idea of the level of questions that might work, here's what we've done this semester: -- My students started by reading Gardner's Relativity Simply Explained, which covers both SR and GR at a popularized level. I just assigned them to read one chapter for each meeting, and quizzed them to make sure they did the reading, but we didn't discuss any of it in class. After that, I did SR from scratch at a slightly more mathematical level, using the pedagogical approach typified by Takeuchi and Mermin, i.e., we don't write down a lot of equations, but mostly just draw spacetime diagrams and represent Lorentz transformations as distortions of the Cartesian graph-paper grid overlaid on the space of events. The book is online here: http://lightandmatter.com/area1sn.html (ch. 7). What they know about dynamics is basically the equations p=mγv and E=mγ (in natural units), plus whatever they've soaked up informally from Gardner. I have also introduced them briefly to the energy-momentum four-vector and the relation m2=E2-p2. They know Maxwell's equations and have been informally introduced to electromagnetic waves, but we haven't actually gone into EM waves as solutions of Maxwell's equations in detail. They have been introduced briefly to the light cone, but not the spacetime interval.

Thanks in advance for any suggestions!

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Austin0

How can light have momentum if it has zero mass?
Accelerate a baseball to ultrarelativistic speeds. Does it become a black hole?
What does the universe look like in a frame of reference moving at c?
Where did the Big Bang happen?

Thanks in advance for any suggestions!
How about " How can a box of light have weight on a scale if a photon has no weight or mass?"

PAllen

Has the course work beaten twins to death? (is this possible?)

Pushing on a rigid rod to break light speed limit?

Some variant of barn pole?

Twin's age differs when back together, but what about distance traveled?

pervect

Staff Emeritus
The relativistic rotating disk (Ehrenfest paradox) causes immense confusion, but I'm not sure the class has been prepared for it.

Something about relativistic velocity addition - "If two obsevers are a distance d apart in the lab frame, and one has a velocity of c/2 in the x direction, and one has a velocity of c/2 in the -x direction"

At what time in the lab frame do they meet?
What is their relative velocity (closing velocity?) in the lab frame?
What is the relative velocity in the object's frame?

Dale

Mentor
Aren't relativistic effects just optical illusions?

bcrowell

Staff Emeritus
Gold Member
Lots of good suggestions here -- thanks! We've already done a whole procession of SR paradoxes: twins, rigid pole, Bell's, Ehrenfest, and submarine.

robphy

Homework Helper
Gold Member
I feel that one misconception is that "relativity involves high-speed motion".
Certainly, there are (for example) time-dilation effects even at slow speeds... which you could observe if you use high-precision wristwatches [as one does with the GPS].

Mordred

One that I once had a problem with

If the area within the event horizon is seen from the outside observer as infinite density and zero time how does it have a temperature and grow or change?

Not sure how useful this will be to you I myself found it handy to understand relativity

Dmitry67

Two misconceptions:
1. Only extremely dense objects can become black holes
2. As nothing moves faster than light, and Big Bang happened 13.75 billion years ago, the radius of the visible universe is exactly 13.75 billion light years.

ShayanJ

Gold Member
Are time dilation,length contraction and mass increase in SR,real?
I guess this one is good because it makes the students familiar with the meaning of ”real” in physics and maybe the symmetry between inertial frames

ZikZak

When YOU travel at nearly c, YOUR time slows down.

A lot of students never rid themselves of the absolute rest idea, and still want to assign absolute speeds to bodies, including the observer.

bobc2

I was thinking of the same thing ZikZak was. Some students get the idea that if they are moving relative to someone else, then their own time is slowing. Also, the idea that if they are hovering near a black hole, they see their own clock slowing.

You are to be highly commended for your work with the students in your physics department, bcrowell. And your students are quite fortunate to have someone of your knowledge and expertise as their prof. It is obvious you love teaching, and what a great subject to teach!

And by the way, we are fortunate to have you bringing your no-nonsense knowledge of physics to this forum.

