# Light flash from spaceship

1. Nov 1, 2012

### Saxby

If i was standing on the earth at rest and a spaceship travelling at 0.99c passed by and flashed a light for a very small amount of time would this time appear longer for me or the pilot of the spaceship???

2. Nov 1, 2012

### phinds

You

See www.phinds.com/time dilation/ [Broken]

Last edited by a moderator: May 6, 2017
3. Nov 1, 2012

### Staff: Mentor

What phinds said.
Also, an exercise that you might find helpful: Take a look at 71GA's diagram in post #4 of
this thread, which shows how relative velocity affects lengths , try drawing a similar diagram showing the influence on elapsed time.

4. Nov 1, 2012

### ghwellsjr

It depends on when exactly the flash occurs.

If a flash occurs as the spaceship is approaching the earth, the light will be blue shifted to a frequency that is about 14 times what it was sent at and would last for 1/14th of the time.

We'll assume you have special goggles or equipment to detect it.

If a flash occurs as the spaceship is departing away from the earth, the light will be red shifted to a frequency that is about 1/14th of what it was sent at and would last for 14 times longer.

Clearly, there is a point at which the flash could occur where it would last just as long for you as for the pilot, somewhere just overhead. But even then, the first part of it will be blue shifted and the latter part of it will be red shifted.

By the way, the previous answers are talking about time dilation which is not something that can be seen, only calculated, depending on the arbitrary frame of reference that you select.

5. Nov 1, 2012

### Jeronimus

Once again you confuse what people see with what is actually the case physically.

According to you, if an observer on earth has his friend 1km away also at rest on earth, and the rocket was just between them when the flash happens, one of them would measure a higher time-interval while the other would measure a shorter time interval.
This confusion occurs because of the difference between SEEING something because of the way photons travel towards us at a limited speed and CONCLUDING when and where something happened (x/t) because you know physics.

Let's say E1(x1/t1) / E2(x2/t2) start/end of flash. The flash lasted Δt = t2-t1 in the rocket frame.
In the frame the observer on earth is at rest, those events will happen at

E1'(x1'/t1') E2(x2'/t2'). Δt' = t2' - t1' = Δt * γ

the well known formula, see here http://en.wikipedia.org/wiki/Time_dilation

Both observers on earth will CONCLUDE, which is not the same as SEE locally (without doing the maths to make up for the rocket motion and photon traveling speed), that the time interval between those two events is LONGER on earth by the SAME amount, no matter if they stand in front, behind or right at where the rocket passes.

Indeed, if the two observers did not know much about photons and were only to rely on their local observation without knowing physics, they would SEE different time intervals for the flashes, depending on if they stand in front or behind, just like you hear a different sound of a racing car incoming vs moving away. Did the motor start making different sounds all of the sudden?

Last edited: Nov 1, 2012
6. Nov 1, 2012

### PAllen

What people see or measure (seeing is a specific form of measuring) is all that is certain.

What people compute/conclude/interpret for predictive or explanatory purposes can never be said to be what is "actually the case physically". Nobody knows, or will ever know, what is "actually the case physically".

Also, note the OP was not ambiguous. It asked appear, not interpret/compute/conclude.

Note that Gwellsjr did not dispute the using an inertial frame where the earth was stationary, you would compute time dilation as other posts indicated.

Last edited: Nov 1, 2012
7. Nov 1, 2012

### PAllen

If I asked you 'what would you hear?', what would you say?

The OP used the word appear.

8. Nov 1, 2012

### ghwellsjr

Thanks, PAllen, I couldn't have said it better myself.

9. Nov 1, 2012

### Jeronimus

I assumed that when OP asked that question, he wanted to know something about relativity, this being the section of the forums, not about Doppler effects which would occur even if space was absolute.

So how would someone ignorant to SR possibly learn anything more than about doppler effects which happen in absolute space just as well, by reading ghwellsjr post?

Or are we to take questions asked by people trying to learn relativity just as an opportunity to stimulate our own ego by delivering the "right" answer?

10. Nov 1, 2012

### PAllen

Relativistic Doppler is different from classical doppler. In fact, one of the way to derive the time dilation is to factor out light speed and classical Doppler. You are left with time dilation.

I think a key aspect of relativity is that clock rates of clocks at different positions and relative speeds; and simultaneity; cannot be spoken of in any absolute way. Thinking of time dilation as something real that happens to clocks leads to, at best, preferred frame interpretations of the equations of SR. This is a possible philosophy (that can lead to correct physical predictions), but it is certainly not Einstein's approach.

[Note: I am not implying you hold such a philosophy ]

11. Nov 2, 2012

### ghwellsjr

I also assumed that the OP's question was about relativity and my specific answer would have been wrong "if space was absolute", as you put it.