# Travelling at speed of light and looking in the mirror

1. May 18, 2010

### seto6

suppose if you can travel at the speed of light. and you travel at the speed of light and look in the mirror can you see yourself in the mirror?

people say you can but no one bothered to give me a explanation or they are just wrong

i have no clue

2. May 18, 2010

### Staff: Mentor

You can't travel at the speed of light.

Now, if you mean that you are travelling at .999999 c then the answer depends on how fast the mirror and the light source are travelling.

3. May 19, 2010

### Fredrik

Staff Emeritus
Adding to what DaleSpam said: If the mirror has zero velocity relative to you, you get the same result no matter what your speed is relative to something else (like the house you live in). We don't have to worry about the speed of the "light source" too much, because in this case you are the light source. The light you see in the mirror is light that's been scattered off your clothes and the parts of your body that aren't covered with clothes. If the only light that hits you comes from a lamp that has a very high velocity relative to you, the wavelength of the light might be shifted out of the range that your eyes can detect, but in this case, you won't see your hand in front of you either.

It isn't possible to completely explain why you can see yourself. Questions about how the world works can only be answered by theories*, and every theory is based on a set of assumptions that defines the theory. In this case, the fact that the speed of the light that scatters off you will have speed c relative to you, regardless of your speed relative to anything else, is something that follows almost immediately from those assumptions. There's also some freedom in how those assumptions are chosen, and if we want to, we can have this fact be one of the assumptions. The ultimate justification for it is that the theory makes predictions that are more accurate than the predictions of the theories in which space and time have properties that agree with your intuition.

*) Contrary to popular belief, a theory isn't "a guess that might be true". It's a set of statements that can be used to make predictions about results of experiments. If the predictions are accurate, it's a good theory. If they're not, it's a bad theory. If it doesn't make predictions (like the claim "God exists"), it's not a theory.

4. May 19, 2010

### Phrak

When you look in a mirror and see yourself, light from you goes to the mirror and comes back. If you are going the speed of light, your light going to the mirror is moving just as fast. You, and the light from you will both hit the mirror at the same time for someone watching you collide with the mirror. What do you think you will see?

5. Nov 18, 2010

### nkrazor

you will probably see a still image of yourself from moments before you reached speed of light

6. Nov 18, 2010

### John232

If you where traveling at a constant speed close to the speed of light you would be able to look in a mirror and appear normal just as though you where at rest. You would measure the speed of light to still remain the same between you and the mirror.

7. Nov 18, 2010

### Passionflower

You will always see yourself in the mirror regardless of your relative speed with something else. But there do exist extreme cases you cannot see yourself in the mirror when you and the mirror are accelerating.

8. Nov 19, 2010

### bcrowell

Staff Emeritus
This is incorrect. As DaleSpam correctly pointed out, you can't travel at the speed of light.

9. Nov 19, 2010

### nadroj

Einstein stated that light is not relative therefore it is a constant in vacou. So if you and the mirror are moving at the speed of light (neglecting the fact that you cannot reach the speed of light in our universe), but for this situation we can through thought experiment. The light around you is not travelling any faster than c in your frame of reference and also to observers in another frame. So you will see an image of yourself in the mirror because inside your frame of reference everything will be completely normal, time will seem the same to you as well as light. But with the observers time will appear slow, but light will be the same.

Last edited: Nov 19, 2010
10. Nov 20, 2010

### michelcolman

In theory, it's not entirely impossible to travel at the speed of light. The only speed you cannot reach, is the speed of light in vacuum. But who says we're in a vacuum?

11. Nov 20, 2010

### Aaron_Shaw

We do. That's good enough.
And it's not so much that we can't reach the speed of light in a vaccum. More that we cannot reach C. Which just happens to be the same value as that of the speed of light in a vaccum.

12. Nov 20, 2010

### jamesfirst

Einstein postulated that reflection would be shown because he believe Principle of Relativity (Inertial frame cannot be detected without an outside frame of reference).

If you were traveling at speed of light and mirror relative to you also does then you would because if there is no reflection it means Principle of relativity is violated since no reflection would be a form of detecting a motion

13. Nov 21, 2010

### DrGreg

No, any "frame" travelling at the speed of light would not be inertial. People, mirrors, or anything else with mass cannot travel at the speed of light (in vacuum).

14. Nov 23, 2010

### mauriceb

Well no one in the history of mankind has ever travelled the speed of light, so how would anyone really know? see, these are the problems with our physics theories. They are all based upon theories and no credible evidence to support anything.

15. Nov 24, 2010

### Fredrik

Staff Emeritus
You need to stop making false claims here. The forum has rules against that. If you would like to learn something, just ask a question.

16. Nov 24, 2010

### michelcolman

By the same logic, how do we know the earth's core isn't made out of chocolate? After all, nobody's ever been down there, right?

Every experiment that has ever been done with relativity, has confirmed the theory. Time dilation has been confirmed experimentally with actual atomic clocks. If faster than light travel was possible, we could use it to travel back in time, kill our grandfather, etc. That would seem to be a pretty solid argument against it.

Also, the required energy to reach light speed would be infinite. Those equations, too, have been experimentally confirmed. We know exactly how much energy we need to let particles reach a certain speed.

Think about the Large Hadron Collider, for example. They are sending around particles at .99c, .999c, even 0.999999991c. If it was possible to make them go faster than light, don't you think they would have gotten there by now?

17. Nov 24, 2010

### xmavidis

I have a question regarding travelling at the speed of light.

Suppose that we were massless, so we could travel at the speed of light for a stationary observer. What speed we would measure for light in this case in our reference frame? According to the postulates of special relativity our reference frame is equivalent with all others, so we are going to measure the speed of light equal to c.

But, if we are moving with the speed of light how is this possible?

18. Nov 24, 2010

### Fredrik

Staff Emeritus
There's no inertial coordinate system in which a massless particle is at rest, so the "point of view" of a massless particle is undefined. More about "points of view" here.

The postulate you're referring to is a statement about inertial frames. It doesn't say that there's a way to associate an inertial frame with the motion of a massless particle.

An actual measuring device (or a person, or any other object with non-zero mass) can't be accelerated to the speed of light in any inertial frame.

19. Nov 27, 2010

### Bob S

Are you talking about traveling at near the speed of light toward a stationary ideal mirror and seeing your reflection from the mirror? In your reference frame, the mirror is traveling toward you at near the speed of light. This is actually very similar to laser photons (≈visible light) being Compton-backscattered off of a fast approaching electron (relativistic parameters β and γ) approaching at velocity βc. (This experiment has actually been done.) The backscattered photons from the electron (or mirror) have an energy boost (blue shift) = 4γ2. As the electron (or mirror) velocity approaches β → 1, γ → ∞. So the backscattered photon energy has been boosted (blue shifted) by a factor 4γ2 = ∞.

Bob S

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