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haushofer

A common misconception I've encountered more than once is that SR can only deal with inertial observers, and that for accelerating observers one needs GR. But to actually make homework problems with accelerating observers perhaps is a bit too much if I read the level in your OP.

The common misconception about "time slows down if you move" is also nice to threat, indeed. Two last suggestions (perhaps they have already been mentioned) is how a black hole can form if it takes an infinite time for an outside observer to see his dear friend falling into the black hole, or how a universe can be infinitely large but not infinitely old.

nitsuj

When YOU travel at nearly c, YOUR time slows down.

A lot of students never rid themselves of the absolute rest idea, and still want to assign absolute speeds to bodies, including the observer.
travel at nearly c is relative to something, the same can be said for time/length.

How is that a misconception. The only issue I see is semantics with regard to "YOUR time". I can only assume your interpretation is proper time. And I assume the student(s) is referring to the comparative result...which is their time is slower. (the traveler already determined the rest frame (0,0)from which they measure themselves to be moving at nearly c,)

While I am sure the idea of no absolute rest is tricky, I think you are underestimating the understanding " A lot of students" have with regard to a rest frame.

To say it different, a lot of students may not use the terminology properly. With something like physics discussions terminology is important of course, but I think it is unrelated to understanding the mechanics.

jedishrfu

Mentor
How about the actual observation of length contraction appearing more as a rotation? ie Penrose-Terrell rotation.

Another area would be how sci-fi shows portray relativity. On Stargate SG-1, they opened a portal to a world being sucked into a blackhole and they couldn't close the portal. The relativistic part was the time dilation effect as you got closer to the portal your voice would drop down in pitch...

People outside the complex said they were there waiting for months whereas people in the command center thought it was only hours. Needless to say the physics was hard to believe that they could easily move from the command center to outside without any struggle against the immense gravity.

But it was still cool to watch.

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andrien

The most common one is the saying that 'all is relative'.

arindamsinha

On a different note, how about 'Relativity was a concept developed only by Einstein?'

Dmitry67

BTW is it true that E=mc^2 was discovered first by Oliver Heaviside?

robphy

Homework Helper
Gold Member
On a different note, how about 'Relativity was a concept developed only by Einstein?'
Yes, indeed... that's a misconception.
A more specific variation of that misconception is that "Einstein came up with the idea of space-time."

Another is that "Einstein was responsible for the atomic bomb."

ShayanJ

Gold Member
BTW is it true that E=mc^2 was discovered first by Oliver Heaviside?
In the book "Feynman lectures on physics",it is said that the formula arised from the concept of electromagnetic mass before einstein proposed it
It can also be found at en.wikipedia.org/wiki/electromagnetic mass

ZikZak

travel at nearly c is relative to something, the same can be said for time/length.

How is that a misconception.
That isn't a misconception, because you happen to know the solution. The misconception is that students of relativity frequently DON'T understand that travel at nearly c is relative to anything, but rather absolute in some sense.

FalseVaccum89

2. As nothing moves faster than light, and Big Bang happened 13.75 billion years ago, the radius of the visible universe is exactly 13.75 billion light years.
That's a good one!

FalseVaccum89

robphy;4140524A said:
more specific variation of that misconception is that "Einstein came up with the idea of space-time."
Quite. His former instructor (the one who called him a 'lazy dog'), Hermann Minkowski came up with the idea. At first, Einstein didn't even like it, but he was eventually convinced of space-time's utility.

grav-universe

If the sun became a black hole, the Earth and planets would be torn apart and pulled in.

As mentioned in another thread, that M-M demonstrates SR, since M-M can be easily explained ballistically, with the source of the apparatus at rest with the receiver, but rather it was originally the aberration of stars that demonstrated that light travels isotropically in a non-ballistic, non-Galilean manner regardless of the motion of the source and receiver.

harrylin

Quite. His former instructor (the one who called him a 'lazy dog'), Hermann Minkowski came up with the idea. At first, Einstein didn't even like it, but he was eventually convinced of space-time's utility.
In fact, it was Poincare who came up with the invariant space-time interval in 1905.

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