List of common misconceptions about relativity?

In summary: A twin paradox with acceleration, e.g. constantly accelerating rocket ships- The constancy of the speed of light and what happens if you turn on a flashlight while runningIn summary, the teacher plans to have the students discuss specific misconceptions, questions, and confusions about relativity in lab groups. The questions assigned to each group include topics such as light momentum, black holes, the universe in a frame of reference moving at c, and the location of the Big Bang. The teacher is also looking for suggestions to add to the list of questions. The students have been reading a popularized book on relativity and have been introduced to concepts such as dynamics and the energy-momentum four-vector. Common misconceptions mentioned include the belief that
  • #1
bcrowell
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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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  • #2
bcrowell said:
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?"
 
  • #3
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?
 
  • #4
The relativistic rotating disk (Ehrenfest paradox) causes immense confusion, but I'm not sure the class has been prepared for it.

Bell's paradox might work?

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?
 
  • #5
Aren't relativistic effects just optical illusions?
 
  • #6
Lots of good suggestions here -- thanks! We've already done a whole procession of SR paradoxes: twins, rigid pole, Bell's, Ehrenfest, and submarine.
 
  • #7
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].
 
  • #8
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
 
  • #9
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.
 
  • #10
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
 
  • #11
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.
 
  • #12
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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  • #13
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.
 
  • #14
ZikZak said:
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.
 
  • #15
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 black hole 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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  • #16
The most common one is the saying that 'all is relative'.
 
  • #17
On a different note, how about 'Relativity was a concept developed only by Einstein?'
 
  • #18
BTW is it true that E=mc^2 was discovered first by Oliver Heaviside?
 
  • #19
arindamsinha said:
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."
 
  • #20
Dmitry67 said:
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
 
  • #21
nitsuj said:
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.
 
  • #22
Dmitry67 said:
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!
 
  • #23
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.
 
  • #24
How about

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.
 
  • #25
FalseVaccum89 said:
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. :wink:
 
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  • #26
You are going to use student group discussions leading to their presentation...

The concept problems will involve the most students (with their varying experience) if the issues are simple to grasp, foundational to relativity, and yet tough enough to challenge them...

Is there absolute motion?
If two objects are in relative motion with respect to each other, can't we conclude that at least one of them must be in some kind of absolute, true, or real motion?
Won't this hold true for any and all possible frames of reference?

Is relativity "real"?
Is relativity a theory of measurement, or a theory of existence?
Is it just a way to transform data from a distant/fast source into what makes sense to us locally, or does it actually describe the distant/fast situation?

How does gravity get out of a black hole?
How is there gravitational attraction outside of the black hole?
If it comes from inside the event horizon, how does it get out?
If from outside the event horizon, where is the source mass?
Does gravitational attraction pass through a black hole from one side to the other, or is it blocked?

Why aren't photons allowed to carry flashlights?
Why is there no inertial frame of reference for c?
Why don't velocities add up like ordinary numbers in simple addition?
 
  • #27
Is aberration classical or relativistic effect?
 
  • #28
zonde said:
Is aberration classical or relativistic effect?

Both. It has a derivation according to:

(1) corpuscular theory
(2) moving aether theory
(3) relativistic derivation

They don't agree exactly, but (last I knew) the differences were actually too small to detect.
 
  • #29
For a myth, how about "SR has proven the aether does not exist." I'm not advocating for aether, but there's a big difference between proving that something cannot exist in a theory versus merely acknowledging that it isn't necessary in that theory.
 
  • #30
PAllen said:
Both.
Right. It's just that aberration is usually explained classically but for me it was quite enlightening to find out how it can be worked out in relativity.
 
  • #31
rjbeery said:
For a myth, how about "SR has proven the aether does not exist." I'm not advocating for aether, but there's a big difference between proving that something cannot exist in a theory versus merely acknowledging that it isn't necessary in that theory.
As I see this is rather complicated question. Obviously it's a myth. But at the time people where probably hung on the idea of finding preferred frame. So such a myth would ban useless discussions.
But nowadays I would say that such a myth promotes kind of magical thinking. That's because dimensional modelling is very powerful tool for consistency checking but that myth kind of prevents using it. But that's my viewpoint.
 
  • #32
And another thing that I considered quite interesting (somewhat related to relativistic interpretation of aberration). This is actually a paradox kind of thing but it of course is related to some misconception (if you have it).

Let's say we have situation like this:
2vcdl3k.jpg

Observers "1" and "2" are observing box with an "A" printed on it's side. We view this situation in rest frame of the box and first observer. Second observer is in motion in that frame. Neither observer can see the "A" on the side of the box when both observers are side by side.

Now let's change to rest frame of second observer:
mcgoxw.jpg

The box and first observer is in motion in that frame. Because the box is in motion and light doesn't get to observer instantly second observer see the box in it's past position. So should the "A" be visible for him?
 

1. What is relativity and why is it important?

Relativity is a theory proposed by Albert Einstein in the early 20th century that explains the relationship between space and time. It is important because it revolutionized our understanding of the universe and has been confirmed by numerous experiments and observations.

2. Is the theory of relativity just a theory?

While the word "theory" may suggest uncertainty, the theory of relativity is a well-established and extensively tested scientific framework. It has been confirmed by countless experiments and is widely accepted by the scientific community.

3. What is the difference between special and general relativity?

Special relativity deals with the relationship between space and time in the absence of gravity, while general relativity includes the effects of gravity. Special relativity is based on the principle of the constancy of the speed of light, while general relativity introduces the concept of curved spacetime.

4. Can the theory of relativity be proven wrong?

Like all scientific theories, the theory of relativity is subject to revision and refinement as new evidence and technologies become available. However, it has been extensively tested and confirmed by numerous experiments and observations, making it highly unlikely to be proven wrong.

5. What are some common misconceptions about relativity?

Some common misconceptions about relativity include the idea that it only applies to large objects or that it can explain everything about the universe. Another misconception is that it is too complex for the average person to understand. In reality, relativity has been shown to apply to all objects, and while it is a complex theory, its basic principles can be understood by anyone with a basic understanding of physics.

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